Excretion Overview

Metabolism generates waste. Respiration produces carbon dioxide; the breakdown of amino acids produces ammonia; the breakdown of red blood cells produces bile pigments; and many drugs and hormones must be inactivated and removed. If these wastes were allowed to accumulate, they would poison the organism. Excretion is the process by which such metabolic waste products are removed from the body. This lesson introduces the categories of waste that mammals must excrete, the organs involved, and why excretion is essential. The content maps to OCR A-Level Biology A specification module 5.1.2(a)–(b).

Key Definitions:

• Excretion — the removal of metabolic waste products from the body.
• Egestion — the removal of undigested food material from the gut. This is not excretion.
• Metabolic waste — substances produced as by-products of reactions occurring inside the body.
• Deamination — the removal of the amino group (−NH₂) from an amino acid, producing ammonia.

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Excretion Versus Egestion

A common early-career error is to describe defecation as excretion. It is not. Egestion is the removal of material that was never part of the body's metabolism — undigested food, dead gut bacteria, shed intestinal cells — through the anus. Excretion is the removal of substances that have been made inside the body and which, if retained, would be harmful. Urine, sweat, exhaled CO₂ and bile pigments are excreted; faeces are egested.

Process	What is removed	From where
Excretion	Metabolic waste (e.g., CO₂, urea, bilirubin)	Body tissues via blood
Egestion	Undigested material from food	Gut lumen via anus

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The Main Metabolic Wastes in a Mammal

Mammals produce several categories of metabolic waste. OCR expects you to know the origin, the route of excretion, and why each must be removed.

1. Carbon Dioxide

• Origin: Produced continuously by aerobic respiration in every cell. About 200 ml min⁻¹ is generated in a resting adult human.
• Transport: Around 70 % is carried in the blood as hydrogencarbonate ions (HCO₃⁻) after the reaction CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ in red blood cells. About 23 % is bound loosely to haemoglobin as carbaminohaemoglobin, and the remaining 7 % is dissolved in plasma.
• Route of excretion: Diffuses across the alveolar epithelium in the lungs and is exhaled.
• Why removed: High CO₂ concentrations lower blood pH by forming carbonic acid, denaturing enzymes and disrupting the affinity of haemoglobin for oxygen (the Bohr effect). Chronic hypercapnia causes acidosis.

2. Nitrogenous Waste (Urea)

• Origin: Mammals cannot store excess amino acids. When the diet contains more protein than required, the surplus amino acids are deaminated in the liver. This produces highly toxic ammonia (NH₃), which is immediately converted to the less toxic urea ((NH₂)₂CO) via the ornithine cycle. Urea is carried in the blood to the kidneys.
• Route of excretion: Filtered by the kidneys and excreted in urine.
• Why removed: Ammonia is extremely toxic because it disrupts enzyme function and raises cellular pH. Urea is much less toxic but would still disturb osmotic balance if allowed to accumulate. Humans excrete about 25–30 g of urea per day.

3. Bile Pigments

• Origin: Old or damaged red blood cells are broken down in the liver and spleen. The haem group of haemoglobin is split to release iron (recycled) and a tetrapyrrole ring, which is converted into bilirubin and related bile pigments.
• Route of excretion: Bile pigments are secreted in bile from the liver into the duodenum. They pass through the gut and colour faeces brown.
• Why removed: Bilirubin is toxic to cells and, if it accumulates in the blood, causes jaundice (yellowing of the skin and sclera). Neonatal jaundice is common because the infant liver is not yet fully able to conjugate bilirubin.

4. Other Wastes

Mammals also excrete: