Gas Exchange

What is Minute ventilation (V_E)?

The total air entering the lungs per minute.

• V_E = TV x Breathing Frequency

• V_E = 500 mL x 12 breaths/min = 6 L/min

What is Alveolar ventilation (V_A)?

Accounts for air reaching the alveoli that’s available for gas exchange.

• V_A = (TV - Dead space) x Breathing frequency

• V_A = (500 mL - 150 mL) x 12 breaths/min = 4.2 L/min

What is dead space (DS)?

It is the fraction of tidal volume that does not participate in gas exchange:

• Anatomical DS: about 150 mL, parts of the respiratory system that does not partake in gas exchange due to their anatomical position, like the trachea and bronchi

• Physiological DS: includes anatomical dead space, and alveolar dead space = alveoli not partaking gas exchange due to dysfunctionality/poor blood flow

  • In healthy individuals, alveolar DS is 0

Barometric pressure of gases

Barometric (atmospheric) pressure at sea level is around 760 mmHg, consisiting of atmospheric gases: N2 (79%) and O2 (21%)

• Partial pressure = Absolute pressure x gas (%)

• PN2 = 760 × 0.79 = 600 mmHg

• PO2 = 760 × 0.21 = 160 mmHg

At increasing altitudes, the barometric pressure decreases:

• 4000m → 460 mmHg

• 8800m → 253 mmHg

What are the alveolar gas pressures?

Alveolar PO2 = 195 mmHg

Alveolar PCO2 = 40 mmHg

Why do alveolar partial pressures not match barometric?

Alveolar partial pressures does not match ambient air partial pressures because our ventilation frequency is not large enough to renew the alveolar air with each breath fast enough, and tissues consume O2 and produce CO2.

What is Hypoventilation and Hyperventilation?

Hypoventilation: when metabolic CO2 exceeds what alveolar ventilation can eliminate

Hyperventilation: when alveolar ventilation exceeds the rate of metabolic CO2 production

• Not the same as increased ventilation during exercise, where alveolar ventilation matches the demand of expelling CO2 production

What does Henry’s Law state?

Gases dissolve in liquid in proportion to their partial pressure, moving from high to low. At equilibrium, partial pressures are equal.

Gas exchange: Alveoli ←→ Blood

Deoxygenated blood arriving from pulmonary arteries:

• PO2: 40 mmHg

• PCO2: 46 mmHg

Oxygenated blood leaving from pulmonary veins:

• PO2: 100 mmHg

• PCO2: 40 mmHg

In healthy individuals, rates of O2 and CO2 diffusion are high enough and blood flow slow enough to reach equilibrium. Net diffusion stops when capillary and alveoli partial pressures are equal (oxygenated blood gas pressures = alveolar gas pressures).

Gas transport: Blood ←→ Tissues

• Tissue metabolism consumes O2 and produces CO2, creating a concentration gradient of low PO2 and high PCO2 in tissues

• Mitochondria uses O2, lowest PO2 <5 mmHg

• O2 travels from blood to tissue, down its partial pressure gradient

  • Capillary → interstitial fluid → cell → mitochondria

• CO2 diffuses oppositely, from tissue to blood

  • Mitochondria → cell → interstitial fluid → capillary

What is ventilation-perfusion mismatching, and what are two mechanisms to counter it?

Ventilation-perfusion mismatching is the cause for inadequate gas transfer, and is countered by:

• Oxygen driven vasoconstriction

  • In tissues, low PO2 leads to vasodilation to increase perfusion and O2 delivery to working cells

  • However, in alveoli, low PO2 leads to vasoconstriction to divert blood to better-ventilated alveoli with higher PO2

• CO2-driven bronchoreactivity

  • If blood flow to the lungs decreases, e.g. due to blood clot, the drop in PCO2 triggers bronchoconstriction to decrease air flow and reduce ventilation to poorly perfused areas

How does high altitude cause lung edema?

1. At high altitudes, the barometric pressure (and thus PO2) is decreased, leading to reduced alveolar and arterial PO2

2. In response, pulmonary arterioles constrict

   • Normally, this helps to divert blood to better ventilated alveoli, but at high altitudes, all alveoli are hypoxic

3. Widespread vasoconstriction raises resistance in pulmonary circulation and leads to elevated pulmonary blood pressure

4. High pressure forces fluid out of capillaries into the interstitial space and into alveoli, and its accumulation results in lung edema