Describe the structure and function of the human excretory system.

The human excretory system is responsible for removing metabolic wastes and maintaining homeostasis by regulating the composition and volume of body fluids. It consists of the kidneys, ureters, urinary bladder, urethra, skin, and lungs, each contributing to waste elimination.

Structure and Function

1. Kidneys

• Structure: Paired, bean-shaped organs located in the abdominal cavity. Each kidney has an outer cortex, an inner medulla (containing renal pyramids), and a central renal pelvis that collects urine.
• Function: Filter blood to form urine, removing urea, excess salts, water, and metabolic wastes via nephrons (functional units). They also regulate blood pressure, pH, and electrolyte balance by reabsorbing useful substances.

2. Ureters

• Structure: Two narrow muscular tubes (~25 cm long) connecting the kidneys to the urinary bladder.
• Function: Transport urine from the renal pelvis to the bladder via peristaltic movements.

3. Urinary Bladder

• Structure: A hollow, muscular sac located in the pelvis, lined with mucous membrane and surrounded by the detrusor muscle.
• Function: Stores urine until it is expelled from the body. The internal urethral sphincter (involuntary) and external urethral sphincter (voluntary) control urine release.

4. Urethra

• Structure: A tubular structure (~3–4 cm in females, ~20 cm in males) leading from the bladder to the outside.
• Function: Conveys urine out of the body. In males, it also serves as the passage for semen.

5. Skin (Sweat Glands)

• Structure: Contains eccrine and apocrine sweat glands distributed across the body.
• Function: Excretes water, salts (Na⁺, Cl⁻), urea, and lactic acid via sweat, helping regulate body temperature and electrolyte balance.

6. Lungs

• Structure: Spongy organs with a rich blood supply.
• Function: Excrete carbon dioxide (CO₂) and water vapor as byproducts of cellular respiration.

Key Processes

• Filtration: Blood plasma is filtered in the glomerulus of nephrons.
• Reabsorption: Useful substances (glucose, amino acids, water) are reabsorbed into the blood.
• Secretion: Additional wastes (e.g., potassium, hydrogen ions) are secreted into the filtrate.
• Excretion: Urine is formed and expelled through the ureters → bladder → urethra.

The excretory system ensures the removal of metabolic wastes while maintaining homeostasis, acid-base balance, and fluid volume regulation.