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All vertebrates have kidneys. However, there are differences in the structure and functioning of various vertebrate kidneys that adapt them to the environment in which the animals live.
All animals that live in fresh water must cope with a continual inflow of water from their hypotonic environment. In order to maintain homeostasis of its extracellular fluid (ECF), the freshwater fish must excrete this excess water. Contraction of its heart (powered by ATP) provides the pressure to force the water, small molecules, and ions into the glomerulus as nephric filtrate. The essential ingredients are then reclaimed by the tubules, returning to the blood in the capillaries surrounding the tubules. The blood in these capillaries comes from the glomerulus (as in humans) and also from the renal portal veins which drain the posterior part of the fish's body.
After solute reabsorption is complete, the urine is little more than water. Most of the nitrogenous wastes (including large amounts of ammonia, NH3) leave by diffusion out of the gills. So, the kidney is mostly a device for maintaining water balance in the animal, rather than an organ of excretion.
The amphibian kidney also functions chiefly as a device for excreting excess water. The permeable skin of the frog provides an easy route for the fresh water of its pond to enter by osmosis. But, as their name suggests, amphibians also spend time on land. Then the problem is to conserve water, not eliminate it.
The frog adjusts to the varying water content of its surroundings by adjusting the rate of filtration at the glomerulus. When blood flow through the glomerulus is restricted, a renal portal system is present to carry away materials reabsorbed through the tubules. The frog is also able to use its urinary bladder to aid water conservation. When in water, the frog's bladder quickly fills up with a hypotonic urine. On land, this water is reabsorbed into the blood helping to replace water lost through evaporation through the skin. The reabsorption is controlled by a hormone similar to mammalian ADH.
Lizards and Snakes
Many reptiles live in dry environments (e.g., rattlesnakes in the desert). Among the many adaptations to such environments is their ability to convert waste nitrogen compounds into uric acid. Uric acid is quite insoluble and so can be excreted using only a small amount of water. Thus we find that reptile glomeruli are quite small and, in fact, some reptiles have no glomeruli at all. Those with glomeruli filter just enough fluid to wash the uric acid, which is secreted by the tubules, into the cloaca. Most of this moisture is reabsorbed in the cloaca. Emptying the cloaca deposits feces (brown) and uric acid (a white paste). The cloaca is a chamber through which the feces and the gametes, as well as urine, pass on the way to the outside. The name comes from the Latin word for sewer. These water conservation mechanisms can allow the reptile to forgo drinking water. The water content of its food plus the water produced by cellular respiration is usually sufficient.
Bird kidneys function like those of reptiles (from which they are descended). Uric acid is also their chief nitrogenous waste. Most birds have a limited intake of fresh water. However, they need filter only enough to wash a slurry of uric acid into the cloaca where enough additional water is reclaimed to convert the uric acid into a semisolid paste. It is the whitish material that pigeons leave on statues.
All mammals share our use of urea as their chief nitrogenous waste. Urea requires much more water to be excreted than does uric acid. Mammals produce large amounts of nephric filtrate but are able to reabsorb most of this in the tubules. But even so, humans lose several hundred ml each day in flushing urea out of the body. Some mammals have more efficient kidneys than ours. The kangaroo rat of the desert can produce a urine 17 times more concentrated that its blood. (The best we can do is 3-4 times as concentrated.) The efficiency of the kangaroo rat kidney enables it to survive without drinking water — simply depending on the water content of its food and that produced by cellular respiration.
We like to think of ourselves as highly advanced. Why don't we have kidneys as efficient as those of the reptiles and birds? It is the luck of our inheritance. The line of vertebrate evolution that produced the mammals split off before the evolution of the diapsids whose ability to convert nitrogenous wastes into uric acid was passed on to all their descendants, including the lizards, snakes, and birds.
Marine fishes face just the opposite problem from that of freshwater fishes. The salt content of sea water (~3%) is so hypertonic to that of their extracellular fluid that they are in continual danger of dehydration. The two major groups of marine fishes have solved this dilemma differently.
