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Why is flight so important to birds?
One of the defining traits of many birds is the ability to fly. Obviously, flight is a major evolutionary advantage. But together with the ability to fly must come a number of structural modifications. What do you think these might be?
Structure and Function in Birds
Birds are endothermic tetrapod vertebrates. They are bipedal, which means they walk on two legs. Birds also lay amniotic eggs with hard, calcium carbonate shells. Although birds are the most recent class of vertebrates to evolve, they are now the most numerous vertebrates on Earth. Why have birds been so successful? What traits allowed them to increase and diversify so rapidly? Birds can vary considerably in size, as you can see from the world’s smallest and largest birds, pictured in Figure below. The tiny bee hummingbird is just 5 centimeters (2 inches) long, whereas the ostrich towers over people at a height of 2.7 meters (9 feet). All modern birds have wings, feathers, and beaks. They have a number of other unique traits as well, most of which are adaptations for flight. Flight is used by birds as a means of locomotion in order to find food and mates and to avoid predators. Although not all modern birds can fly, they all evolved from ancestors that could.
Range of Body Size in Birds. The bee hummingbird is the smallest bird. The ostrich is the largest.
Wings and Feathers
Wings are an obvious adaptation for flight. They are actually modified front legs. Birds move their wings using muscles in the chest. These muscles are quite large, making up as much as 35 percent of a bird’s body weight.
Feathers help birds fly and also provide insulation and serve other purposes. Birds actually have two basic types of feathers: flight feathers and down feathers. Both are shown in Figure below. Flight feathers are long, stiff and waterproof. They provide lift and air resistance without adding weight. Down feathers are short and fluffy. They trap air next to a bird’s skin for insulation.
Types of Bird Feathers. These two types of bird feathers have different uses. How is each feather’s structure related to its function?
Organ Systems Adapted for Flight
Birds need a light-weight body in order to stay aloft. Even so, flying is hard work, and flight muscles need a constant supply of oxygen- and nutrient-rich blood. The organ systems of birds are adapted to meet these needs.
- Birds have light-weight bones that are filled with air. They also lack a jaw, which in many vertebrates is a dense, heavy bone with many teeth. Instead, birds have a light-weight keratin beak without teeth.
- Birds have air sacs that store inhaled air and push it into the lungs like bellows. This keeps the lungs constantly filled with oxygenated air. The lungs also contain millions of tiny passages that create a very large surface area for gas exchange with the blood (see Figure below).
- Birds have a relatively large, four-chambered heart. The heart beats rapidly to keep oxygenated blood flowing to muscles and other tissues. Hummingbirds have the fastest heart rate at up to 1,200 beats per minute. That’s almost 20 times faster than the human resting heart rate!
- Birds have a sac-like structure called a crop to store and moisten food that is waiting to be digested. They also have an organ called a gizzard that contains swallowed stones. The stones make up for the lack of teeth by grinding food, which can then be digested more quickly. Both structures make it easier for the digestive system to produce a steady supply of nutrients from food.
Organ System Adaptations for Flight. The intricate passageways in a bird’s lung are adapted for efficient gas exchange. Find the crop and gizzard in the digestive tract diagram. What are their functions? Bird Lung (left), Bird Digestive Tract (right)
Nervous System and Sense Organs
Birds have a large brain relative to the size of their body. Not surprisingly, the part of the brain that controls flight is the most developed part. The large brain size of birds is also reflected by their high level of intelligence and complex behavior. In fact, birds such as crows and ravens may be more intelligent than many mammals. They are smart enough to use objects such as twigs for tools. They also demonstrate planning and cooperation. Most birds have a poor sense of smell, but they make up for it with their excellent sense of sight. Predatory birds have especially good eyesight. Hawks, for example, have vision that is eight times sharper than human vision.
- Birds are endothermic tetrapod vertebrates. They are bipedal and have wings and feathers.
- Bird organ systems are adapted for flight. For example, they have light-weight air-filled bones and a large four-chambered heart.
- Birds also have relatively large brains and a high level of intelligence.
- Why do birds fly?
- List two functions of feathers in birds.
- Describe the bird crop and gizzard. What are their functions?
- How do birds keep their lungs filled with oxygenated air?
- Give an example of bird behavior that shows their relatively great intelligence.
Excretory System of Birds: Structure and Elements
He Excretory system of birds Is composed of kidneys, ureters and cloaca. All three are responsible for eliminating waste from the blood of these animals.
The kidneys are responsible for filtering the nitrogen and uric acid residues from the blood. These are sent through the ureters to one of the cloacal chambers, since they are expelled to the outside (EncyclopediaBritannica, 2013).
Excretory system of a bird. Photo retrieved from: people.eku.edu
If one of these three organs fails, the bird dies quickly because of blood poisoning by high levels of urea (Melissa Belliwski, 2017).
The main functions of the bird excretory system are: to maintain the electrolyte balance, to maintain the water balance and to eliminate residues of the metabolic process, in particular nitrogenous products such as uric acid.
Photosynthetic complex I is a key element in photosynthetic electron transport, but little has been known about it so far.
An international team of researchers has solved the structure and elucidated the function of photosynthetic complex I. This membrane protein complex plays a major role in dynamically rewiring photosynthesis. The team from the Max Planck Institute for Biochemistry, Osaka University and Ruhr-Universität Bochum together with their collaboration partners report the work in the journal “Science”, published online on 20 December 2018. “The results close one of the last major gaps in our understanding of photosynthetic electron transport pathways,” says Associate Professor Dr. Marc Nowaczyk, who heads the Bochum project group “Cyanobacterial Membrane Protein Complexes”.
Biology’s electrical circuits
Complex I is found in most living organisms. In plant cells it is used in two places: one is in mitochondria, the cell’s power plants, the other is in chloroplasts, where photosynthesis occurs. In both instances, it forms part of an electron transport chain, which can be thought of as biology’s electrical circuit. These are used to drive the cells molecular machines responsible for energy production and storage. The structure and function of mitochondrial complex I as part of cellular respiration has been well investigated, whereas photosynthetic complex I has been little studied so far.
Using cryoelectron microscopy, the researchers were able to solve for the first time the molecular structure of photosynthetic complex I. They showed that it differs considerably from its respiratory relative. In particular, the part responsible for electron transport has a different structure, since it is optimised for cyclic electron transport in photosynthesis.
Cyclic electron transport represents a molecular short circuit in which electrons are reinjected into the photosynthetic electron transport chain instead of being stored. Marc Nowaczyk explains: “The molecular details of this process have been unknown and additional factors have not yet been unequivocally identified.” The research team simulated the process in a test tube and showed that the protein ferredoxin plays a major role. Using spectroscopic methods, the scientists also demonstrated that the electron transport between ferredoxin and complex I is highly efficient.
Molecular fishing rod
In the next step, the group analysed at the molecular level which structural elements are responsible for the efficient interaction of complex I and ferredoxin. Further spectroscopic measurements showed that complex I has a particularly flexible part in its structure, which captures the protein ferredoxin like a fishing rod. This allows ferredoxin to reach the optimal binding position for electron transfer.
“This enabled us to bring the structure together with the function of the photosynthetic complex I and gain a detailed insight into the molecular basis of electron transport processes,” summarises Marc Nowaczyk. “In the future, we plan to use this knowledge to create artificial electron transport chains that will enable new applications in the field of synthetic biology.”
