Control of Blood Pressure (MD3001) PDF

7. Control of blood pressure MD3001 Dr Alun Hughes 1 Lecture overview Feedback systems Baroreceptors MCVC centre Fine control 2 Integration and control of the heart and blood vessels Required to maintain tissue perfusion across whole of the body – To keep a relatively constant arterial blood...

7. Control of blood pressure MD3001 Dr Alun Hughes 1 Lecture overview Feedback systems Baroreceptors MCVC centre Fine control 2 Integration and control of the heart and blood vessels Required to maintain tissue perfusion across whole of the body – To keep a relatively constant arterial blood pressure • Too low, blood flow to organs would fail • Too high, damage to vessels and organs – To control distribution of the total cardiac output • 5L/min not sufficient to perfuse entire body • Needs to respond to tissue demands • Satisfied by local control mechanisms Nervous control of arterial pressure is rapid Can increase arterial pressure to 2x normal within 5-10s Can decrease arterial pressure to 50% normal within 10-40s 4 Reflex control of blood pressure Fundamental components of a reflex control system 1) Internal variable to be maintained 2) Receptors sensitive to change in the variable 3) Afferent pathways from the receptors 4) An integrating center for the afferent inputs 5) Efferent pathways from the integrating center 6) Target effectors that alter their activities Baroreceptors Guyton 206 12th ed, p220 13th ed 6 Feedback control of mean arterial pressure Main baroreceptor locations – Walls of aorta • Afferent fibers follow vagus (Xth cranial) nerve – Carotid artery • Afferent fibers follow (IXth cranial) glossopharyngeal nerve Baroreceptor activity – “Stretch receptors” – Firing rate ↑when BP ↑ – Firing rate ↓when BP ↓ – Sensitive around a “set-point” • Can change, e.g. hypertension Rate of baroreceptor firing Normal Normal resting value (“set point”) Guyton p206 12th ed, p220 13th ed Berne and Levy p388 Baroreceptor control of blood pressure Primary purpose of baroreceptor reflex control is to reduce the minute-tominute variations or arterial pulse Guyton p207 12th ed, p221 13th ed Changes in mean aortic pressure (mean +/- SE) in response to an 8% blood loss in a group of eight dogs. Left panel, The carotid sinus baroreceptor reflexes were intact and the aortic reflexes were interrupted. Middle panel, The aortic reflexes were intact and the carotid sinus reflexes were interrupted. Right panel, All sinoaortic reflexes were abrogated. Cardiopulmonary baroreceptors Cardiopulmonary baroreceptors (“low-pressure receptors”) sense central blood volume – Atria, ventricles, veins and pulmonary vessels If rate of cardiopulmonary baroreceptors firing ↓ (signaling ↓ blood volume) then: – sympathetic nerve activity to the heart and blood vessels ↑ – parasympathetic nerve acCvity to the heart ↓ Atrial reflex control of BP (Bainbridge Reflex) Sympathetic-mediated reflex in response to increased blood in atria – ↑HR and ↑ contracClity – prevents damming of blood in veins etc If aortic/carotid baroreceptors sense high pressure, Bainbridge reflex can over-ride – How is integration of these signals controlled? Integrated control of BP Medullary cardiovascular control (MCVC) “vasomotor” centre – Sensory area • Input from baroreceptors – Lateral portion • Efferent sympathetic nerves – Medial portion • Efferent parasympathetic (vagal) nerves Target effectors in reflex control of blood pressure Sympathetic and parasympathetic effect the heart – Both control heart rate and normally function simultaneously • At rest parasympathetic = predominate tone • Sympathetic can significantly effect stroke volume and rate Target effectors in reflex control of blood pressure Sympathetic effects on blood vessels – Continuous low-level tone affects total peripheral resistance • “sympathetic vasoconstrictor tone” exerts “vasomotor tone” on vessels •  kept partially constricted – Remember veins also innervated by sympathetic • ↓ capacitance  ↑ venous return  ↑ stroke vol  ↑ cardiac output Medullary cardiovascular control (CVC) centre Baroreceptor discharge rate Parasympathetic (vagal) output Cardiac output Arterial blood pressure Sympathetic output Heart rate Adrenal gland Stroke volume Venous return Peripheral resistance CNS ischemic response Emergency pressure control system – “last ditch stand” When blood flow to the medullary CVCC is ↓↓↓ – ↑ peripheral vasoconstricCon • almost completely occludes some peripheral vessels – ↑ sympatheCc sCmulaCon of heart – ↑↑ systemic arterial pressure • As high as 250 mmHg for 10 min Fine control of blood flow Local control superimposed on organ dist. of CO – Organs auto-regulate blood flow • Reactive and active hyperemia • Independent of innervation / hormonal control Intrinsic ability to maintain blood flow safely if BP ↑ – Myogenic theory (acute auto-regulation) • Stretch-induced vascular depolarisation of smooth muscle due to ↑ arterial pressure – Metabolic theory (acute auto-regulation) • ↑ arterial pressure increases O2 and “washes out” local factors Remember not all capillaries in an organ perfused simultaneously Cardiac output summary Total peripheral resistance summary Main points Neuronal reflex mechanisms exist to maintain BP in the immediate/short term These are integrated in the MCVC centre Fine control of local blood flow still occurs Long term regulation is via blood volume Learning outcomes To identify the components of the feed-back system involved in the reflex control of mean arterial blood pressure (including the receptors, integrating centre, target effectors and associated afferent and efferent pathways), where they are located and how they contribute to regulation of MABP. To predict how the cardiovascular system will reflexively respond to physiological changes to maintain mean arterial blood pressure. To explain how tissue demands for blood flow are balanced against the requirement to maintain a mean arterial blood pressure.