The principles of homeostasis and negative feedback, the structure and function of the kidney, ultrafiltration, selective reabsorption and osmoregulation.

What this dot point is asking

WJEC wants you to explain the principles of homeostasis and negative feedback, describe the structure of the kidney and nephron, and explain ultrafiltration, selective reabsorption and the control of water potential by ADH.

Homeostasis and negative feedback

Negative feedback keeps variables such as blood glucose, temperature and water potential within narrow limits. The opposite, positive feedback, amplifies a change (as in the surge of oxytocin during birth) and is rarer because it is destabilising.

The kidney and ultrafiltration

The kidney is made of about a million nephrons. Blood enters each nephron through an afferent arteriole into the glomerulus, a knot of capillaries inside the cup-shaped Bowman's capsule. The afferent arteriole is wider than the efferent arteriole leaving it, which raises the pressure inside the glomerulus.

Reabsorption and osmoregulation

In the proximal convoluted tubule, selective reabsorption returns all the glucose, most ions and much water to the blood; glucose is taken up by co-transport with sodium, and water follows by osmosis. The walls here have microvilli and many mitochondria to power active uptake. The loop of Henle acts as a counter-current multiplier, building a high salt concentration deep in the medulla so water can be reabsorbed from the collecting duct by osmosis as urine passes through it.

Osmoregulation controls blood water potential. When the blood is too concentrated, osmoreceptors in the hypothalamus trigger release of ADH from the posterior pituitary; ADH inserts aquaporins, making the distal tubule and collecting duct more permeable to water, so more water is reabsorbed and a small volume of concentrated urine forms. When blood is too dilute, less ADH is released and large volumes of dilute urine are made.

Examples in context

Example 1. Diabetes insipidus. In this condition the body cannot make or respond to ADH, so the collecting ducts stay impermeable to water. The patient produces huge volumes of dilute urine and is constantly thirsty, a clear clinical demonstration of ADH's role in osmoregulation.

Example 2. The desert kangaroo rat. This animal has an extremely long loop of Henle, building a very steep salt gradient in the medulla so it reabsorbs almost all its water and excretes tiny amounts of highly concentrated urine. It can survive without drinking, showing how nephron structure is adapted to water availability.

Try this

Q1. Name the process by which small molecules are forced out of the glomerulus into the Bowman's capsule. [1 mark]

• Cue. Ultrafiltration.

Q2. Explain why glucose is normally absent from urine. [2 marks]

• Cue. All the glucose is selectively reabsorbed by co-transport in the proximal convoluted tubule before urine forms.

Q3. The kidneys filter $180 \text{ dm}^3$ per day and produce $1.8 \text{ dm}^3$ of urine. Calculate the percentage of filtrate reabsorbed. [2 marks]

• Cue. Reabsorbed $= 180 - 1.8 = 178.2$; $\frac{178.2}{180} \times 100 = 99\%$.