Circulation in humansOxford AQA IGCSE Biology: Revision notes
Section 1
What is the structure of the heart and how do its chambers and valves work?
The heart is a muscular pump divided into four chambers: two atria (upper chambers) and two ventricles (lower chambers). The left atrium receives oxygenated blood from the pulmonary vein, while the right atrium receives deoxygenated blood from the vena cava (superior and inferior). The left ventricle pumps oxygenated blood to the body via the aorta, and the right ventricle pumps deoxygenated blood to the lungs via the pulmonary artery.
Two types of valves prevent backflow of blood:
- Atrioventricular valves (tricuspid in right side, bicuspid/mitral in left side) lie between atria and ventricles
- Semilunar valves lie in the aorta and pulmonary artery and prevent blood flowing back into the ventricles
The coronary arteries branch from the aorta and supply oxygenated blood to the heart muscle itself, ensuring the heart has its own blood supply.
When describing the heart, always specify LEFT and RIGHT. Examiners look for this precision. Saying 'the aorta leaves the ventricle' loses marks—say 'the aorta leaves the LEFT ventricle'.
Think of the atria as collection tanks and the ventricles as pump chambers. Valves are one-way gates that only let blood move in one direction.
Section 2
How does the double circulatory system work?
The human circulatory system is a double circulation consisting of two circuits connected at the heart:
Pulmonary circulation:
- Right atrium receives deoxygenated blood from the vena cava
- Right ventricle pumps this blood to the lungs via the pulmonary artery
- Blood becomes oxygenated in the lungs
- Oxygenated blood returns to the left atrium via the pulmonary vein
Systemic circulation:
- Left atrium receives oxygenated blood from pulmonary vein
- Left ventricle pumps this blood to the body via the aorta
- Blood delivers oxygen to body tissues and collects carbon dioxide
- Deoxygenated blood returns to right atrium via the vena cava
The key advantage of the double circulation is that the heart can maintain high pressure in the systemic circulation to efficiently deliver oxygenated blood throughout the body, whilst the pulmonary circulation operates at lower pressure suitable for delicate lung tissue.
Examiners expect you to name both vessels involved in each circuit: 'pulmonary artery to lungs' and 'pulmonary vein from lungs' for pulmonary circulation; 'aorta to body' and 'vena cava from body' for systemic circulation.
A red blood cell in the right atrium: enters right ventricle → pumped through pulmonary artery → travels to lungs → becomes oxygenated → returns via pulmonary vein to left atrium → enters left ventricle → pumped through aorta to body → delivers oxygen → returns deoxygenated via vena cava to right atrium. This is ONE complete circuit.
Section 3
What happens during the cardiac cycle and how does the heart beat?
The cardiac cycle is a sequence of contractions and relaxations that pumps blood through the heart in one heartbeat. It consists of three main phases:
Atrial systole (0.1 seconds):
- Both atria contract simultaneously, pushing blood into the ventricles
- Atrioventricular valves open; semilunar valves remain closed
- Ventricles remain relaxed (diastole)
Ventricular systole (0.3 seconds):
- Both ventricles contract simultaneously, forcing blood out of the heart
- Atrioventricular valves slam shut (preventing backflow), producing the first heart sound 'lub'
- Blood is pushed into the aorta and pulmonary artery
- Semilunar valves open to allow blood exit
Diastole (0.4 seconds):
- Both atria and ventricles relax and refill with blood
- All valves closed
- As ventricles relax, semilunar valves snap shut, producing the second heart sound 'dub'
- Blood flows into atria from veins
- Cycle repeats
The heart beats approximately 70 times per minute at rest. Each beat takes about 0.8 seconds.
Students often say 'the whole heart contracts' or forget that atrial systole happens separately from ventricular systole. Remember: atria contract FIRST (pushing blood in), then ventricles contract (pushing blood out). They don't contract at the same time.
When explaining the cardiac cycle, link valve movements to each phase: name WHICH valves open and close, and WHY (e.g. 'semilunar valves close when ventricular pressure drops below aortic pressure, preventing backflow').
Section 4
How are arteries, veins and capillaries adapted to their functions?
| Feature | Arteries | Capillaries | Veins |
|---|---|---|---|
| Wall thickness | Thick elastic and muscular wall | Single layer of endothelial cells | Thin wall with less muscle |
| Lumen diameter | Small | Very small (7-10 μm) | Large |
| Pressure | High pressure | Low pressure | Low pressure |
| Valves | None | None | Valves prevent backflow |
| Blood flow | Rapid, smooth, pulsatile | Slow, allows exchange | Slow, steady |
| Function | Carry oxygenated blood away from heart under high pressure | Exchange of substances (O₂, CO₂, glucose, urea) between blood and tissue fluid | Return deoxygenated blood to heart under low pressure |
How structure relates to function:
Arteries: The thick muscular wall can withstand high pressure from ventricular contractions. Elastic fibres allow stretching and recoil to smooth blood flow (pressure remains high). Small lumen concentrates pressure to maintain high velocity delivery of oxygenated blood to tissues.
Capillaries: The single-cell-thick wall allows rapid diffusion of oxygen, carbon dioxide, glucose and other small molecules across the wall. Small diameter and slow blood flow allow time for exchange. Their narrow bore increases surface area relative to volume, maximising exchange efficiency. No valves are needed as pressure is very low.
Veins: The thin wall reflects low pressure in venous return. The large lumen offers low resistance to flow. Valves prevent backflow of blood, crucial since pressure is insufficient to push blood uphill (in limbs) against gravity. Smooth muscle contraction and surrounding skeletal muscle (skeletal muscle pump) help return blood to the heart.
