Info

Total Peripheral Resistance

Arteriolar Radius

Heart Rate Stroke Volume

Parasympathetic Sympathetic —»-Venous Sympathetic Vasoactive Local metabolic activity activity return activity substances activity

Blood Skeletal muscle Respiratory volume activity activity

Figure 15.3 Factors that affect mean arterial pressure. Mean arterial pressure is determined by cardiac output and total peripheral resistance. Important factors that influence these two variables are summarized in this figure.

Baroreceptors n >,

Chemoreceptors —► Center

Low pressure receptors Center

Parasymptheti^Hear^ COX j activity^^^, ^

^ Sympathetic Blood tpr ^ activity vessels

MEAN ARTERIAL PRESSURE

Figure 15.4 Effects of the autonomic nervous system on mean arterial pressure. The baroreceptors, chemoreceptors, and low-pressure receptors provide neural input to the vasomotor center in the brainstem. The vasomotor center integrates this input and determines the degree of discharge by the sympathetic and parasympathetic nervous systems to the cardiovascular system. Cardiac output and total peripheral resistance are adjusted so as to maintain mean arterial pressure within the normal range.

enhances stroke volume (SV). The increases in HR and SV cause an increase in CO and therefore MAP (Figure 15.5).

The sympathetic system also innervates vascular smooth muscle and regulates the radius of the blood vessels. All types of blood vessels except capillaries are innervated; however, the most densely innervated vessels include arterioles and veins. An increase in sympathetic stimulation of vascular smooth muscle causes vasoconstriction and a decrease in stimulation causes vasodilation. Constriction of arterioles causes an increase in TPR and therefore MAP. Constriction of veins causes an increase in venous return (VR) which increases end-diastolic volume (EDV), SV (Frank-Starling law of the heart), CO, and MAP.

Sympathetic nerves are distributed to most vascular beds. They are most abundant in the renal, gastrointestinal, splenic, and cutaneous circulations. Recall that these tissues receive an abundant blood flow, more than is necessary simply to maintain metabolism. Therefore, when blood is needed by other parts of the body, such as working skeletal muscles, sympathetic vasoconstrictor activity reduces flow to the tissues receiving excess blood so that it may be redirected to the muscles. Interestingly, there is no sympathetic innervation to cerebral blood vessels. In fact, these vessels do not have a1-adrenergic receptors, so they cannot be affected by circulating catecholamines. No physiological circumstance exists in which blood should be directed away from the brain.

Vasomotor center. Autonomic nervous activity to the cardiovascular system is regulated by the vasomotor center (see Figure 15.4). Located in the lower pons and the medulla of the brainstem, the vasomotor center is an integrating center for blood pressure regulation. It receives several sources of input, processes this information, and then adjusts sympathetic and para-sympathetic discharge to the heart and blood vessels accordingly.

Sympathetic nerves going to the arterioles are tonically active. In other words, these nerves discharge continuously, causing vasomotor tone. As a result, under resting conditions, arterioles are partially constricted. This vasomotor tone is important because it helps to maintain MAP in the range

Table 15.1 Summary of Major Cardiovascular Principles

Pulse Pr essure = PsystoUc - ?ijastolk

Notes: CO: cardiac output; VR: venous return; HR: heart rate; SV: stroke volume; EDV: end-diastolic volume; ESV: end-systolic volume; Q: blood flow; DP: pressure gradient; R: resistance; r: vessel radius; Psystolic: systolic pressure; Pdiastolic: diastolic pressure; MAP: mean arterial pressure; TPR: total peripheral resistance, Pv: venous pressure; PRA: right atrial pressure; Rv: venous resistance.

of 90 to 100 mmHg. Without this partial vasoconstriction of the arterioles, MAP would fall precipitously and blood flow to vital organs would be compromised. Another physiological advantage of vasomotor tone is that the degree of vasoconstriction can be increased or decreased. In this way, blood flow to the tissue can be increased or decreased. Without tone, the vessels could only constrict and blood flow to the tissue could only decrease.