12.21: Bird Structure and Function - Biology
This page has been translated into Belorussian by Paul Bukhovko and is available at www.movavi.com/opensource/birdrespiration-be
This page has been translated into Swedish by Johanne Teerink and is available at
The avian respiratory system delivers oxygen from the air to the tissues and also removes carbon dioxide. In addition, the respiratory system plays an important role in thermoregulation (maintaining normal body temperature). The avian respiratory system is different from that of other vertebrates, with birds having relatively small lungs plus nine air sacs that play an important role in respiration (but are not directly involved in the exchange of gases).
|( A). Dorsal view of the trachea (circled) and the lung of the Ostrich (Struthio camelus). The lungs are deeply entrenched into the ribs on the dorsolateral aspects (arrowhead). Filled circle is on the right primary bronchus. Note that the right primary bronchus is relatively longer, rather horizontal and relatively narrower than the left primary bronchus. Scale bar, 1 cm. (B) Close up of the dorsal aspect of the lung showing the deep costal sulci (s). Trachea indicated by an open circle filled circle = right primary bronchus. Scale bar, 2 cm (Maina and Nathaniel 2001).|
Avian respiratory system
(hd = humeral diverticulum of the clavicular air sac adapted from Sereno et al. 2008)
The air sacs permit a unidirectional flow of air through the lungs. Unidirectional flow means that air moving through bird lungs is largely 'fresh' air & has a higher oxygen content. In contrast, air flow is 'bidirectional' in mammals, moving back and forth into and out of the lungs. As a result, air coming into a mammal's lungs is mixed with 'old' air (air that has been in the lungs for a while) & this 'mixed air' has less oxygen. So, in bird lungs, more oxygen is available to diffuse into the blood (avian respiratory system).
Pulmonary air-sac system of a Common Teal (Anas crecca). a. Latex injection (blue) highlighting the location of air sacs.
b, Main components of the avian flow-through system. Abd, abdominal aire sac Cdth, caudal thoracic air sac Cl, clavicular
air sac Crth, cranial thoracic air sac Cv, cervical air sac Fu, furcula Hu, humerus Lu, lung Lvd, lateral vertebral diverticula
Pv, pelvis and Tr, trachea (From: O'Connor and Claessens 2005).
The alveolar lungs of mammals (Rhesus monkey A) and parabronchial lungs of birds (pigeon B) are subdivided into large
numbers of extremely small alveoli (A, inset) or air capillaries (radiating from the parabronchi B, inset). The mammalian respiratory
system is partitioned homogeneously, so the functions of ventilation and gas exchange are shared by alveoli and much of the lung volume.
The avian respiratory system is partitioned heterogeneously, so the functions of ventilation and gas exchange are separate in the air sacs
(shaded in gray) and the parabronchial lung, respectively. Air sacs act as bellows to ventilate the tube-like parabronchi (Powell and Hopkins 2004).
Comparison of the avian 'unidirectional' respiratory system (a) where gases are exchanged between the lungs and the blood in the parabronchi, and the bidirectional respiratory system of mammals (b) where gas exchange occurs in small dead-end sacs called alveoli (From: West et al. 2007).
Credit: Zina Deretsky, National Science Foundation
Bird-like respiratory systems in dinosaurs -- A recent analysis showing the presence of a very bird-like pulmonary, or lung, system in predatory dinosaurs provides more evidence of an evolutionary link between dinosaurs and birds. First proposed in the late 19 th century, theories about the animals' relatedness enjoyed brief support but soon fell out of favor. Evidence gathered over the past 30 years has breathed new life into the hypothesis. O'Connor and Claessens (2005) make clear the unique pulmonary system of birds, which has fixed lungs and air sacs that penetrate the skeleton, has an older history than previously realized. It also dispels the theory that predatory dinosaurs had lungs similar to living reptiles, like crocodiles.
The avian pulmonary system uses "flow-through ventilation," relying on a set of nine flexible air sacs that act like bellows to move air through the almost completely rigid lungs. Air sacs do not take part in the actual oxygen exchange, but do greatly enhance its efficiency and allow for the high metabolic rates found in birds. This system also keeps the volume of air in the lung nearly constant. O'Connor says the presence of an extensive pulmonary air sac system with flow-through ventilation of the lung suggests this group of dinosaurs could have maintained a stable and high metabolism, putting them much closer to a warm-blooded existence. "More and more characteristics that once defined birds--feathers, for example--are now known to have been present in dinosaurs, so, many avian features may really be dinosaurian," said O'Connor. A portion of the air sac actually integrates with the skeleton, forming air pockets in otherwise dense bone. The exact function of this skeletal modification is not completely understood, but one explanation theorizes the skeletal air pockets evolved to lighten the bone structure, allowing dinosaurs to walk upright and birds to fly.
Some hollow bones are providing solid new evidence of how birds evolved from dinosaurs.
- one interclavicular sac
- two cervical sacs
- two anterior thoracic sacs
- two posterior thoracic sacs
- two abdominal sacs
Air sacs and axial pneumatization in an extant avian. The body of bird in left lateral view, showing the cervical (C), interclavicular (I), anterior thoracic (AT), posterior thoracic (PT), and abdominal (AB) air sacs. The hatched area shows the volume change during exhalation. The cervical and anterior thoracic vertebrae are pneumatized by diverticula of the cervical air sacs. The posterior thoracic vertebrae and synsacrum are pneumatized by the abdominal air sacs in most taxa. Diverticula of the abdominal air sacs usually invade the vertebral column at several points. Diverticula often unite when they come into contact, producing a system of continuous vertebral airways extending from the third cervical vertebra to the end of the synsacrum. Modified from Duncker 1971 (Wedel 2003).
|Computerized axial tomogram of an awake, spontaneously breathing goose air is darkest. A large percentage of the bird's body is filled with the several air sacs. Upper left: At the level of the shoulder joints (hh, humeral head) is the intraclavicular air sac (ICAS), which extends from the heart cranially to the clavicles (i.e., furcula or wishbone). S, sternum FM, large flight muscles with enclosed air sac diverticula, arrowheads t, trachea. Upper right: At the level of the caudal heart (H) is the paired cranial thoracic air sacs (TAS). Arrowhead points to the medial wall of the air sac (contrast enhanced with aerosolized tantalum powder). The dorsal body cavity is filled with the lungs, which are tightly attached to the dorsal and lateral body wall. V, thoracic vertebrae. Lower left: At the level of the knees (K) is the paired caudal thoracic air sacs (PTAS) and paired abdominal air sacs, with the abdominal viscera (AV) filling the ventral body cavity. The membrane separating the abdominal air sacs from one another (arrowhead) and from the caudal thoracic air sacs (arrows) can be seen. Lower right: At the level of the caudal pelvis, the abdominal air sacs, which extend to the bird's tail, can be seen. Arrow, membrane separating abdominal air sacs (Brown et al. 1997).|
Birds can breathe through the mouth or the nostrils (nares). Air entering these openings (during inspiration) passes through the pharynx & then into the trachea (or windpipe). The trachea is generally as long as the neck. However, some birds, such as cranes, have an exceptionally long (up to 1.5 m) trachea that is coiled within the hollowed keel of the breastbone (shown below). This arrangement may give additional resonance to their loud calls (check this short video of calling Sandhill Cranes).
Sandhill Cranes calling in flight
The typical bird trachea is 2.7 times longer and 1.29 times wider than that of similarly-sized mammals. The net effect is that tracheal resistance to air flow is similar to that in mammals, but the tracheal dead space volume is about 4.5 times larger. Birds compensate for the larger tracheal dead space by having a relatively larger tidal volume and a lower respiratory frequency, approximately one-third that of mammals. These two factors lessen the impact of the larger tracheal dead space volume on ventilation. Thus, minute tracheal ventilation is only about 1.5 to 1.9 times that of mammals (Ludders 2001).
Examples of tracheal loops found in Black Swans (Cygnus atratus), Whooper
Swans (Cygnus cygnus), White Spoonbills (Platalea leucorodia), Helmeted Curassow (Crax pauxi),
and Whooping Cranes (Grus americana).
The trachea bifurcates (or splits) into two primary bronchi at the syrinx. The syrinx is unique to birds & is their 'voicebox' (in mammals, sounds are produced in the larynx). The primary bronchi enter the lungs & are then called mesobronchi. Branching off from the mesobronchi are smaller tubes called dorsobronchi. The dorsobronchi, in turn, lead into the still smaller parabronchi. Parabronchi can be several millimeters long and 0.5 - 2.0 mm in diameter (depending on the size of the bird) (Maina 1989) and their walls contain hundreds of tiny, branching, & anastomosing 'air capillaries' surrounded by a profuse network of blood capillaries (Welty and Baptista 1988). It is within these 'air capillaries' that the exchange of gases (oxygen and carbon dioxide) between the lungs and the blood occurs. After passing through the parabronchi, air moves into the ventrobronchi.
