Transport in Humans
SEAB syllabus topic 5: how the blood, the blood vessels and the heart carry materials around the body, with the why, the traps and the exam checks.
Arteries carry blood away from the heart, veins carry blood towards the heart, and capillaries link the two.
Blood transports substances around the body, and its white blood cells and platelets help to defend the body.
The heart pumps the blood in a double circulation, and its valves keep the blood flowing in one direction.
Main blood vessels
identify the main blood vessels to and from the heart, lungs, liver and kidney
Blood travels to and from the heart, lungs, liver and kidneys in named blood vessels, and each vessel carries blood either towards or away from an organ.
- Aorta
- Aorta: the largest artery, which carries oxygenated blood from the left ventricle to the body
- Vena cava
- Vena cava: the large vein that carries deoxygenated blood from the body back to the right atrium
- Superior and inferior vena cava
- Superior and inferior vena cava: the superior vena cava brings blood from the head and arms, and the inferior vena cava brings blood from the parts of the body below the heart supports K325-5(a)
- Pulmonary artery
- Pulmonary artery: the artery that carries deoxygenated blood from the right ventricle to the lungs
- Pulmonary vein
- Pulmonary vein: the vein that carries oxygenated blood from the lungs to the left atrium
- Hepatic artery
- Hepatic artery: the artery that carries oxygenated blood to the liver
- Hepatic portal vein
- Hepatic portal vein: the vein that carries blood from the gut to the liver; after a meal, this blood contains a high concentration of absorbed food substances such as glucose and amino acids
- Hepatic vein
- Hepatic vein: the vein that carries blood away from the liver to the vena cava
- Renal artery
- Renal artery: the artery that carries oxygenated blood from the aorta to the kidney, and this blood contains urea for the kidney to remove
- Renal vein
- Renal vein: the vein that carries blood away from the kidney to the vena cava, and this blood contains less urea than the blood in the renal artery
- Oxygenated and deoxygenated blood
- Oxygenated and deoxygenated blood: oxygenated blood has a high concentration of oxygen and is bright red, while deoxygenated blood has a low concentration of oxygen and is darker red supports K325-5(a)
- Double circulation
- Double circulation: a circulation in which the blood passes through the heart twice in one complete circuit of the body supports K325-5(a)
- Pulmonary circulation
- Pulmonary circulation: the route of the blood from the heart to the lungs and back to the heart supports K325-5(a)
- Systemic circulation
- Systemic circulation: the route of the blood from the heart to the rest of the body and back to the heart supports K325-5(a)
Vessels to and from each organ
| Organ | Vessels that bring blood to the organ | Vessels that take blood away from the organ |
|---|---|---|
| Heart | The vena cava brings deoxygenated blood from the body, and the pulmonary vein brings oxygenated blood from the lungs. | The aorta takes oxygenated blood to the body, and the pulmonary artery takes deoxygenated blood to the lungs. |
| Lungs | The pulmonary artery brings deoxygenated blood from the heart. | The pulmonary vein takes oxygenated blood back to the heart. |
| Liver | The hepatic artery brings oxygenated blood, and the hepatic portal vein brings blood from the gut. | The hepatic vein takes blood away from the liver to the vena cava. |
| Kidney | The renal artery brings oxygenated blood that contains urea. | The renal vein takes away blood that contains less urea. |
Following the blood: the double circulation
- 1In a double circulation, the blood passes through the heart twice in one complete circuit of the body.
- 2The pulmonary circulation carries blood from the right side of the heart to the lungs and back to the left side of the heart, and the systemic circulation carries blood from the left side of the heart to the rest of the body and back to the right side of the heart.
Blood from the gut goes to the liver through the hepatic portal vein before it goes to the heart; it does not go directly to the heart.
This lets the liver change the amount of absorbed substances, such as glucose, in the blood before the blood reaches the rest of the body. supports K325-5(a)
Depth: why and how
- Why is the pulmonary artery an exception among the arteries? It carries deoxygenated blood, because it takes blood from the heart to the lungs, where the blood picks up oxygen.
