Blood vessels and blood

Last time

Before we start: last time, in Xylem, phloem and water uptake by roots, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Roads for the blood

If you laid out all your blood vessels end to end, they would go around the Earth twice. There are three kinds: arteries, veins and capillaries, each built for its job. And the blood itself has four parts. Today: both.

Three vessels

Here are the three kinds of blood vessel in cross-section. An artery with a thick wall and narrow lumen, a vein with a thin wall and wide lumen, and a capillary only one cell thick.

Arteries

Arteries carry blood away from the heart at high pressure. They have thick walls with muscle and elastic tissue, to withstand and even out the pressure. They have a narrow lumen, which keeps the pressure high.

Veins

Veins carry blood back to the heart at low pressure. They have thinner walls and a wide lumen, so blood flows easily. They have valves to stop the low-pressure blood flowing backwards. Squeezing by nearby muscles helps push blood back up the legs.

Capillaries

Capillaries are tiny vessels that pass close to every cell. Their walls are only one cell thick, so substances diffuse in and out quickly. Oxygen and glucose diffuse out to the cells; carbon dioxide and waste diffuse in.

Compare the vessels

Here is the comparison. Wall, lumen, valves and pressure. The examiner often asks which has valves: only veins.

Named vessels

The coronary arteries supply the heart muscle. The hepatic artery and hepatic portal vein go to the liver; the hepatic vein leaves it. The renal artery takes blood to each kidney, and the renal vein brings it back.

Blood

Plasma is the liquid part. It carries glucose, amino acids, carbon dioxide, urea, hormones and heat. Red blood cells contain haemoglobin, which carries oxygen. They have no nucleus. White blood cells fight disease: phagocytes engulf pathogens, and lymphocytes make antibodies. Platelets help the blood clot.

Clotting

When a blood vessel is cut, platelets start the clotting process. Soluble fibrinogen in the plasma is changed into insoluble fibrin, which forms a mesh. The mesh traps red cells, forming a clot that stops blood loss and keeps pathogens out.