Osmoregulation and the kidney

MDCAT Biology · Biology · Homeostasis. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

The kidneys remove nitrogenous waste and keep the water and salt content of the blood steady. A kidney has an outer cortex, an inner medulla and a pelvis that leads to the ureter. Its working unit is the nephron: Bowman's capsule with its glomerulus, proximal convoluted tubule, loop of Henle, distal convoluted tubule, then a collecting duct.

Urine is made in two main steps. Ultrafiltration forces small molecules out of the glomerulus under high pressure. Selective reabsorption then takes back what the body needs.

The loop of Henle makes the medulla salty. ADH decides how much water the collecting ducts give back to the blood, and so how concentrated the urine is.

Rules to remember

The afferent arteriole is wider than the efferent arteriole, so the blood pressure in the glomerulus is high
Filtrate contains water, glucose, amino acids, salts and urea; blood cells and plasma proteins are too large to pass
Proximal convoluted tubule: reabsorbs all the glucose and amino acids and most of the water and salts
Descending limb of the loop is permeable to water; ascending limb moves salts out and is impermeable to water
Less water in the blood → more ADH from the posterior pituitary → collecting ducts more permeable to water → small volume of concentrated urine
Too little ADH = diabetes insipidus: large volumes of dilute urine with no glucose
Kidney stones are mostly calcium oxalate (lithotripsy breaks them with shock waves); in kidney failure, dialysis removes urea by diffusion across a partially permeable membrane

Common mistake

ADH does not act on the glomerulus and does not change how much filtrate is formed. It acts on the distal tubule and collecting duct and changes how much water is reabsorbed.

Worked example

In a glomerulus the blood pressure is 55 mm Hg. The osmotic pressure of the plasma is 30 mm Hg and the pressure of the fluid already in Bowman's capsule is 15 mm Hg. Find the net filtration pressure.

Blood pressure pushes fluid out of the glomerulus; the other two pressures oppose it.
Opposing pressure = 30 + 15 = 45 mm Hg.
Net filtration pressure = 55 − 45 = 10 mm Hg.

Answer: 10 mm Hg, directed from the glomerulus into Bowman's capsule, so filtrate forms.

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