Blood and Oxygen Your circulatory and respiratory system Why do we need a circulatory system? The human body is large and so it needs a system to transport what it needs around. This system is The Circulatory System and it does 3 things; – Supplies oxygen – Supplies nutrients – Removes wastes (eg Carbon dioxide) What is it made of? The circulatory system is made up of; – Vessels. Eg. Arteries, veins and capillaries. – A pump. In the case of mammals a heart. The heart (the pump) In humans the heart is located behind the lungs in the centre of the chest slightly leaning to the left. The heart has two atria (chambers on the top of the heart) and two ventricles (very muscular chambers on the bottom) The Vessels Blood vessels are not all the same size. They get smaller in diameter as you move further from the heart. There are three main types of blood vessels; – Arteries – Veins – Capilliaries Arteries Arteries carry blood away from the heart, delivering oxygen and nutrients to cells and tissues. Arteries have very thick walls which contain a lot of elastic tissue and muscle to help them withstand the pressure produced by the heart. Veins Veins carry blood back from the cells and tissues to the heart so it can be pumped into the lungs for re-oxygenation. Veins have thinner walls than arteries, which contain less elastic tissue and muscle. Veins contain one way valves which prevent blood flowing the wrong way. These valves are important in returning blood from the legs against gravity. Capillaries Capillaries are very narrow they are only wide enough for one red blood cell to pass through at a time. The walls of capillaries are only one cell thick. This means that in capillaries, substances are able to be transferred across the cell membrane into and out of the blood stream.