Chapter 11
Transport in Humans
Cambridge O Level Biology 5090 Topic 11 revision chapter covering transport in humans: the circulatory system as one connected pump-vessel-blood system, double circulation and its pressure advantage, the structure of the heart, the complete blood route through the heart, heart-wall thickness, the heartbeat and the action of valves, methods of monitoring heart activity, the effect of physical activity on heart rate and how to investigate it safely, coronary arteries and coronary heart disease, modifiable and non-modifiable risk factors, the nine named major blood vessels, the structure and function of arteries, veins and capillaries, the components of blood and their functions, blood clotting, and the formation and role of tissue fluid. The chapter is taught as a single causal system rather than as four separate fact lists: the heart creates pressure, valves keep flow one-way, arteries carry blood away from the heart, capillaries allow exchange with the tissues, veins return blood to the heart, blood carries the dissolved substances, and tissue fluid forms the final short link between a capillary and a body cell. Double circulation is defined precisely as blood passing through the heart twice during one complete circuit of the body, and the common misreading that it means blood travels round the body twice is corrected directly. The pulmonary circuit is traced from the right side of the heart through the pulmonary artery to the lungs and back through the pulmonary vein to the left side, and the systemic circuit from the left side through the aorta to the body organs and back through the vena cava to the right side, with the pressure difference between them explained in terms of protecting the thin-walled lung capillaries while still delivering blood rapidly to every body tissue. Red and blue are used strictly as diagrammatic oxygenation cues and never as literal blood colours, and every figure carries a written label as well, because the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood, so artery and vein are defined by direction relative to the heart and never by oxygen content. The heart is drawn in full cross-section with the right and left atria, the right and left ventricles, the septum, the muscular wall, the atrioventricular and semilunar valves, the vena cava, the pulmonary artery, the pulmonary veins, the aorta and the coronary arteries on the surface of the cardiac muscle, and the diagram orientation is stated explicitly so that the anatomical right side appears on the viewer's left. The complete route from vena cava to aorta is traced with every valve named in sequence, and the role of the septum in keeping oxygenated and deoxygenated blood separate is explained. Wall thickness is explained entirely through workload and the pressure that must be generated: the atria push blood only into the ventricles, the right ventricle pumps to the nearby lungs at lower pressure, and the left ventricle pumps to the whole body and therefore has the thickest muscular wall, an explanation that never depends on oxygen content. The heartbeat is set out as a three-stage pressure sequence in which the atrioventricular valves close when ventricular pressure rises above atrial pressure and the semilunar valves open when ventricular pressure rises above arterial pressure, with the standing rule that valves are opened and closed by pressure differences and never contract. Monitoring is covered through pulse rate, heart sounds produced mainly by valve closure and the electrocardiogram, with the limits of each stated. Exercise is explained as a causal chain from increased muscle activity through increased respiration to a greater demand for oxygen and glucose and a greater production of carbon dioxide, and a full investigation of pulse rate and recovery is written out with its variables, controls, repeats, ethics, safety stopping rules and worked calculations including beats per minute, means, change from rest, percentage increase and recovery time. Coronary heart disease is explained mechanistically: coronary arteries supply cardiac muscle itself rather than the chambers, narrowing or blockage reduces the delivery of oxygen and glucose, aerobic respiration in the cardiac muscle is limited, and the muscle may be damaged. Risk factors are separated into modifiable and non-modifiable groups and a risk factor is defined as something that changes probability rather than something that guarantees disease. The nine required vessels are mapped organ by organ, with the hepatic portal vein distinguished carefully from the hepatic vein. Arteries, veins and capillaries are compared feature by feature with each structural point linked to the pressure it experiences and the job it does. The components of blood are identified from their appearance and matched to their functions, clotting is traced from damaged tissue through the conversion of soluble fibrinogen into insoluble fibrin to a clot that reduces blood loss and pathogen entry, and tissue fluid is explained as plasma forced out at the arterial end of a capillary, leaving red blood cells and large plasma proteins behind, bathing the cells, exchanging substances with them by diffusion and largely returning to the capillary as pressure falls.Show moreShow less
Syllabus 50902026-2028
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What is Transport in Humans about?
