Blood Vessels
Blood Vessels
We will now examine the structure and function of arteries, capillaries, veins and valves.
Arteries
Arteries carry blood Away from the heart. The pressure created by the pumping heart forces blood through the arteries. Arteries have three layers. They have an outside layer made up of connective tissue; a middle layer made up of smooth muscle, to allow contraction of the arteries in order to regulate the pressure of blood flow, and an inside layer of tightly connected simple squamous endothelial cells. The large arteries close to the heart branch into smaller arterioles (smaller arteries) and eventually branch into capillaries.
Did You Know?
Laughing is good exercise for your heart. Whenever you laugh, the blood vessels dilate (open up), causing the blood flow to increase, thus keeping your heart healthy.
Micrograph of artery.
Capillaries
Capillaries are little more than a single layer of endothelial cells. Capillaries form intricate networks throughout the tissues. They allow water, nutrients and gases to diffuse out of the blood and waste materials to diffuse into the blood. This exchange occurs between the blood and the tissue fluid. The tissue fluid is the fluid surrounding the cells. The blood cells never come into contact with the cells. The blood and tissue fluid exchange material, and the tissue fluid then exchanges material with the cells.
Veins
The intricate networks formed by the capillaries eventually converge to form venules, (small veins). The venules then converge to form veins which return the blood to the heart. Vein walls only consist of two layers. The outer layer is made up of connective tissue whereas the inner layer is made up of endothelial cells.
Schematic diagram of a vein.
Diagram representing the branching of an artery into arterioles. These subsequently form the capillary bed which empties into several venules, leading to the vein.
Valves
Once the blood has passed through the capillaries very little blood pressure remains to return blood to the heart.Instead of pressure from the heart veins use a series of valves to force blood to return to the heart. Contraction of the muscles squeezes the veins, pushing the blood through them. The valves cause the blood to flow in only one direction, back to the heart.
Valves ensure that blood flows one only way through veins.
This lesson is part of:
Animal Systems