Blood Pressure: Definition, Normal Values, and Regulation by Baroreceptors

1. Definition of Blood Pressure

• Systolic BP (SBP) — the peak pressure in the arteries during ventricular contraction (systole).
• Diastolic BP (DBP) — the minimum pressure during ventricular relaxation (diastole).

MAP = Diastolic pressure + 1/3 × Pulse pressure
or
MAP = 1/3 SBP + 2/3 DBP

BP = Cardiac Output (CO) × Total Peripheral Resistance (TPR)

2. Normal Values of Blood Pressure

Adults

• Optimal BP: 115/75 mm Hg — associated with minimal vascular mortality.
• Normal SBP range: 90–140 mm Hg.
• Normal DBP range: 60–90 mm Hg.
• Normal MAP: ~93 mm Hg (at a BP of ~120/80 mm Hg).

3. Regulation of Blood Pressure by Baroreceptors

3.1 Overview of Neural Regulation

• Short-term (seconds to minutes): Neural pathways — primarily baroreceptor reflexes — targeting the heart, vessels, and adrenal medulla.
• Long-term (hours to days): Hormonal and renal mechanisms (renin–angiotensin–aldosterone system, fluid balance).

3.2 The Baroreceptor Reflex Arc

3.3 The Baroreceptors (Sensors)

1. Carotid sinus — a dilation of the internal carotid artery just above the bifurcation of the common carotid. These receptors respond to both increases and decreases in arterial pressure.
2. Aortic arch — receptors here are primarily responsive to increases in arterial pressure.

• An increase in arterial pressure stretches the vessel wall → depolarizes the mechanoreceptor membrane (receptor potential) → increases action potential firing frequency in afferent nerves.
• A decrease in arterial pressure → less stretch → decreased firing rate (approaches zero below ~40–60 mm Hg).
• Baroreceptors are more sensitive to pulsatile (changing) pressure than to constant pressure. The strongest stimulus is a rapid change in arterial pressure.
• The response is biphasic: an initial large dynamic depolarization followed by a smaller, sustained static depolarization — encoding both rate-of-change and absolute magnitude of pressure.

Threshold for carotid sinus nerve activity: ~50 mm Hg
Maximum activity: ~200 mm Hg
At normal mean BP (~100 mm Hg), bursts of action potentials appear during systole with few spikes in early diastole.

Fig. 4.31 — Baroreceptor reflex pathway showing CN IX/X inputs to the nucleus tractus solitarius, parasympathetic (cardiac decelerator) and sympathetic (cardiac accelerator + vasoconstrictor) outputs to the heart and blood vessels. (Costanzo Physiology 7th Ed.)

3.4 Afferent Pathways

• Carotid sinus baroreceptors → carotid sinus nerve (branch of CN IX, glossopharyngeal nerve) → medulla.
• Aortic arch baroreceptors → aortic depressor nerve (branch of CN X, vagus nerve) → medulla.

3.5 The Central Coordinating Center (Medulla)

1. Nucleus Tractus Solitarius (NTS): First relay station. Glutamatergic input from baroreceptors → excites NTS neurons → activates CVLM.
2. Caudal Ventrolateral Medulla (CVLM): Receives excitatory input from NTS → releases GABA to inhibit RVLM.
3. Rostral Ventrolateral Medulla (RVLM): The major vasomotor center — source of tonic sympathetic outflow to the cardiovascular system. When inhibited by CVLM (in response to high BP), sympathetic tone is reduced.
4. Nucleus Ambiguus / Dorsal Motor Nucleus: Receive excitatory projections from NTS → increase vagal (parasympathetic) outflow to the heart.

Neural pathways for BP control: baroreceptor afferents → NTS (glutamate) → CVLM (glutamate) → RVLM (GABAergic inhibition) → IML of thoracic cord → sympathetic neurons → heart/arterioles/adrenal medulla. (Ganong's Review of Medical Physiology 26th Ed.)

3.6 Efferent Responses

3.7 Integrated Baroreceptor Response

Response to INCREASED Arterial Pressure (e.g., hypertensive stimulus):

1. ↑ Pa → ↑ stretch on carotid sinus and aortic arch baroreceptors
2. ↑ Firing frequency in CN IX and CN X afferents
3. NTS senses Pa > set-point → coordinates response:
   • ↑ Parasympathetic outflow (vagus → SA node) → ↓ Heart rate
   • ↓ Sympathetic outflow to SA node → further ↓ Heart rate
   • ↓ Sympathetic outflow to myocardium → ↓ Contractility → ↓ Stroke volume
   • ↓ Sympathetic outflow to arterioles → Vasodilation → ↓ TPR
   • ↓ Sympathetic outflow to veins → Venodilation → ↑ unstressed volume → ↓ stressed volume → ↓ venous return
4. Net effect: ↓ CO + ↓ TPR → ↓ MAP back to set-point

Response to DECREASED Arterial Pressure (e.g., hemorrhage):

1. ↓ Pa → ↓ stretch on baroreceptors → ↓ afferent firing
2. NTS senses Pa < set-point:
   • ↓ Parasympathetic outflow → ↑ Heart rate
   • ↑ Sympathetic outflow → ↑ Heart rate + ↑ Contractility → ↑ CO
   • ↑ Sympathetic outflow to arterioles → Vasoconstriction → ↑ TPR
   • ↑ Sympathetic outflow to veins → Venoconstriction → ↑ venous return → ↑ CO (Frank-Starling)
3. Net effect: ↑ CO + ↑ TPR → ↑ MAP back to set-point

Baroreceptor control of arterial pressure as a negative-feedback loop. An increase in MAP activates baroreceptors → medullary coordinating center → bradycardia and vasodilation → MAP returns to normal. (Medical Physiology, Boron & Boulpaep)

3.8 Sensitivity and Resetting

• Baroreceptors are more sensitive to rapid changes in pressure than to gradual or sustained changes.
• In chronic hypertension, the baroreceptors undergo resetting — they adapt to the elevated baseline and no longer recognize it as abnormal. The set-point in the medullary centers is effectively raised. This explains why the baroreceptor reflex maintains rather than corrects chronic hypertension.
• A decline in pulse pressure (without change in mean pressure) reduces baroreceptor discharge and provokes a compensatory rise in BP and tachycardia.

3.9 Clinical Test: Valsalva Maneuver

1. ↑ Intrathoracic pressure → ↓ Venous return → ↓ CO → ↓ Pa
2. If reflex intact: baroreceptors detect ↓ Pa → ↑ sympathetic + ↓ parasympathetic outflow → ↑ Heart rate (detected clinically)
3. On maneuver release: venous return rebounds → ↑ CO → ↑ Pa → baroreceptors detect rise → ↓ Heart rate

4. Summary Table

• Costanzo Physiology, 7th Ed., Ch. 4 — Baroreceptor Reflex
• Medical Physiology, Boron & Boulpaep — Ch. 23: Neural Regulation of Blood Pressure
• Ganong's Review of Medical Physiology, 26th Ed. — Ch. 32: Baroreceptors
• Roberts and Hedges' Clinical Procedures in Emergency Medicine — Normal BP Values
• Comprehensive Clinical Nephrology, 7th Ed. — Table 34.2: JNC 7 BP Classification