Egan’s Ch. 12 Gas Exchange and Transport Quiz

Purpose and Mechanics of Respiration

• Primary purpose of respiration is to exchange gases: oxygen intake and carbon dioxide removal.
• Gas movement between lungs and tissues mainly occurs through diffusion driven by concentration gradients.

Gaseous Composition and Movement

• Oxygen has the highest partial pressure in human cells.
• Carbon dioxide (CO2) moves from cells into venous blood, balancing internal production and arterial release.

CO2 Production and Levels

• If CO2 production increases while alveolar ventilation (VA) remains constant, PaCO2 levels rise.
• Normal range for arterial PACO2 is typically 35-45 mmHg.

Alveolar Air Equation and Composition

• Alveolar air equation accounts for the partial pressures of gases in alveoli, reflecting discrepancy between alveolar and arterial oxygen tensions.
• Partial pressure of nitrogen (N2) in alveolar gas is determined by its concentration in inspired air and the total atmospheric pressure.

Gas Ratios and Roles

• Normal CO2 to oxygen ratio in the lungs is around 1:4.
• Nitrogen has no role in gas exchange but occupies space and exerts pressure in the lungs.

Determinants of Gas Exchange

• Prime determinant of PACO2 in the lungs is the rate of CO2 production relative to its elimination.
• Main barrier to gaseous diffusion in the lung is the alveolar-capillary membrane.

Requirements and Influencing Factors

• Gases like CO2 or O2 require a concentration gradient and sufficient membrane surface area for diffusion between alveoli and pulmonary capillary blood.
• According to Fick's law, diffusion increases with larger surface area, steeper gradients, and lower molecular weight gases.

Diffusion Dependence and Limits

• In normal lungs, diffusion primarily depends on the surface area and distance across the alveolar-capillary membrane.
• During hyperventilation, PaO2 may not exceed 120 mmHg due to limitations in oxygen solubility and transport.

Pressure Gradients and V/Q Relationships

• Pulmonary diffusion occurs along pressure gradients between alveolar gas and blood.
• Approximately three-quarters of the normal difference between PAO2 (alveolar oxygen pressure) and PaO2 (arterial oxygen pressure) is attributed to shunting and ventilation-perfusion ratios.

Ventilation-Perfusion Ratios

• Conditions arise when venous blood enters areas with ventilation-perfusion (V/Q) ratios of 0, leading to hypoxemia.
• Regional variations in V/Q in the normal lung are mainly caused by gravity, affecting blood flow distribution.
• Exchange units with V/Q values of 0 primarily occur in dependent lung regions.

High V/Q Ratios and Effects

• The apex of the lungs typically exhibits high V/Q ratios, often above 1.
• Mixing of well-oxygenated arterial blood with poorly oxygenated blood causes dilution of oxygen content in the arteries.

Gas Solubility and Diffusion

• Carbon dioxide has a higher solubility in plasma than oxygen, facilitating faster diffusion across the alveolar-capillary membrane.
• Blood normally spends around 0.75 to 1 second in pulmonary capillaries to allow for gas equilibration with alveolar gas.

Equilibration Dynamics

• Typical PO2 of blood leaving tissue capillaries after equilibration is around 40 mmHg.
• Carbon dioxide diffuses from blood into the alveolus due to higher concentration in the blood relative to the alveolar air.

Anatomic Shunts and Cellular Dynamics

• Primary cause of anatomic shunts leading to lower PaO2 than calculated PAO2 is blood bypassing ventilated alveoli.
• Blood returns from tissue capillaries into cells after equilibration due to concentration gradients favoring nutrient and gas exchange.

Test your knowledge on gas exchange, transport of O2 and CO2, and the process of respiration in the context of Egan’s Chapter 12. Explore concepts such as diffusion of gases between tissues and blood, as well as the movement of O2 from the lungs to the tissues.