Hormone Regulation of Urine Formation

Antidiuretic Hormone ( ADH)

• Antidiuretic Hormone is produced by the Hypothalamus. ADH is a peptide hormone, and consists of nine amino acids.

• It is stored in the posterior pituitary gland

• The single most important effect of antidiuretic hormone is to conserve body water by reducing the loss of water in urine.

• Antidiuretic hormone binds to receptors on cells in the collecting ducts of the kidney and promotes reabsorption of water back into the circulation. Antidiuretic hormone stimulates water reabsorbtion by stimulating insertion of "water channels" or aquaporins into the membranes of kidney tubules. These channels transport water through tubular cells and back into blood.

• This leads to a decrease in plasma osmolarity and an increase osmolarity of urine.

Control of Antidiuretic Hormone Secretion

• The most important variable regulating antidiuretic hormone secretion is plasma osmolarity, or the concentration of solutes in blood.

• Osmolarity is sensed in the hypothalamus by neurons known as an osmoreceptors, and those neurons, in turn, stimulate secretion from the neurons that produce antidiuretic hormone.

• When plasma osmolarity is below a certain threshold, the osmoreceptors are not activated and secretion of antidiuretic hormone is suppressed. When osmolarity increases above the threshold, the osmoreceptors recognize this and stimulate the neurons that secrete antidiuretic hormone

• The hypothalamic osmoreceptors also create the sensation of thirst. The osmotic threshold for antidiuretic hormone secretion is considerably lower than for thirst, as if the hypothalamus is saying "Let's not bother him by invoking thirst unless the situation is bad enough that antidiuretic hormone cannot handle it alone."

• Secretion of antidiuretic hormone is also stimulated by decreases in blood pressure and volume, conditions sensed by stretch receptors in the heart and large arteries. Changes in blood pressure and volume are not nearly as sensitive a stimulator as increased osmolarity, but are nonetheless potent in severe conditions. For example, Loss of 15 or 20% of blood volume by hemorrhage results in massive secretion of antidiuretic hormone.

• Another strong stimulus of antidiuretic hormone is nausea and vomiting, both of which are controlled by regions in the brain with links to the hypothalamus.

Aldosterone

• Aldosterone is primarily concerned with the regulation of blood pressure. It acts to regulate sodium levels in the blood plasma, and blood volume.

• This pathway involves the release of three hormones: renin, angiotensin and aldosterone.

• Juxtaglomerular (JG) cells associated with the afferent arteriole entering the renal glomerulus are the primary site of renin storage and release. A reduction in afferent arteriole pressure causes the release of renin from the JG cells, whereas increased pressure inhibits renin release.

Juxtaglomerular apparatus

• Specialized cells (macula densa) of distal tubules lie adjacent to the JG cells of the afferent arteriole.

• The macula densa senses the concentration of sodium and chloride ions in the filtrate fluid. When NaCl is elevated in the filtrate, renin release is inhibited. In contrast, a reduction in tubular NaCl stimulates renin release by the JG cells.

• When renin is released into the blood, it acts upon a circulating substrate, angiotensinogen, and cleaves it to angiotensin I.

• Vascular endothelium, particularly in the lungs, has an enzyme, angiotensin converting enzyme (ACE), that further cleaves angiotensin I to angiotensin II (AII

• Angiotensin II:• Constricts resistance vessels which

increases blood pressure• Stimulates sodium transport (reabsorption) at

several renal tubular sites, thereby increasing sodium and water retention by the body

• Acts on the adrenal cortex to release aldosterone, which in turn acts on the kidneys to increase sodium and fluid retention

• Stimulates the release of vasopressin (antidiuretic hormone, ADH) from the posterior pituitary, which increases fluid retention by the kidneys

• Stimulates thirst centers within the brain