Excretion and the Kidneys

What is Excretion?

Excretion is the removal of metabolic waste products from the body. It is NOT the same as egestion (removal of undigested food). Key excretory products:

• CO₂ — from respiration; excreted by lungs
• Urea — produced in the liver from excess amino acids (deamination); excreted by kidneys in urine
• Water — excreted by lungs (water vapour), skin (sweat), and kidneys (urine)

Kidney Structure and Function

The kidneys are the main excretory organs. Each kidney contains about one million nephrons (functional units).

Key structures:

• Cortex (outer region): contains Bowman's capsule and glomerulus (filtration), and convoluted tubules (reabsorption).
• Medulla (inner region): contains loops of Henle and collecting ducts.
• Pelvis: collects urine → ureter → bladder → urethra.

Nephron process:

1. Ultrafiltration (Bowman's capsule + glomerulus): high blood pressure forces small molecules (glucose, water, urea, mineral ions) out of capillaries into Bowman's capsule. Large molecules (proteins, blood cells) remain in blood.
2. Selective reabsorption: useful substances (all glucose, some water, some mineral ions) are actively reabsorbed into the blood in the proximal convoluted tubule.
3. Osmoregulation (collecting duct): final adjustment of water content under ADH (anti-diuretic hormone) control.

Osmoregulation and ADH

ADH (anti-diuretic hormone) is released by the pituitary gland when blood water content is too low (e.g. after exercise, sweating, or drinking too little water).

• ADH → collecting duct walls become more permeable to water → more water reabsorbed → small volume of concentrated urine.
• Low ADH (e.g. high water intake) → collecting duct less permeable → large volume of dilute urine.

This is another example of negative feedback.

Kidney Failure — Dialysis

When kidneys fail, urea and waste build up in blood. Two treatments:

Haemodialysis (blood dialysis):

• Blood passed through dialysis machine (artificial kidney).
• Blood flows one side of a partially permeable membrane; dialysis fluid (same concentration as healthy blood for useful substances) on the other.
• Urea and excess minerals diffuse out of blood into dialysis fluid (concentration gradient).
• Glucose and ions diffuse in/out until equilibrium (dialysis fluid prevents loss of useful substances).
• Typically 3 sessions per week, 4–6 hours each.

Kidney transplant:

• Donor kidney implanted.
• Patient must take immunosuppressant drugs for life (to prevent rejection).
• Better quality of life; more cost-effective long-term.
• Limited donor organs available; risk of rejection.

Water Balance

The body regulates water content through urine concentration. Factors increasing urine concentration (less water lost as urine):

• High temperature / sweating → more ADH released
• Eating salty food → lowers blood water potential → more ADH
• Dehydration / insufficient fluid intake

Factors producing more dilute urine: high fluid intake → less ADH.