The human circulatory system is a muscular pump (the heart), a network of blood vessels and the blood itself, with one-way valves that keep blood moving in a single direction. Blood passes through the heart twice during one complete circuit of the body, which is what makes the system a double circulation: a low-pressure pulmonary circuit to the lungs and a high-pressure systemic circuit to the rest of the body.
Key ideas to remember
- One sentence to hold the topic together: pressure creates flow, valves give it direction, and structure follows pressure.
- If you have five minutes and nothing else: say the route from vena cava to aorta out loud, with every valve. It touches the chambers, the valves, the two circuits and the named vessels all at once.
What you need to be able to do
- Describe the circulatory system as a system of blood vessels with a pump and valves that maintain one-way blood flow.
- Describe double circulation in terms of a low-pressure circulation to the lungs and a high-pressure circulation to the body tissues, and explain the advantage of that arrangement.
- Identify the muscular wall, septum, atria, ventricles, atrioventricular valves, semilunar valves and coronary arteries on a diagram of the heart.
- Trace blood from the vena cava to the aorta, naming every chamber and every valve in sequence.
- Explain the relative thickness of the atrial wall, the right ventricle wall and the left ventricle wall in terms of workload and pressure.
- Describe the heartbeat as a sequence of pressure changes, and explain how those changes open and close the valves.
- State methods of monitoring heart activity: pulse rate, listening to the sounds of valve closure, and the electrocardiogram (ECG).
- Investigate and describe the effect of physical activity on heart rate, including the controls, repeats, safety rules and calculations that make the result trustworthy.
- Explain the effect of coronary artery blockage on the heart muscle, and discuss the risk factors for coronary heart disease and the measures that reduce modifiable risk.
- Name the vessels entering and leaving the heart, the lungs, the liver and the kidneys, and describe what the hepatic portal vein does.
- Identify an artery, a vein and a capillary from a diagram, and relate the structure of each to the pressure it carries and the function it performs.
- Identify red blood cells, lymphocytes, phagocytes, platelets and plasma, and state the function of each.
- Describe the formation of tissue fluid at the arterial end of a capillary and explain how substances are exchanged between blood, tissue fluid and body cells.
Key terms in Transport in Humans
- Heart Rate
- Heart rate is the number of times the heart contracts in one minute, measured in beats per minute. It increases during physical activity because the muscles respire faster, so they need oxygen and glucose delivered more quickly and carbon dioxide and other wastes removed more quickly; a faster heart rate moves more blood past the muscles in a given time. After activity stops, heart rate falls gradually back to the resting value as the extra demand disappears.
- Double Circulation
- Double circulation is an arrangement in which blood passes through the heart twice during one complete circuit of the body: once through the right side on the low-pressure pulmonary circuit to the lungs, and once through the left side on the high-pressure systemic circuit to the body tissues. The low pressure in the pulmonary circuit protects the thin-walled lung capillaries, while the high pressure in the systemic circuit delivers blood rapidly to every organ.
- Circulatory System
- The human circulatory system is a transport system made of a muscular pump (the heart), a branching network of blood vessels (arteries, capillaries and veins) and the blood itself, with one-way valves that keep blood flowing in a single direction. It carries oxygen, carbon dioxide, glucose, amino acids, ions, hormones, urea, blood cells and heat between the exchange surfaces of the body and every living cell.
- Pulse Rate
- Pulse rate is the number of pulses felt in an artery near the surface of the body in one minute. Each pulse is the stretch and recoil of the artery wall caused by a surge of blood as the left ventricle contracts, so pulse rate is normally used as an estimate of heart rate. It is measured by pressing two fingertips gently over an artery, counting the pulses for a measured time and converting the count to beats per minute.
- Vein
- A vein is a blood vessel that carries blood towards the heart. By the time blood reaches a vein it has passed through a capillary bed, so it is at low pressure and no longer surges. A vein therefore has a thin wall with little muscle or elastic tissue, a wide lumen that offers little resistance to flow, and valves along its length that prevent the low-pressure blood from flowing backwards.
