Unit 8
Human Biology
Chapter 8.4
Excretory SystemClass 12 Biology – Excretory System Notes PDF
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Introduction
The kidneys remove nitrogenous wastes, regulate water and electrolyte balance, contribute to acid-base balance and help maintain internal homeostasis.
1. Human Urinary System
Fig. 1 – Human Urinary System
2. Internal Structure of Kidney
Each kidney has an outer cortex, inner medulla, renal pyramids and a collecting region that drains urine toward the renal pelvis and ureter.
Fig. 2 – Longitudinal Section of Kidney
3. Nephron – Structural and Functional Unit
A nephron consists of the renal corpuscle and renal tubule. The corpuscle contains the glomerulus enclosed by Bowman’s capsule.
Fig. 3 – Labelled Nephron
4. Formation of Urine
4.1 Glomerular Filtration
Blood pressure forces water and small solutes from glomerular capillaries into Bowman’s capsule, producing filtrate. Blood cells and most large plasma proteins remain in the blood.
4.2 Selective Reabsorption
Useful substances such as glucose, amino acids, ions and much water are returned from tubular fluid to the blood.
4.3 Tubular Secretion
Additional substances are transferred from peritubular blood into the tubule, helping remove wastes and regulate ion and acid-base balance.
Fig. 4 – Three Basic Processes of Urine Formation
5. Concentration of Urine
The loop of Henle and collecting ducts help establish and use an osmotic gradient in the kidney medulla. This allows water to be conserved when the body needs to produce concentrated urine.
Fig. 5 – Countercurrent Arrangement in Loop of Henle
6. Hormonal Regulation and Osmoregulation
ADH
Antidiuretic hormone increases water reabsorption in the distal nephron/collecting ducts, helping conserve water.
Aldosterone
Promotes sodium reabsorption and influences potassium handling, helping regulate salt and water balance.
7. Common Kidney Problems
- Kidney stones may obstruct urinary passages.
- Renal failure reduces the ability to filter and regulate body fluids.
- Dialysis can partially replace filtration functions when kidneys fail severely.
- Urinary tract infections may affect urethra, bladder or higher urinary organs.
Fig. 6 – Principle of Haemodialysis
8. Quick Revision & Exam Points
- Draw and label the human urinary system.
- Describe the internal structure of kidney.
- Draw and label a nephron.
- Explain glomerular filtration, reabsorption and secretion.
- How is concentrated urine produced?
- Write the role of ADH.
- Define osmoregulation.
- Explain the basic principle of haemodialysis.
- Kidney = major excretory organ.
- Nephron = functional unit.
- Filtration occurs at renal corpuscle.
- Reabsorption returns useful substances.
- Secretion adds selected substances to tubule.
- Loop of Henle helps concentrate urine.
- ADH increases water conservation.
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