Semi-schematic drawing of the lung-air sac system in situ. The cranial half of the dorsobronchi (4) and the parabronchi (6) has been removed. 1 = trachea, 2 = primary bronchus, 3 = ventrobronchi with the connections into (A) cervical, (B) interclavicular and (C) cranial thoracic air sacs, 5 = laterobronchi and the caudal primary bronchus open into the (D) posterior thoracic and (E) abdominal air sacs (From: Duncker 2004).
Avian respiratory system showing the bronchi located inside the lungs. Dorsobronchi and ventrobronchi branch off of the primary bronchus parabronchi extend from the dorsobronchi to the ventrobronchi. Light blue arrows indicate the direction of air flow through the parabronchi. The primary bronchus continues through the lung and opens into the abdominal air sac. (Source: http://www.ivis.org/advances/Anesthesia_Gleed/ludders2/chapter_frm.asp)
Birds exhibit some variation in lung structure and, specifically, in the arrangement of parabronchi. Most birds have two sets of parabronchi, the paleopulmonic (&lsquoancient lung&rsquo) and neopulmonic (&lsquonew lung&rsquo) parabronchi. However, the neopulmonic region is absent in some birds (e.g., penguins) and poorly developed in others (e.g., storks [Ciconiidae] and ducks [Anatidae]). In songbirds (Passeriformes), pigeons (Columbiformes), and gallinaceous birds (Galliformes), the neopulmonic region of the lung is well-developed (Maina 2008). In these latter groups, the neopulmonic parabronchi contain about 15 to 20% of the gas exchange surface of the lungs (Fedde 1998). Whereas airflow through the paleopulmonic parabronchi is unidirectional, airflow through the neopulmonic parabronchi is bidirectional. Parabronchi can be several millimeters long and 0.5 - 2.0 mm in diameter (depending on the size of the bird) (Maina 1989) and their walls contain hundreds of tiny, branching, and anastomosing air capillaries surrounded by a profuse network of blood capillaries.
Differences among different birds in the development of the neopulmonic region of the lung. (a) Penguin lungs are entirely paleopulmonic.
(b) Some birds, such as ducks, have a relatively small neopulmonic region. (c) Songbirds have a well-developed neopulmonic region.
1, trachea, 2, primary bronchus, 3, ventrobronchus, 4, dorsobronchus, 5, lateral bronchus, 6, paleopulmonic parabronchi,
7, neopulmonic parabronchi A, cervical air sac, B, interclavicular air sac, C, cranial thoracic air sac, D, caudal thoracic air sac,
E, abdominal air sac. The white arrows indicate changes in volume of the air sacs during the respiratory cycle (From: McLelland 1989).
So, how does air flow through the avian lungs & air sacs during respiration?
Air flow through the avian respiratory system during inspiration (a) and expiration (b).
1 - interclavicular air sac, 2 - cranial thoracic air sac, 3 - caudal thoracic air sac, 4 - abdominal air sac
(From: Reese et al. 2006).
A schematic of the avian respiratory system, illustrating the major air sacs and their connections to the lung. (A) The lateral and dorsal direction of motion of the rib cage during exhalation is indicated by arrows. (B) The direction of airflow during inspiration. (C) The direction of flow during expiration (From: Plummer and Goller 2008).
Avian respiratory cycle
This Flash diagram shows the paths that air takes through the respiratory system when a bird breathes.
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- During inspiration:
- the sternum moves forward and downward while the vertebral ribs move cranially to expand the sternal ribs and the thoracoabdominal cavity (see diagrams below). This expands the posterior and anterior air sacs and lowers the pressure, causing air to move into those air sacs.
- Air from the trachea and bronchi moves into the posterior air sacs and, simultaneously,
- air from the lungs moves into the anterior air sacs.
Changes in the position of the thoracic skeleton during breathing in a bird. The solid lines represent
thoracic position at the end of expiration while the dotted lines show the thoracic position
at the end of inspiration (Source: http://www.ivis.org/advances/Anesthesia_Gleed/ludders2/chapter_frm.asp).
Drawing of a bird coelom in transverse section during expiration (gray bones) and inspiration (white bones). Dashed lines illustrate the
horizontal septum that separates the pleural cavity (PC) where the lungs are located from the subpulmonary cavity (SP) where most
of the air sacs are located (except the abdominals that are in the peritoneal cavity), and the oblique septum that separates the air sacs from
the abdominal cavity (AC) and digestive viscera. Both septa insert on the ventral keel of vertebrae. The volume of the pleural cavity changes
very little with respiratory rib movements, but the volume of the subpulmonary cavity (and the air sacs) is greatly increased when the oblique
septum is stretched during inspiration (Adapted from: Klein and Owerkowicz 2006). The increase in volume lowers air pressure and draws air
into the air sacs.
Schematic representation of the lungs and air sacs of a bird and the pathway of
gas flow through the pulmonary system during inspiration and expiration. For purposes of clarity, the neopulmonic lung
is not shown. The intrapulmonary bronchus is also known as the mesobronchus. A - Inspiration. B - Expiration
- During expiration:
- the sternum moves backward and upward & the vertebral ribs move caudally to retract the sternal ribs and reduce the volume of the thoracoabdominal cavity. The reduces the volume of the anterior & posterior air sacs, causing air to move out of those sacs.
- Air from the posterior sacs moves into the lungs &, simultaneously,
- air from the anterior sacs moves into the trachea & out of the body.
So, air always moves unidirectionally through the lungs and, as a result, is higher in oxygen content than, for example, air in the alveoli of humans and other mammals.
Variation in length of uncinate processes -- Birds with different forms of locomotion exhibit morphological differences in their rib cages: (A) terrestrial (walking) species, Cassowary (Casuaris casuaris) (B) a typical flying bird, Eagle Owl (Bubo bubo) and (C) an aquatic, diving species, Razorbill (Alca torda). Uncinate processes are shorter in walking species, of intermediate length in typical birds, and relatively long in diving species (scale bar, 5 cm). Muscles attached to uncinate processes (appendicocostales muscles) help rotate the ribs forwards, pushing the sternum down and inflating the air sacs during inspiration. Another muscle (external oblique) attached to uncinate processes pulls the ribs backward, moving the sternum upward during expiration. The longer uncinate processes of diving birds are probably related to the greater length of the sternum and the lower angle of the ribs to the backbone and sternum. The insertion of the appendicocostales muscles near the end of the uncinate processes may provide a mechanical advantage for moving the elongated ribs during breathing (Tickle et al. 2007).
Ward presented his ideas at the 2003 annual meeting of the American Geological Society in Seattle. See: http://www.nature.com/nsu/031103/031103-7.html
In the avian lung, oxygen diffuses (by simple diffusion) from the air capillaries into the blood & carbon dioxide from the blood into the air capillaries (shown in this figure and in figures below ). This exchange is very efficient in birds for a number of reasons. First, the complex arrangement of blood and air capillaries in the avian lung creates a substantial surface area through which gases can diffuse. The surface area available for exchange (SAE) varies with bird size. For example, the ASE is about 0.17 m 2 for House Sparrows (about 30 gms Passer domesticus), 0.9 m 2 for Rock Pigeons (about 350 gms Columba livia), 3.0 m 2 for a Mallard (about 1150 gms Anas platyrhynchos), and 8.9 m 2 for a male Graylag Goose (about 3.7 kg Anser anser) (Maina 2008). However, smaller birds have a greater SAE per unit mass than do larger birds. For example, the SAE is about 90 cm 2/gm for Violet-eared Hummingbirds (Colibri coruscans Dubach 1981), about 26 cm 2/gm for Mallards, and about 5.4 cm 2/gm for Emus (Dromaius novaehollandiae Maina and King 1989). Among mammals, there is also a negative relationship between SAE and body size, with smaller mammals like shrews having a greater SAE per unit mass than larger mammals. However, for birds and mammals of similar size, the SAE of birds is generally about 15% greater (Maina et al. 1989).
