Blood Vessels

Blood is transported around the body in a network of vessels which although similar in some respects are adapted to their particular functions.

Arteries and arterioles (smallest arteries) carry blood away from the heart, capillaries supply blood to the tissues permitting the exchange of materials and veins return blood to the heart.

Arteries carry oxygen-rich blood to the tissues and organs in the body. Blood flows through arteries under greater force than in veins, so the walls of arteries are thicker. Arteries get smaller and smaller as they get farther away from the heart. At their smallest point, arteries become capillaries. Capillaries connect arteries to veins and carry blood to and from all tissues in the body. Capillaries deliver oxygen and nutrients to all cells. Waste products and carbon dioxide from the cells passes through the walls of capillaries back into the bloodstream. Capillaries lead into veins which carry blood with less oxygen back to the heart. The walls of veins are much thinner than artery walls because blood flows through veins at a lower pressure.

Both arteries and veins have a similar structure which is made up of three basic layers:

The relative thickness and composition of each layer varies depending on the vessels function. The middle layer shows the greatest variation.

Capillaries are comprised solely of the inner layer described above, their walls consist of endothelium, a single layer of squamous epithelium through which substances in blood are exchanged for substances in the tissues fluid.

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