Homeostasis and the kidney: the principle of negative feedback; the structure of the nephron; ultrafiltration and selective reabsorption; the role of the loop of Henle; and osmoregulation by ADH.

What this dot point is asking

Eduqas wants you to explain negative feedback, describe the structure of the nephron, explain ultrafiltration and selective reabsorption, explain the role of the loop of Henle, and explain osmoregulation by ADH. This is the homeostasis core of Component 3.

Negative feedback

The nephron and ultrafiltration

A nephron is the kidney's functional unit: the renal (Bowman's) capsule with its glomerulus, then the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule and the collecting duct.

Ultrafiltration occurs in the renal capsule: blood enters the glomerulus through a wide afferent arteriole and leaves by a narrower efferent arteriole, creating high hydrostatic (blood) pressure. This forces water and small molecules (glucose, ions, urea) out through the capillary wall, the basement membrane and the podocytes into the capsule. The basement membrane retains large molecules (plasma proteins, cells), so the filtrate contains water, glucose, ions and urea but no proteins or cells.

Selective reabsorption

In the proximal convoluted tubule, useful substances are reabsorbed into the blood: all the glucose and amino acids (by co-transport with sodium), most ions, and much water (by osmosis). The cells are adapted with microvilli (surface area) and many mitochondria (ATP for active transport). Normally no glucose appears in the urine.

The loop of Henle and osmoregulation

Osmoregulation is the control of the water potential of the blood. Osmoreceptors in the hypothalamus detect changes: if blood water potential falls (too concentrated), the posterior pituitary releases more ADH (antidiuretic hormone), which makes the collecting duct more permeable to water (inserting aquaporins), so more water is reabsorbed and a small volume of concentrated urine is made. If blood water potential rises, less ADH is released, less water is reabsorbed, and dilute urine is made. This is negative feedback.

Examples in context

Example 1. Desert mammals and long loops. The kangaroo rat has very long loops of Henle, making its medulla extremely salty so it reabsorbs almost all its water and produces tiny amounts of highly concentrated urine, a classic Eduqas adaptation example.

Example 2. Glucose in the urine and diabetes. In untreated diabetes, blood glucose is so high that not all of it can be reabsorbed in the proximal tubule, so glucose appears in the urine, linking selective reabsorption to a clinical sign.

Try this

Q1. State what is meant by negative feedback. [2 marks]

• Cue. A change from the set point is detected and a response reverses it, returning the factor towards the set point.

Q2. Explain why plasma proteins are not found in the glomerular filtrate. [2 marks]

• Cue. They are too large to pass through the basement membrane during ultrafiltration, so they remain in the blood.

Q3. State what happens to ADH release when the water potential of the blood rises (the blood is too dilute). [2 marks]

• Cue. Less ADH is released, so the collecting duct is less permeable to water, less water is reabsorbed, and a large volume of dilute urine is produced.