SEC G3 Combined Science Biology component K327/K328
B5: Transport in Humans
Connect blood, vessels, heart action, valves, double circulation, and coronary heart disease.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
The human transport system matches each vessel, blood component, and heart structure to a movement or protection function.
Named vessels and vessel design
The aorta carries blood from the heart to the body and the vena cava returns blood to the heart. The pulmonary artery carries blood from the heart to the lungs and the pulmonary vein returns it. The hepatic artery supplies the liver, the hepatic vein drains it to the vena cava, and the hepatic portal vein carries absorbed nutrients from the small intestine to the liver. The renal artery supplies each kidney and the renal vein returns blood from it.
Arteries carry blood away from the heart and withstand high pressure with thick elastic muscular walls. Veins carry blood towards the heart at lower pressure through wider lumens and use valves against backflow. Capillaries have walls one cell thick for short exchange distance. Specific muscle-layer names are not required.
Blood components
Red blood cells use haemoglobin to carry oxygen. Plasma transports cells, ions, soluble food, hormones, carbon dioxide, urea, vitamins, and plasma proteins.
White blood cells defend by phagocytosis and antibody formation and take part in tissue rejection. Platelets initiate conversion of fibrinogen to fibrin so a clot forms.
Heart action and coronary disease
The vena cava returns blood to the right atrium, the right ventricle sends it through the pulmonary artery to the lungs, the pulmonary vein returns it to the left atrium, and the left ventricle sends it through the aorta to the body. Atria receive blood and contract before the thicker-walled ventricles pump it out. The septum separates the two sides, and valves prevent backflow as muscular contraction creates pressure. Detailed heart-muscle histology, nerve names, and transmitter substances are not required.
Coronary heart disease occurs when coronary arteries become narrowed or blocked, reducing heart-muscle blood supply. Unhealthy diet, inactivity, and smoking increase risk; balanced diet, regular activity, and avoiding smoking reduce it.
Formulae and relationships
This chapter is assessed mainly through models, field patterns and explanations. Build the causal chain before adding any calculation.
Worked examples
Example 1: Explain why capillaries are suited to exchange with tissues.
- Their walls are one cell thick.
- This creates a short diffusion distance.
- Their narrow network brings blood close to many cells.
Answer: Thin walls and close contact support rapid diffusion between blood and tissue.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Explain Named vessels and vessel design with the named terms, evidence, and causal links kept distinct.
- Use Blood components to interpret the evidence given and justify each conclusion.
- Apply Heart action and coronary disease to a new example, then check the conclusion against the information given.
Official outcome coverage
K327 B5: 5 mapped outcomes, references B5(a), B5(b), B5(c), B5(d), B5(e). Check the official K327 syllabus.
K328 B5: 5 mapped outcomes, references B5(a), B5(b), B5(c), B5(d), B5(e). Check the official K328 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Observe or interpret blood-vessel, blood-cell or heart representations and record structural evidence before inferring function. For pulse or exercise data, standardise the activity and measurement interval, repeat readings, and explain biological variation.
Exam traps and retrieval check
Avoid these traps
- Assuming every artery carries oxygenated blood.
- Giving plasma the oxygen-carrying role of haemoglobin.
- Explaining valves as structures that create pressure.
Check from memory
Which vessel carries blood from heart to lungs?
The pulmonary artery.
What converts to fibrin during clotting?
Fibrinogen.
What tissue is deprived in coronary heart disease?
Heart muscle.
Official Combined Science scope
This shared Combined Biology owner serves both K327 and K328. Named vessels, vessel structure, blood components, heart contraction and valves, and coronary heart disease.