Respiratory System: Gas Exchange, Volumes, and Capacities 23-2

Respiratory Volumes and Capacities

Overview of Lung Volumes and Capacities

Respiratory volumes and capacities are essential measurements for assessing ventilation and diagnosing pulmonary disorders. They describe the amount of air the lungs can hold and the changes in lung volume during different phases of breathing.

• Tidal Volume (TV): The amount of air taken into or expelled out of the lungs during a quiet breath.

• Inspiratory Reserve Volume (IRV): The amount of air taken into the lungs during a forced inspiration, following a quiet inspiration. IRV is a measure of lung compliance.

• Expiratory Reserve Volume (ERV): The amount of air expelled from the lungs during a forced expiration, following a quiet expiration. ERV is a measure of lung and chest wall elasticity.

• Residual Volume (RV): The amount of air left in the lungs following a forced expiration.

Capacities Derived from Volumes

Lung capacities are combinations of two or more lung volumes and represent the total ability to inspire or expire air.

• Inspiratory Capacity: TV + IRV; total ability to inspire.

• Functional Residual Capacity: ERV + RV; amount of air typically left in lungs after quiet expiration.

• Vital Capacity: TV + IRV + ERV; maximum amount of air that can be forcefully expired after a forced inspiration.

• Total Lung Capacity: TV + IRV + ERV + RV; maximum amount of air the lungs can hold.

Volume	Definition	Typical Value (Male)	Typical Value (Female)
Tidal volume (TV)	Amount of air taken in or expelled during quiet breath	500 mL	500 mL
Inspiratory reserve volume (IRV)	Amount of air taken in during forced inspiration after quiet inspiration	3100 mL	1900 mL
Expiratory reserve volume (ERV)	Amount of air expelled during forced expiration after quiet expiration	1200 mL	700 mL
Residual volume (RV)	Amount of air left in lungs after forced expiration	1200 mL	1100 mL

Capacity	Formula	Definition	Typical Value (Male)	Typical Value (Female)
Inspiratory capacity	TV + IRV	Total ability to inspire	3600 mL	2400 mL
Functional residual capacity	ERV + RV	Amount of air typically left in lungs after quiet expiration	2400 mL	1800 mL
Vital capacity	TV + IRV + ERV	Maximum amount of air that can be forcefully expired after forced inspiration	4800 mL	3100 mL
Total lung capacity	TV + IRV + ERV + RV	Maximum amount of air the lungs can hold	6000 mL	4200 mL

Four Processes of Respiration

Overview of Respiration

Respiration consists of four integrated processes that ensure the exchange and transport of gases necessary for cellular function.

1. Pulmonary Ventilation: Movement of air into (inspiration) and out of (expiration) the lungs.

2. External Respiration: Diffusion of gases between alveoli and blood.

3. Transport of Respiratory Gases: Movement of O2 and CO2 in the blood between lungs and tissues.

4. Internal Respiration: Diffusion of gases between blood and tissue cells.

Chemical Principles of Gas Exchange

Partial Pressure and Dalton’s Law

Gas exchange is governed by the partial pressures of gases and Dalton’s law. Partial pressure is the pressure exerted by each gas in a mixture, and each gas moves independently down its partial pressure gradient.

• Dalton’s Law: The total pressure of a mixture of gases is equal to the sum of the partial pressures of each gas.

• Formula: $\text{Partial pressure of gas} = \text{Total pressure} \times \text{Fraction of gas in mixture}$

• Example: Nitrogen is 78.6% of air; at 760 mm Hg, its partial pressure is $760 \times 0.786 = 597$ mm Hg.

Gas Exchange in the Lungs and Tissues

External Respiration (Pulmonary Gas Exchange)

Pulmonary gas exchange is the movement of O2 and CO2 between alveolar air and pulmonary capillary blood, driven by partial pressure gradients.

• Oxygen: PO2 in alveoli = 104 mm Hg; PO2 in blood entering capillaries = 40 mm Hg. Oxygen diffuses from alveoli to blood.

• Carbon Dioxide: PCO2 in alveoli = 40 mm Hg; PCO2 in blood = 45 mm Hg. CO2 diffuses from blood to alveoli.

Internal Respiration (Tissue Gas Exchange)

Tissue gas exchange is the movement of O2 and CO2 between systemic capillary blood and tissue cells.

• Oxygen: PO2 in systemic capillaries = 95 mm Hg; PO2 in cells = 40 mm Hg. Oxygen diffuses from blood to cells.

• Carbon Dioxide: PCO2 in cells = 45 mm Hg; PCO2 in capillaries = 40 mm Hg. CO2 diffuses from cells to blood.

Comparison of Gas Partial Pressures in Atmosphere and Alveoli

Atmospheric vs. Alveolar Gas Composition

The composition of gases in the alveoli differs from atmospheric air due to gas exchange, humidification, and mixing with air in the dead space.

Gas	Approximate Percentage (Atmosphere)	Partial Pressure (Atmosphere, mm Hg)	Approximate Percentage (Alveoli)	Partial Pressure (Alveoli, mm Hg)
N2	78.6%	597	74.9%	569
O2	20.9%	159	13.7%	104
CO2	0.04%	0.3	5.2%	40
H2O	0.46%	3.7	6.2%	47
Total	100%	760	100%	760

Factors Influencing Gas Exchange

Ventilation-Perfusion Coupling

Ventilation-perfusion coupling is the matching of alveolar ventilation with pulmonary blood perfusion. It ensures optimal gas exchange by adjusting bronchiolar and arteriolar diameters in response to local PO2 and PCO2 levels.

• Perfusion: Blood flow reaching alveoli.

• Ventilation: Amount of gas reaching alveoli.

• Regulation: PO2 controls perfusion by changing arteriolar diameter; PCO2 controls ventilation by changing bronchiolar diameter.

• Example: Increased PCO2 causes bronchioles to dilate; decreased PO2 causes arterioles to constrict.

Additional info: Ventilation-perfusion mismatch can lead to impaired gas exchange and hypoxemia.