Kidneys, ultrafiltration, reabsorption and ADH

What you’ll learn

• Relate nephron structure to filtration and reabsorption.
• Explain the loop of Henle as a countercurrent multiplier.
• Trace negative feedback in plasma water balance.

Understand the idea

The kidney contains an outer cortex and inner medulla draining through the pelvis to the ureter. Nephrons contain a glomerulus within Bowman’s capsule, proximal convoluted tubule, loop of Henle, distal tubule and collecting duct. Excess amino acids are deaminated in the liver; toxic ammonia is converted to urea for excretion. In the glomerulus, high hydrostatic pressure drives small solutes and water through a filtration barrier, while blood cells and most large proteins remain in the blood.

The proximal tubule selectively reabsorbs useful substances. Sodium gradients maintained by active transport support co-transport of glucose and other solutes; water follows by osmosis. Microvilli increase surface area and many mitochondria provide ATP. In the loop of Henle, the ascending limb transfers salts to the medulla and is relatively impermeable to water, while the descending limb loses water to the concentrated surroundings. Opposite flow multiplies the gradient, enabling water recovery from collecting ducts.

If plasma becomes more concentrated and its water potential falls, hypothalamic osmoreceptors stimulate increased ADH release from the posterior pituitary. ADH is made in the hypothalamus and increases collecting-duct water permeability by promoting aquaporin insertion. More water returns to the blood, producing a smaller volume of concentrated urine. Dilute plasma reduces ADH release. The response helps regulate plasma concentration and blood volume through negative feedback; urea still requires excretion even during water conservation.

Key formulae

$$\text{fractional reabsorption}=\frac{\text{amount filtered}-\text{amount excreted}}{\text{amount filtered}}$$

$$\text{percentage reabsorbed}=\text{fractional reabsorption}\times100\%$$

Worked example

A person becomes dehydrated. Explain the sequence from plasma change to urine change.

1. Loss of water makes plasma more concentrated and lowers its water potential.
2. Osmoreceptors stimulate increased ADH release from the posterior pituitary.
3. Collecting ducts become more permeable to water, so more water moves by osmosis into the medullary tissue and blood.

Answer: A smaller volume of more concentrated urine helps conserve water and oppose the original plasma change.