Without pressure the blood would not move at all, since a certain amount of pressure is necessary to keep the blood flowing throughout the body. Blood is always under pressure, but the amount of pressure varies in different blood vessels. High blood pressure in the arteries gradually decreases in the capillaries and veins. Blood entering the heart, into both atria at the same time, when full the pressure causes the tricuspid and bicuspid valves to open and the blood flows into the ventricles. The ventricles contract (systole) and the atria relax (diastole) and the pressure closes the valves allowing deoxygenated blood to enter the heart to oxygenate and a higher pressure when it reaches the aorta oxygenates to allow it to circulate throughout the body. Without blood pressure, the blood would not move. Blood itself carries oxygen, nutrients, hormones and enzymes throughout the body. It transports carbon dioxide and waste materials from the body to be excreted by various organs. It helps us fight infections, coagulates after accidents and regulates body temperature. Without the pressure pushing blood throughout the body, none of these actions would take place