Blood Pressure

Like any pump, the heart produces pressure. When the heart contracts, it produces a wave of fluid pressure in the arteries. The force of the blood on the arteries' walls is known as blood pressure. Blood pressure decreases when the heart relaxes, but the system still remains under pressure. It's a good thing, too. Without that pressure, blood would stop flowing through the body.

Medical workers can measure blood pressure with a device called a sphygmomanometer (sfig-moh-muh-NAHM-uh-tur). A cuff is wrapped around the upper arm. Air is pumped into the cuff until blood flow through an artery is blocked. As the pressure is released, the worker listens to the pulse with a stethoscope and records two numbers from a pressure gauge. The first number is the systolic pressure—the force felt in the arteries when the ventricles contract. The second number is the diastolic pressure—the force of the blood felt in the arteries when the ventricles relax. A typical blood pressure reading for a healthy person is 120/80.

The body normally regulates blood pressure in two ways. Sensory receptors at several places in the body detect the level of blood pressure, sending impulses to the medulla oblongata region of the brain stem. When blood pressure is too high, the autonomic nervous system releases neurotransmitters that cause the smooth muscles in blood vessel walls to relax, lowering blood pressure. When blood pressure is too low, neurotransmitters are released that elevate blood pressure by causing these smooth muscles to contract.

The kidneys, which remove water from the blood, also help to regulate blood pressure. Hormones produced by the heart and other organs cause the kidneys to remove more water from the blood when blood pressure is high. This action reduces blood volume, thereby lowering the blood pressure.