This I a plan of the basic design of the human circulatory system. This diagram is drawn as though you were looking at a person who is facing you. The pump, the heart, pushes a fluid, blood, along tubes called vessels. As the blood passes twice through the heart on this single complete journey around the system it is called a double circulatory system. By returning to the heart after picking up oxygen in the lungs there is enough pressure to ensure that every cell in the body gets enough oxygen. A fish only has a single circulatory system. After picking up oxygen at the gills it delivers it directly to the cell/tissues
A blood vessel is a tube with a space in the centre, called the lumen.
Mammals have three types of blood vessels. Arteries carry blood away from the heart Veins carry blood towards the heart Linking the two, forming networks of tiny tubes in the tissues are capillaries. Arterioles are small arteries . As an artery gets closer to the tissue to which it is delivering blood it divides to form many smaller vessels, which are similar to arteries (less elastic as blood flow has become smoother) Venules are small veins which collect blood after it has flowed through a capillary bed.
So blood leaves the heart through arteries, which in turn give way to arterioles and then capilliaries where the oxygen is delivered and exchanged for carbon dioxide. The capillaries then become larger venules and then veins which then deliver the blood back to the heart.
Now lets have a look at the structure of these vessels There are three layers of tissue in both arteries and veins 1 - tunica intima 2 - tunica media 3 - tunica externa (or adventitia) The capillaries are made of a single layer of endothelial cells.
The tunica externa is the strong outer covering of arteries and veins. It is composed of connective tissue as well as collagen and elastic fibers . These fibers allow the arteries and veins to stretch to prevent overexpansion due to the pressure that is exerted on the walls by blood flow. The tunica media is the middle layer of the walls of arteries and veins. It is composed of smooth muscle and elastic fibers. The tunica intima is the inner layer of arteries and veins.
Blood pumped out of the heart is at a very high pressure, so the structure of the arteries must be adapted to this. They can withstand high pressure by having very thick walls made up of elastic fibres and smooth muscle. The elastic fibres increase its elastic strength. The smooth muscle can change the diameter of the lumen, especially in the smaller arteries. (higher students The smaller arteries and arterioles are more muscular and can contract ( vasoconstriction ) to close off the capillary beds to which they lead; or relax ( vasodilation ) to open up the capillary bed. ) . Lumen is relatively narrow. The tunica media layer is thicker in arteries than in veins tunica intima In arteries this layer is composed of an elastic membrane lining and smooth endothelium that is covered by elastic tissues.
This is a normal coronary artery.
ELASTIC ARTERY (AORTA) Stained with orsein 1 - tunica intima 2 - tunica media 3 - tunica externa
The coronary artery shown here has narrowing of the lumen due to build up of atherosclerotic plaque. Severe narrowing can lead to angina, ischemia, and infarction.
Veins carry blood from every tissue in the body to the heart. As the pressure is so much lower in the veins than in the arteries, there is little need for the elastic fibres and smooth muscle in the walls. they have a larger lumen that arteries, to reduce the resistance to flow
They also have semi-lunar valves to stop the blood flowing backwards
It is particularly difficult for blood to flow upwards through the legs to heart, and the flow is helped by contractions of the leg and abdominal muscles (see animation in favourites) The body relies on constant contraction of these muscles to get the blood back to the heart, and this explains why soldiers standing still on parade for long periods can faint, and why sitting still on a long flight can cause swelling of the ankles and Deep Vein Thrombosis (DVT or “ economy class syndrome ” ), where small blood clots collect in the legs
This is a medium sized vein, recognizable as such by its scanty wall and the presence of a valve inside it. The valve flaps are marked by arrows.
Artery and a vein
Capillaries are where the transported substances actually enter and leave the blood No exchange of materials takes place in the arteries and veins, whose walls are too thick and impermeable. Capillaries are very narrow and thin-walled, but there are a vast number of them (10 8 m in one adult!), so they have a huge surface area : volume ratio, helping rapid diffusion of substances between blood and cells. Capillaries are arranged in networks called capillary beds feeding a group of cells, and no cell in the body is more than 2 cells away from a capillary.
A capillary wall is very thin and composed of (endothelium only) single layer of cells as it does not have to withstand high internal pressure. A capillary wall is often highly permeable, partly because its very thin and partly because of holes in and between cells in some capillaries (particularly those with high demand of exchange eg endocrine glands)
Capillary diameter is very small. A typical small artery or vein may be 5mm (in diameter in a human) But a capillaries are approx 10 micrometer about the size of a red blood cell (erythrocyte)
A capillary bed
Blood Vessels <ul><li>Learning Objectives: </li></ul><ul><li>Compare and contrast the structure and function of </li></ul><ul><ul><li>Arteries </li></ul></ul><ul><ul><li>Veins </li></ul></ul><ul><ul><li>Capillaries </li></ul></ul>
Task Draw and complete a table to compare structure and function of arteries, veins and capillaries. E.g. Artery Veins Capillary Function
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