- Why is the pulmonary vein an exception among the veins? It carries oxygenated blood, because it brings blood that has picked up oxygen in the lungs back to the heart.
- Why does a kidney need both a renal artery and a renal vein? The renal artery brings blood that contains urea to the kidney, and the renal vein takes away blood from which urea has been removed.
- Why does the blood pass through the heart twice in one circuit? The blood loses a lot of pressure when it flows through the capillaries of the lungs, so it returns to the heart to be pumped again before it goes to the body. The double circulation also keeps oxygenated and deoxygenated blood separate, and the lungs receive blood at lower pressure while the body receives it at high pressure.
- Cross-link to Nutrition in humans: glucose and amino acids absorbed into the capillaries of the villi are carried in the hepatic portal vein to the liver.
- Cross-link to Excretion in humans: the kidneys remove urea from the blood that arrives in the renal artery.
Common mistakes
All arteries carry oxygenated blood and all veins carry deoxygenated blood.
Arteries carry blood away from the heart and veins carry blood towards the heart; the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood.
Blood from the intestines goes straight to the heart.
Blood from the intestines goes to the liver in the hepatic portal vein before it goes to the heart.
The vena cava takes blood from the heart to the body.
The vena cava brings blood from the body to the right atrium, and the aorta takes blood from the left ventricle to the body.
The hepatic vein brings blood to the liver.
The hepatic vein takes blood away from the liver, and the hepatic artery and the hepatic portal vein bring blood to the liver.
Name the vessel by what it joins: a vessel that brings blood to an organ ends at that organ, and a vessel that takes blood away starts from it, so the hepatic portal vein is the one that reaches the liver from the gut.
Exam check
Q1Which vessel carries oxygenated blood from the lungs to the heart?
- AAorta
- BPulmonary artery
- CPulmonary vein
- DVena cava
Show answer
MCQ answer key C (Pulmonary vein): the pulmonary vein carries oxygenated blood from the lungs to the left atrium, whereas the pulmonary artery carries deoxygenated blood to the lungs, the aorta carries oxygenated blood to the body and the vena cava carries deoxygenated blood from the body.
Q2Blood leaves the capillaries in the villi of the small intestine after a meal. Which vessel does it enter next on its way to the liver?
- AHepatic vein
- BHepatic portal vein
- CRenal vein
- DVena cava
Show answer
MCQ answer key B (Hepatic portal vein): the hepatic portal vein carries blood from the small intestine to the liver, and the blood returns to the heart later through the hepatic vein and the vena cava.
Q3Which vessel brings blood that contains urea to the kidney?
- ARenal vein
- BRenal artery
- CHepatic vein
- DPulmonary artery
Show answer
MCQ answer key B (Renal artery): the renal artery brings blood that contains urea to the kidney, whereas the renal vein takes away blood that contains less urea.
StructuredBlood flows through the heart, lungs and liver.
- Name the vessel that carries blood from the right ventricle to the lungs. [1]
- State two ways in which the blood in the pulmonary vein differs from the blood in the pulmonary artery. [2]
- Name the vessel that carries blood from the gut to the liver, and state one reason why the blood goes to the liver before it goes to the heart. [2]
Total: 5 marks
Show mark scheme
(a) 1 mark
- Mark scheme: pulmonary artery
(b) 2 marks
- Mark scheme: the blood in the pulmonary vein has a higher concentration of oxygen
- Mark scheme: the blood in the pulmonary vein has a lower concentration of carbon dioxide
(c) 2 marks
- Mark scheme: hepatic portal vein
- Mark scheme: so that the liver can change the amount of absorbed substances, such as glucose, in the blood before it reaches the rest of the body
Arteries, veins and capillaries
relate the structures of arteries, veins and capillaries to their functions (specific names of muscle layers in arteries and veins are not required)
Arteries, veins and capillaries have different structures, and each structure is adapted to the job of the vessel.
