9.3 Blood vessels
- Syllabus
- 0610–2026–2027
- Topic
- 9.3
- Level
- —
Learning objectives
9.3.1Structure of arteries, veins• Describe the structure of arteries, veins and capillaries, limited to: relative thickness of wall, diameter of the lumen and the presence of valves in veins9.3.2Functions of capillaries• State the functions of capillaries9.3.3Main blood vessels• Identify in diagrams and images the main blood vessels to and from the: (a) heart, limited to: vena cava, aorta, pulmonary artery and pulmonary vein (b) lungs, limited to: pulmonary artery and pulmonary vein (c) kidney, limited to: renal artery and renal vein9.3.4Structure of arteries and veins is• Explain how the structure of arteries and veins is related to the pressure of the blood that they transport9.3.5Structure of capillaries is related• Explain how the structure of capillaries is related to their functions9.3.6Main blood vessels• Identify, in diagrams and images, the main blood vessels to and from the liver as: hepatic artery, hepatic veins and hepatic portal vein
Compare the structures of arteries, veins and capillaries
Blood vessels can be distinguished by wall thickness, lumen diameter and the presence of valves.
| Vessel | Relative wall thickness | Lumen | Valves |
|---|---|---|---|
| artery | thick | relatively narrow | absent |
| vein | thin | relatively wide | present |
| capillary | one cell thick | extremely narrow | absent |
In cross-section, an artery keeps a smaller, more regular lumen inside a thick wall; a vein has a larger lumen and thinner wall; a capillary is only wide enough for cells to pass close to its wall.
Compare relative dimensions: a drawn vessel's absolute size is unreliable when diagrams are not to scale.
State the functions of capillaries
Capillaries form networks that bring blood close to cells and provide the exchange surface between blood and tissues.
| Direction | Examples |
|---|---|
| blood → tissues | oxygen, glucose and other dissolved nutrients |
| tissues → blood | carbon dioxide and other waste products |
Capillaries connect the small branches of arteries to the small branches of veins, allowing exchange throughout tissues.
Capillaries are exchange vessels; they do not pump blood and are not the same as lymphatic vessels.
Identify the main vessels of the heart, lungs and kidneys
Identify a vessel from the organ it connects and the direction of blood flow, not from oxygen content alone.
| Organ connection | Vessel | Direction |
|---|---|---|
| heart/body | aorta | heart → body |
| heart/body | vena cava | body → heart |
| heart/lungs | pulmonary artery | heart → lungs |
| lungs/heart | pulmonary vein | lungs → heart |
| heart/kidney | renal artery | heart/body artery → kidney |
| kidney/heart | renal vein | kidney → vena cava/heart |
Use artery = away from the heart and vein = towards the heart, then attach the organ name for pulmonary and renal vessels.
The pulmonary artery is deoxygenated and the pulmonary vein oxygenated, so oxygen content is not the definition of artery or vein.
Explain how artery and vein structure matches blood pressure
Arteries carry high-pressure, pulsatile blood from the heart; veins return lower-pressure blood.
| Vessel feature | Pressure-related function |
|---|---|
| artery: thick muscular wall | withstands high pressure |
| artery: elastic tissue | stretches and recoils as pressure changes |
| artery: relatively narrow lumen | helps maintain pressure |
| vein: wide lumen | offers low resistance to low-pressure flow |
| vein: valves | prevent backflow when pressure is low |
| vein: thinner wall | sufficient because the blood pressure is lower |
An explanation must link each structural difference to high or low pressure, rather than merely list features.
Valves do not raise venous pressure; they stop low-pressure blood reversing direction.
Explain how capillary structure supports exchange
Capillaries are built for rapid exchange over a short distance and across a large total surface area.
| Structural feature | Functional consequence |
|---|---|
| wall one cell thick | short diffusion distance |
| very narrow lumen | blood cells pass close to the wall and blood flow is slower |
| extensive branching network | large total surface area and close contact with many cells |
| gaps or pores between wall cells | water and small dissolved substances can move between blood and tissue fluid |
These features allow useful substances to leave blood and wastes to enter it efficiently by diffusion or fluid movement.
A capillary wall is one cell thick; saying each wall cell has a thin cell wall confuses animal cell membranes with plant cell walls.
Identify the main blood vessels of the liver
The liver has two main blood inputs and one main blood output.
| Vessel | Direction | Distinguishing role |
|---|---|---|
| hepatic artery | heart/body artery → liver | supplies oxygenated blood |
| hepatic portal vein | small intestine → liver | carries absorbed nutrients to the liver |
| hepatic vein | liver → vena cava/heart | drains blood from the liver |
On a diagram, find the vessel from the intestine to identify the hepatic portal vein; the remaining inflow is the hepatic artery and the outflow is the hepatic vein.
The hepatic portal vein is a vein because it carries blood towards the heart eventually, even though it first connects one organ's capillaries to the liver.