2.13 Baroreceptors and Volume Receptors

Baroreceptors are mechanoreceptors that play a crucial role in the short-term and long-term regulation of blood pressure and blood volume. They are classified into two main types:

• High-pressure arterial baroreceptors
• Low-pressure baroreceptors (Volume receptors)

High-pressure Arterial Baroreceptors

Location: Primarily found in the walls of the aorta (aortic arch) and the carotid sinus (at the base of the internal carotid arteries). These Receptors→ are strategically placed to monitor blood pressure flowing to the brain and the rest of the body.

Function: These mechanoreceptors sense the arterial blood pressure and relay this information to the cardiovascular center in the medulla oblongata to maintain Blood Pressure→.

Mechanism (Negative Feedback - Baroreflex):

• Response to High Blood Pressure:

  • When mean arterial pressure increases, the baroreceptors are stretched and increase their firing rate.
  • This stimulates the parasympathetic nervous system and inhibits the sympathetic system.
  • Leads to widespread vasodilatation and a reduction in Heart Rate→.
  • Consequence: Reduction in blood pressure back to normal levels.

• Response to Low Blood Pressure:

  • Decreased stretch reduces the firing rate of the baroreceptors.
  • This triggers increased nerve impulse transmission along sympathetic nerves.
  • Causes body-wide vasoconstriction and increased cardiac output.
  • Consequence: A rise in blood pressure.

The system acts immediately as part of a negative feedback system known as the baroreflex, providing rapid adjustments to maintain homeostasis.

Low-Pressure Baroreceptors or Volume Receptors

Location: Found primarily in the atria of the heart (especially the right atrium) and also in the pulmonary trunk.

Function: These receptors detect changes in blood volume by sensing the degree of stretch in the low-pressure areas of the circulatory system.

Mechanism (ADH-mediated volume regulation):

1. Detection: When receptors detect a blood volume decrease (low venous return) in the atria.
2. Signal Transmission: A signal is transmitted from these receptors to the hypothalamus.
3. Hormonal Response: The hypothalamus increases the production and release of antidiuretic hormone (ADH) from the posterior pituitary.
4. Kidney Action: ADH acts on the Kidney→ to increase water reabsorption.
5. Outcome: This process increases the overall blood volume, subsequently leading to an increase in blood pressure.

Conversely, if blood volume is too high, atrial stretch triggers the release of Atrial Natriuretic Peptide (ANP), which promotes salt and water excretion to lower volume.