Excretion

Definition

Excretion is the process by which metabolic wastes and excess substances are removed from the body to maintain homeostasis.

Importance of Excretion

• Removes toxic metabolic wastes (urea, CO₂, excess salts).
• Maintains osmotic balance and water balance.
• Helps regulate body pH.
• Supports homeostasis.

Human Excretory System

• Kidneys: filter blood to produce urine.
• Ureters: transport urine from kidneys to bladder.
• Bladder: stores urine until excretion.
• Urethra: conducts urine from bladder to outside.
• Skin: removes water and salts through sweat.
• Liver: removes nitrogenous wastes via deamination.

Urine Formation

• Filtration: Blood enters glomerulus under pressure, water, salts, glucose, and urea are filtered into Bowman's capsule.
• Reabsorption: Useful substances (glucose, some water, salts) are reabsorbed into blood.
• Secretion: Additional wastes (H⁺, K⁺) are secreted into tubules.
• Final urine contains mainly water, urea, and salts.

Practice Questions

• Define excretion and explain its importance. (4 marks)
• Describe the structure and functions of the human excretory system. (6 marks)
• Explain the stages of urine formation. (6 marks)
• How do the skin and liver contribute to excretion? (4 marks)
• Short answer: Name the tube that carries urine from kidney to bladder. (1 mark)