STRUCTURE AND FUNCTION OF ARTERIES, CAPILLARIES AND VEINS
What are Arteries?
Arteries carry blood away from the heart. They carry oxygenated blood except pulmonary artery which carries deoxygenated blood to the lungs.
Arteries have a thick, muscular wall, which has elastic and collagen fibres that resist the pressure of the blood flowing in them.
The high pressure is due to the pumping action of the heart.
The pressure in the arteries originates from the pumping action of the heart.
The pulse or number of times the heart beats per minute can be detected by applying pressure on an artery next to the bone.
g. by placing the finger/thumb on the wrist.
The innermost layer of the artery is called endothelium which is smooth.
It offers least possible resistance to blood flow.
Have a narrow lumen.
The aorta forms branches which supply blood to all parts of the body.
These arteries divide into arterioles which further divide to form capillaries.
Capillaries
Capillaries are small vessels whose walls are made of endothelium which is one cell thick.
This provides a short distance for exchange of substances.
Capillaries penetrate tissues,
The lumen is narrow therefore blood flowing in capillaries is under high pressure.
Pressure forces water and dissolved substances out of the blood to form tissue fluid.
Exchange of substances occurs between cells and tissue fluid.
Part of the tissue fluid pass back into capillaries at the venule end.
Excess fluid drains into small channels called lymph capillaries which empty their contents into lymphatic vessels.
Capillaries join to form larger vessels called venules which in turn join to form veins which transport blood back to the heart.
Veins
Veins carry deoxygenated blood from the tissues to the heart (except pulmonary vein which carries oxygenated blood from the lungs to the heart).
Veins have a wider lumen than arteries.
Their walls are thinner than those of arteries.
Blood pressure in the veins is low.
Forward flow of blood in veins is assisted by contraction of skeletal muscles, hence the need for exercise.
Veins have valves along their length to prevent backflow of blood.
This ensures that blood flows towards the heart.
The way the valves work can be demonstrated on the arm.
By pressing on one vein with two fingers, leaving one and pushing blood toward the heart then releasing the latter finger, it can be observed that the part in between is left with the vein not being visible.
This is because bleed does not flow back towards the first finger.
See also