Quick questions on Osmoregulation, the nephron and ADH (QCE Biology Unit 2)

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

The whole kidney function is built from four sequential processes.

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

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.

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.

1. Bowman's capsule. Cup-shaped structure surrounding the glomerulus, a capillary tuft. 2.

1. Stimulus. Blood osmolarity rises above the set point (around 300 mOsm per kg). 2.

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

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

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