Functions

The kidneys perform the following specific functions:

1. Maintaining water balance in the body. The kidney has the ability to concentration urine. It is done by the urea recycling in the deep inner renal medulla. It involves different urea transporters i.e. UT-A1/3, UT-A2 and UT-B.

UT-A1 and -A3 are located in the tubular cells in the collecting duct and they are vasopressin-sensitive. Urea is facilitated diffused along its concentration gradient through UT-A1 and -A3 and enter the interstitial fluid.

UT-A2 in the wall of the descending limb of the loop of Henle allow urea to diffuse back into the nephron and enter the tubule fluid which will then recycle back to collecting duct.

UT-B is found in the endothelium of vasa recta, which is one of the branches of efferent arteriole. It is a loop-like structure that allows solutes exchange from the ascending limb to the descending limb; this mechanism is called countercurrent exchange. Urea in the inner medulla interstitium is taken up by the vasa recta via UT-B. As a result, the concentration of urea is maintained through countercurrent exchange mechanism and this generates a high interstitial osmolality and a deep osmotic gradient which will draw water from the tubular fluid under the regulation of vasopressin. Finally, concentrated urine is produced and much water is retained in the body in the antidiuretic condition.

When the water level is high in the body, the level of vasopressin is decreased. As a result, the expression of UT-A1 and -A3 will decline. Less urea is reabsorbed and it shallows the osmotic gradientant and water is kept in the tubular fluid and finally excreted in the urine. Diluted and low osmolality urine is produced.

Figure 1: urea recycling is illustrated in here.

2. Regulating the quantity and concentration of most ECF ions. It includes Na⁺, Cl^-, K⁺, H⁺, HCO₃^-, Ca²⁺, Mg²⁺, SO₄^2- and PO₄^3-. Even minor fluctuations in the ECF concentrations of some electrolytes can have profound effects. For example, changes in potassium ions will lead to hypokalemia or hyperkalemia which will cause cardiac dysfunction; increases in calcium ions will lead to hypercalcaemia which wil lead to abnormal heart rhythms.

3. Hormone productions. Many important hormones are produced by the kidney. It includes renin, erythropoietin and vitamin D. Renin is a protein released by the JGA, it results in the formation of angiostensin II and it promotes sodium retention via the action of aldosterone. It activates hormonal mechanisms that control blood pressure and electrolyte balance. Erythropoietin is a hormone which stimulate red blood cells formation in bone marrow. Vitamin D is synthesised from calcitriol and contribute to calcium homeostasis.

4. Filtering blood plasma. Kidney separates wastes from the useful chemicals, and eliminate the wastes while returning the rest to the bloodstream.

5. Gluconeogenesis. In times of starvation, they carry out gluconeogenesis; they deaminate amino acids (removing the -NH₂ group), excrete the amino group as ammonia, and synthesize glucose from the rest of the molecule.

6. Acid-base balance. The kidney functions with the lungs to regulate the carbon dioxide concentration in the blood and maintaining the blood pH.