Giraffe have unusually high heart rates for their size, at 150 beats per minute. When the animal lowers its head the blood rushes down fairly unopposed and a rete mirabile in the upper neck, with its large cross sectional area, prevents excess blood flow to the brain. When it raises again, the blood vessels constrict and direct blood into the brain so the animal does not faint. The jugular veins contain several (most commonly seven) valves to prevent blood flowing back into the head from the inferior vena cava and right atrium while the head is lowered. Conversely, the blood vessels
Blood vessel diffuse vascular supply it is called an anastomosis. Anastomoses provide critical alternative routes for blood to flow in case of blockages. There is a layer of muscle surrounding the arteries and the veins which help contract and expand the vessels. This creates enough pressure for blood to be pumped around the body. Blood vessels are part of the circulatory system, together with the heart and the blood. The biggest difference in the structure of arteries and veins is the presence of valves. Backflow of blood is prevented in arteries by the heart. However in veins, one-direction valves are used to