Cartilaginous Fishes (Chondrichthyes)
The cartilaginous fishes such as sharks, skates, and rays have developed high levels of urea in their blood. Shark's blood may contain 2.5% urea in contrast to the 0.01-0.03% in other vertebrates. This high level makes sharks blood isotonic to sea water, so the shark lives in osmotic balance with its environment and has a kidney that functions like ours with the exception that far more urea is reabsorbed in the shark's tubules than in ours.
Bony Fishes (Osteichthyes)
Marine bony fishes have solved the problem differently. They do lose water continuously but replace it by drinking sea water and then desalting it. The salt is returned to the sea by active transport at the gills. Living in constant danger of dehydration by the hypertonic sea, there is no reason to pump out large amounts of nephric filtrate at the glomerulus. The less water placed in the tubules, the less that has to be reabsorbed. So it is not surprising that many bony fishes have small glomeruli and some have no glomeruli at all. With a reduction in the filtration-reabsorption mechanism, the marine bony fishes rely more on tubular secretion for eliminating excess or waste solutes. Tubular secretion requires a good blood supply to the tubules. Lacking efficient glomeruli, the renal portal system must carry most of the burden.
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Excretion, the process by which animals rid themselves of waste products and of the nitrogenous by-products of metabolism. Through excretion organisms control osmotic pressure—the balance between inorganic ions and water—and maintain acid-base balance. The process thus promotes homeostasis, the constancy of the organism’s internal environment.
Every organism, from the smallest protist to the largest mammal, must rid itself of the potentially harmful by-products of its own vital activities. This process in living things is called elimination, which may be considered to encompass all of the various mechanisms and processes by which life forms dispose of or throw off waste products, toxic substances, and dead portions of the organism. The nature of the process and of the specialized structures developed for waste disposal vary greatly with the size and complexity of the organism.
Four terms are commonly associated with waste-disposal processes and are often used interchangeably, though not always correctly: excretion, secretion, egestion, and elimination.
Excretion is a general term referring to the separation and throwing off of waste materials or toxic substances from the cells and tissues of a plant or animal.
The separation, elaboration, and elimination of certain products arising from cellular functions in multicellular organisms is called secretion. Though these substances may be a waste product of the cell producing them, they are frequently useful to other cells of the organism. Examples of secretions are the digestive enzymes produced by intestinal and pancreatic tissue cells of vertebrate animals, the hormones synthesized by specialized glandular cells of plants and animals, and sweat secreted by glandular cells in the skins of some mammals. Secretion implies that the chemical compounds being secreted were synthesized by specialized cells and that they are of functional value to the organism. The disposal of common waste products should not, therefore, be considered to be of a secretory nature.
Egestion is the act of excreting unusable or undigested material from a cell, as in the case of single-celled organisms, or from the digestive tract of multicellular animals.
As defined above, elimination broadly defines the mechanisms of waste disposal by living systems at all levels of complexity. The term may be used interchangeably with excretion.
Development of Kidney in Various Vertebrates | Zoology
The kidneys develop within the postero-dorsal mesoderm of the embryo. At the onset of development this mesoderm expands to form a nephric ridge (Fig. 2.48). The next structure that appears is the paired nephrotome. The nephrotome is usually segmental and contains the nephrocoel, which is a coelomic chamber that may open via a ciliated peritoneal funnel into the coelom.
From the median part of nephrotome develops glomerulus and from the lateral end grows ducts. Several ducts fuses to form a common nephric duct. At this point the nephrotome is called nephric tubule, which is the fundamental structure of the urinary system.
The nephric tubule opens at one end into the coelom and on the other end into the nephric duct, with a glomerulus in between. In adult vertebrate kidney, the opening of nephric tubule in the coelome is rarely found. The fundamental organization is changed during development.
Tripartite Concept of Kidney Organisation:
During development, the nephric tubules differ structurally within the nephric ridge. This difference is described by different authors as tripartite concept. According to this concept the nephric tubules develop in three regions of the nephric ridge.
Subsequent merger, loss or replacement of these tubules constitutes the developmental basis for the definitive adult kidneys. Nephric tubules that arise at the anterior region is called pronephros, tubules arise at the middle region is called mesonephros and which arise at the posterior region is called metanephros (Fig. 2.49).
