Controlling Blood Water Potential

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The Kidneys Regulate the Water Potential of the Blood

Water is essential to keep the body functioning, so the amount of water in the blood (and so the water potential of the blood) needs to be kept constant.

Mammals excrete urea (and other waste products) in solution, which means water is lost during excretion. Water is also lost in sweat. The kidneys regulate the water potential of the blood (and urine) so the body has just the right amount of water - this is called osmoregulation.

If the water potential of the blood is too low (the body is dehydrated), more water is reabsorbed by osmosis into the blood from the tubules of the nephrons. This means the urine is more concentrated so less water is lost during excretion.

If the water potential of the blood is too high (the body is too hydrated), less water is reabsorbed by osmosis into the blood from the tubules of the nephrons. This means urine is more dilute, so more water is lost during excretion.

Water is reabsorbed into the blood along almost all of the nephron, but regulation of water potential mainly takes place in the Loop of Henle, DCT and collecting duct. The volume of water reabsorbed by the DCT and collecting duct is controlled by hormones.

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The Loop of Henle Maintains a Sodium Ion Gradient

The Loop of Henle is located in the medulla (inner layer) of the kidneys. It's made up of two limbs' - the descending limb and the ascending limb. The limbs control the movement of sodium ions so that water can be reabsorbed by the blood.

Near the top of the ascending limb, Sodium ions are pumped out into the medulla using active transport. The ascending limb is impermeable to water so the water stays inside the tubule. This creates a low water potential in the medulla, because there's a high concentration of ions.

Because there's a lower water potential in the medulla than in the descending limb, water moves out of the descending limb (which is permeable to water) into the medulla by osmosis. This makes the filtrate more concentrated (the ions can't diffuse out - the descending limb isn't permeable to them). The water in the medulla is reabsorbed into the blood through the capillary network.

Near the bottom of the ascending limb sodium ions diffuse out into the medulla further lowering the water potential in the medulla. The ascending limb is impermeable to water, so it stays in the tubule.

Water moves out of the distal convoluted tubule (DCT) by osmosis and is reabsorbed into the blood.

The first three stages massively increase the ion concentration in the medulla, which lowers the water potential. This causes water to move out of the collecting duct by osmosis. As before, the water in the medulla is reabsorbed into the blood through the capillary network.

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Water Reabsorption is Controlled by Hormones

The water potential of the blood is monitored by cells called osmoreceptors in a part of the brain called the hypothalamus.

When the water potential of the blood decreases, water will move out of the osmoreceptor cells by osmosis. This causes the cells to decrease in volume. This sends a signal to other cells in the hypothalamus, which send a signal to the posterior pituitary gland. This causes the posterior pituitary to release a hormone called antidiuretic hormone (ADH) into the blood.

ADH makes he walls of the DCT and collecting duct more permeable to water.

This means more water is reabsorbed from these tubules into the medulla and into the blood by osmosis. A small amount of concentrated urine is produced, which means less water is lost from the body.

Here's how ADH changes the water content of the blood when it's too low or too high:

BLOOD ADH LEVEL RISES WHEN YOU'RE DEHYDRATED

Dehydration is what happens when you lose water, e.g. by sweating during exercise, so the water content of the blood needs to be increased:

The water content of the blood drops, so it's water potential drops.

This is detected by osmoreceptors in the hypothalamus.

The posterior pituitary gland is stimulated to release more ADH into the blood.

More ADH means that the DCT and collecting duct are more permeable, so more water is reabsorbed into the blood by osmosis.

A small amount of highly concentrated urine is produced and less water is lost.

BLOOD ADH LEVEL FALLS WHEN YOU'RE HYDRATED

If you're hydrated, you've taken in lots of water, so the water content of the blood needs to be reduced:

The water content of the blood rises, so it's water potential rises.

This is detected by the osmoreceptors in the hypothalmus.

The posterior pituitary gland released less ADH into the blood.

Less ADH means that the DCT and collecting duct become less permeable, so less water is reabsorbed into the blood by osmosis.

A large amount of dilute urine is produced and more water is lost.