Physiology Question-Based Learning by Hwee Ming Cheng

Author:Hwee Ming Cheng
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

4. Why does the prolonged, increased voluntary breathing produce dizziness in a person?

Answer

Cerebral vasoconstriction due to the induced hypocapnia.

Concept

Increased ventilation produces a respiratory alkalosis, since the CO2 is “blown” off by the lungs. The cerebral arterioles are particularly sensitive to changes in blood CO2. A reduced partial pressure of CO2 will constrict the cerebral vessels. This lessens the cerebral blood flow and gives the sensation of giddiness.

This arteriolar response to CO2 is also part of the intrinsic autoregulatory mechanism of cerebral blood flow. The cerebral vasculature has the inherent ability to maintain a relatively constant perfusion in spite of blood pressure fluctuations over a certain autoregulatory range (60–160 mmHg). If there is an acute decrease in cerebral flow, soon the local CO2 in the brain tissues builds up. The cerebral arterioles will then vasodilate and blood flow is sustained. This is an example of a physiologic role of CO2, a gas which should be viewed more than just a metabolic end product that needs to be eliminated by the lungs during expiration.

If the decrease in cerebral blood flow is due to hypovolemia/hypotension, it is useful to contrast the local compensatory cerebral vasodilation with the need to sustain an adequate head-driving arterial blood pressure. Concurrent with the reduced cerebral vascular resistance, there will be a systemic increase in the total peripheral resistance (TPR). The compensatory increase in TPR is effected by baroreflex activated sympathetic nerve action on selective arteriolar vasoconstriction (including skin, splanchnic, renal).

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