A second reason why gas exchange in avian lungs is so efficient is that the blood-gas barrier through which gases diffuse is extremely thin. This is important because the amount of gas diffusing across this barrier is inversely proportional to its thickness. Among terrestrial vertebrates, the blood-gas barrier is thinnest in birds. Natural selection has favored thinner blood-gas barriers in birds and mammals because endotherms use oxygen at higher rates than ectotherms like amphibians and reptiles. Among birds, the thickness of the blood-gas barrier varies, with smaller birds generally having thinner blood-gas barriers than larger birds. For example, the blood-gas barrier is 0.099 &mum thick in Violet-eared Hummingbirds and 0.56 &mum thick in Ostriches (West 2009).
Comparison of the mean thickness of the blood-gas barrier of 34 species of birds, 37 species of mammals,
16 species of reptiles, and 10 species of amphibians revealed that birds had significantly thinner blood-gas
barriers than the other taxa (West 2009).
Also contributing to the efficiency of gas exchange in avian lungs is a process called cross-current exchange. Air passing through air capillaries and blood moving through blood capillaries generally travel at right angles to each other in what is called cross-current flow (Figure below Makanya and Djonov 2009). As a result, oxygen diffuses from the air capillaries into the blood at many points along the length of the parabronchi, resulting in a greater concentration of oxygen (i.e., higher partial pressures) in the blood leaving the lungs than is possible in the alveolar lungs of mammals (Figures below).
(A) Micrograph of lung tissue from a Brown Honeyeater (Lichmera indistincta) showing (a) parabronchi, (b) blood vessel, and (c) exchange tissue (bar, 200 micrometers). (B) Electron micrograph from the lung of a Welcome Swallow (Hirundo neoxena) showing (a) blood-air barrier, (b) air capillary, (c) blood capillary, and (d) red blood cell in the blood capillary (bar, 2 micrometers). (From: Vitali and Richardson 1998).
A) Medial view of the lung of a domestic chicken (Gallus gallus domesticus). p, primary bronchus v, ventrobronchus d, dorsobronchus r, parabronchi. Scale bar, 1 cm. (B) An intraparabronchial artery (i) giving rise to blood capillaries (c) in the lung of an Emu (Dromiceus novaehollandiae). a, air capillaries. Scale bar, 15 &mum. (C) Air capillaries closely associated with blood capillaries (arrows) in a chicken lung. Scale bar, 10 &mum. (D) Blood capillaries (c) closely associated with air capillaries (spaces) in a chicken lung. Scale bar, 12 &mum. (From: Maina 2002).
An individual air capillary (AC) surrounded by a dense network of blood
capillaries (asterisk) in a chicken lung. The blood capillaries drain into a
larger vein (V6) adjacent to an infundibulum (IF). Note that the general direction
of blood flow through the blood capillaries is perpendicular to the flow of air through
the air capillaries, i.e., cross-current flow (From: Makanya and Djonov 2009).
In birds, the thickness of the blood-gas barrier in the 7.3-g Violet-eared Hummingbird ( Colibri coruscans ) is 0.099 µm, whereas that of an immature 40-kg Ostrich ( Struthio camelus ) is 0.56 µm (Maina and West 2005).
Relationship between the harmonic mean thickness of the blood-gas barrier (the thickness of the barrier that affects the diffusion of oxygen from air capillaries into blood capillaries) against body mass in the lungs of bats, birds, and non-flying mammals. Birds have particularly thinner barriers than bats and non-flying mammals
Light micrographs of a portion of the lung of a chicken (A) and rabbit (B).
Note the small diameter of the air capillaries in the chicken lung vs. that of the rabbit alveoli (same magnification).
(A) In the chicken lung, pulmonary capillaries are supported by 'struts' of epithelium (arrows). (B) In the rabbit lung,
pulmonary capillaries are suspended in the large spaces between alveoli (Watson et al. 2007).
Top: Air flow (large arrows) and blood flow (small arrows) illustrating the cross-current gas-exchange mechanism operating
in the avian lung (between the blood capillaries and air capillaries). Note the serial arrangement of blood capillaries running from the periphery to the lumen of the parabronchus and the air capillaries radially extending from the parabronchial lumen. The exchange of gases (simple diffusion of O2 and CO2) occurs only between blood capillaries and air capillaries. As air moves through a parabronchus and each successive air capillary, the partial pressure of oxygen (PO2) declines (as indicated by the decreased density of the stippling) because oxygen is diffusing into the blood capillaries associated with each air capillary. As a result of this diffusion, the partial pressure of oxygen in the blood leaving the lungs (pulmonary vein) is higher than that in blood entering the lungs (pulmonary artery) (as indicated by the increased density of the stippling).
Bottom: Relative partial pressures of O2 and CO2 (1) for air entering a parabronchus (initial-parabronchial, PI) and air leaving a parabronchus (end-parabronchial, PE), and (2) for blood before entering blood capillaries in the lungs (pulmonary artery, PA) and for blood after leaving the blood capillaries in the lungs (pulmonary vein, PV). The partial pressure of oxygen (PO2) of venous blood (PV) is derived from a mixture of all serial air capillary-blood capillary units. Because of this cross-current exchange the partial pressure of oxygen in avian pulmonary veins (PV) is greater than that of the air leaving the parabronchus (PE) air that will be exhaled. In mammals, the partial pressure of oxygen in veins leaving the lungs cannot exceed that of exhaled air (end-expiratory gas, or PE) (Figure adapted from Scheid and Piiper 1987). Importantly, the partial pressure of oxygen in blood leaving the avian lung is the result of 'mixing' blood from a series of capillaries associated with successive air capillaries along the length of a parabronchus is mixed as the blood leaves the capillaries and enters small veins. As a result, the direction of air flow through a parabronchus does not effect the efficiency of the cross-current exchange (because gases are only exchanged between blood capillaries and air capillaries, not between the parabronchus and the blood). So, in above diagram, reversing the direction of air flow would obviously mean that the air capillary on the far left would have the highest partial pressure of oxygen rather than the air capillary on the far right (so the stippling pattern that indicates the amount of oxygen in each air capillary would be reversed). However, because of the 'mixing' of blood just mentioned, this reversal would have little effect on the PV, the partial pressure of oxygen in blood leaving via pulmonary veins (the PO2 would likely be a bit lower because some oxygen would have been lost the first time air passed through the neopulmonic parabronchi). This is important because most birds have neopulmonic parabronchi as well as paleopulmonic parabronchi and, although air flow through paleopulmonic parabronchi is unidirectional, air flow through neopulmonic parabronchi is bidirectional.
Diagram showing the flow of air from the parabronchial lumen (PL) into the air capillaries (not shown) and arterial blood from the periphery of the
parabronchus into the area of gas exchange (exchange tissue, ET). The orientation between the flow of air along the parabronchus and that of blood into
the exchange tissue (ET) from the periphery is perpendicular or cross-current (dashed arrows). The exchange tissue is supplied with arterial blood
by interparabronchial arteries (IPA) that give rise to arterioles (stars) that terminate in blood capillaries. After passing through the capillaries, blood flows
into the intraparabronchial venules (asterisks) that drain into interparabronchial veins (IPV). These in turn empty into the pulmonary vein which returns the
blood to the heart. (From: Maina and Woodward 2009).
Control of Ventilation:
Ventilation and respiratory rate are regulated to meet the demands imposed by changes in metabolic activity (e.g., rest and flight) as well as other sensory inputs (e.g., heat and cold). There is likely a central respiratory control center in the avian brain, but this has not been unequivocally demonstrated. As in mammals, the central control area appears to be located in the pons and medulla oblongata with facilitation and inhibition coming from higher regions of the brain. It also appears that the chemical drive on respiratory frequency and inspiratory and expiratory duration depend on feedback from receptors in the lung as well as on extrapulmonary chemoreceptors, mechanoreceptors, and thermoreceptors (Ludders 2001).
