Blood vessels

Biology · Transport in humans. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

There are three types of blood vessel, and the structure of each fits the pressure of the blood inside it.

Arteries carry blood away from the heart at high pressure. They have thick walls with a lot of muscle and elastic tissue, and a narrow lumen. The elastic wall stretches with each surge of blood and then recoils. Veins carry blood back to the heart at low pressure. They have thin walls, a wide lumen and valves to stop backflow.

Capillaries join arteries to veins. Their walls are one cell thick, so substances pass quickly between the blood and the cells. Vessels are named after the organ: hepatic means liver, renal means kidney, pulmonary means lungs.

Rules to remember

Artery: thick wall, much muscle and elastic tissue, narrow lumen, no valves along its length, high pressure
Vein: thin wall, little muscle and elastic tissue, wide lumen, valves, low pressure
Capillary: wall one cell thick, very narrow lumen, no valves; site of exchange
Heart and body: aorta leaves the left ventricle; vena cava returns blood to the right atrium
Lungs: pulmonary artery carries deoxygenated blood to them; pulmonary vein carries oxygenated blood back
Liver: hepatic artery in (oxygenated), hepatic portal vein in (from the gut, rich in absorbed food), hepatic vein out
Kidneys: renal artery in; renal vein out (less urea)

Common mistake

"Arteries always carry oxygenated blood" is wrong. The pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood. Arteries are defined by direction: away from the heart.

Worked example

Urea is made in the liver and removed by the kidneys. Name, in order, the main blood vessels a urea molecule passes through on its way from the liver to a kidney.

It leaves the liver in the hepatic vein and enters the vena cava, which takes it to the right side of the heart.
It goes to the lungs in the pulmonary artery and returns to the left side of the heart in the pulmonary vein.
The left ventricle pumps it into the aorta, and a branch of the aorta, the renal artery, takes it to the kidney.

Answer: Hepatic vein → vena cava → pulmonary artery → pulmonary vein → aorta → renal artery.

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Transport in Humans

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