The cardiovascular system is centrally regulated by autonomic reflexes. These work with local mechanisms (see Chapter 23) and the renin – angiotension – aldsterone and antidiuretic hormone systems (see Chapter 29) to minimize fluctuations in the mean arterial blood pressure (MABP) and volume, and to maintain adequate cerebral and coronary perfusion. Intrinsic reflexes, including the baroreceptor, cardiopulmonary and chemoreceptor reflexes, respond to stimuli originating within the cardiovascular system. Less important extrinsic reflexes mediate the cardiovascular response to stimuli originating elsewhere (e.g. pain, temperature changes). Figure 27 illustrates the responses of the baroreceptor and cardiopulmonary reflexes to reduced blood pressure and volume, as would occur, for example, during haemorrhage.

Cardiovascular reflexes involve three components:

Intrinsic Cardiovascular Reflexes

The Baroreceptor Reflex

This reflex acts rapidly to minimize moment-to-moment fluctuations in the MABP. Baroreceptors are afferent (sensory) nerve endings in the walls of the carotid sinuses (thin-walled dilatations at the origins of the internal carotid arteries) and the aortic arch. These mechanoreceptors sense alterations in wall stretch caused by pressure changes, and respond by modifying the frequency at which they fire action potentials. Pressure elevations increase impulse frequency; pressure decreases have the opposite effect.

When MABP decreases, the fall in baroreceptor impulse frequency causes the brain to reduce the firing of vagal efferents supplying the sinoatrial node, thus causing tachycardia. Simultaneously, the activity of sympathetic nerves innervating the heart and most blood vessels is increased, causing increased cardiac contractility and constriction of arteries and veins. Stimulation of renal sympathetic nerves increases renin release, and consequently angiotensin II production and aldosterone secretion (see Chapter 29). The resulting tachycardia, vasoconstriction and fluid retention act together to raise MABP. Opposite effects occur when arterial blood pressure rises.

There are two types of baroreceptors. A fibres have large, myelinated axons and are activated over lower levels of pressure. C fibres

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