Regulation of Kidney Function

Kidney function is not left to run on its own — it is closely monitored and adjusted by hormonal feedback loops. Three players are chiefly involved: the hypothalamus, the juxta glomerular apparatus (JGA) and, to some extent, the heart.

The ADH (vasopressin) feedback loop

Special sensors called osmoreceptors in the body respond to changes in blood volume, body fluid volume and ionic concentration.

• When the body loses too much fluid, these receptors are activated and prompt the hypothalamus to release antidiuretic hormone (ADH), also called vasopressin, from the neurohypophysis.
• ADH promotes the reabsorption of water from the later parts of the tubule, which prevents excessive loss of water as urine (diuresis).
• When body fluid volume rises again, the osmoreceptors are switched off and ADH release is suppressed, completing the feedback loop.

ADH has a further effect: it constricts blood vessels, which raises blood pressure. A rise in blood pressure increases the flow of blood through the glomerulus and so raises the GFR.

The Renin-Angiotensin mechanism

The JGA plays a more complex regulatory role. If glomerular blood flow, glomerular blood pressure or GFR falls, the JG cells release renin. This sets off a chain of events:

• Renin converts angiotensinogen in the blood into angiotensin I, which is then converted further into angiotensin II.
• Angiotensin II is a powerful vasoconstrictor; it raises the glomerular blood pressure and thereby the GFR.
• Angiotensin II also stimulates the adrenal cortex to release aldosterone.
• Aldosterone causes reabsorption of sodium ions and water from the distal parts of the tubule, which again raises blood pressure and GFR.

Together these steps make up the Renin-Angiotensin mechanism. …