Tubule that develops within the anterior part of the nephric ridge is called pronephric tubules. Pronephros usually develops and retained for a short duration, during developmental stages of vertebrates. The pronephric tubules join to form a common pronephric duct (Fig. 2.49A).
This duct grows posteriorly and reaches and opens into the cloaca. The glomerulus protrudes from the roof of the coelom and filters fluid from coelom. Pronephric tubules then take up this coelomic fluid via the ciliated peritoneal funnels and excrete as urine.
Pronephric tubules and glomeruli form functional kidney in larval cyclostomes, in some adult fishes and in the embryos of most of the lower vertebrates. In most other vertebrates, the embryonic pronephros regresses, and as it does so, it is replaced by a second type of embryonic kidney, the mesonephros.
Middle part of the nephric ridge gives rise to the tubules of mesonephric kidney (Fig. 2.49B). These mesonephric tubules do not form a separate new duct for excretion instead tap into the existing pronephric duct. Now this duct is coined as mesonephric duct.
The mesonephros usually remains functional in the embryonic condition. But in some adult fishes and amphibians it persists. In that case, it is modified by the incorporation of additional tubules arising within the additional posterior tubules. This is termed as opisthonephros (Fig. 2.49D). In amniotes, the mesonephros is replaced in later development by a third type of embryonic kidney, the metanephros.
In case of metanephros, metanephric duct appears first as metanephric duct, as a ureteric diverticulum, at the base of preexisting mesonephric duct. This ureteric diverticulum grows dorsally into the posterior part of the nephric ridge and stimulates the growth of metanephric tubules.
These tubules and ducts form the metanephric kidney. This type of kidney is present in the adult amniotes and the metanephric duct is called as ureter (Fig. 2.49C).
Kidney in Lower Vertebrates — Pro and Meso­nephros:
In hagfish, pronephric tubules unite successively with one another, forming a pronephric duct (Fig. 2.50A). Anterior tubules lack glomeruli but open to the coelome via peritoneal funnels. Posterior tubules are associated with the glomeruli but lack connec­tion to the coelom.
In adult, aglomerular tubules together with several glomerular tubules become the compact pronephros. This pronephros contribute to the formation of coelomic fluid. However, in adult hagfish mesonephros is the functional kidney.
In lampreys, the ammocoetes larva posse­sses pronephric kidney, consisting of three to eight coiled tubules served by a single compacted bundle of capillaries called a glomus. A glomus differs from a glomerulus in that each vascular glomus services several tubules (Fig. 2.50B).
Kidney in Amphibians — Opisthonephros:
In amphibians, early embryonic pro­nephros is usually succeeded by the larval mesonephros, which upon metamorphosis is replaced by an opisthonephros. The anterior kidney tubules also transport sperm in adult, illustrating the dual use of the ducts that serve as both genital and urinary system.
Kidney in Other Vertebrates:
In most of the amniotes, the mesonepnros is completely replaced by metanephros in adults. The metanephros drain its products by a new urinary duct, the ureter. A metanephric tubule is long and well differentiated into proximal, intermediate and distal regions. In mammals, in particular, the intermediate section of the tubule is specially elongated, constituting the major part of the loop of Henle (Fig. 2.52 and 2.53).
The loop of Henle of the metanephric tubule occurs only in the group of animals who are capable of producing concentrated urine. Among vertebrates only mammals and birds can produce urine, which is much concentrated than their blood. Therefore, loop of Henle is present in these groups only. The ability to produce concentrated urine is directly correlated with the length of the loop.
The term loop of Henle refers to both a positional and structural feature of nephron. Positionally, the loop refers to the part of the tubule which comes out of the cortex region and enters into the medulla of the kidney. The entering part of the tubule, from glomerulus, to medulla, is called descending limb. The out­going part from the medulla to cortex is called the ascending limb (Fig. 2.53).
Structurally, the loop of Henle has three regions — these are:
(i) The straight portion of the proximal tubule.
(ii) The thin-walled intermediate region.
(iii) The straight portion of the distal tubule. Except the intermediate region, the other two regions are thick-walled. The terms thick and thin refer here to the height of the epithelial cells forming the loop. Cuboidal cells are thick and squamous cells are thin.