Other regions of the vasomotor center transmit impulses to the heart via sympathetic nerves or the vagus nerves. An increase in sympathetic activity to the heart typically occurs concurrently with an increase in sympathetic activity to blood vessels and a decrease in vagal stimulation of the heart. Therefore, the resulting increases in CO and TPR work together to elevate MAP more effectively. Conversely, an increase in vagal stimulation of the heart typically occurs concurrently with a decrease in sympathetic activity to the heart and blood vessels. Therefore, decreases in CO and TPR work together to decrease MAP more effectively. The vasomotor center receives input from multiple sources (summarized in Table 15.2) including:

Baroreceptors Chemoreceptors Low-pressure receptors

Table 15.2 Cardiovascular Receptors and Their Stimuli

Baroreceptors Blood pressure

Chemoreceptors Blood gases (0 O2, T CO2, 0 pH)

Low-pressure receptors Blood volume

Baroreceptors. The baroreceptors provide the most important source of input to the vasomotor center; these receptors monitor blood pressure in the systemic circulatory system. They are found in two locations: the arch of the aorta and the carotid sinuses. As the aorta exits the left ventricle, it curves over the top of the heart, forming an arch, and then descends through the thoracic and abdominal cavities. The coronary arteries, which supply the cardiac muscle, branch off the aorta in this most proximal portion of the aorta. The left and right common carotid arteries also branch off the aortic arch and ascend through the neck toward the head. Each common carotid artery bifurcates, or divides, forming an external carotid artery, which supplies the scalp, and an internal carotid artery, which supplies the brain. The carotid sinus is located at the bifurcation of each common carotid artery. Because blood flow to a tissue is dependent in large part upon blood pressure, baroreceptors are ideally located to monitor blood pressure in regions of the circulatory system responsible for delivering blood to the heart and brain, the two most vital organs in the body.

Because baroreceptors respond to stretch or distension of the blood vessel walls, they are also referred to as stretch receptors. A change in blood pressure will elicit the baroreceptor reflex, which involves negative feedback responses that return blood pressure to normal (see Figure 15.6). For example, an increase in blood pressure causes distension of the aorta and carotid arteries, thus stimulating the baroreceptors. As a result, the number of afferent nerve impulses transmitted to the vasomotor center increases. The vasomotor center processes this information and adjusts the activity of the autonomic nervous system accordingly. Sympathetic stimulation of vascular smooth muscle and the heart is decreased and parasympathetic stimulation of the heart is increased. As a result, venous return, CO, and TPR decrease so that MAP is decreased back toward its normal value.

On the other hand, a decrease in blood pressure causes less than normal distension or stretch of the aorta and carotid arteries and a decrease in baroreceptor stimulation. Therefore, fewer afferent nerve impulses are transmitted to the vasomotor center. The vasomotor center then alters autonomic nervous system activity so that sympathetic stimulation of vascular smooth muscle and the heart is increased and parasympathetic stimulation of the heart is decreased. As a result, venous return, CO, and TPR increase so that MAP is increased back toward its normal value. The effects are summarized in Figure 15.5.

It is important to note that the baroreceptor reflex is elicited whether blood pressure increases or decreases. Furthermore, these receptors are

PARASYMPATHETIC SYSTEM

SYMPATHETIC SYSTEM

Heart

4. Heart rate

Heart

Cardiac ' Output

Heart

Arterioles

Veins t Heart rate

ÎContractility

Î Vasoconstriction Î Vasoconstriction

ÎContractility

Î Vasoconstriction Î Vasoconstriction

Figure 15.5 Effects of sympathetic and parasympathetic nervous activity on mean arterial pressure. The parasympathetic nervous system innervates the heart and therefore influences heart rate and cardiac output. The sympathetic nervous system innervates the heart and veins and thus influences cardiac output. This system also innervates the arterioles and therefore influences total peripheral resistance. The resulting changes in cardiac output and total peripheral resistance regulate mean arterial pressure.

