Mechanisms of Pulmonary Ventilation and Atmospheric Pressure

Concepts of Ventilation Versus Respiration

• Ventilation is defined as the movement of gas, which can be thought of as inhaling oxygen ($O_2$) to replenish the body and exhaling carbon dioxide ($CO_2$) to remove it.

• Respiration is distinct from ventilation and refers to the actual gas exchange occurring within the body. There are two primary types of respiration:

  • External Respiration: The gas exchange occurring between the alveoli in the lungs and the surrounding capillaries.

  • Internal Respiration: The gas exchange occurring between the capillaries and the body tissues.

• Relationships between movement and exchange: The specific pattern, depth, and rate of ventilation (how quickly and how much gas is taken in, and how long it remains) directly affect the efficiency of both internal and external respiration.

• Goals for Future Respiratory Therapists (RRTs): Understanding mechanisms of pulmonary ventilation requires a solid grasp of atmospheric gases, pressure units, the elastic properties of the lung and chest wall, and the dynamic characteristics of moving gas.

Composition of Atmospheric Air

• Room Air: The air in the current atmosphere consists specifically of $20.95\%$ oxygen ($O_2$). In clinical practice, this is standardly referred to as $21\%$. This level is sufficient for normal bodily function.

• Supplemental Oxygen: When patients experience lung trauma or blood-oxygen transport issues, respiratory therapists provide supplemental oxygen via nasal cannula or endotracheal tube. This supplementation involves delivery of oxygen levels exceeding the $21\%$ found in room air.

• Nitrogen ($N_2$): Nitrogen makes up the vast majority of the air we breathe, comprising approximately $78\%$ of the atmosphere.

• Argon: This gas comprises $0.9\%$ of the atmospheric composition.

• Trace Gases: Various other gases are present in the atmosphere at concentrations less than $1\%$. These include:

  • Carbon Dioxide ($CO_2$): Referenced in various sources as approximately $0.02\%$ to $0.04\%$.

  • Neon ($Ne$)

  • Helium ($He$)

  • Methane ($CH_4$)

  • Krypton ($Kr$)

  • Nitric Oxide ($NO$)

Atmospheric Layers and the Gravitational Force

• Stratification: Atmospheric gases are divided into distinct layers based on temperature gradients. These layers are held in place around the Earth due to the gravitational pull.

• Atmospheric Pressure: Defined as the weight of gas molecules exerting a force on the Earth and everything on it. Even when not explicitly perceived, the gases above an individual are constantly exerting pressure.

Altitudes and High-Elevation Physiology

• Sea Level versus Elevation: Locations such as Shreveport, Louisiana, or New Orleans are considered to be at sea level. High-altitude locations include cities like Denver (Colorado), regions of Montana, and mountain ranges like those in Lima, Peru or Mount Kilimanjaro.

• The Concept of "Thin Air":

  • At high altitudes, the percentage of oxygen remains constant at $21\%$.

  • However, the air is called "thin" because there is less atmospheric pressure exerting force on those oxygen molecules. There is a smaller "column" of pressure above the individual at higher elevations.

• Physiological Impact of Altitude:

  • Breathing becomes difficult because there is less pressure pushing oxygen across the alveolar-capillary membrane and into the blood.

  • High altitudes result in less force behind the oxygen, which can lead to altitude sickness or a requirement for supplemental oxygen, even in individuals with healthy lungs.

Dalton’s Law of Partial Pressures

• Verbatim Definition: Dalton’s Law states that each gas in a mixture exerts a partial pressure proportional to its fractional concentration in a vacuum or air.

• Total Atmospheric Pressure: At sea level, the total pressure of all atmospheric gases is $760\,mmHg$ (millimeters of mercury). This value can also be expressed as $760\,torr$, where $1\,torr = 1\,mmHg$.

• Calculating Partial Pressure: To find the pressure exerted by a specific gas, convert its percentage to a decimal (fraction) and multiply by the total barometric pressure ($P_B$):

  • $P_{gas} = F_{gas} \times P_B$

• Pressure of Nitrogen at Sea Level:

  • $760\,mmHg \times 0.78 = 592.8\,mmHg$ (approximately $593\,mmHg$).

• Pressure of Oxygen at Sea Level:

  • $760\,mmHg \times 0.21 = 159.6\,mmHg$ (approximately $160\,mmHg$).

