O-Level and SEC G3 Biology K325
B5: Transport in Humans
Connect blood components, vessels, the heart, circulation, tissue exchange, and disease risk.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Transport in Humans connects blood, vessels, the heart, and tissue exchange as one double-circulation system. Explanations should trace direction of flow, compare structures through pressure and function, and separate evidence about heart rate from evidence about blood pressure or disease risk.
Blood and tissue exchange
Plasma transports dissolved nutrients, carbon dioxide, urea, hormones, antibodies, and heat. Red blood cells carry oxygen using haemoglobin and have a biconcave shape, no nucleus, and a flexible membrane. White blood cells defend against pathogens, while platelets help form clots that reduce blood loss and entry of pathogens.
At capillaries, high hydrostatic pressure forces some plasma components into spaces around cells, forming tissue fluid. Large proteins and blood cells remain in the vessels. Useful substances diffuse to cells, wastes diffuse back, and excess tissue fluid enters lymph vessels before returning to the blood.
Blood vessels and ABO transfusion
Arteries carry blood away from the heart under relatively high pressure and have thick elastic muscular walls. Veins carry blood towards the heart at lower pressure, have wider lumens, and contain valves. Capillaries have walls one cell thick and narrow lumens for efficient exchange. Trace the named vessels serving the heart, lungs, liver, and kidneys.
ABO transfusion compatibility depends on antigens on donor red blood cells and antibodies in recipient plasma. For red-cell transfusion within the ABO system, group O cells can donate across ABO groups, group AB recipients can receive across ABO groups, and same-group donation is compatible. Use the direction from donor cells to recipient plasma and do not reverse it.
Cardiac cycle and coronary disease
Pressure differences open and close heart valves. During atrial systole, atria contract and move blood into ventricles. Ventricular systole raises ventricular pressure and sends blood into arteries. During diastole, chambers relax and refill.
Coronary arteries supply heart muscle with oxygen and glucose. Fatty deposits can narrow them, reducing blood flow and increasing the risk of a clot, heart attack, or tissue damage. Smoking, prolonged high blood pressure, inactivity, and an unsuitable diet can increase risk, but risk factors are not guarantees for one person.
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: A group A patient needs a red-cell transfusion. Explain why group A and group O donor cells are compatible within the ABO system, but group B cells are not.
- Group A recipient plasma contains anti-B antibodies.
- Group A donor cells have A antigens and group O donor cells have neither A nor B antigens.
- Group B donor cells carry B antigens and can be agglutinated by the recipient's anti-B antibodies.
Answer: Group A and O red cells avoid a B-antigen reaction, while group B red cells do not.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Relate blood components to transport, defence, and clotting functions.
- Compare arteries, veins, and capillaries, then trace the named vessels serving the heart, lungs, liver, and kidneys.
- Apply ABO red-cell compatibility and explain the cardiac cycle and coronary heart disease.
Official outcome coverage
K325 B5: 8 mapped outcomes, references B5(a), B5(b), B5(c), B5(d), B5(e), B5(f), B5(g), B5(h). Check the official K325 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Measure pulse or recovery data with a safe activity, standardised timing, repeats, and a clear distinction between heart rate and blood pressure.
Exam traps and retrieval check
Avoid these traps
- Saying arteries always carry oxygenated blood instead of defining them by direction from the heart.
- Reversing donor and recipient when checking ABO red-cell compatibility.
- Listing a risk factor without explaining how reduced coronary blood flow damages heart muscle.
Check from memory
Which blood component carries most oxygen?
Red blood cells using haemoglobin.
Which ABO recipient can receive red cells from all ABO groups?
A group AB recipient, considering the ABO system alone.
What do coronary arteries supply?
The heart muscle itself.
Pure versus Combined scope
Combined Biology shares the core idea but assesses a narrower outcome set. Use the K327 or K328 component checklist to set the exact boundary.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K325. Open the related IP explanation.