Sample Notes: The Circulatory System
AS Level Biology – Detailed Notes: The Circulatory System
1. Overview of the Mammalian Circulatory System
- The mammalian circulatory system is a closed, double circulatory system, meaning:
- Closed: Blood remains within vessels throughout its journey.
- Double: Blood passes through the heart twice in one full circulation (once through pulmonary, once through systemic circulation).
- Components:
- Heart – a muscular pump.
- Blood – transport medium.
- Blood vessels – arterial and venous network including:
- Arteries
- Arterioles
- Capillaries
- Venules
- Veins
2. Pulmonary and Systemic Circulations
Pulmonary Circulation
- Carries deoxygenated blood from the heart to the lungs and back.
- Main vessels:
- Pulmonary artery: from right ventricle → lungs (deoxygenated).
- Pulmonary vein: from lungs → left atrium (oxygenated).
Systemic Circulation
- Carries oxygenated blood from the heart to the body and returns deoxygenated blood.
- Main vessels:
- Aorta: largest artery, from left ventricle → body.
- Vena cava: brings deoxygenated blood from body → right atrium.
3. Structure and Function of Blood Vessels
| Vessel Type | Structure | Function |
|---|---|---|
| Arteries | Thick walls, small lumen, elastic fibres, no valves | Carry high-pressure oxygenated blood away from the heart |
| Arterioles | Narrower than arteries, smooth muscle control | Regulate blood flow to capillaries |
| Capillaries | One-cell-thick wall, no muscle, very small diameter | Site of gas/nutrient/waste exchange |
| Venules | Small veins, thin walls | Drain capillaries into veins |
| Veins | Thin walls, large lumen, valves present | Carry low-pressure deoxygenated blood to the heart |
4. Arteries vs Veins – Microscopic and Functional Analysis
Artery
- Muscular arteries: Distribute blood; have thick tunica media.
- Elastic arteries: Near the heart; rich in elastin to accommodate pressure surge.
Vein
- Larger lumen.
- Valves prevent backflow.
- Thin walls due to low pressure.
Capillaries
- Endothelial layer only (single cell thick).
- Enables efficient diffusion.
5. Blood Cell Identification and Function
| Cell Type | Appearance | Function |
|---|---|---|
| Red Blood Cells (RBCs) | Biconcave, no nucleus | Transport oxygen via haemoglobin |
| Neutrophils | Multilobed nucleus | Phagocytosis of pathogens |
| Lymphocytes | Large round nucleus | Produce antibodies |
| Monocytes | Kidney-shaped nucleus | Differentiate into macrophages |
- Can be recognized in microscope slides and photomicrographs.
- RBCs are the most numerous.
- White blood cells (WBCs) are larger but less in number.
6. Properties of Water in Transport
Water as the main component of blood and tissue fluid
- ~90% of plasma is water.
Key Properties:
- Solvent: Dissolves ions, glucose, amino acids, oxygen, carbon dioxide.
- High Specific Heat Capacity:
- Maintains stable temperature.
- Prevents harmful fluctuations affecting enzymes and metabolic rate.
7. Tissue Fluid and its Formation
Tissue Fluid
- Formed from plasma that exits capillaries (minus proteins).
- Surrounds cells for exchange of substances.
Formation Process:
- At the arterial end of capillaries:
- Hydrostatic pressure > osmotic pressure
- Plasma forced out → tissue fluid
- At the venous end:
- Osmotic pressure > hydrostatic pressure
- Some fluid re-enters capillaries.
- Excess drained into lymphatic system.
Functions:
- Bathes cells in nutrients and oxygen.
- Removes waste.
- Maintains tissue hydration.
8. Key Definitions
| Term | Definition |
|---|---|
| Closed circulation | Blood remains in vessels at all times. |
| Double circulation | Blood passes through the heart twice per full body circuit. |
| Capillary | Narrowest blood vessel for substance exchange. |
| Tissue fluid | Fluid that bathes body cells, formed by filtration from plasma. |
| Osmotic pressure | Force drawing water into capillaries due to plasma proteins. |
| Hydrostatic pressure | Pressure exerted by blood against vessel walls. |
9. Example Exam Questions with Key Concepts
- Explain why the left ventricle has a thicker wall than the right.
- It pumps blood to the whole body → higher pressure needed.
- Describe how RBC structure is adapted for oxygen transport.
- Biconcave for larger surface area.
- No nucleus → more space for haemoglobin.
- What is the role of lymphocytes in immunity?
- Recognize antigens and produce antibodies.
- Compare composition of plasma and tissue fluid.
- Plasma contains proteins; tissue fluid does not.
10. Diagrams to Practice Drawing
- Transverse Section (TS) of artery and vein
- Plan diagram of capillary network
- Red blood cell and types of WBCs
- Tissue fluid movement diagram