Other Cardiovascular Reflexes and Responses

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Stimuli acting upon receptors other than baroreceptors can initiate reflexes that cause changes in arterial pressure. For example, the following stimuli all cause an increase in blood pressure: decreased arterial oxygen concentration; increased arterial carbon dioxide concentration; decreased blood flow to the brain; increased intracranial pressure; and pain originating in the skin (in contrast, pain originating in the viscera or joints may cause marked decreases in arterial pressure).

Many physiological states such as eating and sexual activity are also associated with changes in blood pressure. For example, attending business meetings raises mean blood pressure by 20 mmHg, walking increases it 10 mmHg, and sleeping lowers it 10 mmHg. Mood also has a significant effect on blood pressure, which tends to be lower when people report that they are happy than when they are angry or anxious.

These changes are triggered by input from receptors or higher brain centers to the medullary cardiovascular center or, in some cases, to pathways distinct from these centers. For example, the fibers of certain neurons whose cell bodies are in the cerebral cortex and hypothalamus synapse directly on the sympathetic neurons in the spinal cord, bypassing the medullary center altogether.

There is a marked degree of flexibility and integration in the control of blood pressure. For example, in an experimental animal, electrical stimulation of a discrete area of the hypothalamus elicits all the usually observed neurally mediated cardiovascular responses to an acute emotional situation. Stimulation of other brain sites elicits cardiovascular changes appropriate to the maintenance of body temperature, feeding, or sleeping. It seems that such outputs are "preprogrammed." The complete pattern can be released by a natural stimulus that initiates the flow of information to the appropriate controlling brain center.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

III. Coordinated Body Functions

14. Circulation

© The McGraw-Hill Companies, 2001

Circulation CHAPTER FOURTEEN

Circulation CHAPTER FOURTEEN

f Arterial pressure

I Cardiac output

Cardiac muscle

I Stroke volume

Kidneys

I Urinary loss of sodium and water

I End-diastolic volume

I Plasma volume

| Venous return

I Venous pressure

I Blood volume

f Blood volume f Venous pressure f Venous return i Plasma volume

I End-diastolic volume

Kidneys t Urinary loss of sodium and water

Cardiac muscle

I Stroke volume f Car output

I Arterial pressure

FIGURE 14-60

Causal reciprocal relationships between arterial pressure and blood volume. (a) An increase in arterial pressure due, for example, to an increased cardiac output induces a decrease in blood volume by promoting fluid excretion by the kidneys, which tends to restore arterial pressure to its original value. (b) An increase in blood volume due, for example, to altered kidney function induces an increase in arterial pressure, which tends to restore blood volume to its original value by promoting fluid excretion by the kidneys. Because of these relationships, blood volume is a major determinant of arterial pressure.

PART THREE Coordinated Body Functions

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions

SECTION E SUMMARY

Mean arterial pressure, the primary regulated variable in the cardiovascular system, equals the product of cardiac output and total peripheral resistance.

The factors that determine cardiac output and total peripheral resistance are summarized in Figure 14-54.

Baroreceptor Reflexes

I. The primary baroreceptors are the arterial baroreceptors, including the two carotid sinuses and the aortic arch. Nonarterial baroreceptors are located in the systemic veins, pulmonary vessels, and walls of the heart.

II. The firing rates of the arterial baroreceptors are proportional to mean arterial pressure and to pulse pressure.

III. An increase in firing of the arterial baroreceptors due to an increase in pressure causes, by way of the medullary cardiovascular center, an increase in parasympathetic outflow to the heart and a decrease in sympathetic outflow to the heart, arterioles, and veins. The result is a decrease in cardiac output and total peripheral resistance and, hence, a decrease in mean arterial pressure. The opposite occurs when the initial change is a decrease in arterial pressure.

Blood Volume and Long-Term Regulation of Arterial Pressure

I. The baroreceptor reflexes are short-term regulators of arterial pressure but adapt to a maintained change in pressure.

II. The most important long-term regulator of arterial pressure is the blood volume.

SECTION E KEY TERMS

total peripheral resistance arterial baroreceptors

(TPR) medullary cardiovascular carotid sinus baroreceptors center aortic arch baroreceptor

SECTION E REVIEW QUESTIONS

Write the equation relating mean arterial pressure to cardiac output and total peripheral resistance. What variable accounts for mean pulmonary arterial pressure being lower than mean systemic arterial pressure?

Draw a flow diagram illustrating the factors that determine mean arterial pressure. Identify the receptors, afferent pathways, integrating center, efferent pathways, and effectors in the arterial baroreceptor reflex.

When the arterial baroreceptors decrease or increase their rate of firing, what changes in autonomic outflow and cardiovascular function occur? Describe the role of blood volume in the long-term regulation of arterial pressure.

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