Renal physiology: Counter current multiplication

What is countercurrent multiplication?

Your kidneys have a remarkable mechanism for reabsorbing water from the tubular fluid, called countercurrent multiplication.
  • Countercurrent multiplication in the kidneys is the process of using energy to generate an osmotic gradient that enables you to reabsorb water from the tubular fluid and produce concentrated urine. This mechanism prevents you from producing litres and litres of dilute urine every day, and is the reason why you don’t need to be continually drinking in order to stay hydrated.

Where does it happen?

The kidneys contain two types of nephrons, superficial cortical nephrons (70-80%) and juxtamedullary nephrons (20-30%). These names refer to the location of the glomerular capsule, which is either in the outer cortex of the kidney, or near the corticomedullary border. Nephrons can be thought of in sections, each with a different structure and function. These are the glomerulus, the proximal tubule, the loop of Henle, the distal tubule, and the collecting duct. The loop of Henle is a hairpin-like structure comprised of a thin descending limb, a thin ascending limb and a thick ascending limb. While the loops of Henle of cortical nephrons penetrate only as far as the outer medulla of the kidney, those of the juxtamedullary nephrons penetrate deeply within the inner medulla.
Although both cortical and juxtamedullary nephrons regulate the concentrations of solutes and water in the blood, countercurrent multiplication in the loops of Henle of juxtamedullary nephrons is largely responsible for developing the osmotic gradients that are needed to concentrate urine. Fluid leaving the ascending limb of the loop of Henle enters the distal convoluted tubule, where its composition is further adjusted, and then drains into collecting tubules. These tubules empty into collecting ducts that descend back through the medulla, and eventually connect to the ureter, which transports urine to the bladder.
Cross section of the kidney with various labeled segments such as the cortex, papilla, and inner medulla.
Labeled diagram of the nephron.
Although the loops of Henle are essential for concentrating urine, they do not work alone. The specialized blood capillary network (the vasa recta) that surrounds the loops are equally important. The vasa recta capillaries are long, hairpin-shaped blood vessels that run parallel to the loops of Henle. The hairpin turns slow the rate of blood flow, which helps maintain the osmotic gradient required for water reabsorption.
Illustration of the vasa recta which run alongside nephrons.

How does countercurrent multiplication work?

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:
  1. 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.
  2. 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.
Illustration of the nephron with various pressures highlighted along different parts of it.
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.

Urea recycling

Urea recycling in the inner medulla also contributes to the osmotic gradient generated by the loops of Henle. Antidiuretic hormone increases water permeability, but not urea permeability in the cortical and outer medullary collecting ducts, causing urea to concentrate in the tubular fluid in this segment. In the inner medullary collecting ducts it increases both water and urea permeability, which allows urea to flow passively down its concentration gradient into the interstitial fluid. This adds to the osmotic gradient and helps drive water reabsorption.

Consider the following:

The kidneys are able to separate the reabsorption of water and solutes in the loop of Henle, distal nephron and collecting ducts. This means urine can be made more concentrated or more dilute than plasma, depending on how hydrated you are. This process is mainly controlled by antidiuretic hormone, a hormone that is made in the hypothalamus of the brain and stored in the pituitary gland. The release of antidiuretic hormone by the pituitary gland is controlled by sensors in your heart and blood vessels that detect drops in blood pressure, or increased concentrations of salt in your bloodstream that may occur when you are dehydrated. If you have ever felt dehydrated after having a few glasses of wine or beer, this is because alcohol inhibits the release of antidiuretic hormone, which increases urine production.
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