Gas Exchange: Physical Principles

Physical Principles of Gas Exchange

• Pressure is proportional to the concentration of gas molecules. The sum of all gas particles' force of impact in the airways and alveoli equals the amount of pressure.
• Diffusion occurs in response to concentration gradients, with particles moving from high to low concentration areas.
• Diffusion in response to a pressure gradient happens when a gas's diffusion rate is directly proportional to the pressure it creates, also known as partial pressure.
• Room air consists of 79% Nitrogen and 21% Oxygen.
• At sea level, total pressure is 760 mmHg, barometric pressure is the sum of the atmospheric gases' partial pressures.
• Partial pressure calculation: Partial Pressure ∝ Patm × fraction of gas, where Patm equals 760 mm Hg.
• Total pressure is the sum of each gas's partial pressure, for example O₂, N₂, CO₂, and H₂O.
• Gas diffusion across a membrane relies on the partial pressure gradient across it.

Partial Pressure of Gas Dissolved in Fluid

• Partial Pressure is equal to the Concentration of dissolved gas divided by the Solubility Coefficient
• Hydrophilic molecules dissolve more without increasing solution pressure.
• Hydrophobic molecules develop higher partial pressure at lower concentrations.
• The solubility coefficient depends on a molecule's hydrophilic or hydrophobic properties.
• CO₂ is much more soluble than oxygen.
• The higher the partial pressure, the more fat a substance can dissolve

CO₂ Solubility

• CO₂ is 20 times more soluble than O₂.
• A partial pressure of CO₂ is 1/20th of O₂'s partial pressure at the same concentration due to high solubility.
• Net diffusion depends on each gas's partial pressure.
• In order for O₂, diffusion into blood, and CO₂, diffusion into the alveoli is favored
• Air humidification results in a 47 mm Hg vapor pressure.
• Vapor pressure is exerted by water molecules escaping into gas phase.
• Higher temperature increases the kinetic activity of water molecules and increases vapor pressure, while colder temperatures lower vapor pressure.
• Solubility denotes that CO₂ dissolves more easily in liquids like blood than O₂.
• Consequently, at the same concentration, CO₂ exerts a much lower partial pressure than O₂ because it dissolves more readily.

Impact of Net Diffusion of Gases

• Gases diffuse from areas of higher to lower partial pressure.
• O₂ has higher pressure in the alveoli than in the blood, so it diffuses into the blood.
• CO₂ has higher pressure in the blood than in the alveoli, so it diffuses into the alveoli to be exhaled.

Vapor Pressure

• Air is humidified upon breathing, adding water vapor and adding water vapor pressure (47 mm Hg) to the total pressure in the lungs.
• Higher temperature imparts more energy, making escape to gas phase more likely, hence higher vapor pressure.
• Lower temperature equates to less energy, less escape, and lower vapor pressure.

Fick's Law of Diffusion

• Fick’s Law of Diffusion describes how gases diffuse across membranes.
• Diffusion = (P₁-P₂) * Area * Solubility / (Distance * √MW)
• In accordance with Fick's Law, diffusion rate increases with greater pressure gradient, area, and solubility, but decreases with greater distance and molecular weight.

Key Points of Fick's Law

• Pressure gradient drives diffusion. The higher the number of molecules moving to an area of less pressure.
• Cross-sectional area refers to the fluid, so more area equates to more space for diffusion, and faster diffusion.
• The shorter the distances, the quicker diffusion.
• Solubility and molecular weight are fixed, as temperature affects diffusion.
• Temperature is constant in the body so is less important

Diffusion Factors

• The average thickness of the respiratory membrane is 0.6 micrometers
• The respiratory membrane has a large surface area (about 70 sq meters in a healthy man)
• Lung capillaries contain approximately 60 to 140 ml of blood.
• Each capillary is roughly 5 micrometers in diameter.

Factors Affecting Diffusion

• Thickness of the respiratory membrane may increase with edema or pulmonary fibrosis.
• The surface area of the respiratory membrane will decrease in diseases like emphysema, which diminishes gas exchange.
• Diffusion coefficient depends on gases solubility and the square root of its molecular weight.
• Pressure difference across the respiratory membrane.