- Pulmonary Artery
- The pulmonary artery is the blood vessel that carries deoxygenated blood from the right ventricle of the heart to the lungs. It is an artery because it carries blood away from the heart, not because of the oxygen content of that blood, and together with the pulmonary vein it is the standard exception to the assumption that arteries always carry oxygenated blood.
- Capillary
- A capillary is the smallest blood vessel, connecting small arteries to small veins as it passes through a tissue. Its wall is one cell thick and its lumen is only about the width of one red blood cell, so the distance substances must diffuse between the blood and the body cells is extremely short. Capillaries branch to form a dense network, giving a very large total surface area for exchange, and blood flows through them slowly and at low pressure, allowing time for oxygen, glucose, carbon dioxide and other substances to be exchanged.
- Left Ventricle
- The left ventricle is the chamber of the heart that pumps oxygenated blood into the aorta and around the whole body. It has the thickest muscular wall of the four chambers because it must generate the highest pressure: the systemic circuit is long and offers a great deal of resistance, so blood must leave the left ventricle with enough pressure to reach every organ from the brain to the feet.
- White Blood Cell
- A white blood cell is a blood cell involved in defending the body against pathogens. There are two types required at this level: a lymphocyte, which has a large round nucleus and very little cytoplasm and produces specific antibodies, and a phagocyte, which has a lobed nucleus and an irregular outline and engulfs and destroys pathogens by phagocytosis. Unlike red blood cells, white blood cells keep their nuclei.
- Coronary Artery
- A coronary artery is one of the arteries that branch from the aorta just above the aortic valve and run over and into the wall of the heart, supplying the cardiac muscle itself with oxygen and glucose. Because cardiac muscle contracts continuously it respires aerobically at a high rate, so any narrowing or blockage of a coronary artery reduces the delivery of oxygen and glucose, limits aerobic respiration in that region of muscle, and may damage it.
- Artery
- An artery is a blood vessel that carries blood away from the heart. The blood inside it is at high pressure that surges with each contraction of the ventricle, so an artery has a thick wall containing a great deal of smooth muscle and elastic tissue and a relatively narrow lumen. The thick wall withstands the pressure without bursting, the elastic tissue stretches and recoils to smooth out the surges and help maintain pressure between beats, and the smooth muscle can alter the diameter of the lumen.
- Septum
- The septum is the thick muscular partition that divides the heart into a right side and a left side. Because it is complete in a healthy heart, deoxygenated blood returning from the body on the right side never mixes with oxygenated blood returning from the lungs on the left side, so blood delivered to the body tissues carries the maximum possible amount of oxygen.
- Red Blood Cell
- A red blood cell is the cell that transports oxygen in the blood. It is a biconcave disc, thinner in the middle than at the edges, and a mature red blood cell has no nucleus, which leaves more room for haemoglobin. Haemoglobin combines with oxygen in the lungs, where the oxygen concentration is high, and releases it in respiring tissues, where the oxygen concentration is low. The cell is flexible enough to squeeze through capillaries only one cell wide.
- Heart
- The heart is the muscular pump of the human circulatory system. It is divided by a septum into a right side, which receives deoxygenated blood from the body and pumps it to the lungs, and a left side, which receives oxygenated blood from the lungs and pumps it to the rest of the body. Each side has a thin-walled atrium above and a thick-walled ventricle below, separated by an atrioventricular valve, and each ventricle empties through a semilunar valve into a great artery.
- Plasma
- Plasma is the straw-coloured liquid part of the blood, in which the blood cells and platelets are suspended. It is mostly water but is not pure water: dissolved in it are glucose, amino acids, mineral ions, hormones, urea, vitamins, carbon dioxide and plasma proteins such as fibrinogen. Plasma transports all of these around the body and also distributes heat from active organs such as the liver and the muscles.
- Risk Factor
- A risk factor is something that increases the probability that a person will develop a particular disease, without guaranteeing that they will. For coronary heart disease the recognised risk factors include diet, physical inactivity, smoking, prolonged stress, inherited predisposition, increasing age and biological sex. Some of these can be changed by a person's own choices and are described as modifiable; others, such as age and inheritance, cannot.
