Describe the function of baroreceptors and to relate this knowledge to common clinical situations.
Baroreceptors are stretch receptors which monitor changes in arterial pressure. Arterial pressure is monitored by receptors in the:
- Aortic arch
- Innervated by CNX
- Carotid sinus
Small dilation of the ICA at the level of the bifurcation.
- Innervated by CNIX
- Remember the carotid sinus is a baroreceptor, the carotid body is a chemoreceptor
Low-pressure stretch receptors:
- Respond to increased venous return
- Are inhibited by positive pressure ventilation
- Act by stretch and typically described as volume receptors
- Are located in the:
- Atrial walls
- SVC and IVC
- Pulmonary circulation
Afferent fibres from CNIX and CNX travel to the NTS in the medulla. Effector neurons from the RVLM are GABAergic and therefore inhibitory, i.e. increased baroreceptor discharge reduces tonic sympathetic tone and increases vagal tone.
Increased baroreceptor activity therefore results in:
- Arterial and venous vasodilation
- Decreased cardiac output
- Decreased respiratory rate
Increased low-pressure stretch receptor activity results in an increase rather than a decrease in heart rate.
- More sensitive to pulsatile pressure than constant pressure
A decrease in pulse pressure without a change in MAP will decrease baroreceptor firing.
- Active throughout the cardiac cycle
Rapid compensatory responses are vital in the short-term control of blood pressure, e.g. with posture.
- Active over the range from 50mmHg to 200mmHg
- This curve is left-shifted in children and neonates, and right-shifted in chronic hypertension, though this is reversible
Atrial stretch receptors also excrete ANP, which reduces blood pressure in the following ways:
- Increased GFR
ANP constricts the efferent arteriole and dilates of the afferent arteriole. This subsequently inhibits renin secretion through increased hydrostatic pressure at the JGA and increased Na+ and Cl- delivery to the macula densa.
- Decreased aldosterone
ANP inhibits aldosterone secretion.
ANP causes vasodilation of peripheral smooth muscle.
- Barrett KE, Barman SM, Boitano S, Brooks HL. Ganong's Review of Medical Physiology. 24th Ed. McGraw Hill. 2012.
- CICM September/November 2014
- ANZCA July/August 2000