Cambridge O Level Biology · Syllabus 5090 · Transport in Humans
Vein
What is 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.
This definition is part of the Transport in Humans chapter in Cambridge O Level Biology.
Common mistakes with Vein
- “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.
- “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.
- “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.
Examiner tips on Vein
- 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.
Questions students ask about Vein
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.
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.