Concentration and dilution of urine, Lecture notes of Physiology

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Concentration and

Dilution of Urine

General Concept

• (^) In human beings, urine can be formed with an osmolality ofas high as

1400 mosm/kg of H2O and as low as 30 mosm/kgof H2O. Such a

capability of kidneys to pass urine with extremes of osmolality is a

fundamental necessity for volume and electrolyte homeostasis.

• (^) Though glomerular filtrate is about 180 liters a day, only about 1.

liters of urine is excreted daily.

• (^) Therefore, normally, kidney excretes a concentrated urine to prevent

volume depletion from the body, in which osmolality of urine

is an index of its concentrating and diluting

capacity.

Requirements of a Countercurrent

Mechanism

• (^) The mechanism by which urine is concentrated is known as countercurrent mechanism. In medulla of the kidney, there is an increasing gradient of osmolality from outer region to inner region of medulla with highest osmolality at the tip of the renal papillae. This osmolality gradient, which is produced and maintained by the counter current mechanisms, is essential for urine concentration. Countercurrent means flow of fluid in opposite direction in adjacent structures. For any countercurrent mechanism to operate, it requires three conditions :
• (^) 1. There should be two tubes (inflow and outflow tubes) that should run parallel to each other.
• (^) 2. The movement of fluid in one tube should be in opposite direction to the other.
• (^) 3. The tubes should be in close proximity to each other and should be selectively permeable.

In Kidney

• (^) countercurrent mechanisms of simila nature operate. Two countercurrent processes work in the medulla of the kidney: countercurrent multiplication system and countercurrent exchange system. Loop of Henle (LOH) acts as countercurrent multiplier and vasa recta as countercurrent exchanger.
• (^) 1. In LOH, fluid flows deep into the medulla along the descending limb and from medulla into the cortex along the ascending limb. The descending and ascending limbs for their selective permeabilities for water and solutes, establish an osmotic gradient in the medulla.
• (^) 2. In vasa recta , blood flows in opposite direction in descending and ascending vessels during which water and solutes are exchanged passively between capillary blood vessels. This passive exchange in vasa recta maintains the osmotic gradient.
• (^) 3. Collecting duct is juxtaposed to LOH and vasa recta , and acts as an osmotic equilibrating device.

• (^) More axial gradient: - Slower the fluid flow, larger the single effect and greater the loop length, the bigger is the axial gradient.
• (^) Less axial gradient :- Conversely, faster the fluid flow, smaller the single effect and shorter LOH reduces the axial gradient.
• (^) Rate of flow is Important : When the rate of flow is slower , time taken for transport of solutes in the ascending limb of LOH is more, andtherefore, the strength of single effect and consequently the magnitude of axial gradient is enhanced.
• (^) When LOH islonger , a greater axial gradient is generated as the chance for multiplication becomes higher.

Countercurrent Exchange System

• (^) The solute gradient created in the medulla by LOH is maintained by vasa recta.
• 1. The exchange of solutes and water takes place purely passively between descending and ascending limbs of vasa recta and this process of exchange decreases the dissipation of the solute gradient from medullary interstitium.
• (^) 2. Water is removed from descending limb of vasa recta into the hyperosmolal interstitium, which is taken back by ascending limb of vasa recta.
• (^) 3. Thus, vasa recta prevents dilution of osmolality of interstitium.
• 4. Moreover, solutes come out of ascending limb and enter the descending limb of LOH, which ensures trapping of solutes in the medulla.
• (^) 5. Thus, the process of exchange of water and solutes between descending and ascending limbs of vasa recta maintains the gradient of osmolality established by LOH.

Countercurrent Mechanism

• (^) The countercurrent mechanism for urine concentration depends on

generation and maintenance of an increasing

osmolality gradient along the medullary pyramid from its outer

layer to its deepest part. As discussed above, two systems operate to

generate and maintain the high osmolality gradient in the medullary

interstitium.

• (^) 1. The osmolality gradient is generated by the loop of

Henle , which acts as countercurrent multiplier, and maintained

by vasa recta , which acts as countercurrent exchanger.

• 2. However, collecting duct contributes in this process

by equilibrating the gradient (acts as equilibrating device).

Thus, renal countercurrent mechanism depends on integrated

functioning of loop of Henle, vasa recta and collecting duct.

Osmotic gradient across the medulla of kidney