High-Yield Exam Focus
Circulation and Excretion are tested in both Section A (structured diagram questions) and Section B (essay questions) of ECZ Biology Paper 2. Practice labeling cross-sections of the heart and nephron tubule.
Table of Contents
- 1. Mammalian Heart Anatomy & Blood Flow Pathway
- 2. Double Circulation & Blood Vessel Structural Comparison
- 3. Blood Composition, Plasma & Immune Defense
- 4. Kidney Structure & Nephron Ultrafiltration
- 5. Selective Reabsorption & ADH Osmoregulation
- 6. Kidney Failure: Dialysis Machine vs Renal Transplant
1. Mammalian Heart Anatomy
The human heart is a muscular four-chambered double pump composed of cardiac muscle:
- Right Atrium & Ventricle: Receives deoxygenated blood from the vena cava and pumps it to the lungs via the pulmonary artery. Separated by the tricuspid valve.
- Left Atrium & Ventricle: Receives oxygenated blood from the lungs via pulmonary veins and pumps it to the body via the aorta. Separated by the bicuspid (mitral) valve.
- Muscular Wall Thickness: The left ventricle has a much thicker muscular wall than the right ventricle because it must generate high pressure to pump blood to systemic body extremities.
- Septum: Prevents mixing of oxygenated and deoxygenated blood.
2. Double Circulation & Blood Vessels
Mammals possess a closed double circulatory system consisting of:
- Pulmonary Circuit: Heart $\to$ Lungs (gas exchange) $\to$ Heart.
- Systemic Circuit: Heart $\to$ Body tissues (nutrient delivery) $\to$ Heart.
| Feature | Arteries | Veins | Capillaries |
|---|---|---|---|
| Lumen Size | Narrow lumen | Wide lumen | Microscopic (one RBC wide) |
| Wall Thickness | Thick, muscular & elastic | Thin, less muscular | Single layer of endothelial cells |
| Blood Pressure | High and pulsating | Low and steady | Decreasing pressure |
| Valves | Absent (except aorta/pulmonary root) | Present (prevent backflow) | Absent |
3. Blood Composition & Immunity
- Red Blood Cells (Erythrocytes): Biconcave discs without nuclei; packed with hemoglobin for oxygen transport.
- White Blood Cells (Leukocytes): Phagocytes (engulf bacteria by phagocytosis) and Lymphocytes (produce specific antibodies and antitoxins).
- Platelets (Thrombocytes): Cell fragments essential for blood clotting via fibrinogen conversion to fibrin mesh.
- Plasma: Pale yellow liquid transporting dissolved nutrients, carbon dioxide, urea, hormones, and heat.
4. Kidney Structure & Ultrafiltration
Excretion is the removal of toxic metabolic waste products from the body. The kidney's functional filtering unit is the Nephron:
Ultrafiltration at the Glomerulus
Blood enters the glomerulus via the wider afferent arteriole and leaves through the narrower efferent arteriole. This diameter difference creates high hydrostatic pressure, forcing water, glucose, mineral salts, amino acids, and urea through the basement membrane into Bowman's capsule, forming the glomerular filtrate. Red blood cells and large plasma proteins are retained in the blood.
5. Selective Reabsorption & ADH
As the filtrate passes through the nephron tubule, essential substances are reclaimed into capillary networks:
- Proximal Convoluted Tubule (PCT): $100\%$ of glucose and amino acids are reabsorbed by active transport against concentration gradients, along with most water by osmosis.
- Loop of Henle: Concentrates salts in the renal medulla, enabling water conservation.
- Osmoregulation (Role of ADH): When body fluids are concentrated (dehydration), the hypothalamus signals the pituitary gland to release Antidiuretic Hormone (ADH). ADH makes the collecting duct walls permeable to water, causing more water to be reabsorbed into blood, producing concentrated urine.
Teacher Marking Advice
Train students never to confuse excretion (removal of metabolic waste like urea and carbon dioxide) with egestion (expulsion of undigested dietary fiber as feces from the alimentary canal). Mixing these up costs key definition marks.