- Platelet
- A platelet is a small fragment of a cell found in the blood. When a blood vessel is damaged, platelets gather at the site and set off the clotting process, in which the soluble plasma protein fibrinogen is converted into insoluble fibrin. Fibrin forms a mesh of fibres across the wound that traps blood cells and forms a clot, which reduces the loss of blood and reduces the entry of pathogens through the damaged skin.
- Tissue Fluid
- Tissue fluid is the fluid that surrounds and bathes the body cells. It is formed at the arterial end of a capillary, where the pressure of the blood forces water and small dissolved substances such as glucose, oxygen, amino acids and ions out through the capillary wall. Red blood cells and large plasma proteins are too large to pass through and remain in the capillary, so tissue fluid differs in composition from plasma. Substances are exchanged between tissue fluid and the body cells by diffusion, and much of the fluid returns to the capillary at the venous end as the pressure falls.
- Atrioventricular Valve
- An atrioventricular valve is a one-way valve between an atrium and the ventricle below it. It is pushed open when atrial pressure is higher than ventricular pressure, allowing blood to flow into the ventricle, and is pushed shut when the ventricle contracts and ventricular pressure rises above atrial pressure, which prevents blood flowing backwards into the atrium. The valve is passive: it is moved by pressure differences and does not contract.
Common mistakes to avoid
- “Double circulation means the blood goes round the body twice.” CorrectBlood passes through the heart twice during one complete circuit of the body: once on the way to the lungs and once on the way to the rest of the body.
- “Arteries carry oxygenated blood; veins carry deoxygenated blood.” CorrectArteries carry blood away from the heart and veins carry blood towards it. The pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood, and both are named correctly.
- “The right side of the heart is on the right of the diagram.” CorrectIn a standard front-facing diagram you are looking at the person from the front, so their anatomical right appears on your left. Check which side the aorta arches away from before you label anything.
- “The left ventricle wall is thick because it holds oxygenated blood.” CorrectWall thickness follows workload. The left ventricle generates the pressure needed to push blood all the way round the body, so its muscle is thickest. Oxygen content has nothing to do with it.
- “The valves contract to push the blood along.” CorrectValves are passive flaps. They are pushed open and slammed shut by differences in pressure on their two sides. Only cardiac muscle contracts.
- “Smoking causes coronary heart disease, so a smoker will get it.” CorrectSmoking is a risk factor: it raises the probability of the disease across a population. It does not guarantee the disease in any individual, and non-smokers can develop it too.
- “Double circulation means the blood goes round the body twice.” Why wrongIt confuses the number of trips round the body with the number of visits to the pump. WriteBlood passes through the heart twice during one complete circuit of the body: once on the pulmonary circuit and once on the systemic circuit.
- “Arteries always carry oxygenated blood.” Why wrongIt takes a pattern that happens to hold in the systemic circuit and treats it as a definition. WriteAn artery carries blood away from the heart. The pulmonary artery carries deoxygenated blood to the lungs.
- “Veins always carry deoxygenated blood.” Why wrongSame error, the other way round. WriteA vein carries blood towards the heart. The pulmonary vein carries oxygenated blood from the lungs.
- “The left side of the heart contains deoxygenated blood.” Why wrongIt usually comes from reading the diagram the wrong way round. WriteThe left side receives oxygenated blood from the lungs through the pulmonary veins and pumps it to the body.
- “The right side of the heart is on the right of the diagram.” Why wrongA heart diagram is drawn as if you are facing the person, so their right is opposite your right. WriteThe anatomical right side appears on the viewer's left. Confirm it by finding the thickest ventricle wall, which is always the left ventricle.
- “The atria have thicker walls than the ventricles because they fill first.” Why wrongFilling requires no force; pumping does. WriteThe atria have the thinnest walls because they push blood only a short distance into the ventricles below them.
- “The left ventricle wall is thick because of the oxygen in the blood.” Why wrongOxygen content has no mechanical effect on the muscle. WriteThe left ventricle pumps blood to the whole body, so it must generate the highest pressure, so it needs the thickest layer of cardiac muscle.
- “The valves contract to push the blood along.” Why wrongValves are flaps of tissue, not muscle. WriteValves are opened and closed by differences in pressure on their two sides. Only the cardiac muscle contracts.
