# Human Physiology — The Blood System (IB)
The blood system (Topic 6.2) transports substances around the body. IB requires knowledge of heart structure and function, blood vessel types, the cardiac cycle, and the causes and consequences of coronary heart disease.
1. Heart Structure and Double Circulation
- Four chambers: right atrium, right ventricle, left atrium, left ventricle
- Double circulation: pulmonary (right heart → lungs → left heart) and systemic (left heart → body → right heart)
- Left ventricle wall thicker than right (higher pressure for systemic circulation)
- Septum separates left and right sides
- Atrioventricular (AV) valves (tricuspid right, bicuspid/mitral left) prevent backflow from ventricles to atria
- Semi-lunar valves at base of aorta and pulmonary artery prevent backflow into ventricles
- Coronary arteries supply heart muscle with oxygen and glucose
2. The Cardiac Cycle
The cardiac cycle is the sequence of contraction (systole) and relaxation (diastole) of the heart chambers.
Stages
- Atrial systole: Atria contract, pushing blood into ventricles through open AV valves
- Ventricular systole: Ventricles contract; AV valves close ("lub" — first heart sound); blood pushed through semi-lunar valves into aorta and pulmonary artery
- Diastole: Ventricles relax; semi-lunar valves close ("dub" — second heart sound); blood flows from veins into atria
Control of Heart Rate
- Sinoatrial (SA) node ("pacemaker"): in right atrium; initiates each heartbeat; sends electrical impulses
- Impulse spreads across atria, causing them to contract
- Atrioventricular (AV) node: receives impulse; delays it slightly (allows atria to finish contracting)
- Impulse passes down the Bundle of His and Purkinje fibres → ventricles contract from the bottom up
- Heart rate modulated by medulla oblongata via sympathetic (speeds up) and parasympathetic/vagus (slows down) nerves
- Adrenaline increases heart rate
3. Blood Vessels
| Feature | Arteries | Capillaries | Veins |
|---|---|---|---|
| Wall thickness | Thick (muscle + elastic) | One cell thick | Thin |
| Lumen | Narrow | Very narrow | Wide |
| Pressure | High | Low (decreasing) | Low |
| Valves | No | No | Yes |
| Function | Carry blood from heart | Exchange of substances | Return blood to heart |
4. Blood Composition
| Component | Function |
|---|---|
| Erythrocytes (red blood cells) | Transport O₂ (haemoglobin); biconcave, no nucleus |
| Leucocytes (white blood cells) | Immune defence (phagocytes + lymphocytes) |
| Platelets | Blood clotting (form platelet plug, release clotting factors → fibrin mesh) |
| Plasma | Liquid transport medium: glucose, amino acids, CO₂, urea, hormones, antibodies, heat |
5. Coronary Heart Disease (CHD)
- Caused by atherosclerosis: build-up of fatty plaques in coronary arteries
- Narrowed lumen → reduced blood flow to heart muscle → reduced O₂ delivery
- Can lead to angina (chest pain) or myocardial infarction (heart attack)
- Risk factors: high blood cholesterol, smoking, hypertension, diabetes, obesity, sedentary lifestyle, genetics
Worked Example
Question: Explain how the structure of the heart ensures one-way flow of blood. (3 marks)
Solution: The heart contains atrioventricular valves between the atria and ventricles that open when atrial pressure exceeds ventricular pressure and close when ventricles contract, preventing backflow to the atria. Semi-lunar valves at the base of the aorta and pulmonary artery open during ventricular systole and close during diastole, preventing blood flowing back into the ventricles. The septum prevents mixing of oxygenated and deoxygenated blood.
Practice Questions
- Draw and label a diagram of the human heart. (4 marks)
- Describe the role of the SA node and AV node in controlling the heartbeat. (4 marks)
- Compare the structure and function of arteries, capillaries, and veins. (4 marks)
- Explain the causes and consequences of atherosclerosis. (4 marks)
Answers
- Diagram should show: four chambers (RA, RV, LA, LV), septum, aorta, pulmonary artery, pulmonary vein, vena cava (superior and inferior), AV valves, semi-lunar valves, coronary arteries.
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Summary
- The heart has four chambers; double circulation ensures efficient oxygen delivery.
- The SA node initiates the heartbeat; AV node, Bundle of His, and Purkinje fibres coordinate contraction.
- Arteries (thick walls, high pressure), capillaries (thin walls, exchange), veins (valves, low pressure).
- Blood: erythrocytes (O₂), leucocytes (defence), platelets (clotting), plasma (transport).
- CHD: atherosclerosis narrows coronary arteries → reduced blood flow → angina or heart attack.