Blood pressure is an important diagnostic index, especially of circulatory function.

Any condition that dilates or contracts the blood vessels or affects their elasticity, or any disease of the heart that interferes with its pumping power, 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.

Blood pressure is read at two points: the high point at which heart contracts to empty its blood into circulation, called systole; and the low point at which the heart relaxes to fill with blood returned by the circulation, called diastole.

Pressure is measured in millimeters (mm) of mercury by an instrument called a sphygmomanometer, consisting of an inflatable rubber cuff connected to a pressure-detecting device with a dial, or mercury column.  

The blood pressure is most often measured by using a device called a sphygmomanometer, a stethoscope, and a blood pressure cuff. 

The cuff is placed around the upper arm and filled with air, tightening to stop the blood from flowing through the artery in the arm. 

The stethoscope is placed over the artery in front of the elbow and the pressure in the cuff is slowly released. No sound is heard until the cuff pressure falls below the systolic pressure in the artery; at that point a pulse is heard. As the cuff pressure continues to fall slowly, the pulse continues; first becoming louder, then dull and muffled. 

These sounds, called the sounds of Korotkoff, are caused by the disturbance of the blood flowing through the vessel. 

The cuff pressure at which the first sound is heard is the systolic blood pressure, and the cuff pressure at which the sounds stop is the diastolic blood pressure. The pressure in both arms is sometimes taken. A major difference between the two readings may mean there is a blockage of the vessels. 

The blood pressure may also be taken using the thigh with the stethoscope held behind the knee. A larger cuff is used when taking the blood pressure of an overweight person or when using on the thigh. 

Any factor that increases resistance of the vessels to the flow of blood or that affects the amount of blood pumped by the heart will change the blood pressure. Strong emotion, for example, tends to do both; therefore the blood pressure reading is usually taken when the person is resting. 

Blood pressure increases with age, mainly because the veins do not expand as well. As a person grows older an increase in systolic pressure comes before an increase in diastolic pressure.

The cuff is wrapped around the upper left arm and inflated by squeezing a rubber bulb connected to it by a tube.

Meanwhile, the physician making the examination listens to a stethoscope applied to an artery in the lower arm.

As the cuff expands, it gradually compresses the artery. As the cuff is slowly deflated, at the point when the circulation is restored, a spurting sound can be heard as the heart contraction forces blood through 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.

The cuff is then allowed gradually to deflate further until the blood is flowing smoothly again.

A point at which the sound disappears is measured as Diastolic pressure.

Reading at this point shows the diastolic pressure that occurs during relaxation of the heart.

Usually both measurements are given as a ratio expression of the highest over the lowest, for example, 140/80.

During a single cardiac cycle or heartbeat, the blood pressure varies from maximum during systole to minimum during diastole.

In healthy persons, 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 that, in younger people, 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, is lowest during sleep, and is influenced by a wide range of factors.

Abnormally high blood pressure, is known as hypertension, is considered a risk factor for heart attacks and stroke.

Abnormally low pressure, or hypotension, is observed in infectious and wasting diseases, hemorrhage, and collapse. A systolic pressure of much lower than 80 is usually associated with shock.