Blood vessels, tissue fluid and lymph

Last time

Before we start: last time, in Transpiration, root pathways and xerophytes, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Three kinds of pipe

Blood leaves the heart through arteries, travels through tiny capillaries, and returns in veins. Each kind of vessel has a different job, so each has a different wall. And at the capillaries, some of the liquid in blood leaks out to bathe the cells. That leaked liquid is tissue fluid, and getting it back is the second half of this lesson.

Arteries

Arteries carry blood away from the heart at high pressure. The wall is thick, with three layers around a narrow lumen. The inner layer is the endothelium, a smooth layer of flat cells that reduces friction. The middle layer, the tunica media, has smooth muscle and elastic fibres. Elastic fibres stretch with each pulse and recoil, keeping blood flowing. The outer layer is tough collagen, which stops the artery bursting.

Veins and capillaries

Veins carry blood back to the heart at low pressure. The wall is thinner, the lumen is wide, and there are valves to stop backflow. Muscles squeezing the veins push the blood along, and the valves keep it moving in one direction. Capillaries are one cell thick, a single layer of endothelium. They have tiny gaps, and are close to every cell, so exchange is quick.