3. It stimulates the adrenal cortex to release Aldosterone , a hormone that in turn stimulates principal cells in the collecting ducts to reabsorb more Na+ and Cl- and secrete more K+. The osmotic consequence of reabsorbing more Na++ and Cl- is excreting less water, which increases blood volume.
3) Antidiuretic Hormone (ADH)
Recall that this hormone is released by the posterior pituitary and it regulates water reabsorption by increasing the water permeability of principal cells in the last part of the DCT and throughout the collecting duct. In the absence of ADH, the apical membranes of principal cells have very low permeability to water. Within principal cells are tiny vesicles containing many copies of a water channel protein known as Aquaporin-2 . ADH stimulates insertion of the aquaporin-2-containing vesicles into the apical membranes via exocytosis. This results in increased water permeability of the apical membranes. A negative feedback system involving ADH regulates water reabsorption. When osmolarity or osmotic pressure of plasma and interstitial fluid increases – when water concentration decreases, osmoreceptors in the hypothalamus detect the change. Nerve impulses then stimulate secretion of more ADH into the blood, and principal cells become more permeable to water. In addition, as blood volume decreases, more ADH is secreted.
4) Atrial Natriuretic Peptide (ANP)
A large increase in blood volume promotes release of ANP from the heart. It can inhibit reabsorption of NA+ and water in the PCT and collecting duct. ANP also suppresses the secretion of aldosterone and ADH. These effects increase excretion of Na+ in urine and increase urine output, therefore decreasing blood volume and blood pressure.
5) Parathyroid Hormone (PTH)
Lower than normal levels of Ca++ in the blood stimulates the parathyroid glands to release PTH. PTH stimulates the cells in the early DCT to reabsorb more Ca++ into the blood.
Production of Dilute and Concentrated Urine
Even with variable fluid intake, the total volume of fluid in your body remains stable. This is in part due to the kidney’s ability to regulate the rate of water loss in urine. Normally, kidneys produce a large volume of dilute urine when fluid intake is high and a small amount of concentrated urine when fluid intake is low, or fluid loss is large. ADH controls whether dilute or concentrated urine is formed. In the absence of ADH, urine is dilute. However, a high level of ADH stimulates reabsorption of more water into blood producing concentrated urine.
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