Gas Diffusion

Flashcards based on a lecture transcript regarding pulmonary gas exchange, including gas laws, the alveolar air equation, and clinical applications of diffusion.

Respiration

Gas exchange that occurs at the level of the alveoli where pulmonary capillaries come in contact.

Ventilation

The bulk flow movement of gas into and out of the lungs.

Diffusion

The movement of gas molecules from an area of high concentration to an area of low concentration.

Diffusion Gradients

Individual gas partial pressures that drive the movement of specific gases.

Nitrogen ($N_2$) percentage

The most abundant gas in the atmosphere, making up $78\%$ of the air.

Oxygen ($O_2$) percentage

The gas making up $21\%$ of the atmospheric gases breathed in.

Atmospheric Carbon Dioxide ($CO_2$) percentage

Makes up a minute portion of the final $1\%$ of atmospheric gas components.

Barometric Pressure at sea level ($P_B$)

$$760\,mmHg$$

Torr equivalence

One millimeter of mercury ($1\,mmHg$) is equivalent to $1\,torr$.

Dalton's Law

States that each gas exerts a partial pressure proportional to its fractional concentration in the air.

Dalton's Law formula

$$P_{gas} = F_{gas} \times P_B$$

Partial Pressure of Oxygen at sea level ($P_{O2}$)

$$159.6\,mmHg$$

Partial Pressure of Nitrogen at sea level ($P_{N2}$)

$$592.8\,mmHg$$

Gravitational Pull effect on Barometric Pressure

Further from Earth's surface, gas molecules are less tightly packed, causing barometric pressure to decrease as altitude increases.

Barometric Pressure at the summit of Mount Everest

Approximately $250\,mmHg$

Atmospheric $P_{O2}$ on Mount Everest

Approximately $43\,mmHg$

Boiling Point Altitude

At $65,000\,feet$, barometric pressure falls below water vapor pressure, and tissue begins to boil or vaporize.

One Atmosphere ($1\,ATM$)

Equivalent to $760\,mmHg$ at sea level.

Underwater Pressure Gradient

For every $33\,feet$ of descent, pressure increases by one additional atmosphere ($760\,mmHg$).

Pressure at $33\,feet$ depth

$1,520\,mmHg$ (or $2\,ATM$)

Pressure at $66\,feet$ depth

$2,280\,mmHg$ (or $3\,ATM$)

Pressure Gradient

The difference in pressure that describes bulk flow, where all gases move together in the same direction.

Gas Diffusion

The movement of individual gas molecules from an area of higher pressure to an area of lower pressure until equilibrium forms.

Humidity at Body Temperature

The body warms air to $37^\circ C$ with $100\%$ relative humidity.

Water Vapor Pressure ($P_{H2O}$) at $37^\circ C$

$$47\,mmHg$$

Absolute Humidity at normal body temperature

$$44\,mg/L$$

Dry Air $P_{O2}$

$$159\,mmHg$$

Alveolar $P_{O2}$ ($P_A O_2$)

Approximately $100\,mmHg$

Arterial Blood $P_{O2}$ ($P_a O_2$)

Approximately $95$ to $100\,mmHg$

Venous Blood $P_{O2}$ ($P_v O_2$)

Approximately $40\,mmHg$

Venous Blood $P_{CO2}$ ($P_v CO_2$)

Approximately $46\,mmHg$

Alveolar Air Equation

$$P_A O_2 = [ (P_B - P_{H2O}) \times F_I O_2 ] - (P_a CO_2 \times 1.25)$$

$F_I O_2$ of Room Air

$21\%$ or $0.21$

Normal $P_a CO_2$ range

$35$ to $45\,mmHg$

Respiratory Exchange Ratio (RER)

The ratio of alveolar $CO_2$ excretion ($250\,mL/min$) to blood oxygen uptake ($200\,mL/min$).

Normal RER value

$$0.8$$

Normal Anatomical Shunt

Small amount of venous blood that mixes with reoxygenated blood, causing $P_a O_2$ to be slightly lower than $P_A O_2$.

