Figure 18.2: Blood is collected in the heart and pumped out to the lungs, where it releases
carbon dioxide and picks up oxygen before it is pumped to the rest of the body. ( 6 )
of the heart muscle causes blood to exert force on the walls of the arteries. This force is
referred to asblood pressure. Blood pressure is highest in the arteries and decreases as the
blood moves into smaller blood vessels. Thick walls help prevent arteries from bursting from
the pressure of blood.
Figure 18.3: Arteries are thick-walled vessels with many layers, including a layer of smooth
muscle. ( 11 )
Every cell in the body needs oxygen, but arteries are too large to bring oxygen and nutrients
to single cells. Further from the heart, arteries form smaller arteries. These smaller arteries
branch into smaller vessels. The smaller blood vessels help to bring nutrients and oxygen
and take away waste from body tissues.
The tiniest blood vessels in the body are calledcapillaries. The walls of capillaries are
only a single layer of cells thick. Capillaries connect arteries and veins together, as shown
inFigure18.4. Capillaries also allow the delivery of water, oxygen and other substances
to body cells. They also collect carbon dioxide and other wastes from cells and tissues.
Capillaries are so narrow that blood cells must move in single file through them.
Acapillary bedis the network of capillaries that supply an organ with blood. The more
metabolically active a tissue or organ is, the more capillaries it needs to get nutrients and