What you need to know
- 1.6–1.11mass transportThe bulk flow of a fluid, such as blood or air, that carries substances over long distances., vessels, cardiac cycleThe sequence of events in one heartbeat: atrial systole, ventricular systole and diastole., haemoglobinA globular protein in red blood cells with four polypeptide chains, each with a haem group containing Fe²⁺ that binds one O₂., atherosclerosisThe hardening and narrowing of arteries caused by atheromas building up in their walls. and clotting.
By the end, you should be able to…
- Retrieve key mechanisms before checking the answers.
- Apply ideas to unfamiliar numerical and experimental examples.
- Identify and correct missing steps in an explanation.
Build your understanding
Use pressure to explain valve behaviour
Name the two compartments being compared, state which has the higher pressure and then state the valve outcome. Do not assign valveA flap of tissue that closes when pressure behind it is higher, so blood can flow in only one direction. an active pumping role.
Connect structure to transport
Use thin capillaryA tiny vessel whose wall is a single layer of endothelium, giving a short diffusion distance for exchange with tissues. walls for short diffusion distanceThe distance molecules have to travel. A shorter distance gives a faster rate of diffusion., elastic arterial walls for pressure changes and valves in veinA vessel that carries blood back to the heart at low pressure. It has a thin wall, a wide lumen and valves. for limiting backflow. Keep oxygenation separate from whether a vessel is an arteryA vessel that carries blood away from the heart at high pressure. It has a thick wall with elastic fibres and smooth muscle. or vein.
Make your reasoning visible
Answer the command word
Describe the pattern, explain the mechanism or evaluate the evidence as requested. A correct fact may still miss the question.
Show workings and units
Write a calculation before its result, and distinguish fractions, percentages and percentage-point changes.
Use answers diagnostically
Attempt the question first, then identify the missing step rather than only checking whether your final number matches.
Avoid this misconception
The claim: A longer answer automatically earns more credit.
The correction: Relevance, accuracy and the requested reasoning matter.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Why does a large mammal need mass transport as well as diffusion?
Show suggested answer
A low surface-area-to-volume ratio and long internal distances make diffusion alone too slow for demand. Circulation moves materials over long distances, leaving short diffusion paths between capillaries and cells.
Practice question
Retrieval and explanation · suggested answer
At one instant, ventricular pressure is above atrial pressure but below arterial pressure. State which valves are closed and why.
Show suggested answer
The atrioventricular valves are closed because ventricular pressure exceeds atrial pressure; semilunar valves are closed because arterial pressure exceeds ventricular pressure. Valves respond to pressure differences.
Practice question
Retrieval and explanation · suggested answer
Explain why tissue with high carbon dioxide production receives more oxygen from haemoglobin at a given oxygen partial pressure.
Show suggested answer
Higher carbon dioxide is associated with lower pH and reduced haemoglobin oxygen affinity: the dissociation curve shifts right. More oxygen is unloaded at the same partial pressure (Bohr effect).
Practice question
Retrieval and explanation · suggested answer
Distinguish an atherosclerotic plaque from a thrombus.
Show suggested answer
A plaque develops in the artery wall through lipid accumulation and inflammation. A thrombus is a blood clot; plaque rupture can trigger clotting and obstruct flow.
Practice question
Retrieval and explanation · suggested answer
Put these conversions in order: fibrinogen to fibrin; prothrombin to thrombin. Explain why the order matters.
Show suggested answer
Prothrombin is converted to thrombin first in the clotting sequence. Thrombin catalyses conversion of soluble fibrinogen to insoluble fibrin, whose fibres form a mesh that stabilises the clot.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
At the same oxygen partial pressure, fetal haemoglobin is more saturated than adult haemoglobin. Explain the benefit without claiming oxygen moves against its partial-pressure gradient.
Show a hint
Separate binding affinity from diffusion direction.
Explore the reasoning
Greater fetal affinity allows binding at lower oxygen partial pressures, helping maintain transfer from maternal blood across the placenta. Oxygen still diffuses down its partial-pressure gradient; the two blood supplies do not need to mix.
Recall this lesson
Lesson recall questions
Question 1 of 5: How does the thick elastic and muscular wall of an artery relate to high blood pressure?
Arteries, veins and capillaries
Practice question
Retrieval and explanation · suggested answer
How does the thick elastic and muscular wall of an artery relate to high blood pressure?
Lesson: 1B 3: Circulation in the Blood Vessels
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Measuring function rather than guessing
1C · Thinking Bigger: Heart FailureNHLBI describes heart failure as a problem with the heart’s ability to supply the body. Measurements help connect symptoms to mechanisms. Heart rate counts beats, whereas cardiac output…Specification 1.6 · 1.8Read & use the scienceClose article
Why this matters
NHLBI describes heart failure as a problem with the heart’s ability to supply the body. Measurements help connect symptoms to mechanisms. Heart rate counts beats, whereas cardiac output describes the volume moved each minute. The same rate can accompany different stroke volumes, and tissues depend on adequate delivery rather than the count alone. Cardiac physiology therefore uses measurements that address pumping and filling as well as rhythm. This is an application of transport biology, not a diagnostic exercise. A numerical change needs a physiological interpretation and a comparison with other observations before it can explain someone’s symptoms.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 1.6 understand why many animals have a heart and circulation (mass transport to overcome the limitations of diffusion in meeting the requirements of organisms)
- 1.8 know the cardiac cycle (atrial systole, ventricular systole and cardiac diastole) and relate the structure and operation of the mammalian heart, including the major blood vessels, to its function
Use the science
Why might measuring heart rate alone miss a pumping problem?
Explore a possible response
It does not measure the volume ejected per beat or whether the blood flow meets tissue needs.
Explore the source
- NHLBI: heart failureNHLBI
What does an oxygen reading really measure?
1B · The Roles of the BloodA pulse oximeter estimates the proportion of haemoglobin carrying oxygen using light. That useful number is not a direct measurement of how much oxygen reaches every tissue. Haemoglobin…Specification 1.9(i) · 1.16Read & use the scienceClose article
Why this matters
A pulse oximeter estimates the proportion of haemoglobin carrying oxygen using light. That useful number is not a direct measurement of how much oxygen reaches every tissue. Haemoglobin concentration and blood flow also influence delivery. Research has additionally found differences in measurement performance across skin pigmentation, prompting the FDA’s 2025 proposals for improved validation studies. This connects the biology of oxygen transport with the design of fair medical technologies. A device should be tested in people representative of those who will use it, and an apparently precise display should not hide uncertainty about the measurement.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 1.9(i) understand the role of haemoglobin in the transport of oxygen and carbon dioxide
- 1.16 be able to evaluate the design of studies used to determine health risk factors, including sample selection and sample size used to collect data that is both valid and reliable
Use the science
Two samples have the same oxygen saturation but different haemoglobin concentrations. Would equal volumes necessarily carry the same amount of oxygen? Explain.
Explore a possible response
No. The same fraction of a smaller amount of haemoglobin carries less bound oxygen; saturation alone does not specify oxygen content.