Need-to-know anatomy

Blood pressure, the amount of tension that blood exerts on the walls of blood vessels as it travels through the circulatory system, rises and falls with each heartbeat. Systolic pressure, the higher number, corresponds with the peak pressure in the arteries when the heart contracts; diastolic pressure, the lower number, reflects the lowest pressure in the arteries as the heart relaxes.

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Blood pressure fluctuates throughout the day under the direct influence of the heart, the arteries, and the kidneys. During exercise, for example, the heart beats faster and more forcefully to raise blood pressure and deliver extra oxygen and nutrients to the muscles; blood pressure drops as the heart slows during sleep. Dilation of small arteries decreases blood pressure, while constriction of these arteries increases blood pressure. The kidneys affect blood pressure by increasing or decreasing the amount of sodium and water excreted in the urine, which affects the volume of blood in the arteries. The heart, arteries, and kidneys control blood pressure through an elaborate network of nerves and hormones.

Normally, this complex regulatory system allows blood pressure to rise and fall as needed while staying within a desirable range. In many people, however, abnormalities in this system lead to chronically elevated blood pressure, or hypertension.