Ventilation-Perfusion Ratio and Gas Exchange

• Ventilation-perfusion balance relates to gas exchange
• Ventilation-perfusion matching is the balance between air and blood flow in the lungs

Ventilation/Perfusion Ratio

• Ventilation is the addition of O2 and removal of CO2
• Perfusion is the removal of O2 and addition of CO2
• The relative relationship between ventilation and perfusion determines the composition of alveolar gas

Effect of Altering V/Q Ratio

• Unit A receives ventilation and blood flow in normal proportions, resulting in normal alveolar gas concentrations
• Unit B receives no ventilation but is perfused, resulting in a V/Q of zero, and the gas exchange does not take place
• Unit B has O2 and CO2 levels of mixed venous blood and behaves as a shunt
• Unit C receives no blood flow but is ventilated, with a V/Q of infinity
• Unit C leads to increased alveolar O2 and decreased CO2, eventually matching inspired gas composition
• Unit C causes "wasted ventilation" or alveolar dead space

Regional V/Q in the Normal Lung

• Ideal lungs would supply each alveolus with equal amounts of air and mixed venous blood during inspiration
• The V/Q of a normal whole lung is approximately 0.8
• V/Q varies regionally due to gravity, from 0.6 at the base to 3.3 at the apex in an upright lung

Apex of the Upright Lung

• There is a decrease in both ventilation and perfusion, perfusion decreases more than ventilation
• The lung at the apex has a V/Q of 3.3 and is more ventilated than perfused
• This results in minimal oxygenation of blood, also called physiologic dead space
• Low PCO2 constricts small airways, redistributing gas to alveoli with better blood flow

Base of the Upright Lung

• There is an increase of both ventilation and perfusion, perfusion increases more than ventilation
• The lung at the base has a V/Q of 0.6 is less ventilated than perfused
• The blood will be less oxygenated due to over perfusion than ventilation, also known as a shunt
• Low PO2 in poorly ventilated alveoli causes arteriolar constriction, redistributing blood flow to well-ventilated alveoli

Diffusion

• Process of gas exchange across the respiratory membrane

Transfer of O2 from Air to Blood

• Transfer of O2 from air to blood occurs as series of steps
• Steps include traveling within the alveoli and across the air to blood components
• Air to blood components include:
• Length of time blood is in capillaries
• Crossing the alveolar/capillary membrane
• Diffusion in blood plasma
• Reaction with hemoglobin, being the limiting step in O2 diffusion

Factors Affecting the Rate of Diffusion

• Fick's Law of Diffusion determines the rate of diffusion through tissues
• Rate of diffusion equals the product of surface area, solubility, and pressure gradient divided by the product of thickness and the square root of molecular weight
• Thickness of the respiratory membrane is normally thin which allows for proper diffusion
• Diffusion through the respiratory membrane is rapid because of the large surface area, extensive capillary plexus, a small amount of blood in pulmonary capillaries, and narrow capillary diameter

Diffusion and Pressure Gradient

• The pressure gradient is the difference between the partial pressure of the gas in the alveoli and the partial pressure in the blood
• The diffusion coefficient of a gas through the respiratory membrane depends on the gas's solubility (CO2 solubility is 24 times greater than O2) and molecular weight (O2 = 32, CO2 = 44)
• The diffusion coefficient of O2 = 1, and CO2 = 20, therefore CO2 diffuses more readily than O2

Diffusing Capacity of the Lung

• The diffusing capacity of the lung or transfer factor, is the volume of gas that diffuses through the membrane each minute with a pressure difference of 1 mm Hg
• At rest, for O2, the standard is 21 ml/min/mm Hg and increases to 65 ml/min/mm Hg during exercise
• Diffusion improves during opening previously dormant capillaries, dilation of open capillaries and due to better ventilation-perfusion ratio
• The diffusion across the respiratory membrane is either perfusion limited or diffusion limited

Perfusion Limited

• Indicates gas equilibrates early along the capillary
• Diffusion increases only if blood flow increases
• This limits diffusion O2, CO2

Diffusion Limited

• Refers to gases not equilibrate by the time blood reaches the end of the capillary
• This limits diffusion of O2 (emphysema, fibrosis), and CO

Transit Time

• There are transit time differences through different regions of the lung
• Transit times less than 0.2 seconds do not allow sufficient time for equilibration, which acts as shunt
• Increased cardiac output during heavy exercise decreases all transit times
• At altitude PatmO2 decreases making exercise at altitude highly detrimental to pulmonary function

Explore ventilation-perfusion (V/Q) matching and its impact on gas exchange. Learn how alterations in the V/Q ratio affect alveolar gas concentrations. Understand the consequences of V/Q mismatch, including shunting and wasted ventilation.