Heart

The four chambers (top to bottom; right to left).

Side	Chamber	Receives blood from	Pumps blood to
Right	Atrium	Vena cava (body)	Right ventricle
Right	Ventricle	Right atrium	Pulmonary artery (lungs)
Left	Atrium	Pulmonary vein (lungs)	Left ventricle
Left	Ventricle	Left atrium	Aorta (body)

The four big vessels.

• Vena cava: HUGE vein. Brings deoxygenated blood from the body INTO the right atrium.
• Pulmonary artery: from RIGHT VENTRICLE → lungs. Carries DEOXYGENATED blood (the exception to "arteries = oxygenated").
• Pulmonary vein: from lungs → LEFT ATRIUM. Carries OXYGENATED blood (the exception to "veins = deoxygenated").
• Aorta: HUGE artery. From left ventricle → body. Carries oxygenated blood at high pressure.

The septum. A solid wall of muscle separating the right and left sides. KEEPS oxygenated and deoxygenated blood SEPARATE.

Coronary arteries. Two small arteries branching off the aorta. They wrap around the heart and supply the HEART MUSCLE ITSELF with oxygen + nutrients. Without them, heart cells would die. A blockage causes a HEART ATTACK.

Worked qualitative. Why can't the heart be supplied with oxygen from the blood inside its chambers?

• The heart wall is too thick — diffusion from the chambers wouldn't reach the outer cells in time.
• Coronary arteries deliver blood directly to the muscle's own capillary network.
• That's why a coronary blockage is so serious — those cells have no other supply.

Cambridge tip. When labelling, remember: in the diagram (heart shown as the patient's view), the LEFT side appears on the RIGHT. Don't flip them.

One full beat takes ~0.8 seconds at rest.

Stage 1 — Diastole (relaxation, ~0.4 s).

• Both atria and ventricles RELAXED.
• Blood from veins (vena cava + pulmonary veins) flows INTO the atria — passive filling.
• AV valves OPEN, blood starts trickling into ventricles.

Stage 2 — Atrial systole (~0.1 s).

• Atria CONTRACT.
• Push blood through OPEN AV valves into ventricles.
• Ventricles fill completely.

Stage 3 — Ventricular systole (~0.3 s).

• Ventricles CONTRACT (powerfully — left more so than right).
• Pressure rises sharply → AV VALVES SLAM SHUT (the 'LUB' sound — first heart sound).
• Pressure pushes open the SEMILUNAR VALVES → blood ejected into aorta and pulmonary artery.
• Atria are relaxed; refilling.

Stage 4 — Diastole again (relaxation).

• Ventricles relax → pressure drops below arteries.
• SEMILUNAR VALVES SLAM SHUT (the 'DUB' sound — second heart sound) preventing backflow from arteries.
• AV valves open → atria refill ventricles. Cycle repeats.

Heart sounds.

• LUB = AV valves closing.
• DUB = semilunar valves closing.
• Listened to via a stethoscope.

Worked qualitative. What happens if a heart valve fails to close properly (a 'leaky valve')?

• Blood flows BACKWARDS during ventricular contraction.
• Each beat pushes less blood forward → heart works harder.
• Long-term: heart enlarges (cardiomyopathy), eventually fails.
• Treatment: surgical valve replacement.

Cambridge tip. When asked about valves, always link CLOSURE to BACKFLOW PREVENTION, not active opening. Valves are passive flaps — pressure opens and closes them.

Compare left vs right ventricle:

Feature	Right ventricle	Left ventricle
Wall thickness	Thin (~3 mm)	Thick (~10 mm)
Pumps blood to	Lungs only	Whole body
Pressure generated	Low (~25 mmHg)	High (~120 mmHg)
Distance covered	~10 cm	~1-2 m

The reason.

• LEFT side has to pump blood through KILOMETRES of body capillaries — high resistance.
• RIGHT side only has to push blood through the lungs — much shorter, less resistance.
• Thicker wall = stronger contraction = higher pressure → reaches all body cells.

Atria are thinner-walled than ventricles because they only need to push blood a short distance into the ventricles.

Worked qualitative. Why does living at high altitude (low O₂) sometimes thicken the right ventricle?

• Low O₂ in air → narrow blood vessels in lungs (a reflex to redirect blood to better-oxygenated areas).
• Narrow lung vessels = high resistance → right ventricle works harder.
• Over time, right ventricle wall THICKENS (compensation).
• Some climbers develop this; usually reverses on returning to lower altitudes.

Cambridge tip. Always link STRUCTURE (thickness) to FUNCTION (pressure for distance). One-word answers like "stronger" miss the mark.

Coronary arteries branch off the AORTA right after it leaves the heart. They wrap around the heart muscle and supply it with oxygen + nutrients.

Why crucial.

• Heart muscle works constantly — needs constant O₂ + glucose.
• It can't get supplies from the blood inside its chambers (too thick to diffuse through).
• Coronary arteries are the only delivery route.

Coronary heart disease (CHD).

• Fatty deposits (atheroma) build up on artery walls.
• Narrow the lumen → less blood reaches the muscle.
• Symptoms: chest pain on exertion (angina).
• Complete blockage → that area of muscle dies → HEART ATTACK (myocardial infarction).
• Massive heart attacks can be fatal.

Risk factors.

• High saturated fat diet.
• High blood cholesterol.
• High blood pressure.
• Smoking.
• Lack of exercise.
• Genetics.
• Stress.

Treatment.

• Lifestyle: better diet, exercise, stop smoking.
• Drugs: statins (lower cholesterol), aspirin (thin blood), beta-blockers (lower pressure).
• Surgery: bypass (graft from another artery), stents (mesh holds artery open).

Worked qualitative. Why does aspirin help prevent heart attacks?

• Aspirin reduces blood CLOTTING (platelet stickiness).
• Heart attacks often start with a clot in a narrowed coronary artery.
• Less stickiness = less clot risk.
• Daily low-dose aspirin reduces second heart attacks in survivors.

Cambridge tip. Cambridge often gives a diagram showing the coronary arteries as branches from the aorta. Don't confuse them with the carotid arteries (to the head).