Question

How is the function of the kidney regulated?

Answer 1

Kidneys are the chief excretory organs which are present between the diaphragm and the intestines. They excrete the toxic wastes out of the body to maintain homeostasis in the body. The functioning of kidneys is regulated by hormonal feedback control mechanism which involves the hypothalamus, juxta glomerular apparatus and to some extent the heart.

Complete explanation:
When there is change in the blood volume, ionic concentration or loss of fluid, osmoreceptors in the hypothalamus are activated and they trigger the release of vasopressin or antidiuretic hormone from the neurohypophysis. The ADH stimulates reabsorption of water by increasing aquaporins on the distal tubules and collecting duct. This process prevents the water loss by diuresis.
In case of excessive body fluid, the osmoreceptors are switched off and so the ADH release is suppressed. The aquaporins move back to the cytoplasm. Due to this, collecting ducts are made impermeable to water and the excess fluid flows down to the collecting duct. Diluted urine is produced to maintain the blood volume. In this way vasopressin secretion and body fluid maintenance is controlled by positive and negative feedback mechanisms.
Juxta glomerular apparatus (JGA) is a specialized tissue in the nephron that consists of macula densa and granular cells. The macula densa cells sense distal tubular flow whereas the granular cells secrete an enzyme called renin. When there is a fall in glomerular blood flow and glomerular filtration rate, the JG cells are activated and they release renin. The renin converts plasma protein, angiotensinogen to angiotensin I which is further converted to angiotensin II.
Angiotensin II stimulates sodium reabsorption in the proximal convoluted tubule by vasoconstriction of the blood vessels and increases the glomerular blood pressure. The sodium excretion is done by Atrial Natriuretic Peptide or a factor released by the heart. By this way, the functioning of the kidney is regulated.

Note:
When the cardiac atrial cells are excessively stretched, it causes an increase in blood flow to the artery of the heart and releases atrial natriuretic factor (ANF). The ANF travels to the kidney and increases $Na^+$ excretion and also increases the blood flow to the glomerulus. The ANF acts as a vasodilator on the afferent glomerular arterioles or as a vasoconstrictor on efferent arterioles. The ANF acts antagonistically to the renin, angiotensin system and vasopressin.