Question

How does the structure of a nephron encourage concentration of urine? What is the name of this process and how does it work?

Answer 1

Let's study it in detailed manner. I explained almost everything . So that you can understand whole process . Thank you. All the best.

FORMATION OF CONCENTRATED URINE
When the water content in body decreases, kidney retains water and excretes concentrated urine. Formation of concentrated urine is not as simple as that of dilute urine.
It involves two processes:
1. Development and maintenance of medullary
gradient by countercurrent system
2. Secretion of ADH.

DEVELOPMENT AND MAINTENANCE OF MEDULLARY GRADIENT
Kidney has some unique mechanism called counter­ current mechanism, which is responsible for the develop­ ment and maintenance of medullary gradient and hyper­ osmolarity of interstitial fluid in the inner medulla.
A countercurrent system is a system of ‘U’­shaped tubules (tubes) in which, the flow of fluid is in opposite direction in two limbs of the ‘U’­shaped tubules.
Divisions of Countercurrent System
Countercurrent system has two divisions:
1. Countercurrent multiplier formed by loop of Henley.
Loop of Henle functions as countercurrent multiplier. It is responsible for development of hyperosmolarity of medullary interstitial fluid and medullary gradient .   
Loop of Henle of juxtamedullary nephrons plays a major role as countercurrent multiplier because loop of these nephrons is long and extends upto the deeper parts of medulla.
Main reason for the hyperosmolarity of medullary interstitial fluid is the active reabsorption of sodium chloride and other solutes from ascending limb of Henle loop into the medullary interstitium. These solutes accumulate in the medullary interstitium and increase the osmolarity.
Now, due to the concentration gradient, the sodium and chlorine ions diffuse from medullary interstitium into the descending limb of Henle loop and reach the ascending limb again via hairpin bend.
Thus, the sodium and chlorine ions are repeatedly re­ circulated between the descending limb and ascending limb of Henle loop through medullary interstitial fluid leaving a small portion to be excreted in the urine.
Apart from this there is regular addition of more and more new sodium and chlorine ions into descending limb by constant filtration. Thus, the reabsorption of sodium chloride from ascending limb and addition of new sodium chlorine ions into the filtrate increase or multiply the osmolarity of medullary interstitial fluid and medullary gradient. Hence, it is called countercurrent multiplier.
Other Factors Responsible for Hyperosmolarity of Medullary Interstitial Fluid
In addition to countercurrent multiplier action provided by the loop of Henle, two more factors are involved in hyperosmolarity of medullary interstitial fluid.
i. Reabsorption of sodium from collecting duct
Reabsorption of sodium from medullary part of collect­ ing duct into the medullary interstitium, adds to the osmolarity of inner medulla.
ii. Recirculation of urea
Fifty percent of urea filtered in glomeruli is reabsorbed in proximal convoluted tubule. Almost an equal amount of urea is secreted in the loop of Henle. So the fluid in distal convoluted tubule has as much urea as amount filtered.
Collecting duct is impermeable to urea. However, due to the water reabsorption from distal convoluted tubule and collecting duct in the presence of ADH, urea concentration increases in collecting duct. Now due to concentration gradient, urea diffuses from inner medullary part of collecting duct into medullary interstitium.
Due to continuous diffusion, the concentration of urea increases in the inner medulla resulting in hyperosmolarity of interstitium in inner medulla.
Again, by concentration gradient, urea enters the ascending limb. From here, it passes through distal convoluted tubule and reaches the collecting duct. Urea enters the medullary interstitium from collecting duct. By this way urea recirculates repeatedly and helps to maintain the hyperosmolarity of inner medullary interstitium. Only a small amount of urea is excreted in urine.
Urea recirculation accounts for 50% of hyper­ osmolarity in inner medulla. Diffusion of urea from collecting duct into medullary interstitium is carried out by urea transporters, UT­A1 and UT­A3, which are activated by AD
2. Countercurrent exchanger formed by vasa recta.   
Vasa recta functions as countercurrent exchanger. It is responsible for the maintenance of medullary gradient, which is developed by countercurrent multiplier.
Vasa recta acts like countercurrent exchanger because of its position. It is also ‘U’­shaped tubule with a descending limb, hairpin bend and an ascending limb. Vasa recta runs parallel to loop of Henle. Its descending limb runs along the ascending limb of Henle loop and its ascending limb runs along with descending limb of Henle loop.
The sodium chloride reabsorbed from ascending limb of Henle loop enters the medullary interstitium. From here it enters the descending limb of vasa recta. Simultaneously water diffuses from descending limb of vasa recta into medullary interstitium.
The blood flows very slowly through vasa recta. So, a large quantity of sodium chloride accumulates in descending limb of vasa recta and flows slowly towards ascending limb. By the time the blood reaches the ascending limb of vasa recta, the concentration of sodium chloride increases very much. This causes diffusion of sodium chloride into the medullary interstitium. Simultaneously, water from medullary interstitium enters the ascending limb of vasa recta. And the cycle is repeated.
If the vasa recta would be a straight vessel without hairpin arrangement, blood would leave the kidney quickly at renal papillary level. In that case, the blood would remove all the sodium chloride from medullary
FIGURE 53.2: Countercurrent exchanger. Numerical indicate osmolarity (mOsm/L)
interstitium and thereby the hyperosmolarity will be decreased. However, this does not happen, since the vasa recta has a hairpin bend.
Therefore, when blood passes through the ascending limb of vasa recta, sodium chloride diffuses out of blood and enters the interstitial fluid of medulla and, water diffuses into the blood.
Thus, vasa recta retains sodium chloride in the medullary interstitium and removes water from it. So, the hyperosmolarity of medullary interstitium is maintained. The blood passing through the ascending limb of vasa recta may carry very little amount of sodium chloride from the medulla.
Recycling of urea also occurs through vasa recta. From medullary interstitium, along with sodium chloride, urea also enters the descending limb of vasa recta. When blood passes through ascending limb of vasa recta, urea diffuses back into the medullary interstitium along with sodium chloride.
Thus, sodium chloride and urea are exchanged for water between the ascending and descending limbs of vasa recta, hence this system is called countercurrent exchanger.

ROLE OF ADH
Final concentration of urine is achieved by the action of ADH. Normally, the distal convoluted tubule and collecting duct are not permeable to water. But the presence of ADH makes them permeable, resulting in water reabsorption. Water reabsorption induced by ADH is called facultative reabsorption of water .
A large quantity of water is removed from the fluid while passing through distal convoluted tubule and collecting duct. So, the urine becomes hypertonic with an osmolarity of 1,200 mOsm/L .