Question

Explain the mechanism of blood pressure homeostasis. Your answer should state the name of the reflex that maintains constant blood pressure and identify all 5 components of the negative feedback loop. Also, describe in detail how the body would compensate for a rise in blood pressure, including the division of the autonomic nervous system stimulated.

Answer 1

Short Term regulation (mins to second )
1) Baroreceptor refex
2) Chemoreceptor refex
3) CNS ischemic response
4) Epinephrine secreted from adrenal medulla

Intermediate term regulation : (mins to hrs)
1) Capillary fuid shift mechanism
2) Stress relaxation and reverse stress relaxation
3) Renin-Angiotensin mediated vasoconstriction

Long-term mechanisms (3–10 days)
1) Renal fuid balance by ADH, ANP,
Aldosterone
2) RAAS system

Baroreceptor Mechanism :

- Baroreptors are Located in the walls of the carotid sinus near bifurcation of the common carotid arteries and
aortic arch.

- Carotid sinus baroreceptors stimulated at pressures above 70 mm Hg, but they respond progressively more rapidly and reach a maximum at about 170 mm Hg. Theworking range 70–170 mm Hg.

- The aortic baroreceptors are similar to those of the carotid baroreceptor, they operateat pressure levels about 30 mm Hg higher. Most sensitive to changing pressure than constant pressure. So, more sensitive to pulse pressure than mean arterial pressure.

Baroreceptor reflex -

Function during changes in body posture

Immediately on standing →decrease BP in upper part of the body → decrease baroreceptors fring→No inhibition to RVLM→RVLM activated → strong sympathetic
discharge and Vasoconstriction and reflex Increase in BP.

If BP rises suddenly ::

Increased BP → Baroreceptors stimulated → stimulation to NTS → stimulation of CVLM →inhibition of RVLM →inhibition of sympathetic system. NTS also stimulates vagal motor neurons in the nucleus ambiguous.

Hence, increased baroreceptor discharge inhibits the tonic discharge of sympathetic nerves and excites the vagal innervation of the heart.

These neural changes produce vasodilation, venodilation, drop in BP, bradycardia, and a decrease in cardiac output.