2.5 The Structure and Function of Arteries, Capillaries and Veins

Blood Vessels

The central space within a blood vessel is called the lumen. The lumen is lined with a layer of cells called the endothelium. The layers of tissue outside the endothelium are different in the different blood vessels.

The Exchange of Materials Between Tissue Fluid and Cells

-Capillaries occur in networks called capillary beds, which run in between cells of the body tissue. -The high pressure of the arteriole blood causes pressure filtration, leading to the capillaries exuding plasma. -This becomes the tissue fluid, which bathes the cells. -Tissue fluid then exchanges material with the cells and returns to the capillaries by osmosis and diffusion. -Excess tissue fluid drains into lymph vessels and is now called lymph.

Arteries

-Carry blood at high pressure away from the heart. -Arterioles are he smallest arteries and supply blood to capillaries. -They have thick walls and a relatively narrow central lumen. -The outer wall is made of tissue with elastic fibres. Inner wall contains smooth muscle with elastic fibres.

Veins

-Carry blood at low pressure back to the heart. -Venules are the smallest veins that remove blood from the capillaries, where exchange of materials with tissues has taken place. -Veins have an outer wall containing elastic fibres but a much thinner inner muscular wall than arteries. -Lumen of a vein is relatively wider than that of an artery. -Veins have valves to prevent backflow of blood flowing to the heart at low pressure.

Contractions of the Heart

-Contractions of the heart force surges of high-pressure blood into the arteries. -The elastic fibres in the outer and middle layers allow the artery walls to stretch as these surges of blood pass through them. -These fibres then recoil between beats, so maintaining the high pressure of blood in the arteries.

Contraction of the Smooth Muscle

-Contractions of the smooth muscle in the middle layer of an artery wall decreases the diameter of the central lumen. -This is called vasoconstriction. -Vasoconstriction of the arteries and arterioles decreases the flow of blood into the caplillaries. -Relaxation of the smooth muscle in the middle layer of an artery wall causes the opposite effectm called vasodilation. -Vasodilation increases the diameter of the central lumen and increases the blood flow into the capillaries.

The Role of Lymph Vessels

-More fluid passes out of the capillaries due to pressure filtration than diffuses back by osmosis and diffusion. -Excess tissue fluid that does not pass into the capillaries entering the lymph vessels is now called lymph. -The pressure of the lymph fluid is even lower than that of the blood in the veins. -Lymph vessels contain valves and surrounding skeletal muscles squeeze the lymph through the valves of the lymph vessels as the body moves. -The lymphatic system returns the lymph fluid back into the bloodstream. -Returning lymph passes through a series of lymph nodes, which contain lymphocytes and other cells of the immune system. -The nodes act as filters, removing foreign particles from the lymph fluid.

Capilalries

Allow exchange of substances carried in the blood with the tissues of the body. -Capillaries have a lumen and very thin walls - only the enothelium. -This allows quick and efficient exchange of materials to occur between he blood inside and the surrounding body tissues.

Capillaries - Function, Structure of Wall, Lumen and Valves

Function - Allow rapid and efficient exchange of substances with tissues Structure of wall - Capillary walls are only one cell thick Lumen - Very narrow Valves - Absent

Arteries - Function, Structure of Wall, Lumen and Valves

Function - Carry blood away from the heart at high pressure Structure of wall - Outer wall of elastic fibres and an inner wall containing smooth muscle with some elastic fibres. Lumen - Relatively narrow Valves - Absent

Veins - Function, Structure of Wall, Lumen and Valves

Function - Carry blood towards the heart at lower pressure Structure of wall - Outer wall of elastic fibres but a much thinner inner muscular wall than arteries. Lumen - Relatively wide Valves - Present to prevent the backflow of blood