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9 out of 9

Peripheral chemoreceptors is stimulated maximally by (DNB Dec-2010)

A Cyanide
B Anaemia

C Hypocapnia
D Alkalosis

Ans. A Cyanide

[A] Peripheral chemoreceptor system for control of Respiratory activity: -

1. These chemoreceptor located in —

a. Carotid bodies Qat the birfurcation of the common carotid artery Q(bilaterally) Afferent fibers via Hering’s Nerve of IX CN Q to the dorsal respiratory area of the medulla.

b. Aortic bodies located in arch of Aorta Qafferent fiber via X CN Q to dorsal respiratory area of the medulla.

2. These receptors are stimulated by: -

a. A rise in PCO2 of arterial blood Q

b. A rise in H+ conc. (iein PH i.e. Metabolic acidosis) of arterial blood and Q

c. A decline in the PO2 of arterial blood. Q

[If these changes in opposite direction, have a slight inhibitory effect on respiration]

3. These carotid and Aortic bodies contain type I cells (glomus) and Type II cells. On stimulation by hypoxia and cyanide release catecholamine Dopamine is the principle transmitter.

4. Blood flow in each 2 mg carotid body is about 0.04 mL/min. or 2000 ml/100gm of tissue/min.

5. Because the blood flow per unit of tissue is so enormous, the O2 needs of the cells can be met largely by dissolved

O2 alone. Therefore, the receptors are not stimulated in conditions such as Anemia and Carbon Monooxide poisoning, in which the amount of dissolved O2 in the blood reaching the receptor is generally normal even though the combined O2 in the blood is markedly decreased.

6. The receptors are stimulated when the arterial PO2 is low (i.e. Hypoxic Hypoxia) or when, because of vascular stasis, the amount of O2 delivered to the receptor perunit time is decreased (i.e. stagnant or Ischemic hypoxia)

7. Powerfullstimulatin of receptors is also produced by drugs such as cyanide, which prevent O2 utilization at the tissue level (Histotoxic hypoxia).