Control of Blood Water Potential – Homeostasis Ep 6

In the first article about the kidneys, we learnt how blood is filtered and substances are reabsorbed in the Bowman’s capsule and the proximal convoluted tubule. Today we will focus on the loop of Henle, distal convoluted tubule, and collecting duct, and look at how blood water potential is controlled.

The loop of Henle, distal convoluted tubule, and collecting duct

The loop of Henle is part of the nephron tubule which loops down into the kidney medulla (check out the diagram of the whole nephron in the last article). It has a descending limb taking the filtrate down into the medulla, and an ascending limb bringing the filtrate back up to the distal convoluted tubule (DCT). The loop of Henle moves sodium (Na⁺) ions into the medulla to allow for reabsorption of water:

1. In the top part of the ascending limb, Na⁺ ions are pumped out of the filtrate into the medulla using active transport. Water is not drawn out by osmosis here because the ascending limb is impermeable to water.
2. At the bottom of the ascending limb, Na⁺ ions move out of the filtrate into the medulla by diffusion.
3. The Na⁺ ions have lowered the water potential in the medulla. Water moves out of the filtrate by osmosis at the descending limb and into the medulla. The water is reabsorbed into the blood through the nearby capillaries.
4. Water moves out of the DCT by osmosis and is reabsorbed.
5. Water moves out of the collecting duct by osmosis and is reabsorbed.

It is the transport of Na⁺ ions that allows osmosis to occur. The water potential gradient must be maintained.

Control of blood water potential by ADH

This is where the homeostasis bit comes in, and a bit of negative feedback.

The hormone which controls the amount of water reabsorbed by the DCT and collecting duct is called antidiuretic hormone (ADH). ADH is produced by the hypothalamus but released by the posterior pituitary gland.

When blood water potential is too low (dehydration):

• Osmoreceptors in the hypothalamus detect the decrease in water potential because water moves out of the osmoreceptor cells by osmosis.
• The hypothalamus sends a nerve impulse to stimulate the posterior pituitary gland to release more ADH into the blood.
• ADH acts to make the DCT and collecting duct more permeable to water.
• More water is reabsorbed into the blood, and less is lost in urine. Smaller amounts of concentrated urine are produced.

When blood water potential is too high (hydration):

• Osmoreceptors in the hypothalamus detect the increase in water potential because water moves into the osmoreceptor cells by osmosis.
• The posterior pituitary gland releases less ADH into the blood.
• The DCT and collecting duct are less permeable to water.
• Less water is reabsorbed into the blood, and more is lost in urine. Larger amounts of dilute urine are produced.

Summary

Here is a summary diagram showing how blood water potential is controlled.