key concept 49.5 The Circulation Is Controlled by Hormonal and Neural Signals

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When we investigate how a physiological process is regulated, we start by identifying the critical components of that process, how they can be controlled, and the information used to govern that control. Because blood flow depends on pressure, we can identify the mean arterial pressure as a critical variable of the circulatory system.

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  • Local blood flow is controlled to meet local metabolic need by autoregulatory responses of arteriole smooth muscles.

  • Mean arterial pressure is regulated through neural and hormonal control of smooth muscles in arterioles.

  • Baroreceptors in the walls of large arteries continuously convey blood pressure information to the cardiovascular regulatory center in the brain, which controls the neural and hormonal mechanisms that influence mean arterial pressure.

Pressure in the aorta oscillates between systole and diastole, and the duration of diastole is usually twice as long as that of systole (see Figure 49.4). Therefore a simple way of estimating the mean arterial pressure (MAP) is to take the diastolic pressure and add to it one-third of the difference between systolic and diastolic pressure. However, to understand the physiological mechanisms that control MAP, we have to consider the factors that determine it. MAP is determined by the cardiac output (CO) and the resistance to flow in the blood vessels, or total peripheral resistance (TPR):

MAP = CO × TPR

Since CO is equal to the heart rate (HR) times how much blood the heart pumps with each beat (stroke volume [SV]), the critical relationships can be expressed as:

MAP = HR × SV × TPR

HR, SV, and TPR are controlled by neural and hormonal mechanisms at both the local and systemic levels.

At the local level, each tissue controls its own blood flow through autoregulatory mechanisms that alter the diameter and therefore the resistance of the arterioles serving it. The collective autoregulatory actions in the arterioles in all tissues of the body influence TPR and therefore MAP. If many arterioles suddenly dilate, TPR goes down and MAP falls. If many arterioles constrict, TPR goes up and MAP goes up. Therefore changes in MAP provide information about changing needs of the body. In addition, as blood flows through capillary beds, its composition changes—its CO2 content goes up and its O2 content goes down. Thus blood composition also provides information the body uses to regulate the circulatory system.

The nervous and endocrine systems respond to changes in MAP and blood composition by changing breathing rate, heart rate, stroke volume, and peripheral resistance to match the metabolic needs of the body.

Activity 49.4 Blood Pressure and Heart Rate Simulation

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