Question

How do antidiuretic hormone (ADH), the rennin-angiotensin-aldosterone system (RAAS), and natriuretic hormones (natriuretic peptides) function in the regulation of fluid and electrolyte balance?

Answer 1

Antidiuretic Hormone (ADH) is a hormone that prevents fluid loss and promotes the conservation of body water. The primary stimulus for ADH release from the posterior pituitary gland is an increase in blood osmolarity . The elevation in blood osmolarity is detected in the hypothalamus by specialized neurons called osmoreceptors. ADH acts by increasing the reabsorption of water in the distal convoluted tubules and collecting ducts of the nephrons in the kidney. The net results of this mechanism is that water is conserved. Under these conditions a small volume of highly concentrated urine is excreted. Another action of ADH is to stimulate thirst. This result in an increase in water intake, which lowers blood osmolarity and helps to restore homeostasis.

Aldosterone is a hormone that regulates blood sodium levels. Aldosterone specifically increase sodium reabsorption in the distal convoluted tubule and collecting duct of the nephrons in the kidneys. Another action of Aldosterone is to increase the secretion of potassium by the kidneys resulting in its decrease in the blood and increase in urine. Aldosterone release is stimulated by the activation of the renin -angiotensin system.

Natriuretic peptide is a hormone that promotes both fluid and sodium loss by the kidneys. Natriuretic peptide release from the atria is stimulated when blood volume and pressure are elevated. Natriuretic peptide has three major effects : 1. It decreases aldosterone release, resulting in a decrease in sodium reabsorption and increased sodium loss in urine, 2. It decreases ADH release which decreases water reabsorption and increases water loss to lower blood volume and pressure, 3. It decreases thirst.