2.2.2: The heart and blood vessels
Not started yet — this one needs some love.
Double circulatory system: right side pumps blood to lungs; left side pumps blood to body.
Blood flow through heart:
Vena cava → right atrium → right ventricle → pulmonary artery (deoxygenated) → lungs.
Lungs → pulmonary vein (oxygenated) → left atrium → left ventricle → aorta → body.
Left ventricle has thicker, more muscular walls (pumps blood further to body at higher pressure).
Pacemaker: group of cells in right atrium; sets the natural resting heart rate; artificial pacemakers correct irregular heartbeat.
Arteries: thick walls (muscle and elastic fibres); small lumen; carry blood away from heart at high pressure; no valves.
Veins: thin walls; large lumen; carry blood to heart at low pressure; have valves to prevent backflow.
Capillaries: walls one cell thick; huge surface area; exchange of O₂, CO₂, glucose, urea between blood and cells.
Alveoli adaptations: large surface area; rich blood supply (capillary network); thin walls (one cell thick) — short diffusion distance for gas exchange.
Common exam mistakes
Pulmonary artery carries deoxygenated blood (right ventricle to lungs); pulmonary vein carries oxygenated blood (lungs to left atrium). Students frequently reverse these.
Coronary arteries supply blood to the heart muscle itself — they do NOT carry blood to the lungs or to the rest of the body.
Blood vessels do NOT have cell walls — that term applies to plants. Use 'muscular wall' or 'fibrous wall'.
When comparing arteries and veins: must make comparative statements (e.g. 'arteries have thicker walls THAN veins') — just describing each separately is not enough.
Veins have valves; arteries do NOT — must state this comparison explicitly.
The moist lining of alveoli is NOT a valid adaptation for gas exchange — examiners do not credit this; focus on large surface area, thin walls, and rich blood supply.
Alveoli have thin walls (one cell thick) — do not say 'thin cell walls' (a plant term) or 'thin membranes'; the correct phrase is 'thin walls' or 'walls one cell thick'.