Sample Notes: Excretion

O Level and IGCSE Biology – Detailed Notes

Chapter 13: Excretion

13.1 Excretion

Definition of Excretion

• Excretion is the removal of toxic materials and waste products of metabolism from organisms.
• It is not the same as egestion, which refers to the removal of undigested food via the anus.
• Excretion eliminates substances produced within the cells of the body as a result of chemical reactions.
• These waste materials must be removed because they may be toxic or disrupt internal balance if allowed to accumulate.

Examples of Waste Products

• Carbon dioxide (CO₂)
  • Waste product of aerobic respiration.
  • Formed in cells during the breakdown of glucose in the presence of oxygen.
  • Carried in the blood to the lungs and excreted during exhalation.
• Urea
  • Formed in the liver from the breakdown of excess amino acids.
  • Toxic if not removed.
  • Carried by blood to the kidneys, then excreted in urine.

13.2 Urinary System

Main Organs and Structures

• Kidneys
  • Filter urea, excess salts, and water from the blood.
  • Produce urine.
• Ureters
  • Muscular tubes that transport urine from kidneys to bladder.
• Bladder
  • Muscular sac that stores urine temporarily.
• Urethra
  • Tube through which urine exits the body.

Function of the Kidney

• Maintains homeostasis by:
  • Removing urea (toxic nitrogenous waste).
  • Removing excess salts and water.
  • Ensuring correct concentration of blood plasma.

The Need for Excretion

Toxicity of Urea

• Urea is formed by the deamination of excess amino acids in the liver.
• If allowed to accumulate:
  • It causes cell damage.
  • Leads to disturbances in osmotic balance.
  • Interferes with enzyme activity.

Structure of the Nephron

Key Parts of the Nephron

• Bowman’s Capsule: Cup-shaped structure that encloses the glomerulus.
• Glomerulus: Network of capillaries inside Bowman’s capsule.
• Tubules: Series of tubes including proximal and distal convoluted tubules for selective reabsorption.
• Loop of Henle: U-shaped tube involved in water reabsorption.
• Collecting Duct: Collects urine and carries it to the renal pelvis for excretion.

Associated Blood Vessels

• Afferent arteriole: Brings blood to glomerulus.
• Efferent arteriole: Carries filtered blood away from the glomerulus.
• Capillary network: Surrounds the tubules for reabsorption and exchange.

Function of the Nephron

(a) Filtration in the Glomerulus

• High pressure forces substances out of the blood into Bowman’s capsule.
• Filtered substances:
  • Water
  • Glucose
  • Urea
  • Ions
• Red blood cells and proteins remain in the blood as they are too large to pass through.

(b) Reabsorption in the Tubules

• All glucose is reabsorbed by active transport.
• Most water is reabsorbed by osmosis.
• Some ions (e.g., sodium) are reabsorbed selectively based on the body’s needs.
• Urea is not reabsorbed, except a small amount passively, which remains to be excreted.

(c) Urine Formation

• Urine contains:
  • Urea
  • Excess water
  • Excess salts/ions
• Final composition of urine depends on the body’s hydration and ion balance.
• Urine passes from the collecting duct → renal pelvis → ureter → bladder → urethra.

Role of Liver in Assimilation of Amino Acids

Assimilation Process

• After digestion, amino acids are absorbed into the blood and carried to the liver.
• In the liver:
  • Useful amino acids are converted into proteins, such as enzymes or structural proteins.
  • These proteins are used by cells for growth, repair, and metabolic activities.

Deamination

Definition

• Deamination is the process by which the nitrogen-containing part of amino acids is removed.
• This occurs in the liver.

Result of Deamination

• Produces urea as a nitrogenous waste product.
• Urea is transported in the blood to the kidneys for excretion.
• The remaining part of the amino acid (carbon skeleton) can be used in respiration or converted to glucose or fat.

Summary Table