- “Coronary arteries carry blood into the heart chambers.” Why wrongIt confuses supplying the muscle with filling the pump. WriteCoronary arteries branch from the aorta and supply the cardiac muscle itself with oxygen and glucose. The chambers are filled by the vena cava and the pulmonary veins.
- “The hepatic vein and the hepatic portal vein are the same vessel.” Why wrongThe similar names hide completely different routes. WriteThe hepatic vein carries blood from the liver to the vena cava. The hepatic portal vein carries blood from the gut to the liver.
- “Veins have thick muscular walls because they have to push the blood back.” Why wrongVeins do no pushing; the heart and the skeletal muscles do. WriteVeins have thin walls and a wide lumen because the blood in them is at low pressure, and valves stop it flowing backwards.
- “Capillaries have no walls, so substances can just leak out.” Why wrong“Very thin” has been read as “absent”. WriteA capillary wall is one cell thick, which gives a very short diffusion distance while still holding the blood cells and large proteins in.
- “Platelets produce antibodies.” Why wrongPlatelets are cell fragments and do not make proteins to order. WriteLymphocytes produce antibodies. Platelets start the clotting process.
- “Lymphocytes engulf pathogens.” Why wrongEngulfing is the phagocyte's job, and the two white cells are being swapped. WritePhagocytes engulf and destroy pathogens by phagocytosis. Lymphocytes produce specific antibodies.
- “Red blood cells leave the capillary to form tissue fluid.” Why wrongThey are far too large to pass through the wall, which is why tissue fluid is not red. WriteTissue fluid is formed from plasma: water and small dissolved substances are forced out, while red blood cells and large plasma proteins remain in the capillary.
- “Plasma is just water.” Why wrongIt is mostly water, but the dissolved substances are the entire reason the blood is worth circulating. WritePlasma is the liquid part of blood, carrying glucose, amino acids, ions, hormones, urea, vitamins, carbon dioxide and plasma proteins, as well as the blood cells and heat.
- “Tissue fluid contains the same large proteins as plasma.” Why wrongThe capillary wall holds them back. WriteLarge plasma proteins are too big to pass through the capillary wall, so they remain in the blood and tissue fluid contains very little of them.
- “Smoking causes coronary heart disease, so every smoker gets it.” Why wrongIt treats a change in probability as a certainty for an individual. WriteSmoking is a risk factor: it increases the probability of coronary heart disease. It does not guarantee it, and people who have never smoked can develop the disease too.
Examiner tips
- The single most valuable habit in this topic. Whenever you state a structural feature, immediately state what it does. “The wall of an artery is thick” is worth nothing on its own. “The wall of an artery is thick, so it can withstand the high pressure of blood leaving the heart without bursting” is the mark.
- Circulation is not respiration. Circulation moves substances around the body. Respiration is the chemical release of energy from glucose inside cells. The circulatory system delivers the raw materials for respiration and removes its waste — it does not carry out respiration, and it does not “produce energy”.
- Orientation first, labels second. A heart diagram is drawn as if you are looking at the person from the front. Their anatomical right side therefore appears on your left. Before you write a single label, find the thickest ventricle wall: that side is always the left ventricle, whichever way the diagram is printed.
- Never write “the valve contracts”. Write “the valve is forced open” or “the valve is pushed shut”, and always say which pressure became the greater. Cardiac muscle contracts; valve flaps are pushed about by the blood.
- Never stop at “the body needs more blood”. That is the observation, not the explanation. The marks are in the middle of the chain: respiration in the muscle cells increases, so more oxygen and glucose are needed and more carbon dioxide is produced.
- Two words that must appear: oxygen and respiration. A great many answers say “the heart does not get enough blood” and stop. Say what the blood was carrying, and say what the muscle cells needed it for.
- The naming rule. “Hepatic” means of the liver; “renal” means of the kidney; “pulmonary” means of the lung. Put those together with artery = away from the heart and vein = towards the heart, and eight of the nine names build themselves. Only the hepatic portal vein has to be learned as a special case.