• Calculation Example (Mount Kilimanjaro):

  • Assuming a barometric pressure ($P_B$) of $345\,torr$:

  • $P_{O_2} = 345 \times 0.21 = 72.45\,torr$.

  • This pressure ($72.45\,torr$) is less than half the oxygen pressure available at sea level, illustrating why gas transfer to the blood is so difficult at high elevations.

Measuring Units and Conversions

• Systems of Measurement:

  • SI Units (International System): Primarily uses kilopascals ($kPa$).

  • Clinical Standards: Respiratory therapists primarily use millimeters of mercury ($mmHg$) and centimeters of water pressure ($cmH_2O$).

• Conversion Constants:

  • $1\,mmHg = 1.36\,cmH_2O$

  • $1\,cmH_2O = 0.74\,mmHg$

• Conversion Examples:

  • Converting $5\,mmHg$ to $cmH_2O$:

    • $5 \times 1.36 = 6.8\,cmH_2O$

  • Converting $5\,cmH_2O$ to $mmHg$:

    • $5 \times 0.74 = 3.7\,mmHg$

Baseline Pulmonary Pressures

• The Zero Baseline: Because we live under constant atmospheric pressure ($760\,mmHg$), we treat this baseline as "zero" when measuring pulmonary pressures. Any pressure we apply or measure is relative to this baseline.

• Positive Pressure (CPAP Example):

  • If a patient is on CPAP with a pressure of $10\,torr$ ($mmHg$), the actual pressure in the lungs is the baseline atmospheric pressure plus the applied pressure.

  • Calculation: $760\,torr + 10\,torr = 770\,torr$.

• Negative Pressure (Suction Example):

  • If a negative pressure of $-8\,mmHg$ is applied to the trachea, the actual pressure inside that space is below atmospheric.

  • Calculation: $760\,mmHg - 8\,mmHg = 752\,mmHg$.

Pressure Gradients and Gas Flow

• Fundamental Rule of Gas Movement: Gas acts like a liquid; it always moves from areas of higher pressure to areas of lower pressure. This is referred to as moving "down" a pressure gradient.

• Pressure Gradient Definition: A pressure gradient is a difference in pressure between two points that allows gas to flow.

• The Primary Principle of Respiration: For air to move, a pressure gradient must be established between the atmosphere (barometric pressure) and the lungs (alveolar pressure).

  • Inspiration Gradient: Alveolar pressure must be lower than barometric pressure.

  • Expiration Gradient: Alveolar pressure must be higher than barometric pressure.

Boyle’s Law

• Verbatim Definition: Boyle’s Law states that a volume of gas varies inversely proportional to its pressure at a constant temperature.

• Mathematical Representation:

  • $P_1 \times V_1 = P_2 \times V_2$

• Mechanics in a Closed System:

  • The lungs can be viewed as a system where temperature remains relatively constant (standardized at $37$ degrees Celsius with $100\%$ relative humidity).

  • If the volume of a container decreases, the pressure of the gas within that container builds up (increases).

  • Conversely, if volume increases, the pressure decreases. This relationship is what allows the creation of the pressure gradients necessary for ventilation.

Questions & Discussion

• Question: What is the gas exchange between alveoli and capillaries called?

• Answer: External respiration.

• Question: What is the gas exchange between capillaries and body tissues called?

• Answer: Internal respiration.

• Question: What percentage of oxygen is in room air?

• Answer: $21\%$ (specifically $20.95\%$).

• Question: What is the most abundant gas in the atmosphere?

• Answer: Nitrogen ($78\%$).

• Question: How does humidity affect breathing?

• Answer: Humidity (water vapor) makes the air feel "heavier" or "thicker," but the oxygen percentage remains standard. In the lungs, water vapor pressure must be accounted for in calculations like the Alveolar Air Equation because the body humidifies air to $100\%$.

• Question: Why is it hard to breathe at high altitudes?

• Answer: There is less pressure pushing the oxygen. It is "not pressured in" with enough force to cross the alveolar-capillary membrane, even though the oxygen percentage is still $21\%$.

• Discussion on Office Hours: The instructor will hold online office hours via Zoom on Thursday at $4:00\,PM$ to answer student questions. Students should come prepared with specific questions rather than asking generally what will be on the test.