Explain the Water balance function of the kidney
Water is one of the most important constituent in the daily diet. You are already aware of the roles it plays in our body. Maintenance of water and electrolyte balance in the body is an equally important consideration. Let us see how kidneys help to regulate and maintain water balance. The body excretes excess water through the kidneys. The balance between intake and output is maintained by the posterior pituitary hormone ADH or anti-diuretic hornzone or vasopressin. Substances that stimulate the formation of urine are termed as diuretics. The hormone ADH causes the suppression of urine. The minimum urinary output is about 500 ml per day average being 1500 ml. There are cells in the hypothalamus (in the brain) which are sensitive to the changes in the osmotic pressure of the blood. These sensory receptors sensing the osmotic pressure are called as osmorecrptors. When the concentration of sodium and other osmotically active substances in the extracellular fluid rises, there is an increased osmotic pressure in the fluid.
Osmoreceptors (supraoptic nucleus) in the hypothalamus are stimulated. The hypothalamus sends message to the posterior pituitary gland and it secretes ADH. The ADH acts on the distal convoluted tubules and collecting ducts of the kidney to cause an increase in the permeability resulting in an increased reabsorption of water which reduces osmotic pressure of the blood. The pore size of the epithelial cell is increased. This is enough for the water molecules to diffuse through, but does not allow most other substances in the tubular fluid to pass through. Thus water is returned to the body fluids whereas the sodium and other solutes are lost in the urine. In this way, the osmotic and water balance is maintained. If there is an increased concentration of dissolved substances in the blood, there is an increased ADH production. The water reabsorption under ADH influence in the DCT and collecting duct is known as facultative absorption. In deficiency of ADH, there is less facultative absorption, hence urine output is increased. This condition is called as diabetes insipidus. On the other hand, if the osmotic pressure of the blood is reduced due to increased plasma volume, the amount of ADH secretion is reduced and diuresis occurs. Two mechanisms that are involved in this process are: the structure and transport properties of the loop of Henle in the nephron, and the anti-diuretic hormone (ADH), also called as vasopressin, secreted by the pituitary gland.