Reabsorption mechanism by the PCT. 

Urine color generally ranges from a pale-yellow color to deep amber. This coloring is primarily caused by the pigment urochrome, also known as urobilin.

The vasa recta helps in reabsorption of important nutrients from medullary interstitial tissues.

As urine collects in the urinary bladder, the muscular walls of the bladder distend to accomodate it.The stretch receptors on the wall of the bladder send signals to the CNS by stimulating the sensory nerve ending in the bladder.It causes an urge to pass out the urine.So if the stretch receptors are totally removed from the bladder, urine will not be collected in the bladder and urination will keep continuing.

The counter current mechanism has been evolved in higher vertebrates (birds and mammals including man) to excrete hypertonic urine(urine is more concentrated than blood) for conserving body water so necessary for land life.

Atrial Natriuretic Factor(ANF) is secreted by the atria of heart, when blood flow increases. It causes vasodilation (dilation of blood vessels) and thereby, decrease the blood pressure. ANF works antagonistically to RAAS.

Birds excrete uric acid in the form of pellet or paste (semi-solid form). It helps is conservation of water.

Excessive loss of fluid from the body activates osmoreceptors, which stimulate the hypothalamus to release ADH from the neurohypophysis. ADH facilitates water reabsorption from posterior parts of tubule.This decreases osmolarity of blood.
An increase in body fluid volume can switch off the osmoreceptors and suppress the ADH release to complete the feedback.ADH also causes constrictory effects on blood vessels.

The nephron and the collecting duct are lined by a transport epithelium that processes the filtrate, forming the urine. The fluid out of the collecting duct is called urine.

NaCl is returned to the medullary interstitium by the ascending part of the vasa recta by reabsorption mechanism.

Micturition : Urine formed by the nephrons is ultimately carried to the urinary bladder where it is stored till a voluntary signal is given by the central nervous system (CNS). This signal is initiated by the stretching of the urinary bladder as it gets filled with urine. In response, the stretch receptors on the walls of the bladder send signals to the CNS. The CNS passes on motor messages to initiate the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter causing the release of urine. The process of release of urine is called micturition and the neural mechanisms causing it is called the micturition reflex.

Counter Current Mechanism. The Henle's loop and vasa recta play a significant role in this. The flow of filtrate in the two limbs of Henle's loop is in opposite directions and thus forms a counter current. The flow of blood through the two limbs of vasa recta is also in a counter current pattern. The proximity between the Henle's loop and vasa recta, as well as the counter current in them help in maintaining an increasing osmolarity towards the inner medullary interstitium, i.e., from 300 mOsmolL^-1 in the cortex to about 1200 mOsmolL^-1 in the inner medulla. This gradient is mainly caused by NaCI and urea. NaCI 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 interstitium by the ascending portion of vasa recta.
Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle's loop which is transported back to the interstitium by the collecting tubule.
The above described transport of substances facilitated by the special arrangement of Henle's loop and vasa recta is called the counter current mechanism. This mechanism helps to maintain a concentration gradient in the medullary interstitium. Presence of such interstitial gradient helps in an easy passage of water from the collecting tubule thereby concentrating the filtrate (urine). Human kidneys can produce urine nearly four times concentrated than the initial filtrate, formed.

ANF is commonly known as Atrial Natriuretic Factor.
ANF is a hormone produced by the walls of the atria of the heart in response to an increase in blood volume and pressure.It increases sodium excretion and decreases blood pressure.
ANF inhibits release of renin from Juxta Glomerular Apparatus(JGA) and thereby, inhibits NaCl reabsorption by the collecting duct and reduces aldosterone release from the adrenal gland.
Function The function of counter current mechanism is to concentrate NaCl in the intestitial fluid and thereby cause water to diffuse out of the collecting tubules and ducts and concentrate the urine.

The layers through which the glomerular capillary blood pressure causes filtration are
i) The endothelium of glomerular blood vessels.
ii) The epithelium of Bowman's capsule.
iii) A basement membrane (present between these two above mentioned layers)

i) Ultrafiltration occurs in Bowman's capsule.
ii) Reabsorption occur in PCT, loop of Henle ,DCT and Collecting ducts.
iii) Secretion occurs in PCT, loop of Henle and DCT.

(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.