The functioning of the kidneys is efficiently monitored and regulated by hormonal feedback mechanisms involving the hypothalamus, JGA and to a certain extent, the heart.
i) Osmoreceptors in the body are activated by changes in blood volume, body fluid volume and ionic concentration. An excessive loss of fluid from the body can activate these receptors which stimulate the hypothalamus to release antidiuretic hormone (ADH)or vasopressin from the neurohypophysis. ADH facilitates water reabsorption from latter parts of the tubule, thereby preventing diuresis.
ii) An increase in body fluid volum can switch off the osmoreceptors and suppress the ADH release to complete the feedback. ADH can also affect the kidney function by its constrictory effects on blood vessels. This causes an increase in blood pressure. An increase in blood pressure can increase the glomerular blood flow and thereby the GFR.
iii) The JGA plays a complex regulatory role. A fall in glomerular blood flow/ glomerular blood pressure/GFR can activate the JG cells to release renin which converts angiotensinogen in blood to angiotensin I and further to angiotensin II. Angiotensin II, being a powerful vasoconstrictor, increases the glomerular blood pressure and thereby GFR. Angiotensin II also activates the adrenal cortex to release Aldosterone. Aldosterone causes reabsorption of Na' and water from the distal parts of the tubule. This also leads to an increase in blood pressure and GFR. This complex mechanism is generally known as the Renin-Angiotensin mechanism.
iv) An increase in blood flow to the atria of the heart can cause the release of Atrial Natriuretic Factor (ANF).ANF can cause vasodilation (dilation of blood vessels) and thereby decrease the blood pressure. ANF mechanism, therefore, acts as a check on the rennin angiotensin mechanism.

Urine formation involves three main processes namely:
1. Glomerular Filtration,
2. Reabsorption and
3. Secretion.
Glomerular Filtration. The glomerular capillary blood pressure causes filtration of blood through three layers, i.e., the endothelium of glomerular blood vessels, the epithelium of Bowman's capsule and a basement membrane between these two layers. The epithelial cells of Bowman's capsule called podocytes are arranged in an intricate manner so as to leave some minute spaces called filtration slits or slit pores. The diameter of efferent arteriole (arteriole bringing blood out of the glomerulus) is less than the diameter of afferent arteriole (arteriole taking blood inside the glomerulus). This difference in diameters creates a pressure which facilitates the filtration. Blood is filtered so finely through these membranes, that almost all the constituents of the plasma, except the proteins pass onto the lumen of the Bowman's capsule. Therefore, it is considered as a process of ultra filtration.
Glomerular Filtration Rate (GFR). The amount of the filtrate formed by the kidneys per minute is called glomerular filtration rate (GFR). GFR in a healthy individual is approximately 125 ml/rninute, i.e., 180 litres per day. On an average, 1100-1200 ml of blood is filtered by the kidneys per minute which constitute roughly 1/ 5th of the blood pumped out by each ventricle of the heart in a minute. Regulation of GFR. The kidneys have built-in mechanisms for the regulation of glomerular filtration rate. One such efficient mechanism is carried out by juxta glomerular apparatus (JGA).JGA is a special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole at the location of their contact. A fall in GFR can activate the JG cells to release renin which can stimulate the glomerular blood flow and thereby the GFR back to normal.
Reabsorption. A comparison of the volume of the filtrate formed per day (180 litres per day) with that of the urine released (1.5 litres), suggest that nearly 99 per cent of the filtrate has to be reabsorbed by the renal tubules. This process is called reabsorption. The tubular epithelial cells in different segments of nephron perform this either by active or passive mechanisms. For example, substances like glucose, amino acids, Na", etc., in the filtrate are reabsorbed actively whereas the nitrogenous wastes are absorbed by passive transport. Reabsorption of water also occurs passively in the initial segments of the nephron. During urine formation, the tubular cells secrete substances like and ammonia into the filtrate. Tubular secretion is also an important step in urine formation as it helps in the maintenance of ionic and acid base balance of body fluids.

In humans, the excretory system consists of a pair of kidneys, one pair of ureters, a urinary bladder and a urethra.
Kidneys
Shape and Size. Kidneys are reddish brown, bean shaped structures situated between the levels of last thoracic and third lumbar vertebra close to the dorsal inner wall of the abdominal cavity. Each kidney of an adult human measures 10-12 cm in length, 5-7 ern in width, 2-3 cm in thickness with an average weight of 120-170 g.
Structure. Towards the centre of the inner concave surface of the kidney is a notch called hilum through which ureter, blood vessels and nerves enter.

