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Blood Circulation




The circulatory system is the continuous system of tubes through which the blood is pumped around the body. It supplies the tissues with their requirements and removes waste products. In mammals and birds the blood circulates through two separate systems - the first from the heart to the lungs and back to the heart again (the pulmonary circulation) and the second from the heart to the head and body and back again (the systemic circulation)


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Diagram 12 - The mammalian circulatory system

The tubes through which the blood flows are the arteries, capillaries and veins. The heart pumps blood into arteries that carry it away from the heart. The arteries divide into very thin vessels called capillaries that form a network between the cells of the body. The capillaries then join up again to make veins that return the blood to the heart


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Arteries



Arteries carry blood away from the heart. They have thick elastic walls that stretch and can withstand the surges of high pressure blood caused by the heartbeat (the pulse, see later). The arteries divide into smaller vessels called arterioles. The hole down the centre of the artery is called the lumen. There are three layers of tissue in the walls of an artery. It is lined with squamous epithelial cells. The middle layer is the thickest layer. It made of elastic fibres and smooth muscle to make it stretchy. The outer fibrous layer protects the artery (see diagram 8.13). The pulse is only felt in arteries.


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Diagram 8.13 - Cross section of an artery




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Capillaries




Arterioles divide repeatedly to form a network penetrating between the cells of all tissues of the body. These small vessels are called capillaries. The walls are only one cell thick and some capillaries are so narrow that red blood cells have to fold up to pass through them. Capillaries form networks in tissues called capillary beds. The capillary networks in capillary beds are so dense that no living cell is far from its supply of oxygen and food (see diagram 8.14).

Note: All arteries carry oxygenated blood except for the pulmonary artery that carries deoxygenated blood to the lungs.



Diagram 8.14 - A capillary bed





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Veins




Capillaries unite to form larger vessels called venules that join to form veins. Veins return blood to the heart and since blood that flows in veins has already passed through the fine capillaries, it flows slowly with no pulse and at low pressure. For this reason veins have thinner walls than arteries although there have the same three layers in them as arteries (see diagram 8.16). As there is no pulse in veins, the blood is squeezed along them by the contraction of the skeletal muscles that lay alongside them.

Veins also have valves in them that prevent blood flowing backwards (see diagram 8.17a and .



Diagram 8.16 - Cross section of a vein



Diagram 8.17 a) and Valves in a vein




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The Formation Of Tissue Fluid And Lymph




The thin walls of capillaries allow water, some white blood cells and many dissolved substances to diffuse through them. These form a clear fluid called tissue fluid (or extracellular fluid or interstitial fluid) that surrounds the cells of the tissues. The tissue fluid allows oxygen and nutrients to pass from the blood to the cells and carbon dioxide and other waste products to be removed from the tissues (see diagram 8.15).


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Diagram 8.15 - The formation of tissue fluid and lymph from blood

Some tissue fluid finds its way back into the capillaries and some of it flows into the blind-ended lymphatic vessels that form a network in the tissues. Once the tissue fluid has entered the lymphatics it is called lymph although its composition remains the same. The lymph vessels have walls that are even thinner than the capillaries. This means that molecules and particles that are larger than those that can pass into the blood stream e.g. cancer cells and bacteria can enter the lymphatic system. These are then filtered out as the lymph passes through lymph nodes







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Important Blood Vessels Of The Systemic (Body) Circulation





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