Gas Composition Changes During Ventilation


(Note "Review of Partial Pressure")

(see figure ==>)

A. Inspired Air

1. Assume breathing dry air: mixture of O2 (21%) and N2 (79%); if air not dry, variable amount of H2O vapor, depending on relative humidity and temperature (generally less than 2%; in tropics up to 6%)  (Also assume total barometric pressure, PB, is 760 mmHg or torr)

B. Tracheal Air (or Inhaled Air, saturated or wet or moist)

1. Inspired air is saturated with water as it passes along the moist airways

2. Water content depends on temperature; at core (deep) body temperature of 37C (typical of lungs), PH2O is 47 mmHg

3. As a result of addition of water, O2 and N2 are slightly diluted (Note: after saturation with water vapor, PB - 47 is left for O2 and N2)

C. Alveolar Air

1. Inspired air is split; some enters the alveoli and some remains in the airways (anatomical dead space)

2. Dead Space

a. Define VD: volume ventilated but not exchanging with pulmonary capillary blood

b. Typical value: 150 ml (about 30% of resting tidal volume)

c. Wasted Ventilation: ventilation of dead space (V'D = f x VD )

3. Alveolar Ventilation (effective ventilation for O2 and CO2 exchange)

a. Define: rate of ventilation of alveoli

b. Equation: V'A = f x ( VT - VD ) = V'EV'D


4. Alveolar gas composition

a. O2 decrease, due to oxygen uptake; usually about 50 mmHg drop

b. CO2 increase, due to carbon dioxide production; generally 40 mmHg rise

c. O2 decrease and CO2 increase depends on metabolic rate versus alveolar ventilation

Metabolism ↑ ⇒ PO2 & PCO2 (assume constant alveolar ventilation)

Ventilation ⇒ PO2 & PCO2 (assume constant metabolism)

Question: What would happen to PO2 and PCO2 if a person began to exercise but alveolar ventilation did not increase?

Question: What would happen to
PO2 and PCO2 if a person's ventilation decreased but his metabolism stayed the same?

Note:  Ideal physiological response to exercise:   Metabolic rate and V'A increase in proportion

D. Exhaled Air

1. Mixture of Alveolar Air and Tracheal Air that remained in the Dead Space

2. End-tidal gas. In a clinical setting, it is possible to estimate the mean alveolar gas concentrations by measuring the composition of the gas exhaled at the end of expiration as this gas comes almost entirely from the alveoli; this is termed the end-tidal gas