- Artery
- Artery: a blood vessel that carries blood away from the heart at high pressure
- Arteriole
- Arteriole: a small branch of an artery that leads to the capillaries supports K325-5(b)
- Capillary
- Capillary: a microscopic blood vessel with a wall one cell thick, where materials are exchanged between the blood and the cells
- Venule
- Venule: a small blood vessel that collects blood from the capillaries and leads to a vein supports K325-5(b)
- Vein
- Vein: a blood vessel that carries blood towards the heart at low pressure
- Lumen
- Lumen: the space inside a blood vessel through which the blood flows supports K325-5(b)
- Elastic fibres
- Elastic fibres: fibres in the wall of a blood vessel that stretch when blood is pushed into the vessel and spring back afterwards supports K325-5(b)
- Valve
- Valve: a flap in a vein that lets blood flow towards the heart but stops it from flowing backwards
Comparing the three vessels
| Feature | Artery | Capillary | Vein |
|---|---|---|---|
| Direction of flow | Carries blood away from the heart. | Carries blood from the arterioles to the venules, close to the cells. | Carries blood towards the heart. |
| Blood carried | Usually oxygenated blood, except in the pulmonary artery. | The blood changes from oxygenated to deoxygenated as it gives up oxygen to the cells, except in the lungs, where the change is the other way round. | Usually deoxygenated blood, except in the pulmonary vein. |
| Wall | Thick wall with elastic fibres and muscle. | Wall one cell thick. | Wall thinner than the wall of an artery, with some elastic fibres and muscle. |
| Lumen | Narrow lumen. | Very narrow lumen, only just wide enough for a red blood cell to pass through. | Wide lumen. |
| Valves | No valves along the artery. | No valves. | Valves at intervals along the vein. |
| Blood pressure | High pressure. | Pressure falls as the blood flows through the capillaries. | Low pressure. |
| Flow of blood | Blood flows in spurts, in time with the heartbeat. | Blood flows slowly. | Blood flows smoothly, without spurts. |
How structure fits function
Artery: built for high pressure
The thick, elastic and muscular wall can withstand the high pressure of the blood that is pumped out of the heart.
When the ventricle pumps blood into an artery, the elastic wall stretches as the blood surges in with each heartbeat (felt as the pulse), and when the ventricle relaxes the wall springs back (recoils), which keeps the blood moving and the pressure up between beats; the spurts come from the contractions of the heart, not from the recoil.
Arteries branch into smaller arterioles, and the arterioles branch into capillaries.
Capillary: built for exchange
Capillaries are found close to almost every cell in the body.
The wall is one cell thick, so the distance for diffusion between the blood and the cells is short.
Capillaries branch many times, which gives a large surface area for the exchange of materials.
The total cross-sectional area of all the capillaries is much larger than that of the artery that supplies them, so the blood flows slowly, which gives more time for the exchange of materials; the blood pressure falls because of the resistance (friction) in the many narrow vessels.
Vein: built to return blood
Capillaries join to form venules, and the venules join to form veins.
The blood is at low pressure in a vein, so valves are needed to prevent the backflow of blood.
The wide lumen gives less resistance to the flow of blood.
When skeletal muscles contract, they squeeze the veins, which helps to push the blood towards the heart, and the valves make sure that the blood moves in one direction only.
Sections through the vessels
Blood pressure along the circulation
Blood pressure falls along the circulation, most steeply across the arterioles: it is highest in the arteries and lowest in the veins.
The pressure falls with increasing distance from the heart, because the blood loses energy as it flows against the resistance of the vessel walls.
The pressure in the arteries fluctuates: it rises when the ventricles contract (ventricular systole) and falls when the ventricles relax (ventricular diastole).
By the time the blood reaches the capillaries and the veins, the pressure is low and no longer rises and falls.
Depth: why and how
- Why do the arteries not burst under high pressure? The wall is thick and has elastic fibres and muscle, so it can stretch and withstand the pressure.