Figure 15.5 Effects of sympathetic and parasympathetic nervous activity on mean arterial pressure. The parasympathetic nervous system innervates the heart and therefore influences heart rate and cardiac output. The sympathetic nervous system innervates the heart and veins and thus influences cardiac output. This system also innervates the arterioles and therefore influences total peripheral resistance. The resulting changes in cardiac output and total peripheral resistance regulate mean arterial pressure.

ÎBlood Pressure

Î Stretch of aorta and carotid arteries

Stimulation of baroreceptors 1

î Number of impulses/sec in afferent nerves to vasomotor cortex

Change in ANS activity to cardiovascular system

I Sympathetic Activity

î Parasympathetic Activity

Vasodilation of veins I Heart rate I Contractility Vasodilation of arterioles

1 Venous return I Cardiac output ITotal peripheral resistance

I Heart rate

ICardiac output /

IBlood Pressure toward normal

Figure 15.6 The baroreceptor reflex. Baroreceptors are the most important source of input to the vasomotor center. The reflex elicited by these receptors is essential in maintenance of normal blood pressure.

most sensitive in the normal range of blood pressures, so even a small change in MAP will alter baroreceptor, vasomotor center, and autonomic nervous system activity. As such, the baroreceptor reflex plays an important role in the short-term regulation of blood pressure. Without this reflex, changes in blood pressure in response to changes in posture, hydration (blood volume), cardiac output, regional vascular resistance, and emotional state would be far more pronounced. The baroreceptor reflex helps to minimize unintentional changes in MAP and maintain adequate blood flow to the tissues.

Chemoreceptors. The peripheral chemoreceptors include the carotid bodies, located at the bifurcation of the common carotid arteries, and the aortic bodies, located in the aortic arch. These receptors are stimulated by a decrease in arterial oxygen (hypoxia), an increase in arterial carbon dioxide (hypercapnia), and a decrease in arterial pH (acidosis). Therefore, as one might expect, chemoreceptors are primarily concerned with regulation of ventilation. A secondary function of these receptors is to influence MAP by providing input to the vasomotor center. A decrease in blood pressure causes a decrease in blood flow to the carotid and aortic bodies.

Assuming a constant rate of metabolism in these tissues (constant oxygen consumption as well as carbon dioxide and hydrogen ion production), then a decrease in blood flow results in hypoxia, hypercapnia, and a decrease in local pH. These conditions stimulate the chemoreceptors and cause an increase in the number of nerve impulses transmitted to the vasomotor center. The vasomotor center processes this input and adjusts activity of the autonomic nervous system accordingly. Sympathetic discharge to the cardiovascular system is increased; the predominant effect is an increase in TPR. As a result of this negative feedback mechanism, MAP is increased and blood flow to the chemoreceptors is increased toward its normal value. Interestingly, the chemoreceptor reflex does not affect the cardiovascular system until MAP decreases below 80 mmHg. Therefore, unlike the baroreceptor reflex, this reflex does not help to minimize the daily variations in MAP. Instead, it supplements activity of the baroreceptor reflex at lower pressures only.

Low-pressure receptors. The low-pressure receptors are located in the walls of the atria and the pulmonary arteries. Similar to baroreceptors, low-pressure receptors are also stretch receptors; however, stimulation of these receptors is caused by changes in blood volume in these low-pressure areas. An overall increase in blood volume results in an increase in venous return; an increase in the blood volume in the atria and the pulmonary arteries; and stimulation of the low-pressure receptors. These receptors then elicit reflexes by way of the vasomotor center that parallel those of barore-ceptors. Because an increase in blood volume will initially increase MAP, sympathetic discharge decreases and parasympathetic discharge increases so that MAP decreases toward its normal value. The simultaneous activity of baroreceptors and low-pressure receptors makes the total reflex system more effective in the control of MAP.

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Essentials of Human Physiology

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