Osmoregulation and ADHEdexcel International A Level Biology: Revision notes

The glomerular filtrate contains useful substances as well as waste, so most of the useful ones are reabsorbed in the proximal convoluted tubule (PCT). The cells lining the PCT are adapted for this: microvilli give a large surface area on the side facing the filtrate, and many mitochondria supply ATP.

The process is:

• Sodium–potassium pumps in the membrane on the blood side actively transport Na⁺ out of the cell, so the Na⁺ concentration inside the cell is low.
• Na⁺ then moves from the filtrate into the cell by facilitated diffusion, through co-transporter proteins that carry glucose or amino acids into the cell at the same time.
• The glucose and amino acids move into the blood by facilitated diffusion.
• Water follows the solutes by osmosis.

All the glucose and amino acids are normally reabsorbed, so none appears in the urine.

The loop of Henle creates a very low water potential in the medulla so that water can be reabsorbed from the filtrate. It works as a countercurrent multiplier: the two limbs carry fluid in opposite directions and the effect of the ion pumping is multiplied.

• The ascending limb actively transports Na⁺ and Cl⁻ out into the medulla. This limb is impermeable to water, so water stays in the filtrate.
• The tissue fluid in the medulla now has a high solute concentration, giving it a low water potential.
• The descending limb is permeable to water but not to ions. Water leaves by osmosis, so the filtrate becomes more concentrated towards the bottom of the loop.
• The concentration gradient steepens deeper into the medulla, so water can also leave the collecting duct by osmosis as it passes through.

Animals living in dry habitats, such as the kangaroo rat, have longer loops of Henle. This gives a steeper gradient, so they reabsorb more water and make a small volume of concentrated urine.

The water potential of the blood is kept near a set point by negative feedback.

• Osmoreceptors in the hypothalamus detect a fall in the water potential of the blood. They lose water by osmosis and shrink.
• The hypothalamus makes antidiuretic hormone (ADH), which passes to the posterior pituitary gland. More ADH is released into the blood.
• ADH binds to receptors on the cells of the collecting duct (and distal convoluted tubule). This inserts more aquaporins into the membrane, so the walls become more permeable to water.
• More water is reabsorbed by osmosis. The urine is a small volume of concentrated urine, and the water potential of the blood rises.

If the water potential of the blood is too high, less ADH is released, fewer aquaporins are present, and a large volume of dilute urine is produced.

Water reabsorption also regulates blood volume. Dehydration lowers both the water potential and the volume of the blood. More ADH means more water is retained, which helps to restore both. After drinking a large amount of water the changes are reversed: less ADH, fewer aquaporins, and a large volume of dilute urine.

The water reabsorbed in the proximal convoluted tubule follows the solutes whatever the ADH level, so ADH only controls the final fine adjustment in the distal convoluted tubule and collecting duct.