A-Level Biology — Salters-Nuffield (SNAB)

The heart and circulation

• A double circulatory system: the right side pumps deoxygenated blood to the lungs (pulmonary), the left side pumps oxygenated blood to the body (systemic) at higher pressure - the left ventricle wall is thicker.
• Cardiac cycle: atrial systole (atria contract, fill ventricles), ventricular systole (ventricles contract, atrioventricular valves shut, semilunar valves open), diastole (relaxation, filling). Valves open and close on pressure differences to prevent backflow.
• Blood vessels: arteries (thick elastic/muscular wall, narrow lumen - maintain high pressure), capillaries (one cell thick - exchange), veins (wide lumen, valves - low-pressure return).

Water as a transport medium

• Water is dipolar and forms hydrogen bonds - a good solvent, with high specific heat capacity and cohesion; these make it an ideal transport medium in blood and for dissolved substances.

Atherosclerosis (the disease process)

1. The endothelium of an artery is damaged (by high blood pressure, toxins from smoking).
2. Inflammation - white blood cells and lipids (cholesterol) build up, forming an atheroma (plaque).
3. The plaque narrows the lumen, raising blood pressure further (a positive feedback loop) and reducing blood flow.
4. Plaque rupture triggers a blood clot (thrombosis) - a coronary artery blockage causes a heart attack; a brain blockage causes a stroke.

Blood clotting

• Damage exposes tissue; thromboplastin triggers a cascade: prothrombin to thrombin, which converts soluble fibrinogen to insoluble fibrin, forming a mesh that traps platelets and cells.

Learn the atheroma cascade as a sequence and link each cardiovascular risk factor to endothelial damage or raised blood pressure.

Diagram