Animal Tissues, Organs and Systems (B2.2)
The circulatory system
Humans have a double circulatory system — blood passes through the heart twice per full circuit:
- Pulmonary circulation: right side of heart → lungs → left side of heart (oxygenation).
- Systemic circulation: left side of heart → body → right side of heart (oxygen delivery).
The heart is a muscular pump that maintains this flow. It is made of cardiac muscle (myogenic — can contract without a nerve signal, controlled by the sino-atrial node, the heart's pacemaker).
Heart structure
| Chamber | Side | Function |
|---|---|---|
| Right atrium | Right | Receives deoxygenated blood from body (via vena cava) |
| Right ventricle | Right | Pumps blood to lungs (pulmonary artery) |
| Left atrium | Left | Receives oxygenated blood from lungs (pulmonary vein) |
| Left ventricle | Left | Pumps blood to body (aorta) — thicker walls because higher pressure needed |
Valves (atrioventricular and semilunar) prevent backflow.
Coronary arteries supply blood to the heart muscle itself.
Blood vessels
| Vessel | Wall | Lumen | Flow | Notes |
|---|---|---|---|---|
| Artery | Thick, muscular, elastic | Narrow | Away from heart | Pulse felt |
| Vein | Thin | Wide | Towards heart | Has valves; blood at low pressure |
| Capillary | One cell thick | Very narrow | Tissue exchange | Site of diffusion to/from cells |
Blood components
- Red blood cells (erythrocytes): carry O₂ via haemoglobin; biconcave; no nucleus.
- White blood cells (leukocytes): immune defence — phagocytosis or antibody production.
- Platelets (thrombocytes): tiny cell fragments; trigger blood clotting.
- Plasma: liquid; carries CO₂, glucose, urea, hormones, proteins.
Coronary heart disease (CHD)
Atherosclerosis — fatty deposits (plaques/atheromas) build up in coronary artery walls → narrowing → reduced blood flow → angina or heart attack (myocardial infarction).
Risk factors: high blood cholesterol, smoking, high blood pressure, obesity, lack of exercise, genetic predisposition.
Treatments:
- Lifestyle changes (diet, exercise, stop smoking)
- Statins (lower blood cholesterol)
- Angioplasty + stent (widens artery, keeps it open)
- Bypass surgery (use vein graft to route blood around blockage)
- Biological pacemaker / artificial pacemaker
- Valve replacement (mechanical or biological)
- Heart transplant (ethical issues, organ availability)
Common exam errors
- Saying the left side of the heart carries deoxygenated blood — it carries oxygenated.
- Confusing pulmonary artery (deoxygenated blood to lungs) with pulmonary vein (oxygenated blood from lungs).
- Missing the valve function when describing heart action.
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