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Blood Pressure 1. Blood pressure is caused by the a. beating of the heart b. recoil of the elastic arteries c. peripheral resistance caused by the narrow capillaries 2. When the ventricles contract pressure is applied to the blood in them forcing it out of the major arteries. This pressure is called the SYSTOLIC PRESSURE and is normally 100 – 140 mm/Hg 3. When the ventricles relax the stretched arteries recoil onto the blood applying pressure known as the DIASTOLIC PRESSURE. This is normally 60 – 90 mm/Hg. 4. Blood pressure is measured using a SPHYGMO-MANOMETER and is recorded this way: 120 80 systolic diastolic 5. Some factors affecting blood pressure: a. elasticity of the vessel walls - this decreases with age due to atherosclerosis. b. rate and force of heart beat CARDIAC OUTPUT = STROKE VOLUME X STROKE RATE c. blood volume – increased by contraction of the spleen Falls in haemorrhage or severe shock d. peripheral resistance – narrow arteries caused by adrenalin e. pain can cause widening of the arteries, loss of BP leading to fainting. D:\565313285.doc Blood pressure in parts of the circulatory system 160 140 Systolic pressure 120 100 80 Diastolic pressure 60 40 20 0 Aorta Arteries Flow rate 200 mm/s Arterioles Capillaries Flow rate 0.5 mm/s Veins Flow rate 133 mm/s 166 000 mm2 Total cross sectional area D:\565313285.doc