Nov 9, 2012



Exchange of oxygen and carbon dioxide in lungs between alveolar air and blood is called external respiration, in contrast to internal respiration occurring at tissue level. Blood (RBC) will take oxygen from lungs to tissue and carbon dioxide from tissue to the lungs for exhalation.

The HCO3-, carbamino compounds, and dissolved CO2 are transported in venous blood to pulmonary capillaries and alveoli. The low pCO2 in alveoli will cause a shift of CO2 from erythrocytes and plasma into the alveoli. On the other hand, the high pO2 in alveoli cause a shift of oxygen into the plasma and erythrocytes.
The respiration is regulated by central chemoreceptors located in medulla that sense change in pH of CSF and peripheral chemoreceptors located in carotid artery and aorta that sense change in pH in blood, in both case caused by excess or deficit CO2 or change in pO2 causing hyperventilation or hypoventilation as required.
Respiratory response to acid-base perturbations:
The respiratory system responds immediately to change in acid-base status. The response becomes maximum once the central and peripheral chemoreceptors are fully stimulated. E.g. in metabolic acidosis plasma pH decreases, so hyperventilation occurs due to stimulation of peripheral chemoreceptors and plasma pCO2 decreases (When HCO3- decreases pCO2 decreases, i.e., regulatory response occurs in one direction). During metabolic alkalosis, pH in plasma increases, this cause depression of peripheral chemoreceptors and thus there is hypoventilation which will retain CO2 in circulation.


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