Everything Life Sciences Grade 10

Figure 8.12: Micrograph of artery.

Capillaries

Capillaries are little more than a single layer or endothelial cells. Capillaries form intricate

networks throughout the tissues. They allow water, nutrients and gases to diffuse out of

the blood and waste materials to diffuse into the blood. This exchange occurs between the

blood and the tissue fluid. The tissue fluid is the fluid surrounding the cells. The blood cells

never come into contact with the cells. The blood and tissue fluid exchange material, and

the tissue fluid then exchanges material with the cells.

Veins

The intricate networks formed by the capillaries eventually converge to form venules, (small

veins). The venules then converge to form veins which return the blood to the heart. Vein

walls only consist of two layers. The outer layer is made up of connective tissue whereas the

inner layer is made up of endothelial cells.

236 8.2. Circulatory systems in animals