Blood pressure and the cardiac reflexes

Vivian Imbriotis | Nov. 20, 2025

Systolic blood pressure is the maximal arterial blood pressure. It is relevant to bleeding / clot disruption, and aneurysm wall stress.

Diastolic pressure is the minimum arterial blood pressure. It is relevant to coronary perfusion (especially LV).

Pulse pressure is their difference; it is proportional to stroke volume.

Mean arterial pressure is the area under the curve divided by the cardiac cycle time. It most closely predicts microvascular flow / organ perfusion.

The total peripheral resistance is the resistance of the peripheral vasculature to a constant laminar flow, \(\frac{8 l \mu}{\pi r^4}\).

Determinants of MAP

Because of Ohm's law for a pipe,

$$\text{MAP} - \text{CVP} = \text{TPR} \cdot \text{CO}$$

Therefore the MAP is determined by preload, afterload, contractility, vessel length, vessel diameter, and blood viscosity.


Determinants of systolic blood pressure

Because \(C = \frac{\Delta V}{\Delta P}\), stroke volume and arterial compliance determine the systolic blood pressure

Reflected pressure waves from impedance mismatch at the arteriole level also increase the systolic pressure.


Determinants of diastolic pressure

Determined by total peripheral resistance, time constants of arterial vessels, and heart rate.

Pulse pressure is given by

$$PP = SBP - DBP = \frac{\text{Stroke volume}}{\text{Arterial compliance}}$$

So the pulse pressure is proportional to stroke volume.

Widened in high-output states (Aortic regurg, AV fistulae, distributive shock) and bradycardia with compensatory high SV.

Narrowed in low output states (AS, high afterload, cardiogenic shock, tachycardia).


Pulse pressure variation is a measure of stroke volume variation.

$$PPV = \frac{PP_{\text{max}} - PP_{\text{min}}}{PP_{\text{mean}}} = \frac{\Delta PP}{\overline{PP}}$$

Recall that \(VR = \frac{\text{MSFP} - \text{CVP}}{\text{TVR}}\)

With inspiration, CVP falls as intrathoracic pressure falls, increasing VR and RV preload. The pulmonary vasculature expands, decreasing LV preload. During normal inspiration, there is some (<10%) PPV.

This effect is exaggerated in:

  1. Tamponade or constrictive pericarditis: The increase in RV filling causes leftward septal movement, decreasing LV filling (ventricular interdependance)
  2. Hypovolaemia: A low MSFP means VR is more affected by variation in CVP
  3. Acute asthma: Very low inspiratory pressures and high expiratory pressures


Baroreceptor reflex

  • Sensors: pressure (carotid sinus and aortic arch)
  • Afferent: vagus and glossopharyngeal nerves
  • Processor: nucleus of the solitary tract and nucleus ambiguus
  • Efferent: vagus nerve and sympathetic chain
  • Effect: increased HR, TPR, and contractility in response to a fall in BP

Bainbridge reflex

  • Sensors: mechanoreceptors in RA
  • Afferent: vagus 
  • Processor: nucleus of the solitary tract and the caudal ventral medulla
  • Efferent: vagus nerve and sympathetic chain
  • Effect: increased RA pressure produces an increased heart rate

Bezold-Jarisch reflex

  • Sensors: C fibres (noxious mechanical or chemical stimulus)
  • Afferent: vagus
  • Processor: nucleus of the solitary tract
  • Efferent: vagus nerve and sympathetic chain
  • Effect: hypotension and bradycardia in response to myocardial ischaemia

Chemoreceptor reflex

  • Afferent: carotid / aortic chemoreceptors (low PaO2 and/or high PaCO2)
  • Processor: nucleus of the solitary tract and nucleus ambiguus
  • Efferent: vagus nerve and sympathetic chain
  • Effect: tachycardia and hypertension in response to hypoxia

Cushing reflex

  • Afferent: Raised ICP - mechanosensors in the rostral medulla?
  • Processor: rostral ventrolateral medulla
  • Efferent: sympathetic fibres to the heart and peripheral smooth muscle
  • Effect: hypertension (to maintain CPP) and baroreflex-mediated bradycardia

Diving reflex

  • Afferent: trigeminal nerve (cold temperature; pressure of immersion)
  • Processor: sensory nucleus of CN V; nucleus of the solitary tract
  • Efferent: vagus nerve and sympathetic chain
  • Effect: vagal bradycardia, systemic vasoconstriction, cerebral vasodilation

Oculocardiac reflex

  • Afferent: trigeminal nerve (pressure to the globe of the eye) 
  • Processor: sensory nucleus of CN V; nucleus of the solitary tract
  • Efferent: vagus nerve and sympathetic chain
  • Effect: vagal bradycardia, systemic vasoconstriction, cerebral vasodilation

Vasovagal reflex

  • Afferent: emotional distress, hypovolaemia
  • Processor: unknown
  • Efferent: vagus nerve and sympathetic chain
  • Effect: bradycardia, systemic vasodilation, hypotension

Respiratory sinus arrhythmia

  • Afferent: central respiratory pacemaker
  • Processor: nucleus ambiguus
  • Efferent: vagus nerve, via the cardiac ganglion
  • Effect: cyclical increase of heart rate during inspiration