What do the kidneys do?
The kidneys are a pair of bean-shaped organs present in all vertebrates. They remove waste products from the body, maintain balanced electrolyte levels, and regulate blood pressure.
The kidneys are some of the most important organs. The Ancient Egyptians left only the brain and kidneys in position before embalming a body, inferring that the held a higher value.
In this article, we will look at the structure and function of the kidneys, diseases that affect them, and how to keep the kidneys healthy.
Share on Pinterest The kidneys play a role in maintaining the balance of body fluids and regulating blood pressure, among other functions.
The kidneys are at the back of the abdominal cavity, with one sitting on each side of the spine.
The right kidney is generally slightly smaller and lower than the left, to make space for the liver.
Each kidney weighs 125–170 grams (g) in males and 115–155 g in females.
A tough, fibrous renal capsule surrounds each kidney. Beyond that, two layers of fat serve as protection. The adrenal glands lay on top of the kidneys.
Inside the kidneys are a number of pyramid-shaped lobes. Each consists of an outer renal cortex and an inner renal medulla. Nephrons flow between these sections. These are the urine-producing structures of the kidneys.
Blood enters the kidneys through the renal arteries and leaves through the renal veins. The kidneys are relatively small organs but receive 20–25 percent of the heart’s output.
Each kidney excretes urine through a tube called the ureter that leads to the bladder.
The main role of the kidneys is maintaining homeostasis. This means they manage fluid levels, electrolyte balance, and other factors that keep the internal environment of the body consistent and comfortable.
They serve a wide range of functions.
The kidneys remove a number of waste products and get rid of them in the urine. Two major compounds that the kidneys remove are:
- urea, which results from the breakdown of proteins
- uric acid from the breakdown of nucleic acids
Reabsorption of nutrients
The kidneys reabsorb nutrients from the blood and transport them to where they would best support health.
They also reabsorb other products to help maintain homeostasis.
Reabsorbed products include:
- amino acids
- chloride, sodium, magnesium, and potassium ions
In humans, the acceptable pH level is between 7.38 and 7.42. Below this boundary, the body enters a state of acidemia, and above it, alkalemia.
Outside this range, proteins and enzymes break down and can no longer function. In extreme cases, this can be fatal.
The kidneys and lungs help keep a stable pH within the human body. The lungs achieve this by moderating the concentration of carbon dioxide.
The kidneys manage the pH through two processes:
- Reabsorbing and regenerating bicarbonate from urine: Bicarbonate helps neutralize acids. The kidneys can either retain it if the pH is tolerable or release it if acid levels rise.
- Excreting hydrogen ions and fixed acids: Fixed or nonvolatile acids are any acids that do not occur as a result of carbon dioxide. They result from the incomplete metabolism of carbohydrates, fats, and proteins. They include lactic acid, sulfuric acid, and phosphoric acid.
Osmolality is a measure of the body’s electrolyte-water balance, or the ratio between fluid and minerals in the body. Dehydration is a primary cause of electrolyte imbalance.
If osmolality rises in the blood plasma, the hypothalamus in the brain responds by passing a message to the pituitary gland. This, in turn, releases antidiuretic hormone (ADH).
In response to ADH, the kidney makes a number of changes, including:
- increasing urine concentration
- increasing water reabsorption
- reopening portions of the collecting duct that water cannot normally enter, allowing water back into the body
- retaining urea in the medulla of the kidney rather than excreting it, as it draws in water
Regulating blood pressure
The kidneys regulate blood pressure when necessary, but they are responsible for slower adjustments.
They adjust long-term pressure in the arteries by causing changes in the fluid outside of cells. The medical term for this fluid is extracellular fluid.
These fluid changes occur after the release of a vasoconstrictor called angiotensin II. Vasoconstrictors are hormones that cause blood vessels to narrow.
They work with other functions to increase the kidneys’ absorption of sodium chloride, or salt. This effectively increases the size of the extracellular fluid compartment and raises blood pressure.
Anything that alters blood pressure can damage the kidneys over time, including excessive alcohol consumption, smoking, and obesity.