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