The human digestive, circulatory and respiratory systems

This is one of the biggest GCSE topics. You need three things: how each system works, the structure-function story for each part, and the lifestyle / disease links.

Digestive system and enzymes

Three classes of digestive enzymes break large food molecules into small soluble ones for absorption:

• Carbohydrase (e.g. amylase, in saliva and pancreas) → starch ⇒ maltose / glucose
• Protease (e.g. pepsin in stomach, trypsin in small intestine) → protein ⇒ amino acids
• Lipase (in pancreas / small intestine) → lipid ⇒ fatty acids + glycerol

Bile, made in the liver and stored in the gall bladder, emulsifies fat (turns big drops into tiny drops) — increasing surface area for lipase. Bile is also alkaline; it neutralises stomach acid so enzymes work in the small intestine.

The lock-and-key model: each enzyme has an active site of complementary shape. Optimum temperature ~37 °C; optimum pH varies (pepsin pH 2; amylase pH 7). Above ~50 °C the active site denatures and the enzyme stops working.

Required practical (food tests):

• Iodine — orange → blue-black with starch
• Benedict's — blue → brick-red with glucose (heat to 80 °C)
• Biuret — blue → purple with protein
• Sudan III / ethanol-emulsion test — for lipids

Circulatory system

A double circulation:

• Right side of heart pumps deoxygenated blood to lungs (pulmonary).
• Left side pumps oxygenated blood to body (systemic).

Blood enters via the vena cava (body) or pulmonary vein (lungs), passes from atria to ventricles, then leaves via pulmonary artery (to lungs) or aorta (to body). Valves stop backflow.

Blood vessels:

• Arteries — thick, elastic, muscular wall; small lumen; carry blood at high pressure away from heart.
• Veins — thinner walls; large lumen; valves; carry blood at low pressure to heart.
• Capillaries — single-cell-thick wall, microscopic, exchange substances with tissues.

Blood components:

• Plasma — straw-coloured liquid; transports CO₂, urea, hormones, nutrients.
• Red blood cells — biconcave, no nucleus, packed with haemoglobin to carry O₂.
• White blood cells — phagocytes engulf, lymphocytes make antibodies.
• Platelets — fragments that help clot blood.

The pacemaker (SAN) generates electrical impulses to time the heartbeat. Artificial pacemakers are implanted if the SAN fails.

Coronary heart disease and treatments

CHD: fatty material builds up in coronary arteries, narrowing them, reducing blood flow to heart muscle. Lifestyle risk factors: poor diet, lack of exercise, smoking, obesity.

Treatments:

• Statins — reduce LDL ("bad") cholesterol synthesis. Pros: effective, oral. Cons: side effects, must be taken long-term.
• Stents — wire mesh keeps artery open. Pros: long-term, immediate. Cons: surgery, infection risk, may re-block.
• Faulty valve — biological or mechanical replacement. Mechanical valves last longer but require blood-thinners.
• Heart transplant — donor heart for severe failure.

Links

Connects to B1.1 (specialised cells — RBCs), B4.2 (respiration provides demand for blood O₂) and B5.6 (hormones in the blood).