YR1 Lecture 1H - Arterial Pressure & Its Control - Dr Alex Burton 2020 PDF

Arterial pressure and its control Alexander Burton Adjunct Fellow School of Medicine Western Sydney University Learning objectives: Recognise that blood pressure is kept relatively constant through the operation of the arterial baroreflex and cardiac baroreflex, both of which are negative feedback l...

Arterial pressure and its control Alexander Burton Adjunct Fellow School of Medicine Western Sydney University Learning objectives: Recognise that blood pressure is kept relatively constant through the operation of the arterial baroreflex and cardiac baroreflex, both of which are negative feedback loops Define the components of the arterial baroreflex: baroreceptors - nucleus tractus solitarius (NTS) - caudal ventrolateral medulla (CVLM) - rostral ventrolateral medulla (RVLM) - muscle sympathetic nerve activity (MSNA) To define the components of the cardiac baroreflex: baroreceptors - nucleus tractus solitarius (NTS) - nucleus ambiguous (NA) and dorsal motor nucleus of the vagus (DMX) - cardiac parasympathetic nerve activity Arterial pressure: The high pressure (peak ~ 120 mmHg) within the arteries required to perfuse the tissues of the body Determined by the pumping of the heart (cardiac output) and the resistance of the blood vessels Cardiac output = stroke volume x heart rate Cardiac output = 70 ml x 70 min-1 = 4.9 l min-1 The arterioles provide most of the peripheral resistance Arterial pressure: The arterioles provide most of the peripheral resistance Arterial pressure: Venous pressure is lower than arterial pressure Arterial pressure: Measurement of systolic and diastolic pressures by auscultation Arterial pressure: Mean arterial pressure is the average pressure over the cardiac cycle (beat-beat) Arterial pressure: Because of the hydrostatic pressure on standing, mean arterial and venous pressures are higher in the foot than in the head Arterial pressure: Accordingly, arterial pressure is measured in the supine (recumbent) position - at heart level Arterial pressure: The gut is highly vascularized: changes in diameter of its arterioles have a large effect on total peripheral resistance Arterial pressure: Skeletal muscle is highly vascularized: changes in diameter of its arterioles have a large effect on total peripheral resistance Arterial pressure: Total peripheral resistance is largely determined by the arteriolar diameter in the muscle vascular beds, controlled by muscle sympathetic vasoconstrictor drive Sympathetic nerve terminals release noradrenaline (norepinephrine), which acts on adrenergic a receptors to cause contraction of the smooth muscle Blood flow through the arterioles causes release of nitric oxide (NO) from the endothelium NO causes vasodilatation, counteracting the neurallymediated vasoconstriction: pharmacological blockade of NO production increases mean BP by ~20mmHg Arterial pressure: Arterial pressure is controlled continuously (beat-beat): Sympathetic vasoconstriction Sympathetic and parasympathetic (cardiac vagal) effects on heart rate and stroke volume The control system (the baroreflex) is a negative feedback loop: an increase in arterial pressure brings about a decrease in arterial pressure (and vice versa), maintaining a constant (set) level Fluctuations in blood pressure are buffered by the baroreflex: falls in blood pressure result in increases in heart rate and sympathetic vasoconstrictor drive ECG systolic pressure diastolic pressure Fluctuations in blood pressure are buffered by the baroreflex: falls in blood pressure result in increases in heart rate and sympathetic vasoconstrictor drive heart rate muscle sympathetic nerve activity Fluctuations in blood pressure are buffered by the baroreflex: falls in blood pressure result in increases in heart rate and sympathetic vasoconstrictor drive fall in arterial pressure Fluctuations in blood pressure are buffered by the baroreflex: falls in blood pressure result in increases in heart rate and sympathetic vasoconstrictor drive increase in heart rate Fluctuations in blood pressure are buffered by the baroreflex: falls in blood pressure result in increases in heart rate and sympathetic vasoconstrictor drive increase in muscle vasoconstrictor drive The baroreflex is a negative feedback loop Arterial baroreceptors, located in the carotid sinus and aortic arch, monitor the arterial pressure Arterial baroreceptors increase their frequency of firing with an increase in arterial pressure The control system is located within the medulla: baroreceptors project to nucleus tractus solitarius (NTS), which then sends excitatory projections to the caudal ventrolateral medulla (CVLM) and nucleus ambiguus (NA) and the dorsal motor nucleus of the vagus (DMX); CVLM sends inhibitory projections to the rostral ventrolateral medulla (RVLM) DMX Conclusions: Arterial pressure is controlled on the beat-to-beat level by the baroreflex - a neural control mechanism that operates on the principle of negative feedback An increase in arterial pressure activates arterial baroreceptors which, via nucleus tractus solitarius (NTS), activate cardiac vagal neurones and inhibits sympathetic vasoconstrictor neurones in the rostral ventrolateral medulla (RVLM) via internuerones in the caudal ventrolateral medulla (CVLM) An increase in cardiac vagal (parasympathetic) activity causes release of acetylcholine (ACh), which slows down the heart and reduces the force of contraction The inhibition of muscle sympathetic vasoconstrictor neurones means that no noradrenaline (norepinephrine) is being released, so the smooth muscle of the arterioles relaxes A fall in arterial pressure causes, via the baroreflex loop, an increase in muscle vasoconstrictor drive leading to an increase in noradrenaline release and hence arteriolar vasoconstriction Unloading of the baroreceptors also causes a withdrawal of parasympathetic (vagal) drive to the heart, thereby causing an increase in rate and force of cardiac contraction Questions? 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