Question

Human Physiology

Explain in detail the process that establishes and maintains the medullary osmotic gradient in the kidneys. Then describe the effect this osmotic gradient has on the water in the renal tubules.

Answer 1

Ans- 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 Henle
2. Countercurrent exchanger formed by vasa recta.

COUNTERCURRENT MULTIPLIER-

Loop of Henle-
Loop of Henle functions as countercurrent multiplier. It
is responsible for development of hyperosmolarity of
medullary interstitial fluid and medullary gradient.

Role of Loop of Henle in Development
of 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 ADH.

COUNTERCURRENT EXCHANGER

Vasa Recta-
Vasa recta functions as countercurrent exchanger. It is
responsible for the maintenance of medullary gradient,
which is developed by countercurrent multiplier.

Role of Vasa Recta in the Maintenance
of Medullary Gradient-
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 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.

When the water content in body decreases, kidney
retains water and excretes concentrated urine. Thus for absorption of water and concentration of urine this medullary intersitituim plays an important role as explained above. The second role is played by the release of ADH which makes distal convoluted tubule and collecting duct permeable to water and results in absorption of water.