The blood that enters the glomerulus is under high pressure, about 75 mmHg. This pressure forces the passage of water and small molecules (amino acids, glucose, salts, urea, etc..) into Bowman's capsule.
The cells (white and red blood cells) and large molecules (proteins) do not cross the glomerular wall. There is a filtration of blood in the glomerulus and the filtered liquid is called glomerular filtrate (or initial urine). Its composition is similar to that of plasma, except for the absence of proteins.
Throughout the renal tubulesGlucose, amino acids, salts and also a small fraction of urea are actively reabsorbed by returning to the blood of the peritubular capillaries along with water.
Once resorption is done, what remains is the urine, hypertonic liquid containing, among other substances, water, salts, urea, uric acid and hemoglobin degradation products, which will be directed to the collecting duct.
In this way the kidneys perform a double function: eliminate substances that should not be used and reabsorb the useful nutrients, returning them to the blood. Thus the kidneys contribute to the maintenance of chemical composition of the internal environment.
The average urinary volume produced by a normal person is 1 L / day.
Water generated in the oxidation of food contributes to water balance. In combustion of 1g of glucose, for example, about 0.6g of water is generated. Each 1g of fat leads to 1.1g of water, while 1g of protein produces about 0.3g of water. By eating 350g of carbohydrate, 100g of fat and 100g of protein daily, a person would generate about 340g of water.
Water resorption regulation
Water reabsorption by the kidneys is under the control of antidiuretic hormone, also known by the acronym ADH.
This hormone is synthesized in the hypothalamus (a region of the brain) and released by the posterior pituitary gland.
ADH acts on the renal tubules, causing increased water reabsorption of the glomerular filtrate.
When we drink little water, the body dehydrates and the tonicity of the blood increases. Certain brain cells perceive the change and stimulate the pituitary to release ADH. As a consequence there is greater reabsorption of water by the renal tubules. The urine becomes more concentrated and the amount of water eliminated decreases.
Ingestion of large amounts of water has the opposite effect. Blood tonicity decreases, stimulating the pituitary to release less ADH. As a result, larger volume of more dilute urine is produced.