Central chemoreceptors affect ventilation in response to changes in arterial P CO 2 and hydrogen ion concentration. Peripheral extrapulmonary chemoreceptors, specifically the carotid bodies (located in the carotid arteries), are influenced by P O 2 and increase their discharge rate as P O 2 decreases, thus increasing ventilation they decrease their rate of discharge as P O 2 increases or P CO 2 decreases. These responses are the same as those observed in mammals. Unlike mammals, birds have a unique group of peripheral receptors located in the lung called intrapulmonary chemoreceptors (IPC) that are acutely sensitive to carbon dioxide and insensitive to hypoxia. The IPC affect rate and volume of breathing on a breath-to-breath basis by acting as the afferent limb of an inspiratory-inhibitory reflex that is sensitive to the timing, rate, and extent of CO 2 washout from the lung during inspiration (Ludders 2001).
Respiration by Avian Embryos
Bernhard, W., A. Gebert, G. Vieten, G. A. Rau1, J. M. Hohlfeld, A. D. Postle, and J. Freihorst. 2001. Pulmonary surfactant in birds: coping with surface tension in a tubular lung. American Journal of Physiology - Regulatory Integrative and Comparative Physiology 281: R327-R337.
Bernhard, W., P. L. Haslam, and J. Floros. 2004. From birds to humans: new concepts on airways relative to alveolar surfactant. American Journal of Respiratory Cell and Molecular Biology 30: 6-11.
Dubach, M. 1981. Quantitative analysis of the respiratory system of the House Sparrow, Budgerigar, and Violet-eared Hummingbird. Respiration Physiology 46: 43-60.
Duncker, H.-R. 1971. The lung air sac system of birds. Advances in Anatomy, Embryology, and Cell Biology 45: 1.
Klein, W., and T. Owerkowicz. 2006. Function of intracoelomic septa in lung ventilation of amniotes: lessons from lizards. Physiological and Biochemical Zoology 79: 1019-1032.
Ludders, J.W. 2001. Inhaled anesthesia for birds. In: Recent advances in veterinary anesthesia and analgesia: companion animals (R. D. Gleed and J. W. Ludders, eds.). International Veterinary Information Service, Ithaca, NY. (www.ivis.org/advances/Anesthesia_Gleed/ludders2/chapter_frm.asp)
Maina, J.N. 1989. The morphometry of the avian lung. Pp. 307-368 in Form and function in birds (A.S. King and J. McLelland, eds.). Academic Press, London.
Maina, J. N. 2002. Structure, function and evolution of the gas exchangers: comparative perspectives. Journal of Anatomy 201: 281-304.
Maina, J. N. 2008. Functional morphology of the avian respiratory system, the lung-air system: efficiency built on complexity. Ostrich 79: 117-132.
Maina, J. N., and A. S. King. 1989. The lung of the Emu, Dromaius novaehollandiae: a microscopic and morphometric study. Journal of Anatomy 163: 67-74.
Maina, J. N., A. S. King, and G. Settle. 1989. An allometric study of the pulmonary morphometric parameters in birds, with mammalian comparison. Philosophical Transactions of the Royal Society of London B 326: 1-57.
Maina, J. N., and C. Nathaniel. 2001. A qualitative and quantitative study of the lung of an Ostrich, Struthio camelus. Journal of Experimental Biology 204: 2313-2330.
Maina, J. N., and J. D. Woodward. 2009. Three-dimensional serial section computer reconstruction of the arrangement of the structural components of the parabronchus of the Ostrich, Struthio camelus lung. Anatomical Record 292: 1685-1698.
Makanya, A. N., and V. Djonov. 2009. Parabronchial angioarchitecture in developing and adult chickens. Journal of Applied Physiology 106: 1959-1969, 2009.
McLelland, J. 1989. Anatomy of the lungs and air sacs. In: Form and function in birds, vol. 4 (A. S. King and J. McLelland, eds.), pp. 221-279. Academic Press, San Diego, CA.
Powell, F.L. 2000. Respiration. Pp. 233-264 in Avian physiology, fifth edition (G. Causey Whittow, ed.). Academic Press, New York, NY.
Powell, F. L. and S. R. Hopkins. 2004. Comparative physiology of lung complexity: implications for gas exchange. News in Physiological Science 19:55-60.
Reese, S., G. Dalamani, and B. Kaspers. 2006. The avian lung-associated immune system: a review. Vet. Res. 37: 311-324.
Scheid, P., and J. Piiper. 1987. Gas exchange and transport. In: Bird respiration, volume 1 (T. J. Seller, ed.), pp. 97-129. CRC Press, Inc., Boca Raton, FL.
Sereno, P. C., R. N. Martinez, J. A. Wilson, D. J. Varricchio, O. A. Alcober, and H. C. E. Larsson. 2008. Evidence for avian intrathoracic air sacs in a new predatory dinosaur from Argentina. PLoS ONE 3(9): e3303.
Tazawa, H. 1987. Embryonic respiration. Pp. 3 - 24 in Bird respiration, vol. 2 (T. J. Seller, ed.). CRC Press, Boca Raton, FL.
Tickle, P. G., A. R. Ennos, L. E. Lennox, S. F. Perry, and J. R. Codd. 2007. Functional significance of the uncinate processes in birds. Journal of Experimental Biology 210: 3955-3961.
Watson, R. R., Z. Fu, and J. B. West. 2007. Morphometry of the extremely thin pulmonary blood-gas barrier in the chicken lung. American Journal of Physiology. Lung Cellular and Molecular Physiology 36: L769-L777.
Wedel, M.J. 2003. Vertebral pneumaticity, air sacs, and the physiology of sauropod dinosaurs. Paleobiology 29: 243.
Welty, J.C. and L. Baptista. 1988. The life of birds, fourth edition. Saunders College Publishing, New York, NY.
West, J. B. 2009. Comparative physiology of the pulmonary blood-gas barrier: the unique avian solution. American Journal of Physiology - Regulatory, Integrative and Comparative Physiology 297: R1625-R1634.
West, J. B., R. R. Watson, and Z. Fu. 2007. The human lung: did evolution get it wrong? European Respiratory Journal 29: 11-17.
Did you ever wonder why there are so many types of bird beaks (scientists call them bills)? The most important function of a bird bill is feeding, and it is shaped according to what a bird eats. You can use the type of bill as one of the characteristics to identify birds. Here are some common bill shapes and the food they are especially adapted to eat:
SHAPE TYPE ADAPTATION Cracker Seed eaters like sparrows and cardinals have short, thick conical bills for cracking seed. Shredder Birds of prey like hawks and owls have sharp, curved bills for tearing meat. Chisel Woodpeckers have bills that are long and chisel-like for boring into wood to eat insects. Probe Hummingbird bills are long and slender for probing flowers for nectar. Strainer Some ducks have long, flat bills that strain small plants and animals from the water. Spear Birds like herons and kingfishers have spear-like bills adapted for fishing. Tweezer Insect eaters like warblers have thin, pointed bills. Swiss Army Knife Crows have a multi-purpose bill that allows them to eat fruit, seeds, insects, fish, and other animals.
Another characteristic that can be used to learn more about birds is feet shapes! The shape of the feet reflects the habitat that the bird will be found in and the type of food it might eat. Here are some common feet shapes and the environment they are especially adapted to live in:
Show/hide words to know
Common ancestor: a single species that gave rise to at least two other species.
Density: the measure of mass per unit of volume. Think of it as how solid something is. If you have two equal size objects, the one with the higher density would weigh more.
Mammal: warm-blooded animal with fur.
Mass: is used to describe how much matter is in an object. If you know the number of atoms, the density of the atoms, and what type of atoms are in an object you can calculate its mass.
Derived from the Greek ἀνατομή anatomē "dissection" (from ἀνατέμνω anatémnō "I cut up, cut open" from ἀνά aná "up", and τέμνω témnō "I cut"),  anatomy is the scientific study of the structure of organisms including their systems, organs and tissues. It includes the appearance and position of the various parts, the materials from which they are composed, their locations and their relationships with other parts. Anatomy is quite distinct from physiology and biochemistry, which deal respectively with the functions of those parts and the chemical processes involved. For example, an anatomist is concerned with the shape, size, position, structure, blood supply and innervation of an organ such as the liver while a physiologist is interested in the production of bile, the role of the liver in nutrition and the regulation of bodily functions. 
