11th Class Biology Excretory Products and their Elimination Homeostatic Regulatory Functions Of Kidneys

By continuously eliminating metabolic wastes and other impurities, and even the surplus quantity of useful materials from blood plasma in the form of urine, kidneys play a vital role in homeostasis. Kidneys also operate certain other homeostatic regulatory mechanisms. Proper maintenance of the internal environment is knows as homeostasis. All regulatory functions of kidneys can be enumerated as follows -

(1) Osmoregulation : Being the universal solvent, water is the actual vehicle in ECF to transport materials between various parts of body. Water volume in ECF tends to vary considerably due to several reason, such as drinking, perspiration, diarrhoea, vomiting, etc. As described in previous pages, the kidneys maintain the water balance in ECF by diluting or concentrating urine.

(2) Regulation of osmotic pressure : Osmolality of cytoplasm is mainly due to proteins and potassium and phosphate ions, whereas that of the ECF is mainly due to sodium, chloride and bicarbonate ions. Inspite of marked difference in chemical composition, the two fluids - intracellular (cytoplasm) and extracellular (interstitium) - must be isotonic, because if ECF becomes hypotonic, cells will absorb water, swell retaining apropriate number, mainly of sodium and chloride ions, kidneys maintain the normal osmolality of ECF.

(3) Regulation of pH : Concentration of hydrogen ions \[(Na{{H}_{2}}P{{O}_{4}})\] in ECF is to be regulated at a constant value usually expressed as pH (minus log of\[{{H}^{+}}\]). The normal pH of ECF is about 7.4. A low pH, i.e. a high \[{{H}^{+}}\] concentration causes acidosis, while a high pH, i.e. a low \[{{H}^{+}}\] concentration causes alkalosis. Both of these conditions severely affect cellular metabolism. Several special control systems, therefore, operate in the body to prevent acidosis and alkalosis. These system are called acid-base buffer system. Kidneys play a key role in maintenance and operation of these systems. Further, the kidneys regulate hydrogen ion concentration in ECF by excreting acidic or basic urine.

(4) Regulation of electrolyte concentrations in ECF : The kidneys regulate, not only the total concentrations of water and electrolytes in ECF, but also the concentrations of individual electrolytes separately. This regulation is complex and is accomplished by tubular reabsorption and secretion under the control of hypothalamic and adrenal hormones.

(5) Regulation of RBC-count in blood : In oxygen deficiency (hypoxia), kidneys secrete an enzyme into the blood. This enzyme reacts with plasma globulin to form erythropoietin. The latter substance stimulates bone marrow to produce more RBCs for enhancing \[{{O}_{2}}-\]intake in lungs.

(6) Regulation of renal blood flow : See (R.A.A.S.).