Chapter 18

• Nervous control of the circulation has global functions, such as redistributing blood flow to different areas of the body, increasing or decreasing pumping activity by the heart, and providing rapid control of systemic arterial pressure
• The autonomic nervous system controls the circulation almost entirely
• The sympathetic nervous system is the most important part of the autonomic nervous system for regulating the circulation
• Sympathetic vasomotor nerve fibers leave the spinal cord through thoracic and lumbar spinal nerves, passing into sympathetic chains and then to the circulation
• Sympathetic nerve fibers innervate most blood vessels, except capillaries
• Sympathetic stimulation increases heart rate and contractility
• Parasympathetic stimulation decreases heart rate and contractility
• The sympathetic nervous system carries vasoconstrictor nerve fibers and only a few vasodilator fibers
• The vasomotor center in the brain controls the vasoconstrictor system and transmits impulses through sympathetic and parasympathetic nerves to regulate arteries, arterioles, and veins
• Under normal conditions, sympathetic vasoconstrictor tone maintains partial constriction in blood vessels, known as vasomotor tone
• The vasomotor center also controls heart activity, increasing or decreasing heart rate and contractility as needed
• Higher nervous centers can excite or inhibit the vasomotor center, with the hypothalamus playing a special role in controlling the vasoconstrictor system
• Norepinephrine is the sympathetic vasoconstrictor neurotransmitter, acting on alpha-adrenergic receptors of vascular smooth muscle to cause vasoconstriction
• Sympathetic impulses to the adrenal medullae cause the secretion of epinephrine and norepinephrine into the blood, affecting blood vessels throughout the body
• Epinephrine and norepinephrine are secreted into the blood stream and act directly on all blood vessels, usually causing vasoconstriction
• In some tissues, epinephrine causes vasodilation by stimulating beta-adrenergic receptors
• Sympathetic nerves to skeletal muscles carry both vasodilator and constrictor fibers
• In primates, vasodilator effect in muscles is believed to be caused by epinephrine exciting specific beta-adrenergic receptors
• The anterior hypothalamus is the principal area of the brain controlling the vasodilator system
• The sympathetic vasodilator system may cause initial vasodilation in skeletal muscles at the onset of exercise
• Emotional fainting involves vasodilatory reaction and vagal cardioinhibitory center signals causing a decrease in heart rate and loss of consciousness
• Nervous control can rapidly increase arterial pressure by vasoconstriction, venoconstriction, and direct stimulation of the heart
• Nervous control of arterial pressure is rapid, beginning within seconds and often increasing the pressure to two times normal within 5 to 10 seconds
• Increases in arterial pressure during muscle exercise result from local vasodilation and sympathetic stimulation of the overall circulation
• Baroreceptor reflexes help maintain arterial pressure by responding to changes in pressure and transmitting signals to reduce pressure back toward normal levels
• Baroreceptors are located in the walls of large systemic arteries and respond rapidly to changes in arterial pressure
• Baroreceptors cause vasodilation and decreased heart rate and strength of heart contraction in response to high pressure, leading to a decrease in arterial pressure
• Circulatory reflex initiated by baroreceptors causes vasodilation and decreased heart rate and strength of heart contraction in response to high pressure, leading to a decrease in arterial pressure
• Carotid sinus reflex causes aortic pressure to fall almost immediately to slightly below normal as a momentary overcompensation and then return to normal in another minute
• Baroreceptors maintain relatively constant arterial pressure in the upper body, important when a person stands up after lying down
• Baroreceptor system opposes increases or decreases in arterial pressure, called a pressure buffer system
• Baroreceptors reduce minute by minute variation in arterial pressure to about one-third that which would occur if the baroreceptor system were not present
• Baroreceptors reset in 1 to 2 days to the pressure level to which they are exposed
• Baroreceptors may contribute to long-term blood pressure regulation by influencing sympathetic nerve activity of the kidneys
• Chronic electrical stimulation of carotid sinus afferent nerve fibers can cause sustained reductions in sympathetic nervous system activity and arterial pressure
• Chemoreceptor reflex becomes important to help prevent further decreases in arterial pressure when pressure falls below 80 mm Hg
• Atrial stretch and activation of low-pressure atrial receptors cause reflex reductions in renal sympathetic nerve activity, decreased tubular reabsorption, and dilation of afferent arterioles in the kidneys
• Chemoreceptor reflex operates similarly to baroreceptor reflex, initiated by chemoreceptors sensitive to low oxygen or elevated carbon dioxide and hydrogen ion levels
• Low-pressure receptors in atria and pulmonary arteries play an important role in minimizing arterial pressure changes in response to changes in blood volume
• Increased atrial pressure may raise heart rate through the Bainbridge reflex, which accelerates the heart rate