The discipline of anatomy can be subdivided into a number of branches including gross or macroscopic anatomy and microscopic anatomy.  Gross anatomy is the study of structures large enough to be seen with the naked eye, and also includes superficial anatomy or surface anatomy, the study by sight of the external body features. Microscopic anatomy is the study of structures on a microscopic scale, along with histology (the study of tissues), and embryology (the study of an organism in its immature condition). 
Anatomy can be studied using both invasive and non-invasive methods with the goal of obtaining information about the structure and organization of organs and systems.  Methods used include dissection, in which a body is opened and its organs studied, and endoscopy, in which a video camera-equipped instrument is inserted through a small incision in the body wall and used to explore the internal organs and other structures. Angiography using X-rays or magnetic resonance angiography are methods to visualize blood vessels.    
The term "anatomy" is commonly taken to refer to human anatomy. However, substantially the same structures and tissues are found throughout the rest of the animal kingdom and the term also includes the anatomy of other animals. The term zootomy is also sometimes used to specifically refer to non-human animals. The structure and tissues of plants are of a dissimilar nature and they are studied in plant anatomy. 
The kingdom Animalia contains multicellular organisms that are heterotrophic and motile (although some have secondarily adopted a sessile lifestyle). Most animals have bodies differentiated into separate tissues and these animals are also known as eumetazoans. They have an internal digestive chamber, with one or two openings the gametes are produced in multicellular sex organs, and the zygotes include a blastula stage in their embryonic development. Metazoans do not include the sponges, which have undifferentiated cells. 
Unlike plant cells, animal cells have neither a cell wall nor chloroplasts. Vacuoles, when present, are more in number and much smaller than those in the plant cell. The body tissues are composed of numerous types of cell, including those found in muscles, nerves and skin. Each typically has a cell membrane formed of phospholipids, cytoplasm and a nucleus. All of the different cells of an animal are derived from the embryonic germ layers. Those simpler invertebrates which are formed from two germ layers of ectoderm and endoderm are called diploblastic and the more developed animals whose structures and organs are formed from three germ layers are called triploblastic.  All of a triploblastic animal's tissues and organs are derived from the three germ layers of the embryo, the ectoderm, mesoderm and endoderm.
Animal tissues can be grouped into four basic types: connective, epithelial, muscle and nervous tissue.
Connective tissue Edit
Connective tissues are fibrous and made up of cells scattered among inorganic material called the extracellular matrix. Connective tissue gives shape to organs and holds them in place. The main types are loose connective tissue, adipose tissue, fibrous connective tissue, cartilage and bone. The extracellular matrix contains proteins, the chief and most abundant of which is collagen. Collagen plays a major part in organizing and maintaining tissues. The matrix can be modified to form a skeleton to support or protect the body. An exoskeleton is a thickened, rigid cuticle which is stiffened by mineralization, as in crustaceans or by the cross-linking of its proteins as in insects. An endoskeleton is internal and present in all developed animals, as well as in many of those less developed. 
Epithelial tissue is composed of closely packed cells, bound to each other by cell adhesion molecules, with little intercellular space. Epithelial cells can be squamous (flat), cuboidal or columnar and rest on a basal lamina, the upper layer of the basement membrane,  the lower layer is the reticular lamina lying next to the connective tissue in the extracellular matrix secreted by the epithelial cells.  There are many different types of epithelium, modified to suit a particular function. In the respiratory tract there is a type of ciliated epithelial lining in the small intestine there are microvilli on the epithelial lining and in the large intestine there are intestinal villi. Skin consists of an outer layer of keratinized stratified squamous epithelium that covers the exterior of the vertebrate body. Keratinocytes make up to 95% of the cells in the skin.  The epithelial cells on the external surface of the body typically secrete an extracellular matrix in the form of a cuticle. In simple animals this may just be a coat of glycoproteins.  In more advanced animals, many glands are formed of epithelial cells. 
Muscle tissue Edit
Muscle cells (myocytes) form the active contractile tissue of the body. Muscle tissue functions to produce force and cause motion, either locomotion or movement within internal organs. Muscle is formed of contractile filaments and is separated into three main types smooth muscle, skeletal muscle and cardiac muscle. Smooth muscle has no striations when examined microscopically. It contracts slowly but maintains contractibility over a wide range of stretch lengths. It is found in such organs as sea anemone tentacles and the body wall of sea cucumbers. Skeletal muscle contracts rapidly but has a limited range of extension. It is found in the movement of appendages and jaws. Obliquely striated muscle is intermediate between the other two. The filaments are staggered and this is the type of muscle found in earthworms that can extend slowly or make rapid contractions.  In higher animals striated muscles occur in bundles attached to bone to provide movement and are often arranged in antagonistic sets. Smooth muscle is found in the walls of the uterus, bladder, intestines, stomach, oesophagus, respiratory airways, and blood vessels. Cardiac muscle is found only in the heart, allowing it to contract and pump blood round the body.
Nervous tissue Edit
Nervous tissue is composed of many nerve cells known as neurons which transmit information. In some slow-moving radially symmetrical marine animals such as ctenophores and cnidarians (including sea anemones and jellyfish), the nerves form a nerve net, but in most animals they are organized longitudinally into bundles. In simple animals, receptor neurons in the body wall cause a local reaction to a stimulus. In more complex animals, specialized receptor cells such as chemoreceptors and photoreceptors are found in groups and send messages along neural networks to other parts of the organism. Neurons can be connected together in ganglia.  In higher animals, specialized receptors are the basis of sense organs and there is a central nervous system (brain and spinal cord) and a peripheral nervous system. The latter consists of sensory nerves that transmit information from sense organs and motor nerves that influence target organs.   The peripheral nervous system is divided into the somatic nervous system which conveys sensation and controls voluntary muscle, and the autonomic nervous system which involuntarily controls smooth muscle, certain glands and internal organs, including the stomach. 
All vertebrates have a similar basic body plan and at some point in their lives, mostly in the embryonic stage, share the major chordate characteristics a stiffening rod, the notochord a dorsal hollow tube of nervous material, the neural tube pharyngeal arches and a tail posterior to the anus. The spinal cord is protected by the vertebral column and is above the notochord and the gastrointestinal tract is below it.  Nervous tissue is derived from the ectoderm, connective tissues are derived from mesoderm, and gut is derived from the endoderm. At the posterior end is a tail which continues the spinal cord and vertebrae but not the gut. The mouth is found at the anterior end of the animal, and the anus at the base of the tail.  The defining characteristic of a vertebrate is the vertebral column, formed in the development of the segmented series of vertebrae. In most vertebrates the notochord becomes the nucleus pulposus of the intervertebral discs. However, a few vertebrates, such as the sturgeon and the coelacanth retain the notochord into adulthood.  Jawed vertebrates are typified by paired appendages, fins or legs, which may be secondarily lost. The limbs of vertebrates are considered to be homologous because the same underlying skeletal structure was inherited from their last common ancestor. This is one of the arguments put forward by Charles Darwin to support his theory of evolution. 
Fish anatomy Edit
The body of a fish is divided into a head, trunk and tail, although the divisions between the three are not always externally visible. The skeleton, which forms the support structure inside the fish, is either made of cartilage, in cartilaginous fish, or bone in bony fish. The main skeletal element is the vertebral column, composed of articulating vertebrae which are lightweight yet strong. The ribs attach to the spine and there are no limbs or limb girdles. The main external features of the fish, the fins, are composed of either bony or soft spines called rays, which with the exception of the caudal fins, have no direct connection with the spine. They are supported by the muscles which compose the main part of the trunk.  The heart has two chambers and pumps the blood through the respiratory surfaces of the gills and on round the body in a single circulatory loop.  The eyes are adapted for seeing underwater and have only local vision. There is an inner ear but no external or middle ear. Low frequency vibrations are detected by the lateral line system of sense organs that run along the length of the sides of fish, and these respond to nearby movements and to changes in water pressure. 
Sharks and rays are basal fish with numerous primitive anatomical features similar to those of ancient fish, including skeletons composed of cartilage. Their bodies tend to be dorso-ventrally flattened, they usually have five pairs of gill slits and a large mouth set on the underside of the head. The dermis is covered with separate dermal placoid scales. They have a cloaca into which the urinary and genital passages open, but not a swim bladder. Cartilaginous fish produce a small number of large, yolky eggs. Some species are ovoviviparous and the young develop internally but others are oviparous and the larvae develop externally in egg cases. 