Structure-function questions are common. Always explain WHY the structure is like that. For example, don't just say 'veins have valves'—explain 'veins have valves because blood pressure is low and cannot overcome gravity, so valves prevent backflow'.
Think of arteries as high-pressure fire hoses (thick walls to withstand pressure), capillaries as permeable paper towels (thin to let things through), and veins as low-pressure drainage pipes (wide to reduce resistance, with one-way valves).
Section 5
What are the composition and functions of blood?
Blood is composed of plasma (the liquid component) and cells suspended within it. A typical blood sample contains:
Red blood cells (erythrocytes): ~5 million per mm³
- Function: Transport oxygen from lungs to tissues using the protein haemoglobin
- Adaptations: Biconcave disc shape increases surface area for oxygen absorption; no nucleus provides extra space for haemoglobin; small and flexible to squeeze through capillaries
White blood cells (leucocytes): ~5,000-10,000 per mm³
- Function: Defence and immune response against pathogens
- Types include phagocytes (engulf bacteria) and lymphocytes (produce antibodies)
Platelets (thrombocytes): ~250,000 per mm³
- Function: Blood clotting (haemostasis) to seal wounds and prevent blood loss
- Fragments of cells that aggregate and trigger clotting cascade
Plasma: ~55% of blood volume, colourless liquid
- Functions: Transport of solutes including glucose, amino acids, hormones, waste products (urea, CO₂) and ions
- Maintains osmotic potential and blood pH
- Contains proteins (albumin, fibrinogen for clotting, antibodies)
The ratio of cells to plasma is approximately 45:55 (packed cell volume).
When asked about red blood cell adaptations, examiners want THREE features with explanations: (1) no nucleus—more space for haemoglobin; (2) biconcave shape—large surface area for oxygen absorption; (3) small size—can squeeze through narrow capillaries.
A glucose molecule enters blood from the digestive system dissolved in plasma. It is transported throughout the body in plasma, delivered to cells that need energy. This shows plasma's transport function. Meanwhile, oxygen molecules bind to haemoglobin in red blood cells and are carried from lungs to respiring tissues—this shows red blood cells' specialised transport role.
Section 6
What is coronary heart disease and how is it treated?
Coronary Heart Disease (CHD) occurs when coronary arteries become narrowed or blocked, restricting blood flow to the heart muscle. This deprives the heart of oxygen, causing chest pain (angina) and potentially a heart attack (myocardial infarction) if blood flow is completely blocked.
Causes of CHD: Narrowing is typically due to atherosclerosis—buildup of fatty deposits (plaques) in artery walls. Key risk factors include:
- Diet high in saturated fat and cholesterol → increases blood cholesterol, promoting plaque formation
- Lack of exercise → weakens heart, increases blood pressure and cholesterol
- Smoking → damages endothelium, increases blood pressure and clotting tendency
- Genetic factors → family history increases susceptibility; some individuals naturally have higher cholesterol
Treatments for CHD:
-
Lifestyle changes: Balanced diet (low saturated fat, high fibre), regular exercise, smoking cessation, stress management—reduce risk factors and slow disease progression
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Statins: Drugs that reduce blood cholesterol levels by inhibiting liver cholesterol production, slowing plaque formation and stabilising existing plaques
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Stents: Small metal tubes inserted into narrowed coronary arteries to keep them open and restore blood flow. Minimally invasive procedure using angioplasty
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Coronary artery bypass surgery (CABG): Surgical grafting of a healthy vessel (taken from leg or chest) to bypass the blocked section of coronary artery, restoring blood supply to the heart muscle
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Heart transplant: Replacement of the damaged heart with a donor heart in severe cases where other treatments are not possible. Requires lifelong immunosuppression to prevent rejection
Early detection through screening and lifestyle modification are crucial to preventing CHD progression.
For CHD risk factors, always link the factor to the mechanism: don't just say 'smoking increases CHD risk'—explain 'smoking damages the artery endothelium and increases blood clotting, promoting atherosclerosis and CHD'. Mechanism explanations score higher marks.
Students often confuse stents with bypass surgery. Remember: stents are placed INSIDE the artery to prop it open (less invasive), while bypass surgery grafts a NEW vessel AROUND the blockage (more invasive surgery).
Must Know
- Heart structure: Four chambers (left/right atria and ventricles), three major vessels (aorta, vena cava, pulmonary artery/vein), two types of valves (atrioventricular and semilunar), coronary arteries supply the heart muscle
- Double circulation: Pulmonary circuit (heart↔lungs) and systemic circuit (heart↔body); the heart maintains high pressure for efficient body perfusion
- Cardiac cycle: Atrial systole pushes blood into ventricles → ventricular systole pumps blood out → diastole allows refilling; atrioventricular valves open during atrial systole, semilunar valves open during ventricular systole
- Vessel adaptation: Arteries have thick muscular walls for high-pressure flow; capillaries have single-cell walls for rapid exchange; veins have valves and large lumens for low-pressure return
- Blood composition: Red blood cells (no nucleus, biconcave, carry O₂ via haemoglobin), white blood cells (immune defence), platelets (clotting), plasma (dissolves and transports substances)
- CHD: Results from atherosclerosis narrowing coronary arteries; risk factors are diet, exercise, smoking, genetics; treatments include lifestyle changes, statins, stents, bypass surgery, and heart transplants
That's the notes covered.
Carry on to the next subtopic.