Diffusion Capacity

• Diffusing capacity is defined as mL of gas diffusing each minute for a pressure difference of 1 mm Hg
• Diffusing capacity for O₂ in a normal healthy male is 21 ml/min per mmHG.
• Normal pressure differential is 11 mm Hg during quiet breathing, but changes with exercise.
• Increase in metabolic rate, will change the pressure gradient by increasing CO₂.
• Diffusing capacity measures alveolar membrane and gas transfer.
• D₁ = Area * diffusion coefficient/thickness
• Diffusion = D₁ * pressure gradient
• DL is diffusion capacity, where thickness equals distance for gas travel.
• The normal diffusing capacity in a healthy male is approximately 21 mL/min/mmHg which mean 21 mL of O₂ is able to diffuse across the lung per minute for every mmHg of pressure difference.
• Exercise can change the diffusion capacity via increased pressure gradient and CO₂ production from metabolism which enhances rate of diffusion.

Alveolar Air Composition

• Alveolar air dilution is slow, even in healthy adults.
• In healthy adults with an FRC of 2300 mL, only 350ml of new air comes in with each breath.
• The air that is inspired goes thru the exhale stage, helping to ensure that inspired+expired airs composition stays the same.

Changes in Alveolar Gas Composition with Ventilation

• Increase in the ventilation rate will then increase the rate of the dilution and the vice versa is also correct..
• A slow rate of change is important for the prevention of fast concentration alterations for gases and stabilizes both O₂ and CO₂. The stabilization of O₂ and CO₂ is helpful for levels of the pH if the respiration is stopped.
• Under the most normal possible conditions, a percentage of about one half of the gas occurs around seventeen seconds.
• Faster breathing (higher ventilation) leads to quicker replacement of “old” gas in the alveoli with “new” air.
• Faster ventilation will lead to faster gas dilution.
• Replacements happen quickly in the lungs, the more you breathe
• Change that has a slower rate is better protection
• A slower rate of change will also prevent any drastic swings in one levels of O2 as well as CO2 and will help also to maintain balance in the pH in the blood ( CO2 is known to affect the pH) Stabilizing is important
• If breathing briefly stops (like during a short apnea) then stable alveolar gasses help to prevent sudden drops in CO2 in the body.
• There are roughly 17 seconds where Half of alveolar gas is exchanged ( if you are under normal breathing).
• Shows the average turnover of gases at rest, meaning maintain a balance while there is normal control.

Main points regarding gas exchange

• An important determinant is that proper ventilation will then be able to affect the gases when it comes to the concentration.
• Faster gas exchange means there is higher ventilation.
• An important note is the protection factor which is when changes will protect you from shifting levels in either gas and pH/blood situations. *Regulate during stress, sleep, or disease, or more breathing factors.

Metabolic Rates and Gas Exchange

• Alveolar ventilation affects partial pressure of oxygen (PAO2).
• The graph shows this at different metabolic rates by the following-
  • Y-axis- Partial pressure of Alveolar oxygen (PAO2) in mmHg
• x axis- Alveolar ventilation ( the speed where the alveoli becomes minute.)
• Red curve means 250 ML and represent a lower metabolic rate
• Blue Curve indicates 1000 ML and represents the rate of metabolic activity which is faster
• A green circle/line will show the operation point which shows if the average minute is above 5 minutes per minute.

Key Concept:

• PAO2 has control, by main factors
  • O2 absorption rate
• Is how blood pulls from the alveoli , known to depend on metabolism,
  • O2 ventilation
• Has control since blood brings to the alveoali and through blood
• Meta rate- low metab with normal PAO2 at normal or stable rate. High metab keeps normal but steadu -Y axis for gas graph is 2. CO2(PACO2),in Hbg -X axis - Ventilation (L/min)
• Decreases -increases PACO2
• A smaller scale for gas means smaller amounts or smaller rate
• And bigger scale for gas equal bigger output

Increasing / Decreasing PACO2

• Decreasing ventilation will increase PAC02 because the body wont push the gases from blood vessels to the alveoli.
• This lowers the amount, leading to alkalosis, because the PH is altered.
• Breathing less requires lower volumes
• Build up causes PAC02.
• Can alter because of low levels/ acid, this alters the levels with ventilation.
• Summarizing-More equal is less c02
• Less equal is more C02