Question

1. Explain how the renal medullary osmotic gradient is generated and maintained. Describe how this is important for the regulation of water balance and urine volume. Include the terms Loop of Henle, aquaporin and antidiuretic hormone.

Please write long answer like an essay.

Answer 1

Generating and Maintaining
a High Medullary Interstitial Osmolality

Counter current multiplication
-Transporters in the loop of Henle generate the vertical osmotic gradient.

-The descending thin limb is PERMEABLE to water but IMPERMEABLE to NaCl

-The thin and thick ascending limbs are permeable to Na+, K+ and Cl- but impermeable to water >Referred to as the DILUTING segment.

-Counter current exchange
The unique blood flow in vasa recta MAINTAINS the osmotic gradient.


-Kidney is capable of producing urine with osmolalities ranging from a low of 50-100 mOs/kg to a high of approximately 1200 mOs/kg.
>At these extremes, typically either a large volume of dilute urine or a small volume of concentrated urine is produced.
-The kidney is uniquely adapted to produce urine with this range of osmolalities.
>A vertical osmotic gradient established in the renal medulla is the cornerstone of the process.
->This vertical gradient must be both established and maintained.


The loop of Henle is responsible for ESTABLISHING the gradient and the vasa recta MAINTAINS the gradient.

IN DETAIL

The three segments of the loops of Henle have different characteristics that enable countercurrent multiplication.

The thin descending limb is passively permeable to both water and small solutes such as sodium chloride and urea. As active reabsorption of solutes from the ascending limb of the loop of Henle increases the concentration of solutes within the interstitial space (space between cells), water and solutes move down their concentration gradients until their concentrations within the descending tubule and the interstitial space have equilibrated. As such, water moves out of the tubular fluid and solutes to move in. This means, the tubular fluid becomes steadily more concentrated or hyperosmotic (compared to blood) as it travels down the thin descending limb of the tubule.

The thin ascending limb is passively permeable to small solutes, but impermeable to water, which means water cannot escape from this part of the loop. As a result, solutes move out of the tubular fluid, but water is retained and the tubular fluid becomes steadily more dilute or hyposmotic as it moves up the ascending limb of the tubule.

The thick ascending limb actively reabsorbs sodium, potassium and chloride. this segment is also impermeable to water, which again means that water cannot escape from this part of the loop. This segment is sometimes called the “diluting segment”.

Countercurrent multiplication moves sodium chloride from the tubular fluid into the interstitial space deep within the kidneys. Although in reality it is a continual process, the way the countercurrent multiplication process builds up an osmotic gradient in the interstitial fluid can be thought of in two steps:

The single effect. The single effect is driven by active transport of sodium chloride out of the tubular fluid in the thick ascending limb into the interstitial fluid, which becomes hyperosmotic. As a result, water moves passively down its concentration gradient out of the tubular fluid in the descending limb into the interstitial space, until it reaches equilibrium.

Fluid flow. As urine is continually being produced, new tubular fluid enters the descending limb, which pushes the fluid at higher osmolarity down the tube and an osmotic gradient begins to develop.

As the fluid continues to move through the loop of Henle, these two steps are repeated over and over, causing the osmotic gradient to steadily multiply until it reaches a steady state. The length of the loop of Henle determines the size of the gradient - the longer the loop, the greater the osmotic gradient.

Absorbed water is returned to the circulatory system via the vasa recta, which surrounds the tips of the loops of Henle. Because the blood flow through these capillaries is very slow, any solutes that are reabsorbed into the bloodstream have time to diffuse back into the interstitial fluid, which maintains the solute concentration gradient in the medulla. This passive process is known as countercurrent exchange.

The concentration of urine is controlled by antidiuretic hormone, which helps the kidneys to conserve water. Its main effects in the renal tubules is to increase water permeability in the late distal tubule and collecting ducts, increase active transport of sodium chloride in the thick ascending limb of the loop of Henle, and enhance countercurrent multiplication and urea recycling, all of which increase the size of the osmotic gradient.