What do baroreceptors (blood pressure receptors) primarily affect?

Baroreceptor Function and Effects

Baroreceptors primarily affect vascular resistance through sympathetic regulation of blood vessel tone, not cardiac output. 1 This is the fundamental mechanism by which baroreceptors maintain blood pressure homeostasis.

Anatomical Location and Sensory Pathway

Baroreceptors are specialized stretch receptors located in:

• Carotid sinus
• Aortic arch
• Great thoracic vessels 1

These receptors function as part of a critical feedback loop that helps maintain blood pressure homeostasis. When activated by increased arterial pressure, information is transmitted via:

• Glossopharyngeal nerves (IX) from the carotid sinus
• Vagus nerves (X) from the aortic arch
• To vasomotor centers in the brainstem 2, 1

Physiological Effects of Baroreceptor Activation

When baroreceptors detect increased blood pressure, they trigger:

1. Decreased vascular resistance (primary effect) 3
2. Decreased heart rate (bradycardia)
3. Decreased cardiac contractility 1

When baroreceptors detect decreased blood pressure, they trigger:

1. Increased vascular resistance (primary effect) 3
2. Increased heart rate
3. Increased cardiac contractility 1

Mechanism of Action

The baroreceptor reflex operates through:

• Afferent pathway: Stretch receptors detect changes in arterial pressure
• Central processing: Brainstem integrates signals
• Efferent pathway: Signals sent via:
  • Parasympathetic nerves to the heart (vagus)
  • Sympathetic nerves to blood vessels and heart 2

Evidence for Primary Effect on Vascular Resistance

Multiple studies confirm that baroreceptors primarily control arterial pressure via reflex changes in vascular resistance rather than cardiac output:

• "The baroreceptor reflex controls arterial pressure primarily via reflex changes in vascular resistance, rather than cardiac output" 3
• "CBR-induced changes in ABP are primarily mediated by alterations in vascular conductance with only minimal contributions from cardiac output" 4
• "The capacity of the CBR to regulate ABP depends critically on its ability to alter vascular tone both at rest and during exercise" 4

Clinical Significance

Baroreceptor dysfunction contributes to:

• Orthostatic hypotension due to impaired vasoconstriction upon standing 2
• Increased risk of sudden cardiac death in post-MI patients 1
• Altered blood flow distribution in heart failure 5

Answer to Question

Based on the evidence, the correct answer is B. Increase vascular resistance. Baroreceptors primarily affect vascular resistance through sympathetic regulation of blood vessel tone, not cardiac output (option A).