The role of the kidney in osmoregulation and in the excretion of metabolic waste. The detailed structure of a nephron and its associated blood vessels. The processes of ultrafiltration and selective reabsorption, the role of the loop of Henle in producing concentrated urine, and the control of blood water potential by antidiuretic hormone (ADH) through negative feedback.

What this dot point is asking

AQA wants you to explain the kidney's role in osmoregulation and excretion, describe the nephron structure, explain ultrafiltration and selective reabsorption, the loop of Henle, and the control of blood water potential by ADH.

The answer

The role of the kidney

The kidneys carry out two key roles:

• Excretion of metabolic waste, especially urea (made in the liver from excess amino acids by deamination, then the ornithine cycle).
• Osmoregulation: controlling the water potential of the blood by adjusting how much water is reabsorbed.

Structure of the nephron

A nephron is the functional unit of the kidney. Its parts, in order, are:

1. Bowman's (renal) capsule, surrounding a knot of capillaries called the glomerulus.
2. Proximal convoluted tubule (in the cortex).
3. Loop of Henle (dipping into the medulla), with a descending and an ascending limb.
4. Distal convoluted tubule.
5. Collecting duct, which carries urine to the ureter.

Associated blood vessels: blood enters the glomerulus via the wide afferent arteriole and leaves via the narrower efferent arteriole; this difference creates high pressure in the glomerulus.

Ultrafiltration

In the Bowman's capsule, ultrafiltration produces a filtrate from the blood:

• Blood in the glomerulus is at high hydrostatic pressure because the efferent arteriole is narrower than the afferent arteriole.
• This forces small molecules (water, glucose, ions, urea) through the basement membrane, which acts as the filter.
• Large molecules (proteins) and blood cells are too big to pass and stay in the blood.
• The filtrate also passes between podocytes (cells with finger-like projections) lining the capsule.

Selective reabsorption

Most of the useful filtrate is reabsorbed back into the blood, mainly in the proximal convoluted tubule (PCT):

• All the glucose and all the amino acids are reabsorbed by co-transport with sodium ions (active transport), then diffuse into the blood.
• The PCT cells are adapted with microvilli (large surface area), many mitochondria (ATP for active transport), and many co-transporter proteins.
• Water follows by osmosis.

The loop of Henle and concentrated urine

The loop of Henle acts as a countercurrent multiplier that creates a very low (very negative) water potential in the medulla, allowing the production of concentrated urine.

• The ascending limb is impermeable to water. Sodium and chloride ions are actively pumped out of the ascending limb into the surrounding tissue (medulla), lowering the water potential of the medulla.
• The descending limb is permeable to water but not to ions. As filtrate flows down through the increasingly low water potential of the medulla, water leaves by osmosis into the blood, concentrating the filtrate.
• Because the limbs run in opposite directions (countercurrent), a steep water potential gradient is maintained down the length of the medulla.
• As filtrate finally passes down the collecting duct through this low water potential medulla, water leaves by osmosis (if the duct is permeable), producing concentrated urine.

Control of blood water potential by ADH

The water potential of the blood is controlled by antidiuretic hormone (ADH) through negative feedback.

If blood water potential is too low (dehydration):

1. Osmoreceptors in the hypothalamus detect the fall in water potential.
2. The posterior pituitary gland releases more ADH into the blood.
3. ADH makes the collecting duct (and distal tubule) more permeable to water by inserting more aquaporins (water channel proteins) into the membranes.
4. More water is reabsorbed by osmosis into the blood, so a small volume of concentrated urine is produced and water potential rises back to normal.

If blood water potential is too high (overhydration):

1. Osmoreceptors detect the rise in water potential.
2. The posterior pituitary releases less ADH.
3. The collecting duct becomes less permeable; less water is reabsorbed.
4. A large volume of dilute urine is produced and water potential falls back to normal.

Examples in context

Example 1. The desert kangaroo rat. The kangaroo rat survives in deserts without drinking by producing extremely concentrated urine. Its very long loops of Henle create an exceptionally steep water potential gradient in the medulla, allowing it to reabsorb almost all the water from the collecting duct. This adaptation directly illustrates how loop length controls urine concentration.

Example 2. Alcohol, caffeine and ADH suppression. Alcohol inhibits ADH release from the posterior pituitary. With less ADH, the collecting ducts become less permeable to water, less water is reabsorbed and a large volume of dilute urine is produced. This is why drinking alcohol leads to increased urination and contributes to the dehydration behind a hangover, a real-world demonstration of ADH control.

Try this

Q1. Explain how ultrafiltration occurs in the Bowman's capsule. [3 marks]

• Cue. The efferent arteriole is narrower than the afferent, raising hydrostatic pressure in the glomerulus; this forces small molecules through the basement membrane; large proteins and cells remain in the blood.

Q2. Explain how the loop of Henle enables the production of concentrated urine. [4 marks]

• Cue. Ions actively pumped out of the impermeable ascending limb lower the medulla water potential; water leaves the permeable descending limb and collecting duct by osmosis; the countercurrent arrangement maintains the gradient.

Q3. Describe how the body responds to a fall in blood water potential. [4 marks]

• Cue. Osmoreceptors in the hypothalamus detect the fall; posterior pituitary releases more ADH; collecting duct becomes more permeable (more aquaporins); more water reabsorbed, producing concentrated urine and raising water potential.