The kidney: excretion, ultrafiltration and reabsorption

Organisms (Continuity and change) · Homeostasis · note 5 of 7

Spec D3.3.7, D3.3.8

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The kidney: excretion, ultrafiltration and reabsorptionSpec D3.3.7, D3.3.8

In short

The kidney carries out both excretion, the removal of metabolic wastes such as urea, and osmoregulation, the regulation of osmotic concentration, measured in osmol L⁻¹. Ultrafiltration in the glomerulus and Bowman's capsule removes small solutes and water from blood plasma; the proximal convoluted tubule then reabsorbs useful substances, leaving toxins and other unwanted solutes to be excreted in urine.

Excretion: Removal from the body of the waste products of metabolism, such as urea, and of toxins and other unwanted solutes.
Osmoregulation: Regulation of the osmotic concentration of body fluids (the amount of water relative to solutes), in osmoles per litre (osmol L⁻¹).

The kidneys do both. Each contains about a million nephrons, the tubules that make urine.

A nephron: blood enters the glomerulus inside Bowman's capsule where ultrafiltration happens, then the proximal convoluted tubule where glucose is reabsorbed, the loop of Henle, the distal convoluted tubule and the collecting duct where water is reabsorbed, with urine going to the ureter. — A nephron. Ultrafiltration happens at the glomerulus; useful substances are reabsorbed along the tubule.

Ultrafiltration: glomerulus and Bowman's capsule

The glomerulus is a knot of capillaries. Its incoming arteriole is wider than the outgoing one, so blood pressure inside is high.
Pressure forces plasma out through gaps in the capillary wall, a fine filter (the basement membrane) and slits between the cells lining Bowman's capsule.
The filtrate contains water, glucose, amino acids, mineral ions, urea and other small solutes. Blood cells and most plasma proteins are too large and stay in the blood.

Selective reabsorption: proximal convoluted tubule

In the proximal convoluted tubule all of the glucose and amino acids are reabsorbed into the blood, by active transport and co-transport with sodium ions.
Most of the mineral ions and most of the water (by osmosis) are also reabsorbed here.
The tubule wall cells have microvilli for a large surface area and many mitochondria to supply ATP for active transport.
Toxins, urea and other unwanted solutes are not reabsorbed (or only partly), so they stay in the filtrate and are excreted in urine.

Common mistake:

Filtration is not selective: it is based on size. The selection happens afterwards, when useful substances are reabsorbed.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

Why does homeostasis use negative feedback rather than positive feedback?

Negative feedback reverses any change from the set point, so it returns a variable to normal whether it rises above or falls below it. Positive feedback amplifies a change away from the set point, which would make an imbalance worse. That is why positive feedback only controls processes that must finish quickly, such as childbirth.

How do insulin and glucagon control blood glucose?

When blood glucose rises, β cells in the pancreatic islets secrete insulin, which makes liver and muscle cells absorb glucose and store it as glycogen. When blood glucose falls, α cells secrete glucagon, which makes liver cells break glycogen down and release glucose. Both hormones travel to their target cells in the blood.

What is the difference between type 1 and type 2 diabetes?

In type 1 diabetes the immune system destroys the insulin-secreting β cells, so little or no insulin is made and insulin must be injected. In type 2 diabetes insulin is still produced but target cells become resistant to it. Type 2 is linked to obesity and inactivity and is managed mainly by diet and exercise.

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