Blood Pressure is a pressure of circulating blood against the walls of the arteries. Blood pressure is an important diagnostic index. This is because the heart can pump into the large arteries a greater volume of blood than can be absorbed by the tiny arterioles and capillaries, the resulting back pressure is exerted against the arteries. Any condition that dilates or contracts the blood vessels or affects their elasticity of the blood vessels then it affects the blood pressure. In a healthy animal the blood pressure normal for its species is maintained within a certain average range with great constancy. Controlled by both cerebrospinal and sympathetic nerve centres, the complex nervous mechanisms that balance and coordinate the activity of the heart and arterial muscles permit great local variation in the rate of blood flow without disturbing the general blood pressure.

Blood pressure is read at two points: the high point at which the heart contracts to empty its blood into the circulation, called systole; and the low point at which the heart relaxes to fill with blood returned by the circulation, called diastole. Pressure is always measured in millimetres (mm) of mercury by an instrument called a sphygmomanometer which consisting of an inflatable rubber cuff connected to a pressure-detecting device with a dial. The cuff is wrapped around the upper left arm and inflated by squeezing a rubber bulb connected to it by a tube. Meanwhile, the doctor making the examination listens to a stethoscope applied to an artery in the lower arm. When the cuff expands then it gradually compresses the artery. The point at which the cuff stops the circulation and at which no pulsations can be heard is read as the systolic pressure. A spurting sound can then be heard as the heart contraction forces blood through the artery. The cuff is then allowed gradually to deflate further until the blood is flowing smoothly again. A reading at this point shows the diastolic pressure that occurs during relaxation of the heart. During a single cardiac cycle or heartbeat the blood pressure varies from maximum during systole to minimum during diastole. Usually both measurements are given as a ratio expression of the highest over the lowest.

In healthy people the blood pressure varies from about 80/45 in infants, to about 120/80 at age 30, to about 140/85 at age 40 and over. This increase occurs when the arteries lose the elasticity which in younger people used to absorbs the shock of heart contractions. Blood pressure varies between individuals and in the same individual at different times. It is generally higher in men than in women and children it is lowest during sleep, and is influenced by a wide range of factors.

Many healthy people have habitual systolic pressures of from about 95 to 115 not associated with symptoms or disease. Abnormally high blood pressure, or hypertension, is considered a contributory cause of arteriosclerosis. Poisons generated within the body cause extreme hypertension in various disorders. Abnormally low pressure also called hypotension, is observed in infectious and wasting diseases. A systolic pressure of much lower than 80 is usually associated with shock.

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