Describe osmoregulation and excretion in mammals, including the structure and function of the nephron and the role of ADH in regulating water balance

What this dot point is asking

QCAA wants you to describe the structure and function of the nephron, identify the four processes occurring in different nephron regions, and explain the role of ADH in adjusting water reabsorption through negative feedback. Diagrams of the nephron appear regularly as stimulus.

The answer

Osmoregulation is the control of water and solute balance in body fluids; excretion is the removal of metabolic wastes (urea, creatinine, excess ions). In mammals, both functions are performed by the kidneys, with the nephron as the functional unit.

The kidney and the nephron

Each human kidney contains around one million nephrons. A nephron is a long tubule served by a tuft of capillaries.

Structure (in order).

1. Bowman's capsule. Cup-shaped structure surrounding the glomerulus, a capillary tuft.
2. Proximal convoluted tubule (PCT). Long coiled section in the cortex; cells lined with microvilli for high surface area.
3. Loop of Henle. Hairpin loop that dips into the medulla; descending and ascending limbs have different permeability.
4. Distal convoluted tubule (DCT). Shorter coiled section in the cortex.
5. Collecting duct. Joins multiple nephrons; runs through the medulla to the renal pelvis.

The four processes

The whole kidney function is built from four sequential processes.

1. Filtration (glomerulus and Bowman's capsule).

Blood enters the glomerulus through the afferent arteriole (wider) and leaves through the efferent arteriole (narrower). The narrowing creates high hydrostatic pressure inside the glomerulus.

Pressure forces water and small solutes (glucose, amino acids, ions, urea) out of the capillaries through fenestrations, the basement membrane and the filtration slits between podocyte foot processes, into the lumen of Bowman's capsule. Blood cells and large proteins are too big and remain in the blood.

The result is the glomerular filtrate, around 180 L per day in humans.

2. Reabsorption (PCT, loop of Henle, DCT, collecting duct).

Useful substances are returned to the blood from the filtrate.

• PCT. Reabsorbs all glucose and amino acids by active transport (and secondary active transport with Na+), most ions (Na+, Cl-, HCO3-), and around 65 percent of water by osmosis. The microvilli and mitochondria-packed PCT cells are adapted for this high-energy reabsorption.
• Loop of Henle. The descending limb is permeable to water but not solutes; water leaves by osmosis. The ascending limb actively pumps out Na+ and Cl- but is impermeable to water; the filtrate is diluted. The net result is a salt gradient in the medulla (more concentrated deeper in the medulla), which is the engine that powers water reabsorption from the collecting duct.
• DCT. Selective ion reabsorption under hormonal control (aldosterone increases Na+ reabsorption).
• Collecting duct. Variable water reabsorption under ADH control (see below).

3. Secretion (mainly DCT).

Active transport of substances from the blood into the filtrate. Hydrogen ions (acid balance), potassium ions, ammonium and some drugs are added to the urine here.

4. Excretion.

The final urine drains from the collecting duct through the renal pelvis, ureter, bladder and urethra. About 1.5 L per day of urine is excreted in humans.

The role of ADH

Antidiuretic hormone (ADH, vasopressin) sets the permeability of the collecting duct to water and is the key short-term regulator of blood osmolarity.

Pathway.

1. Stimulus. Blood osmolarity rises above the set point (around 300 mOsm per kg).
2. Receptor. Osmoreceptors in the hypothalamus detect the rise.
3. Control centre. The hypothalamus signals the posterior pituitary.
4. Effector. Posterior pituitary releases ADH into the blood. ADH binds receptors on collecting duct cells.
5. Response. Aquaporin water channels are inserted into the collecting duct membrane. Water flows out of the duct down the osmotic gradient created by the medullary salt gradient. Urine becomes concentrated and smaller in volume. Blood osmolarity falls toward the set point.

When blood becomes too dilute (water excess), the opposite happens. ADH release stops, aquaporins are removed and large volumes of dilute urine are excreted.

Aldosterone

Aldosterone (from the adrenal cortex) acts on the DCT to increase Na+ reabsorption (and water follows). Released when blood pressure or blood Na+ falls. It is a longer-acting parallel control to ADH.

Nitrogenous waste

Mammals excrete waste nitrogen from protein catabolism mainly as urea, formed in the liver from ammonia via the urea cycle. Urea is less toxic than ammonia and less costly to produce than uric acid. Animals living in dry environments often produce uric acid (birds, reptiles) to save water.

Common traps

Calling filtration "selective". Filtration is passive and non-selective; it lets through anything small enough. The selectivity comes from reabsorption and secretion afterwards.

Forgetting where ADH is released. ADH is made in the hypothalamus but released from the posterior pituitary.

Treating the loop of Henle as a pump that "concentrates urine". The loop establishes a salt gradient in the medulla; the collecting duct uses that gradient (under ADH control) to concentrate urine.

Confusing osmoregulation with excretion. They overlap but are distinct: osmoregulation is solute and water balance, excretion is waste removal. The kidney does both at once.

Cross-link to Year 12 assessment

This dot point is one of the most commonly assessed in EA Paper 1 short-response questions on Unit 2 and is sometimes adapted as IA1 data stimulus when teachers want to test understanding of selective transport. The aquaporin mechanism connects to membrane transport (see movement across membranes).

In one sentence

The kidney maintains water and solute balance through four nephron processes (filtration at the glomerulus, reabsorption through the PCT and loop of Henle, secretion in the DCT, and excretion from the collecting duct), with ADH from the posterior pituitary varying the collecting duct's permeability to water under negative feedback control from hypothalamic osmoreceptors.