FUNCTION OF RENAL (KIDNEY)

RENAL FUNCTION

 

1. Maintenance of water and electrolyte balance of the body The proximal renal tubules reabsorb about 85% of the water filtered by the glomerulus while the rest passes through the loop of Henle where some of the water is reabsorbed. Under normal circumstances all the glucose, amino acids, potassium, bicarbonate and about 70% of the sodium are also reabsorbed by the proximal tubules. The distal renal tubules, on the other hand, control the amount of water (tubular fluid) reabsorbed.

Tubular fluid passes into the collecting ducts which stretch through the renal medulla and empty urine into the renal pelvis. The cells lining the collecting ducts which are normally impermeable to water are rendered permeable by the action of an antidiuretic hormone (ADH, vasopressin). 

Thus water is allowed through the wall of the tubule to re-enter circulation. When the body needs to excrete more water, the level of ADH is reduced, making the tubules impermeable to water, resulting in more water being excreted in the urine (dilute urine).

About 70% of sodium is reabsorbed in the proximal tubule. The remainder of the sodium is reabsorbed in the distal convoluted tubules and collecting ducts under the stimulus of the hormone aldosterone produced by adrenal cortex. 

The presence of aldosterone causes increased reabsorption of sodium by the tubules. In order to maintain electrolyte balance in the body, reabsorption of sodium is linked to potassium and hydrogen ions excretion and chloride and bicarbonate ions reabsorption 

2. Maintenance of blood pH Substances that cause the pH of blood to rise or fall are removed by the kidneys. Substances of acid reaction are the waste products of protein metabolism. These substances are urea, uric acid, and creatinine. 

They are continually formed and so, must be excreted. On the other hand, the alkaline reaction substances are the salts of sodium, potassium, calcium, magnesium and phosphorous. If the concentration of these salts becomes too high, they are removed by the kidneys. 

3. Excretion Waste products of drugs which are left in the body are later on excreted via kidneys. Toxic substances which are rendered harmless in the liver are excreted by the kidneys in the urine. 

4. Filtration The afferent (entering) arteriole carries blood to the glomerulus while the efferent (leaving) arteriole takes the blood from the glomerulus. The walls of the capillaries within the glomerulus, and the walls of the Bowman's capsule act as semipermeable membranes. 

They allow some molecules to pass through while others are held back within the capillary lumen. Since the efferent arteriole is able to constrict, the pressure in the glomerulus is higher than that in the capsule. 

As a result, water, glucose, electrolytes, amino acids and waste products of metabolism like urea, creatinine and uric acid are able to pass from the bloodstream and to the Bowman's capsule. This process is known as ultrafiltration.

The glomerular filtration rate (GFR) measures the efficiency of this filtration and it is determined by urea or creatinine clearance tests. The filtrate passes from the Bowman's capsule into the renal tubule. During this passage, the tubular cells reabsorb those substances that are needed by the body into the bloodstream.

5. Production of erythropoeitin The substance that is necessary for the normal production of red blood cells in the bone marrow is the erythropoietin. It is produced by the kidneys.

6. Reabsorption Desirable elements in the filtrate such as glucose and amino acids are reabsorbed. This reabsorption occurs mainly in the proximal convoluted tubules. 

7. Concentration of urine A large amount of water is filtered out through the kidneys each day. About 180 litres of glomerular filtrate is produced daily, while only 1 or 2 litres is excreted as urine from the body. Most of the water is reabsorbed in the distal and collecting tubules.