Anaes · Applied cardiovascular & respiratory physiology
Control of arterial pressure
Also known as Blood pressure regulation · Mean arterial pressure · Baroreceptor reflex · Systemic vascular resistance · RAAS · Pressure natriuresis
Arterial pressure is the perfusing pressure of every organ, and the body defends it across time-scales from seconds to days. The framework rests on five exam-critical ideas: mean arterial pressure equals cardiac output times systemic vascular resistance (the Ohm's-law analogue), and approximates to diastolic plus one-third of the pulse pressure; the moment-to-moment defence is the baroreceptor reflex, a negative-feedback loop from the carotid sinus and aortic arch stretch receptors through the medulla to the autonomic outputs; medium-term defence uses the renin-angiotensin-aldosterone system and vasopressin to restore volume and tone; long-term defence is the kidney, whose pressure-natriuresis curve sets the eventual steady-state arterial pressure; and individual beds autoregulate, holding flow roughly constant across a range of pressure. Built on the carotid-sinus baroreflex study (Gothelf 2026), the RAAS-in-hypertension review (Albritton 2026), the renal-medulla-in-hypertension review (Cowley 2024), the renal sodium-excretion study (Ostergaard 2026), and the hypothalamic vasopressin blood-pressure-circuit study (Lin 2026).
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Red flags
- Mean arterial pressure equals cardiac output times systemic vascular resistance, and approximates to diastolic plus one-third of the pulse pressure — a low mean arterial pressure means low perfusion regardless of a preserved systolic number.
- The baroreceptor reflex resets within hours to days to the prevailing pressure, so it defends against ACUTE changes but does not set the long-term level — chronic hypertension is not a baroreflex failure.
- Vasopressors raise systemic vascular resistance and therefore mean arterial pressure, but if cardiac output is low they may not restore perfusion — perfusion is delivered by flow, not pressure alone.
- The kidney sets the long-term arterial pressure through pressure-natriuresis: any factor that shifts the curve so sodium is retained at a higher pressure causes chronic hypertension — the basis of nearly all sustained hypertension.
- Autonomic failure (diabetes, ageing, drug-induced) abolishes the baroreflex, producing postural hypotension and labile pressure — the patient who collapses on standing or at induction.
Meet the patient
A 68-year-old on lisinopril for hypertension presents for elective cholecystectomy. You induce with propofol. The MAP drops from 95 to 48 within two minutes. There is no compensatory tachycardia. The trainee reaches for metaraminol and asks: why is this patient so fragile?[1]
Chronic ACE blockade removed one pressor limb. Propofol blunted the baroreflex and venodilated. The reflex that would normally fire back with tachycardia and vasoconstriction is blunted by anaesthesia on top of a pharmacologically weakened RAAS axis. The pressure fell because both determinants — CO and SVR — dropped at once with no backup. (Applied induction reasoning standard teaching; baroreflex limb Gothelf-anchored, RAAS limb Albritton-anchored.)[1][2]
References5ShowHide
- [1]Gothelf I, et al. Predictors of Heart Rate Depression During Carotid Artery Stenting in Presumed Low-Risk Patients: A Retrospective Single-Center Observational Study J Clin Med, 2026.PMID 42356001
- [2]Albritton CF, et al. Myeloid cell renin-angiotensin-aldosterone system in hypertension and inflammation Curr Opin Physiol, 2026.PMID 42358614
- [3]Cowley AW Jr, et al. Renal Medulla in Hypertension Hypertension, 2024.PMID 39344517
- [4]Østergaard AM, Rosenbæk JB, Ejlersen JA, et al. Impact of inorganic nitrate on renal sodium excretion during saline loading in healthy adults; secondary endpoints from a randomized crossover trial Physiol Rep, 2026.PMID 42237694
- [5]Lin W, et al. A hypothalamic VMPO-supraoptic vasopressin circuit mediates procalcitonin-induced fluid imbalance Cell Rep, 2026.PMID 42284142