The bony fish lineage shows more derived anatomical traits, often with major evolutionary changes from the features of ancient fish. They have a bony skeleton, are generally laterally flattened, have five pairs of gills protected by an operculum, and a mouth at or near the tip of the snout. The dermis is covered with overlapping scales. Bony fish have a swim bladder which helps them maintain a constant depth in the water column, but not a cloaca. They mostly spawn a large number of small eggs with little yolk which they broadcast into the water column. 
Amphibian anatomy Edit
Amphibians are a class of animals comprising frogs, salamanders and caecilians. They are tetrapods, but the caecilians and a few species of salamander have either no limbs or their limbs are much reduced in size. Their main bones are hollow and lightweight and are fully ossified and the vertebrae interlock with each other and have articular processes. Their ribs are usually short and may be fused to the vertebrae. Their skulls are mostly broad and short, and are often incompletely ossified. Their skin contains little keratin and lacks scales, but contains many mucous glands and in some species, poison glands. The hearts of amphibians have three chambers, two atria and one ventricle. They have a urinary bladder and nitrogenous waste products are excreted primarily as urea. Amphibians breathe by means of buccal pumping, a pump action in which air is first drawn into the buccopharyngeal region through the nostrils. These are then closed and the air is forced into the lungs by contraction of the throat.  They supplement this with gas exchange through the skin which needs to be kept moist. 
In frogs the pelvic girdle is robust and the hind legs are much longer and stronger than the forelimbs. The feet have four or five digits and the toes are often webbed for swimming or have suction pads for climbing. Frogs have large eyes and no tail. Salamanders resemble lizards in appearance their short legs project sideways, the belly is close to or in contact with the ground and they have a long tail. Caecilians superficially resemble earthworms and are limbless. They burrow by means of zones of muscle contractions which move along the body and they swim by undulating their body from side to side. 
Reptile anatomy Edit
Reptiles are a class of animals comprising turtles, tuataras, lizards, snakes and crocodiles. They are tetrapods, but the snakes and a few species of lizard either have no limbs or their limbs are much reduced in size. Their bones are better ossified and their skeletons stronger than those of amphibians. The teeth are conical and mostly uniform in size. The surface cells of the epidermis are modified into horny scales which create a waterproof layer. Reptiles are unable to use their skin for respiration as do amphibians and have a more efficient respiratory system drawing air into their lungs by expanding their chest walls. The heart resembles that of the amphibian but there is a septum which more completely separates the oxygenated and deoxygenated bloodstreams. The reproductive system has evolved for internal fertilization, with a copulatory organ present in most species. The eggs are surrounded by amniotic membranes which prevents them from drying out and are laid on land, or develop internally in some species. The bladder is small as nitrogenous waste is excreted as uric acid. 
Turtles are notable for their protective shells. They have an inflexible trunk encased in a horny carapace above and a plastron below. These are formed from bony plates embedded in the dermis which are overlain by horny ones and are partially fused with the ribs and spine. The neck is long and flexible and the head and the legs can be drawn back inside the shell. Turtles are vegetarians and the typical reptile teeth have been replaced by sharp, horny plates. In aquatic species, the front legs are modified into flippers. 
Tuataras superficially resemble lizards but the lineages diverged in the Triassic period. There is one living species, Sphenodon punctatus. The skull has two openings (fenestrae) on either side and the jaw is rigidly attached to the skull. There is one row of teeth in the lower jaw and this fits between the two rows in the upper jaw when the animal chews. The teeth are merely projections of bony material from the jaw and eventually wear down. The brain and heart are more primitive than those of other reptiles, and the lungs have a single chamber and lack bronchi. The tuatara has a well-developed parietal eye on its forehead. 
Lizards have skulls with only one fenestra on each side, the lower bar of bone below the second fenestra having been lost. This results in the jaws being less rigidly attached which allows the mouth to open wider. Lizards are mostly quadrupeds, with the trunk held off the ground by short, sideways-facing legs, but a few species have no limbs and resemble snakes. Lizards have moveable eyelids, eardrums are present and some species have a central parietal eye. 
Snakes are closely related to lizards, having branched off from a common ancestral lineage during the Cretaceous period, and they share many of the same features. The skeleton consists of a skull, a hyoid bone, spine and ribs though a few species retain a vestige of the pelvis and rear limbs in the form of pelvic spurs. The bar under the second fenestra has also been lost and the jaws have extreme flexibility allowing the snake to swallow its prey whole. Snakes lack moveable eyelids, the eyes being covered by transparent "spectacle" scales. They do not have eardrums but can detect ground vibrations through the bones of their skull. Their forked tongues are used as organs of taste and smell and some species have sensory pits on their heads enabling them to locate warm-blooded prey. 
Crocodilians are large, low-slung aquatic reptiles with long snouts and large numbers of teeth. The head and trunk are dorso-ventrally flattened and the tail is laterally compressed. It undulates from side to side to force the animal through the water when swimming. The tough keratinized scales provide body armour and some are fused to the skull. The nostrils, eyes and ears are elevated above the top of the flat head enabling them to remain above the surface of the water when the animal is floating. Valves seal the nostrils and ears when it is submerged. Unlike other reptiles, crocodilians have hearts with four chambers allowing complete separation of oxygenated and deoxygenated blood. 
Bird anatomy Edit
Birds are tetrapods but though their hind limbs are used for walking or hopping, their front limbs are wings covered with feathers and adapted for flight. Birds are endothermic, have a high metabolic rate, a light skeletal system and powerful muscles. The long bones are thin, hollow and very light. Air sac extensions from the lungs occupy the centre of some bones. The sternum is wide and usually has a keel and the caudal vertebrae are fused. There are no teeth and the narrow jaws are adapted into a horn-covered beak. The eyes are relatively large, particularly in nocturnal species such as owls. They face forwards in predators and sideways in ducks. 
The feathers are outgrowths of the epidermis and are found in localized bands from where they fan out over the skin. Large flight feathers are found on the wings and tail, contour feathers cover the bird's surface and fine down occurs on young birds and under the contour feathers of water birds. The only cutaneous gland is the single uropygial gland near the base of the tail. This produces an oily secretion that waterproofs the feathers when the bird preens. There are scales on the legs, feet and claws on the tips of the toes. 
Mammal anatomy Edit
Mammals are a diverse class of animals, mostly terrestrial but some are aquatic and others have evolved flapping or gliding flight. They mostly have four limbs but some aquatic mammals have no limbs or limbs modified into fins and the forelimbs of bats are modified into wings. The legs of most mammals are situated below the trunk, which is held well clear of the ground. The bones of mammals are well ossified and their teeth, which are usually differentiated, are coated in a layer of prismatic enamel. The teeth are shed once (milk teeth) during the animal's lifetime or not at all, as is the case in cetaceans. Mammals have three bones in the middle ear and a cochlea in the inner ear. They are clothed in hair and their skin contains glands which secrete sweat. Some of these glands are specialized as mammary glands, producing milk to feed the young. Mammals breathe with lungs and have a muscular diaphragm separating the thorax from the abdomen which helps them draw air into the lungs. The mammalian heart has four chambers and oxygenated and deoxygenated blood are kept entirely separate. Nitrogenous waste is excreted primarily as urea. 
Mammals are amniotes, and most are viviparous, giving birth to live young. The exception to this are the egg-laying monotremes, the platypus and the echidnas of Australia. Most other mammals have a placenta through which the developing foetus obtains nourishment, but in marsupials, the foetal stage is very short and the immature young is born and finds its way to its mother's pouch where it latches on to a nipple and completes its development. 
This Leads to That
Understanding the structure of a biological component provides insight into its function. For instance, researchers at the University of Michigan and Purdue University have investigated how the viruses that cause West Nile fever and dengue fever reproduce. Their goal is to develop vaccines or medications for treatment. By discovering the structure of a protein, they have begun to understand the viruses’ replicating processes, as well as how the viruses affect the immune system.
