=Blood Vessels= Blood vessels are are the branching tube-like structures that carry blood throughout the body. There are three main types of blood vessels: • '''Arteries''', which carry the blood away from the heart • '''Veins''', which carry blood from body tissues back to the heart • '''Cappilaries''', tiny vessels that enable the actual exchange of water and chemicals between the blood and the tissues [image:http://i.imgur.com/7EOQ6Ik.jpg?1] Oxygen rich blood bound for the body tissues leaves the heart through a large artery called the ''aorta''. The arteries continue to branch into smaller vessels called ''arterioles'' (little arteries). These continue to branch into tiny, thin-walled ''capillaries''. Blood travels slowly as the red blood cells squeezes through tiny capillaries. Nutrients, gasses and wastes are exchanged between the blood and the surrounding tissue here. The capillaries then begin together to form small veins called ''venules'' (little veins). The branches continue to combine into large ''veins'' and eventually the blood re-enters the heart through the ''vena cava''. ==Blood Vessel Structure== [image:http://i.imgur.com/ZRtnC6p.png] '''1. Tunica adventitia''' (thickest layer in veins) is a layer of tough connective tissue that help prevent the vessel from rupturing '''2. Tunica media''' (thickest layer in arteries) consists of a combination of elastic fibres and smooth muscle cells. Arteries have a thicker layer of muscle cells that controls the size of the vessel which helps to regulate blood pressure '''3. Tunica intima''' (thinnest layer) a single layer of cells (endothelium) ==Comparison of Arteries & Veins== [image:http://i.imgur.com/uOdSwvi.png?2] ==Capillaries== [image:http://i.imgur.com/TqOdjES.png?2] Capillaries are only one cell thick. Their thin structure allows molecules to diffuse through rapidly. Soluble food molecules (sugars and amino acids) will diffuse into the capillaries in the villi of the small intestine. Oxygen diffuses into capillaries lining the lungs (and carbon dioxide diffuses out). Both Oxygen and Glucose (sugar) will diffuse out of capillaries to surrounding body cells.
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