Inner Structure. Inner to the hilum is a broad funnel shaped space called the renal pelvis with projections called calyces. The outer layer of kidney is a tough capsule. Inside the kidney, there are two zones, an outer cortex and an inner medulla. The medulla is divided into a few conical masses (medullary pyramids) projecting into the calyces (sing .. calyx). The cortex extends in between the medullary pyramids as renal columns called Columns of Bertini.
Nephrons. Each kidney has nearly one million complex tubular structures called nephrons, which are the functional units. Each nephron has two parts - the glomerulus and the renal tubule.
Glomerulus. Glomerulus is a tuft of capillaries formed by the afferent arteriole - a fine branch of renal artery. Blood from the glomerulus is carried away by an efferent arteriole. The renal tubule begins with a double walled cup-like structure called Bowman's capsule, which encloses the glomerulus. Glomerulus along with Bowman's capsule, is called the malpighian body or renal corpuscle.
Tubules. The tubule continues further to form a highly coiled network - proximal convoluted tubule (PCT). A hairpin  shaped Henle's loop is the next part of the tubule which has a descending and an ascending limb. The ascending limb continues as another highly coiled tubular region called distal convoluted tubule (DCT). The DCTs of many nephrons open into a straight tube called collecting duct, many of which converge and open into the renal pelvis through medullary pyramids in the calyces. The Malpighian corpuscle, PCT and DCT of the nephron are situated in the cortical region of the kidney whereas the loop of Henle dips into the medulla.
Cortical Nephrons. In majority of nephrons, the loop of Henle is too short and extends only very little into the medulla. Such nephrons are called cortical nephrons.
Medullary Nephrons. In some of the nephrons, the loop of Henle is very long and runs deep into the medulla. These nephrons are called juxta medullary nephrons.
Vasa Recta. The efferent arteriole emerging from the glomerulus forms a fine capillary network around the renal tubule called the peritubular capillaries. A minute vessel of this network runs parallel to the Henle's loop forming a 'U' shaped vasa recta. Vasa recta is absent or highly reduced in cortical nephrons.

(i) A - Afferent arteriole,
B - Proximal convoluted tubule,
C - Glomerulus,
D - Loop of Henle,
E - Collecting duct
(ii) Following are the functions of each above mentioned part
A - Carry blood to glomerulus for ultrafiltration.
B - Selective reabsorption of useful substances.
C - Ultrafiltration of blood.
D - Maintenance of the high osmolarity of medullary interstitial fluid.
E - Reabsorption of water to form hypertonic urine.

The gradient of increasing hyperosmolarity of medullary interstitium is maintained by a counter current mechanism and the proximity between the Henle's loop and vasa recta.
This gradient is mainly caused by NaCl and urea.The transport of these substances facilitated by the special arrangement of Henle's loop and vasa recta is called the counter current mechanism.
NaCl is transported by the ascending limb of Henle's loop, which is exchanged with the descending limb of vasa recta.NaCl is returned to the medullary interstitium by the ascending part of the vasa recta.
But,contrarily, the water diffuses into the blood of ascending limb of vasa recta and is carried away into the general blood circulation.
Permeability to urea is found only in the deeper parts of thin ascending limbs of Henle's loops and collecting ducts.Urea diffuses out of the collecting ducts and enters into the thin.

Terrestrial adaptation requires the production of less toxic nitrogenous wastes like urea and uric acid for the conservation of water. Aquatic animals excrete ammonia which requires a large amount of water to dissolve. The huge quantity of water to dissolve. The huge quantity of water is easily available to such animals from surroundings. However, for terrestrial animals, such a huge quantity of
water is not available, hence they modify NH₃ to comparatively less toxic products like urea and uric acid which require less amount of water for their excretíon.

Function of Collecting Duct. This long duct extends from the cortex of the kidney to the inner parts of the medulla. Large amounts of water could be reabsorbed from this region to produce concentrated urine. This segment allows passage of small amounts of urea into the medullary interstitium to keep up the osmolarity. It also plays a role in the maintenance of pH and ionic balance of blood by the selective secretion of  \({ H }^{ + }\) and  \({ K }^{ + }\) ions.

Function of Henle's Loop. Reabsorption in this segment is minimum. However, this region plays a significant role in the maintenance of high osmolarity of medullary interstitial fluid. The descending limb of loop of Henle is permeable to water but almost impermeable to electrolytes. This concentrates the filtrate as it moves down. The ascending limb is impermeable to water but allows transport of electrolytes actively or passively. Therefore, as the concentrated filtrate pass upward, it gets diluted due to the passage of electrolytes to the medullary fluid.