- Arteries do not push blood. The heart provides the push. An artery withstands the pressure and, through elastic recoil, maintains it. Writing that the artery wall “squeezes each blood cell forward” loses the mark.
How Transport in Humans is examined
- Transport in humans is one of the most heavily diagrammed topics in the syllabus. Almost every question begins with a picture — a heart in section, a vessel in cross-section, a blood smear or a graph of pulse rate against time — and asks you to read it before you explain anything.
- Most Topic 11 items test a single distinction: artery or vein, atrium or ventricle, hepatic vein or hepatic portal vein, lymphocyte or phagocyte. Read the direction of the arrow before you read the label.
- Structured questions ask for a labelled route, a matched comparison, or an explanation that links structure to pressure to function. Marks come from the link, not the feature.
- Exercise-and-pulse experiments appear as data questions: calculate a mean, read a recovery time from a graph, identify an anomaly, and say which variables were controlled.
- The single most valuable habit in this topic. Whenever you state a structural feature, immediately state what it does. “The wall of an artery is thick” is worth nothing on its own. “The wall of an artery is thick, so it can withstand the high pressure of blood leaving the heart without bursting” is the mark.
- Feature (what is there) → location (where it is) → mechanical or transport effect (what it does physically) → biological advantage (why that matters to the organism). Use it every time and you cannot write a one-word answer by accident.
Frequently asked questions
What is double circulation in humans?
Double circulation means that blood passes through the heart twice during one complete circuit of the body: once through the right side on the low-pressure pulmonary circuit to the lungs, and once through the left side on the high-pressure systemic circuit to the rest of the body.
Why is the wall of the left ventricle thicker than the wall of the right ventricle?
Because the left ventricle pumps blood to the whole body, while the right ventricle pumps it only as far as the lungs. Reaching the whole body requires a much higher pressure, and a thicker layer of cardiac muscle contracts more forcefully to produce it. Oxygen content has nothing to do with it.
Do arteries always carry oxygenated blood?
No. An artery is defined by direction: it carries blood away from the heart. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs, and the pulmonary vein carries oxygenated blood from the lungs back to the heart.
How do heart valves open and close?
By differences in pressure. A valve is pushed open when the pressure behind it is greater than the pressure in front of it, and pushed shut when the pressure in front becomes greater. Valves are passive flaps of tissue and never contract.
What do the coronary arteries do?
They branch from the aorta just above the aortic valve and supply the cardiac muscle itself with oxygen and glucose. They do not deliver blood into the heart's chambers. If one narrows or becomes blocked, that region of muscle receives less oxygen and glucose, aerobic respiration is reduced, and the muscle may be damaged.
What is the difference between the hepatic vein and the hepatic portal vein?
The hepatic vein carries blood away from the liver to the vena cava, and so back towards the heart. The hepatic portal vein carries blood from the gut to the liver, delivering the products of digestion for processing before they reach the rest of the body.
Why are capillary walls only one cell thick?
So that the distance substances must diffuse between the blood and the body cells is as short as possible. A shorter diffusion distance means a faster rate of diffusion, which is what allows oxygen and glucose to reach cells, and carbon dioxide and wastes to leave them, quickly enough to keep up with respiration.
What is tissue fluid and how is it different from blood?
Tissue fluid is the fluid that bathes the body cells. It is formed when pressure forces water and small dissolved substances out of a capillary at its arterial end. Red blood cells and large plasma proteins are too large to pass through the capillary wall, so tissue fluid contains neither, which is why it differs in composition from blood and is not red.
Which blood cell produces antibodies?
The lymphocyte, a white blood cell with a large round nucleus and very little cytoplasm. The other white blood cell required at this level, the phagocyte, has a lobed nucleus and engulfs and destroys pathogens by phagocytosis rather than producing antibodies.
Is a risk factor the same as a cause?
No. A risk factor increases the probability that a person will develop a disease; it does not guarantee it. A person with several risk factors for coronary heart disease may never develop it, and a person with none of the modifiable risk factors still may.
Syllabus reference and sources
Written against: Cambridge O Level Biology (5090) 2026–2028 Syllabus (Subject Content, Topic 11: Transport in humans).
Written by: Academiq Edu Instructor Panel
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