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Mechanism of Urine Formation

When proteins are digested, amino acids are formed. The metabolism of amino acids in the body leads to the formation of ammonia. Ammonia is combined with carbon dioxide to from urea which is the principal from of nitrogenous waste in mammals. The formation of urea takes place in the liver and is carried by the blood to the kidneys where the nephric tubules filter the urea from the blood and eliminate it along with some water and certain other substance in the form of urine.

Urine begins as a simple filtrate of the blood. However, the composition of the initial filtrate in the nephron and that of the urine shows marked differences. Although glucose is generally abundant in the blood, this substance is absent from the final urine of a normal, healthy mammal. Compounds such as urea, on the other hand, are much more abundant in urine than in blood. The concentration of ammonia is 500 times greater than that in the blood, whereas sodium and calcium levels are approximately the same in blood and urine. The total urine volume from beginning to the end of the systems also shows dramatic changes. In a person weighing 70 kilograms, an average of 180 litres of fluid are filtered from blood each day. However, only about one to two litres of urine are actually excreted per day. Approximately, 99 per cent of the water filtered is reabsorbed back to the blood stream.

All these changes are the result of tubular reabsorption and secretion by the different regions of the nephron. It takes the blood filtrate from the glomerulus and reabsorbs and secretes substances into and from the glomerular filtrate to make it the final urine which is excreted away. The formation of urine involves three processes: (a) glomerular filtration, (b) tubular absorption, and (c) tubular secretion.

Glomerular Filtration

The first step in the formation of urine is the filtration of blood. The blood in the glomerulus is separated from the cavity within the Bowman's capsule only by (i) a thin layer of tissue composed of the single-celled endothelial lining, (ii) a layer of material called the basement membrane, and (iii) the single-celled lining of the Bowman's capsule. This barrier permits the filtration of the fluid from the capillaries into the Bowman's capsule. The energy for filtration is derived from the hydrostatic pressure of the blood. The glomerular afferent arteriole has a pressure of +75 mmHg. The osmotic pressure of the plasma proteins (about 30 mmHg) opposes the pressure exerted by the arteriole. The renal and tubular pressure is about 25 mmHg. The normal glomerular filtration rate is 120 ml/minute. The average volume of fluid filtered from the plasma into Bowman's capsule is about 170 litres per day. The composition of the ultrafiltrate in the Bowman's capsule is the same as the composition of blood, except that it does not contain blood cells and proteins. The appearance of red blood cells and protein in the urine indicates a defect in the process of glomerular filtration.

A Magnified View of the Glomerulus and Bowman's Capsule of the Nephron

Tubular Reabsorption and Secretion

The ultra filtrate in the Bowman's capsule contains many substances besides waste products like urea. Most of the useful substances are reabsorbed from the ultrafiltrate as it is being carried to the renal tubule.

In the first part of the nephron, that is, in the proximal convoluted tubule, glucose and sodium ions are actively reabsorbed from the filtrate into the surrounding blood vessels. Reabsorption is an active transport process requiring energy form ATP. Chloride ions (Cl−) are also passively reabsorbed along with sodium ions. However, the reabsorption of these solutes does not change the total solute concentration and the osmotic potential of the glomerular filtrate as its flows through the proximal convoluted tubule. This is because of the absorption of water. The water movement is passive. Movement of glucose and sodium and chloride ions from the glomerular filtrate to the blood tends to make the blood slightly hypertonic to the filtrate, therefore osmosis of water occurs to restore osmotic balance.

Certain substances are transported from the surrounding blood vessels into the nephron, in the region of the proximal convoluted tubule. This process is called secretion. The process of secretion allows these materials to enter the urine in greater quantities than is possible through glomerular filtration alone. The substances secreted include potassium and hydrogen ions, ammonia and various compounds such as certain dyes and drugs. A major function of secretion is the removal of toxic substances. Like the process of active reabsorption of ions, the process of secretion into the nephron also involves active transport of materials.

Just beyond the proximal convoluted tubule, the nephron folds into a loop of Henle that has descending limb which is permeable to water and an ascending limb which is not. Chloride ions (Cl−) are actively transported out of the ascending limb into the surrounding tissues and sodium ions also move likewise. Sodium and chloride ions accumulate in the tissue surrounding the loop so that urine in the loop is hypotonic to the fluid in the tissues. Water moves passively out of the descending limb and some sodium and chloride ions move passively into the descending limb, creating gradients of sodium and chloride ions within the loop of Henle.

In the ascending limb the chloride ions are actively transported out of the nephron and into the surrounding tissue and sodium ions passively follow.

From the loop of Henle, the urine which is now rather dilute, moves to the distal convoluted tubule, and then to a common collecting duct. Each collecting duct serves several nephrons, draining the urine into the pelvis of the kidney. The final changes in the composition of urine occur in the distal convoluted tubule and the collecting duct. Potassium ions, hydrogen ions and ammonia are secreted into the distal convoluted tubule and much water is reabsorbed in the distal convoluted tubule and the collecting tubule. Thus, the process of reabsorption and secretion of specific materials occurring at various places along the nephron, change the composition of the glomerular filtrate resulting in the formation of urine. 

Tubular Reabsorption and Secretion

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