Arteries, veins and capillaries are three kinds of tube with three different jobs, and each one's shape fits the pressure of the blood inside it. Learn to tell them apart, see what capillaries do, and trace the main vessels to and from the heart, lungs and kidneys. Linking structure to pressure and function, and the vessels of the liver, are Extended.
Arteries have thick walls and a narrow lumen; veins have thin walls, a wide lumen and valves; capillaries have walls only one cell thick and a very narrow lumen. Capillaries exchange substances between the blood and the tissues. The main vessels are the vena cava, aorta, pulmonary artery and pulmonary vein (heart and lungs) and the renal artery and renal vein (kidneys).
Switch between artery, vein and capillary. The four readouts list the wall, lumen, blood pressure and valves for each, and the red blood cells show how the blood moves: in pulses in the artery, steadily in the vein, in single file in the capillary.
| Artery | Vein | Capillary | |
|---|---|---|---|
| Wall | thick, with muscle and elastic tissue | thin, little muscle | one cell thick |
| Lumen | narrow | wide | very narrow, about one red blood cell wide |
| Valves | none (except where they leave the heart) | yes | none |
| Blood pressure | high, in pulses | low | low, falling along it |
| Carries blood | away from the heart | back to the heart | through the tissues, from arterioles to venules |
Do not say arteries carry oxygenated blood and veins carry deoxygenated blood. The definition is direction: arteries carry blood away from the heart and veins carry it back. The pulmonary artery carries deoxygenated blood.
Capillaries are where the circulatory system does its real work. In the capillary view of the sim, small particles leave the vessel and move out into the surrounding tissue.
"Capillaries are thin" is too vague. Say the wall is one cell thick, so the diffusion distance is short. The lumen is narrow, which is a different point.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the blood vessels tab, which vessel shows a thin wall, a wide lumen and valves?
What does the sim give as the blood pressure in an artery?
In the capillary view, how do the red blood cells pass along?
In the main vessels tab, which vessel carries blood containing urea to each kidney?
Click a vessel name or the vessel itself to light it up and watch the blood flow along it. Red vessels carry oxygenated blood and blue ones deoxygenated blood, except that the pulmonary artery is blue and the pulmonary vein red. The three liver vessels are Extended.
| Organ | Vessel bringing blood | Vessel taking blood away |
|---|---|---|
| Heart | vena cava (from the body), pulmonary vein (from the lungs) | aorta (to the body), pulmonary artery (to the lungs) |
| Lungs | pulmonary artery | pulmonary vein |
| Kidneys | renal artery (blood containing urea) | renal vein (filtered blood, urea removed) |
| Extended: liver | hepatic artery (oxygenated blood); hepatic portal vein (blood rich in absorbed nutrients from the gut) | hepatic vein, to the vena cava |
The hepatic portal vein runs from the gut to the liver, not from the liver to the heart. Blood leaves the liver in the hepatic vein.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Which feature is found in veins but not in arteries?
Extended: why do arteries have thick, elastic walls?
Extended: which feature of capillaries most directly shortens the diffusion distance?
Extended: which vessel carries blood from the small intestine to the liver?
This skill is now recorded on your profile. Keep the chain going.