CO2 is the most important stimulus for respiration

Receptors for CO2 are found in the medulla of the brain (central chemoreceptors)

Receptors for O2 are found mainly in carotid and aortic bodies

CO2 is the more important gas as the body has more capacity to store CO2 than O2 or hydrogen ions

In normal people at sea level, only 10% of the respiratory drive is due to hypoxic stimulation.

Unlike the central stimulation of hypercapnia, hypoxia causes central depression of the respiratory drive.

Acidosis (high H+ / low blood pH) stimulates respiration; conversely alkalosis depresses it.

For gas exchange, the lungs provide an interface of total surface area about 55 m2 via 700 million alveoli

Alveolar ventilation’ is that part of the total ventilation (i.e. all gas entering the lungs) that participates in gas exchange with pulmonary capillary blood; it is equal to total ventilation minus the ventilation of the conducting airways (i.e. dead-space ventilation).The average alveolar ventilation is about 4 L/min.

The alveolar–arterial oxygen gradient ( P(A-a)O2 ) is a measure of the oxygen that has reached the arterial blood supply as a ratio of the total oxygen in the alveoli. It is a useful index of pulmonary gas exchange function.

This requires that three elements are working correctly:

1. Circulatory anatomy is normal. Anomalies such as ASD & PDA can cause anatomical shunting, i.e. venous blood passes through routes that are not exposed to alveolar air
2. Ventilation and perfusion are matched
3. The respiratory membrane allows sufficient free diffusion of gases between air and blood. A diffusion defect impairs the alveolar–capillary membrane, e.g. in interstitial lung fibrosis

In a healthy individual breathing room air (at FiO2 0.21) the PO2 in alveolar air is 104 mmHg and in arterial blood 95 mmHg . PAO2 exceeds PaO2 by 15 mmHg .Thus, at an FiO2 of 21, the P(A–a)O2 is 15 mmHg

In blood, CO2 is present as:

Dissolved in blood plasma (5.3% in arterial blood)

Bound to haemoglobin as carbaminohaemoglobin within erythrocytes (4.5%)

In the form of bicarbonate attached to a base (90%)

Reference:”Understanding ABGs & Lung Function Tests” Muhunthan Thillai, Keith Hattotuwa