O2 and CO2 exchange - HUBS192 Lecture 18 Notes

Respiratory Volumes and Alveolar Ventilation

Respiratory Minute Volume (VE): rate of air inhaled/exhaled per minute. VE = VT \times f where
- VT = Tidal Volume (volume per breath)
- f = breathing frequency (breaths/min)
Dead Space (VD): air in conducting portions that does not participate in gas exchange (anatomic dead space)
Alveolar Ventilation (VA): portion of ventilation reaching the alveoli for gas exchange. VA = (VT - VD) \times f
Important intuition: large dead space reduces effective gas exchange; small, shallow breaths are not useful for alveolar ventilation
Cardiac output (CO) is a separate flow variable (not part of ventilation), defined as CO = SV \times HR where
- SV = stroke volume, HR = heart rate

Gases move across membranes between alveoli and capillaries by diffusion
Rate of diffusion is determined by three main factors:
- Surface area of the membranes
- Thickness of the membranes
- Pressure difference between the two sides
Alveolar-capillary region has a very large surface area and a thin barrier to facilitate diffusion
Pathologies affecting diffusion:
- Emphysema: dilation and destruction of alveolar walls reduce surface area, impair diffusion
- Pulmonary fibrosis: thickening/scarring of alveolar membranes increases barrier thickness, impeding diffusion
The Blood–Air Barrier distance is very small, facilitating rapid gas exchange

Alveolar O2 partial pressure depends on three determinants:
- Atmospheric O2 partial pressure
- Alveolar ventilation (how well fresh air reaches alveoli)
- Oxygen uptake by blood (blood oxygen tension) / perfusion
Related concept: diffusion occurs down a pressure gradient (partial pressures drive movement)
Dalton’s Law reminder: the pressure of a gas mixture equals the sum of the partial pressures of its components. P{total} = \sumi P_i

Gases diffuse down their pressure gradients across the Blood–Air Barrier
The rate of diffusion is influenced by:
- Surface area of the membrane (more area = more diffusion)
- Membrane thickness (thinner = faster diffusion)
- Pressure difference (larger gradient = faster diffusion)
Oxygen and CO2 exchange are governed by these principles across a thin barrier with high surface area

Alveolar Surface Area: large due to many alveoli and dense capillary network
Membrane Thickness: normally very thin; can thicken in disease (fibrosis)
Diffusion Gradient: driven by partial pressure differences for O2 and CO2
Gas Exchange Efficiency: depends on VA (alveolar ventilation) and perfusion (blood flow)

Emphysema: reduces surface area -> lowers O2 diffusion capacity
Pulmonary Fibrosis: increases barrier thickness -> lowers diffusion rate
Summary implication: diffusion impairment can lower arterial O2 without necessarily changing CO2 unless ventilation or perfusion are disproportionately affected

VE = VT \times f; VA = (VT - VD) \times f; VD is the anatomic dead space air
Diffusion rate across the Blood–Air Barrier depends on surface area, thickness, and pressure difference
Alveolar O2 depends on atmospheric O2, alveolar ventilation, and blood O2 extraction (perfusion)
Gases diffuse according to their partial pressure gradients; total pressure is the sum of partial pressures: P{total} = \sumi P_i