OSMOREGULATION BY KIDNEYS • Kidneys also perform the important function of osmoregulation (regulation of osmolality) by regulating the amount of water in body fluids.
• The normally functioning kidneys produce a large volume of dilute urine when more water is taken, and a small volume of concentrated urine when water intake by the body is poor or there is considerable loss of water from the body.
• This function of kidneys is regulated by the antidiuretic hormone (ADH or Vasopressin) secreted by the posterior lobe of pituitary gland. This hormone increases the water permeability of the last part of distal convoluted tubules and proximal part of the collecting ducts.
• In absence of ADH, these parts are almost impermeable to water and therefore the urine is dilute. But in presence of ADH, these parts become quite permeable to water, so that much of the water present in tubular fluid is reabsorbed and the urine becomes concentrated. Besides ADH, the hormone aldosterone, secreted by adrenal glands, also plays important role in osmoregulatory function of kidneys by increasing Na+, K+ and Cl- reabsorption from the filtrate.
REGULATION OF KIDNEY FUNCTION BY FEEDBACK CIRCUITS
The functioning of the kidneys is efficiently monitored and regulated by hormonal feedback mechanisms involving the hypothalamus, JGA and to a certain extent, the heart.
1. Control by Antidiuretic Hormone (ADH):
• Osmoreceptors in the body are activated by changes in blood volume, body fluid volume and ionic concentration.
• An excessive loss of fluid from the body can activate these receptors which stimulate the hypothalamus to release antidiuretic hormone (ADH) or vasopressin from the neurohypophysis.
• ADH facilitates water reabsorption from latter parts of the tubule, thereby preventing diuresis. An increase in body fluid volume can switch off the osmoreceptors and suppress the ADH release to complete the feedback. ADH can also affect the kidney function by its constrictor effects on blood vessels.
• This causes an increase in blood pressure. An increase in blood pressure can increase the glomerular blood flow and thereby the GFR.
0 Comments