Frequently bought together
"As this book demonstrates in excellently clear drawings and text, a bird is a superbly evolved match of structure with function."―Brian Bertram, Times Literary Supplement
"The book provides an accurate, up-to-date, self-contained course in ornithology for the advanced lay reader."― Library Journal
"A laboratory manual for the academic student, an invaluable reference tool for artists, and a rich source of information for the ordinary birder."― Bird Watcher's Digest
"Here is a volume that has no parallel. . . . Profusely illustrated with drawings by coauthor Lynch, this will be a good reference book for those interested in the details of avian anatomy."―Jerome A. Jackson, Science Books & Films
"A stunning work that integrates a conversational text with abundant, artistic illustrations. . . . A valuable reference work for scientists working with birds, for bird banders and for anatomists."―Edward H. Burtt, Jr., American Scientist
"The text is complete and up-to-date, the authors having incorporated the most recent information available. . . . The writing is clear and easy to follow. I found it easy to retrieve information because the layout is clear and well organized, key words being highlighted in bold face characters. The definitions of ornithological terms are precise, succinct, and carefully worded. . . . It should be consulted by all students enrolled in an ornithology course. I recommend its reading also to amateurs and bird watchers who will find clear explanations about topic often difficult to access elsewhere in the ornithological literature."―Henri Quellet, The Canadian Field Naturalist
"[This book] is richly illustrated with excellent line drawings by Lynch, which cause this manual to stand out from other ornithology texts I have seen. While written primarily as an ornithology lab manual, it would be a valuable reference book in the birder's library with an inexpensive paperback edition making it a practical purchase."― Wildlife Activist
"An impressive collaborative effort between a scientist and an illustrator . . . Profusely and beautifully illustrated, this book provides a wealth of encyclopedic and tabular information on all biological systems of birds, from skeletal to nervous system."― Northeastern Naturalist
"This excellent reference should provide the necessary background information to enable one to pursue further interests in this science. The abundant illustrations form an important background to the book and clearly show anatomical as well as other pertinent information. I recommend this book for any birder who wants to become more than just a watcher."―Charles E. Keller, Indiana Audubon Quarterly
Received an honorable mention in the Biological Science category, Association of American Publishers’ 1993 Professional/Scholarly Publishing Division Award (AAP/PSP)
"A superb contribution to ornithology. It clearly represents the most attractive and accessible contribution to form and function in birds. This book will be of tremendous appeal to academic and lay ornithologists alike."―Steve Zack, Yale University
"Students, their instructors, and other interested readers will welcome availability of the well illustrated Manual of Ornithology now in a paperback edition."―George A. Clark, University of Connecticut
"This is a tremendous book to say that it has no rival among laboratory manuals for ornithology is a dramatic understatement. It is clear, comprehensive, and has the most beautiful as well as useful illustrations possibly ever seen in an ornithology textbook."―Margaret Rubega, University of Connecticut
"A gold mine of facts. . . . Every library and biology department, as well as every birder, should have a copy close at hand."―Roger Tory Peterson, from the foreword
The structure and importance of the nervous system
The nervous system controls and regulates all the vital operations of the body because it receives the information from the environment and from the body, then it interprets this information and makes the body respond to it.
The nervous system is responsible for knowing if the things are hot or cold, sweet or bitter, rough or smooth, The nervous system adjusts the responses that require the emotions, so, it makes you sad or happy, angry or calm.
The nervous system oversees and regulates the multiple functions performed by the human body such as moving, feeding, digestion, breathing, thinking and others.
The structure of the nervous system
The nervous system consists of two major systems which are the central nervous system and the peripheral nervous system.
The building unit of the nervous system is the nerve cell that is called the neuron, and it consists of two main parts which are the cell body and the axon, The cell body contains a nucleus, the cytoplasm and the plasma membrane.
There are some branches extending from the neuron’s body called the dendrites, The dendrites are connected to the neighbouring neurons to form the synapse (the synaptic areas).
The axon is a cylindrical axis covered with a fatty layer called myelin sheath, and it ends with nerve endings called the axon terminals, The axon terminals are connected to the muscles or form a synapse with other neurons.
The nerve cell’s axons are grouped together forming the nerve fibre. and the nerve fibres are grouped together forming the nerve.
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- the sternum moves backward and upward & the vertebral ribs move caudally to retract the sternal ribs and reduce the volume of the thoracoabdominal cavity. The reduces the volume of the anterior & posterior air sacs, causing air to move out of those sacs.
- the sternum moves forward and downward while the vertebral ribs move cranially to expand the sternal ribs and the thoracoabdominal cavity (see diagrams below). This expands the posterior and anterior air sacs and lowers the pressure, causing air to move into those air sacs.
During inhalation, air moves into the posterior air sacs and, simultaneously,
into the lungs and through the parabronchi and into the anterior air sacs.
During exhalation, air moves out of the posterior air sacs into and through the parabronchi and, simultaneously,
out of the anterior air sacs and out of the body via the trachea.
During inhalation, all air sacs expand as inhaled air enters the posterior air sacs and lungs and, simultaneously, air moves out of the lungs
and into the anterior air sacs. During exhalation, the air sacs diminish in volume as air moves (1) from the posterior air sacs through the lungs and
(2) from the anterior air sacs and out of the body via the trachea.
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Respiratory airflow in avian lungs. Filled and open arrows denote direction of air flow during inspiration (filled arrows) and expiration (open arrows), respectively. Relative thickness of the arrows indicates the proportion of air streaming through the different areas of the respiratory system during the respiratory cycle. Dotted arrows indicate the volume changes of air sacs. In bird lungs (A), most air directly enters the caudal air sacs during inspiration (thick black arrow), whereas a lesser part flows through the parabronchi/air capillaries into cranial air sacs (thin black arrows). During expiration the major part of inspired air streams from the reservoirs (caudal air sacs, thick open arrows) through the parabronchi/air capillaries into major distal airways, where it mixes with the deoxygenated respiratory gas stored in cranial air sacs during the inspiratory phase. Consequently, respiratory gas flow through the parabronchi, atria, and the gas-exchanging air capillaries is unidirectional and continuous during both inspiration and expiration. This principle is achieved by cranio-caudal pressure gradients in the respiratory system changing between inspiration and expiration and the consecutive opening and closing of valve systems between mesobronchi/air sacs and the parabronchi (not indicated in the figure). Hence, airflow is constant and high in the parabronchi, atria, and the gas-exchanging air capillaries (From: Bernhard et al. 2004).
Surfactant SP-B (in the figure above) is mixture of phospholipids and specific proteins that functions to maintain airflow through the 'tubes' of the avian respiratory system. Surfactant SP-A has only been detected in the mesobronchi of birds. SP-A plays an important role in innate host defense and regulation of inflammatory processes and may be important in the mesobronchi because air flow is slower and small particles could tend to accumulate there (see figure below). Surfactant SP-C is not found in the avian respiratory system (or, if so, in very small quantities), but is found in the alveoli of mammals along with SP-A and SP-B. Because the mammalian respiratory system (below) includes structures that are collapsible (alveoli) and areas with low airflow, all three surfactants are important for reducing surface tension and innate host defense (Bernhard et al. 2004).
Airflow in mammalian lungs is bidirectional during the respiratory cycle, with highly reduced airflow
in peripheral structures, i.e., bronchioles and, particularly, the gas-exchanging alveoli. Consequently, small particles (< 1 µm)
that enter the alveoli may sediment, making a system of first line of defense necessary, comprising alveolar macrophages
(white blood cells), SP-A, and (phospholipid) regulators of inflammatory processes (From: Bernhard et al. 2004).
A: A high-power view of a foreign particle (p) being engulfed by an epithelial cell (e) in an avian lung.
Arrows, elongated microvilli. B: Surface of an atrium of the lung of the domestic fowl showing red blood
cells with one of them (r) being engulfed by the underlying epithelial cell (arrow): e, epithelial surface m, a free
(surface) macrophage. Scale bars: A = 0.5 µm B = 10 µm (From: Nganpiep and Maina 2002).
Air flow is driven by changes in pressure within the respiratory system: