Key factors about blood vessels

External layer

Middle layer Endothelium (internal layer) Lumen

Fig 5.4 The structure of an artery

Arteries

• Arteries carry blood away from the heart.

• Blood is carried under high pressure.

• Arteries have thick muscular and elastic walls to withstand pressure.

• Arteries have no valves, except at the base of the pulmonary artery where they leave the heart.

• Arteries carry oxygenated blood, except the pulmonary artery, to the lungs.

• Arteries are generally deep-seated, except where they cross over a pulse spot.

• Arteries give rise to small blood vessels called arterioles which deliver blood to the capillaries.

External layer

Middle layer Endothelium (internal layer) Lumen

KEY FACTS

Both arteries and veins have three layers (external, middle and internal layers) but because an artery must contain the pressure of blood pumped from the heart, its walls are thicker and more elastic.

Fig 5.5 The structure of a vein

• Veins carry blood towards the heart.

• Blood is carried under low pressure.

• Veins have thinner muscular walls.

• Veins have valves at intervals to prevent the back flow of blood.

• Veins carry deoxygenated blood, except the pulmonary veins, from the lungs.

• Veins are generally superficial, not deep-seated.

• Veins form finer blood vessels called venules which continue from capillaries.

Capillaries

Fig 5.6 The structure of a capillary

• Capillaries are the smallest vessels.

• Capillaries unite arterioles and venules, forming a network in the tissues.

• The wall of a capillary vessel is only a single layer of cells thick. It is, therefore, sufficiently thin to allow the process of diffusion of dissolved substances to and from the tissues to occur.

• Capillaries have no valves.

• Blood is carried under low pressare but higher than in veins.

• Capillaries are responsible for supplying the cells and tissues with nutrients.

KEY FACTS

The key function of a capillary is to permit the exchange of nutrients and waste between the blood and tissue cells. Substances such as oxygen, vitamins, minerals and amino acids pass through to the tissue fluid to nourish the nearby cells, and substances such as carbon dioxide and waste are passed out of the cell. This exchange of nutrients can only occur through the semi-permeable membrane of a capillary as the walls of arteries and veins are too thick.

Exchange of substances between tissue -cells and blood takes place

Artery to

Arterioles to

Capillaries to

Venules to

Vein

Artery to

Arterioles to

Capillaries to

Venules to

Vein

Fig 5.7 Blood flow from an artery to a vein

Oxygenated blood flowing through the arteries appears bright red in colour due to the oxygen pigment haemoglobin. As it moves through capillaries, it offloads some of its oxygen and picks up carbon dioxide. This explains why blood flow in veins appears darker.

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