Blood Pressure Like any pump, the heart produces pressure. When it contracts, it produces a wave of fluid pressure in the arteries, known as blood pressure. Although blood pressure falls when the heart relaxes between beats, the system still remains under pressure due to the elasticity of the arterial walls. It's a good thing, too. Without that pressure, blood would stop flowing through the body.

Healthcare workers measure blood pressure with a device called a sphygmomanometer (sfig moh muh NAHM uh tur), an inflatable cuff with a pump and a meter. The cuff is wrapped around the upper arm and inflated until blood flow through the artery that runs down the arm is blocked. As the pressure is released, the healthcare worker listens for a pulse with a stethoscope and records a number from the meter. This number represents the systolic pressure—the force in the arteries when the ventricles contract. When the pulse sound disappears, a second number is recorded. This number represents the diastolic pressure—the force in the arteries when the ventricles relax. A typical blood pressure reading for a healthy teen or adult is below 120/80.

The body regulates blood pressure in a number of ways. Sensory receptors in blood vessels detect blood pressure and send impulses to the brain stem. When blood pressure is high, the autonomic nervous system releases neurotransmitters that relax the smooth muscles in blood vessel walls. When blood pressure is low, neurotransmitters are released that cause the smooth muscles in vessel walls to contract.

The kidneys also regulate blood pressure by affecting the volume of blood. Triggered by hormones produced by the heart and other organs, the kidneys remove more water from the blood and eliminate it in urine when blood pressure is high or conserve more water when blood pressure is low.

FIGURE 33–7 Measuring Blood Pressure It's important to have your blood pressure measured because blood pressure that is too high or too low can have serious effects on most body systems.

33.1 Assessment

Review Key Concepts

1. 1. Review List the structures of the circulatory system and explain their roles.

   2. Apply Concepts Why do humans need a circulatory system?

2. 1. Review Describe the two paths of blood circulation through the body.

   2. Relate Cause and Effect How would damage to the sinoatrial node affect the heart's function?

3. 1. Review Describe the functions of three types of blood vessels in the circulatory system.

   2. Infer If you were standing, would you expect the blood pressure to be higher in your arm or in your leg? Explain your answer. (Hint: Think about which area of the body is closer to the source of pressure.)

VISUAL THINKING

4. Trace Figure 33–2. Label the four chambers of the heart. Add arrows and labels to indicate how blood flows through the heart.

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