Pulmonary Shunt

Blood flow without ventilation, such as when blood bypasses under-ventilated or collapsed alveoli.

Atelectasis

The collapse of alveoli leading to reduced surface area for gas exchange.

ACM Total Path Length

$0.2$ to $2.5\,\text{microns}$

Number of ACM layers

9 layers

First layer of the ACM

The surfactant/fluid layer.

Second layer of the ACM

The alveolar epithelium.

Third layer of the ACM

The alveolar basement membrane.

Fourth layer of the ACM

The interstitial space.

Fifth layer of the ACM

The capillary basement membrane.

Sixth layer of the ACM

The capillary endothelium.

Seventh layer of the ACM

The plasma.

Eighth layer of the ACM

The red blood cell (erythrocyte) membrane.

Ninth layer of the ACM

The intracellular erythrocyte fluid.

Normal Transit Time

The time it takes a red blood cell to pass through the pulmonary capillary bed, which is $0.75\,seconds$.

Normal Diffusion Time

The time required for oxygen and carbon dioxide to reach equilibrium, which is $0.25\,seconds$.

Transit Time Cushion

In healthy individuals, only one-third of the available transit time ($0.25\,s$ of $0.75\,s$) is needed for equilibrium.

Transit Time during exercise

Increases in cardiac output can decrease transit time to as low as $0.25\,seconds$.

Fick's Law of Diffusion

States that gas transfer across a tissue sheet is directly proportional to surface area, diffusion constants, and partial pressure difference, and inversely proportional to thickness.

Fick's Law Formula

$$Diffusion \propto \frac{Area \times D \times \Delta P}{Thickness}$$

Henry's Law

States the amount of gas that dissolves in a liquid at a given temperature is proportional to the partial pressure of the gas.

$CO_2$ Solubility comparison

Carbon dioxide is $24$ times more soluble than oxygen.

Graham's Law

States the rate of diffusion through a liquid is directly proportional to the solubility coefficient and inversely proportional to the square root of the gram molecular weight.

$CO_2$ Diffusion Rate comparison

Carbon dioxide diffuses $20$ times faster than oxygen through the alveolar capillary membrane.

ACM $CO_2$ Limitation

The alveolar capillary membrane never limits the outward diffusion of $CO_2$ due to its high solubility and diffusion rate.

Hyperbaric Oxygen Therapy

Oxygen therapy delivered at pressures greater than one atmosphere ($1\,ATM$) to widen the pressure gradient.

Angiogenesis

The development of micro blood vessels stimulated by hyperbaric therapy for tissue repair.

Oxygen Toxicity threshold

Risk develops after breathing concentrations greater than $50\%\,F_I O_2$ for long periods.

Absorption Atelectasis

Alveolar collapse caused by washing out nitrogen with high concentrations of oxygen.

Central Nervous System Oxygen Toxicity effects

Tremors, convulsions, and comatose states.

Classic sign of Pulmonary Edema

Productive cough with frothy white pink secretions.

CPAP

Continuous Positive Airway Pressure used to treat pulmonary edema by driving oxygen across the ACM.

Perfusion Limited Gas Flow

Gas transfer as a function of the amount of blood flowing past the alveoli (e.g., Nitrous Oxide).

Diffusion Limited Gas Flow

Gas transfer as a function of the integrity of the alveolar capillary membrane (e.g., Carbon Monoxide).

Oxygen Flow Classification

Normally perfusion limited but can become diffusion limited with disease.

Emphysema impact on Diffusion

Reduces surface area due to tissue destruction (bullae).

Pulmonary Fibrosis impact on Diffusion

Increases the thickness of the alveolar capillary membrane.

Consolidation in Pneumonia

Increases the distance/thickness gas must travel across the ACM.

DLCO

Diffusion Capacity of the Lung for Carbon Monoxide, a test used to measure the integrity of the ACM.