- Why is slow blood flow useful in the capillaries? The blood stays close to the cells for longer, so there is more time for the exchange of materials by diffusion.
- Why does the blood in the veins need help to return to the heart? The pressure is low by the time the blood reaches the veins, so muscle contraction and the valves help to move the blood towards the heart.
- Cross-link to Movement of substances: the short diffusion distance and the large surface area of the capillaries are the features that make diffusion fast.
- Cross-link to Homeostasis, co-ordination and response in humans: the blood vessels near the surface of the skin can carry more or less blood, which helps to control the body temperature.
- Cross-link to Coronary heart disease: fatty deposits build up in the arteries, where the pressure is high, and narrow the lumen.
Common mistakes
Arteries have thick walls so that they can carry more blood.
Arteries have thick, elastic walls to withstand the high pressure of the blood that is pumped out of the heart.
Capillary walls are thin so that the blood can flow faster.
Capillary walls are one cell thick to give a short diffusion distance, and the blood flows slowly through the capillaries to give time for exchange.
Veins have valves to make the blood flow faster.
Veins have valves to prevent the backflow of blood, because the blood pressure in the veins is low.
The lumen of a vein is narrow so that the pressure stays high.
The lumen of a vein is wide, which reduces the resistance to the flow of blood, and the pressure in a vein is low.
Match each structure to its job: a thick elastic wall for high pressure in arteries, a wall one cell thick for exchange in capillaries, and a wide lumen with valves for the return of blood in veins.
Exam check
Q1Which feature of a vein helps to prevent the backflow of blood?
- AA thick elastic wall
- BValves
- CA narrow lumen
- DA wall one cell thick
Show answer
MCQ answer key B (Valves): valves stop the blood flowing backwards in a vein, where the pressure is low, whereas a thick elastic wall is a feature of an artery and a wall one cell thick is a feature of a capillary.
Q2Which is a reason why the blood flows slowly through the capillaries?
- AThe capillary wall is thick
- BThe total cross-sectional area of the capillaries is large
- CCapillaries have valves
- DCapillaries carry blood at the highest pressure
Show answer
MCQ answer key B (The total cross-sectional area of the capillaries is large): the total cross-sectional area of all the capillaries is very large, so the blood flows slowly; the pressure falls because of the resistance (friction) in the many narrow vessels.
Q3Which statement about an artery is correct?
- AIt has a wide lumen and valves
- BIt has a wall one cell thick
- CIt has a thick, elastic wall that withstands high pressure
- DIt always carries deoxygenated blood
Show answer
MCQ answer key C (It has a thick, elastic wall that withstands high pressure): the thick, elastic and muscular wall of an artery can withstand the high pressure of the blood that is pumped out of the heart.
StructuredArteries, capillaries and veins have different structures.
- Explain how the structure of an artery suits its function. [2]
- Explain how the structure of a capillary allows the exchange of materials with the cells. [2]
- Explain why veins have valves but arteries do not. [2]
Total: 6 marks
Show mark scheme
(a) 2 marks
- Mark scheme: the wall is thick and has elastic fibres and muscle, so it can withstand the high pressure of the blood
- Mark scheme: the wall stretches as blood surges in with each heartbeat and recoils, which keeps the blood moving and the pressure up between beats
(b) 2 marks
- Mark scheme: the wall is one cell thick, so the diffusion distance is short
- Mark scheme: capillaries branch many times, which gives a large surface area, and the slow flow gives time for exchange
(c) 2 marks
- Mark scheme: the blood pressure in a vein is low, so the blood could flow backwards
- Mark scheme: the valves prevent the backflow of blood, so the blood moves towards the heart, whereas the pressure in an artery is high enough to keep the blood moving forwards
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Still to read in this topic:
- Capillaries and tissue fluid
- Components of blood
- ABO blood groups
- Structure of the heart
- The cardiac cycle
- Coronary heart disease
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Notes version 1.0 · Last updated 30 Sept 2026 · Topic 5 of 14