Properties of Air and the Atmosphere

Atmospheric Pressure and Its Origin

Atmospheric pressure is the force exerted by the weight of air molecules on a given area at Earth's surface. It is a fundamental concept in respiratory physiology, as it influences the movement of gases into and out of the lungs.

• Pressure (P): Defined as force (F) per unit area (A):

• Atmospheric Pressure (Patm): The pressure exerted by the air in the atmosphere, typically measured in mm Hg or kPa.

• Evangelista Torricelli: Invented the mercury barometer, demonstrating that atmospheric pressure pushes down on mercury, causing it to rise in a tube.

Example: At sea level, atmospheric pressure is approximately 760 mm Hg.

The Gas Laws

Dalton's Law of Partial Pressures

Dalton's Law states that the total pressure of a mixture of gases is the sum of the pressures of each individual gas. Each gas in a mixture exerts its own pressure, called the partial pressure.

• Equation:

• Application: Used to calculate the partial pressures of oxygen, carbon dioxide, and water vapor in inspired and alveolar air.

Example: In dry air at sea level, mm Hg (21% of 760 mm Hg).

Partial Pressures of Gases in Air as a Function of Temperature

The partial pressures of gases in air change with temperature and humidity. Water vapor pressure increases with temperature, reducing the partial pressures of other gases.

Additional info: As humidity increases, water vapor pressure rises, decreasing the partial pressures of other gases.

Ideal Gas Law

The Ideal Gas Law describes the relationship between pressure, volume, temperature, and the number of molecules in a gas. It is fundamental for understanding gas behavior in the lungs.

• Equation:

• P: Pressure (mm Hg or kPa)

• V: Volume (L)

• n: Number of moles

• R: Gas constant

• T: Temperature (K)

• If is constant, then (Boyle's Law)

Example: Increasing the volume of a container decreases the pressure, assuming temperature and number of molecules are constant.

Boyle's Law

Boyle's Law states that at a constant temperature, the pressure of a gas is inversely proportional to its volume. This law is crucial for understanding pulmonary ventilation.

• Equation:

• Changes in lung volume create changes in pressure.

• Gas flows from areas of high pressure to low pressure.

Example: During inspiration, lung volume increases, pressure decreases, and air flows into the lungs.

Henry's Law

Henry's Law states that at a constant temperature, the amount of gas dissolved in a liquid is proportional to its partial pressure in the gas phase.

• Equation:

• C: Concentration of dissolved gas

• Hep: Henry's solubility constant

• Pgas: Partial pressure of the gas

• CO2 is about 20 times more soluble than O2 in blood.

Example: Oxygen and carbon dioxide exchange in the alveoli is governed by their partial pressures and solubility.

Ventilation and Airway Resistance

Bulk Flow of Air and Airway Resistance

Ventilation depends on differences in gas pressure and resistance to flow in the airways. Air flows from regions of higher pressure to lower pressure, and resistance is determined by airway diameter, length, and viscosity.

• Flow Equation:

• Airway resistance increases as airway diameter decreases.

• Branching of airways increases total cross-sectional area and decreases air flow velocity.

Example: Diseases like asthma or chronic bronchitis increase airway resistance, making breathing more difficult.

Lung Volumes and Capacities

Measurement of Lung Volumes

Lung volumes and capacities are measured using a spirometer, which records the amount and rate of air inhaled and exhaled. These measurements are essential for assessing pulmonary function.

• Tidal Volume (TV): Volume of air inhaled or exhaled in a normal breath.

• Expiratory Reserve Volume (ERV): Additional air that can be exhaled after a normal exhalation.

• Residual Volume (RV): Air remaining in the lungs after maximal exhalation; cannot be measured by simple spirometry.

• Functional Residual Capacity (FRC): Volume of air remaining in the lungs at the end of a normal expiration; FRC = ERV + RV.

• Total Lung Capacity (TLC): Maximum volume of air the lungs can hold.

Example: FRC is typically around 2.5 L in adults.

Measurement of Functional Residual Capacity (FRC)

FRC is measured using the Helium Dilution Technique, which involves adding a known quantity of helium to a known volume of gas and measuring the change in concentration after mixing.

• Helium is used because it is not absorbed by the blood.

• Equation for FRC measurement:

• FRC is a useful index of lung function, affected by various physiological and pathological factors.

Example: FRC decreases in restrictive lung diseases and increases in obstructive diseases.

Summary Table: Lung Volumes and Capacities

Key Points to Consider

• Define pressure and origin of atmospheric pressure.

• Major determinants of bulk flow of air: pressure differences and airway resistance.

• Determinants of the Ideal Gas Equation: pressure, volume, temperature, number of molecules.

• If volume is constant, pressure must also be constant (assuming temperature and number of molecules are constant).

• Boyle's Law relates to lung ventilation: changes in volume create changes in pressure, driving airflow.

• Difference between positive and negative pressure ventilation: positive pressure pushes air into lungs; negative pressure draws air in by expanding the chest cavity.

• Basic composition of air: 21% O2, 78% N2, 0.03% CO2, variable water vapor.

• Dalton's Law: total pressure is sum of partial pressures.

• Effect of humidifying air: increases water vapor pressure, decreases partial pressures of other gases.

• Henry's Law: explains gas exchange in the lungs based on solubility and partial pressure.

• Determinants of airway resistance: diameter, length, viscosity, branching.

• Functions of a spirometer: measures lung volumes and capacities.

• FRC: volume at end of normal expiration; measured by helium dilution.