Two important hormonal controls of the kidney function by negative feedback circuits.
(i) Control by Antidiuretic Hormone (ADH) :
ADH produced in the hypothalamus of the brain and released into the blood stream from the pituitary gland, enhances fluid retention by making the kidneys reabsorb more water. The release of ADH is triggered when osmoreceptors in the hypothalamus detect an increase in the osmolarity of the blood above a set point of 300 mosm L-1
In this situation, the osmoreceptor cells also promote thirst. Drinking reduces the osmolarity of the blood, which inhibits the secretion of ADH, thereby completing the feedback circuit. (Fig. A)
(ii) Control by Juxtaglomerular Apparatus (JGA)
JGA operates a multihormonal Renin-Angiotensin-Aldosterone system (RAAS).The JGA responds to a decrease in blood pressure or blood volume in the afferent arteriole of the glomerulus and releases an enzyme, renin, into the blood stream. In the blood, renin initiates chemical reactions that convert a plasma protein, called angiotensinogen, to a peptide, called angiotensin II, which works as a hormone. Angiotensin II increases blood pressure by causing arterioles to constrict. It also increases blood volume in two ways: firstly, by signalling the proximal convoluted tubules to reabsorb more NaCl and water, and secondly, by aldosterone, a hormone that induces the distal convoluted tubule to reabsorb more Na+ and water. This lead to an increase in blood volume and pressure, completing the feedback circuit by supporting the release renin. (Fig.B)
Still, another hormone, a peptide called Atrial Natriuretic Factor (ANF), opposes the regulation by RAAS. The walls of the atria of the heart release ANF in response to an increase in blood volume and pressure. ANF inhibits the release of renin from the JGA, and thereby inhibits NaCl reabsorption by the collecting duct and reduces aldosterone release from adrenal gland.
Thus ADH , RAAS and ANF provide an elaborate system of checks and balance that regulate the kidney functioning, to control body osmolarity , salt concentrations, blood pressure and blood volume.
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