Chapter 9: Diffusion and Gas Laws

Flashcards covering key vocabulary, definitions, and gas laws from Chapter 9: Diffusion and Gas Laws.

Ventilation

Mechanical movement of air in/out of lungs

Respiration

Gas exchange between alveoli & pulmonary capillaries

Cellular Respiration

Breakdown of glucose inside cells to release energy

Aerobic Respiration

Respiration where O₂ is present; produces CO₂ + H₂O

Anaerobic Respiration

Respiration where no O₂ is present; produces lactic acid

Diffusion

Movement from a high concentration to a low concentration

What is a Concentration Gradient?

Difference in the amount (concentration) of a gas or molecule between two areas.

What is Simple Diffusion?

A type of passive diffusion

What is Facilitated Diffusion?

Diffusion where carrier proteins help transport substances

Osmosis

Diffusion of water across a semipermeable membrane

States of Matter

Solid, Liquid, Gas

Solids

Matter with closely packed molecules, strong bonds, and a fixed shape/volume

Liquids

Matter with loosely packed molecules, weaker bonds, that take the shape of their container

Gases

Matter with very weak bonds, freely moving molecules, no fixed shape/volume, and that is compressible

Kinetic Energy in Gases

Constant random motion of molecules

Effect on molecules from Increased Gas Temperature

Molecules move faster, leading to increased collisions and increased pressure

Effect of Gas Compression

Molecules get closer, leading to increased collisions and increased pressure

Main Gases in Earth’s Atmosphere

N₂: 78%, O₂: 21%, Ar: 0.93%, CO₂: 0.03%

Atmospheric Pressure at Sea Level

1 atm = 760 mmHg (torr)

Partial Pressure Calculation

% of gas × PB (barometric pressure)

Effect of Altitude on PB and Partial Pressures

Both decrease as altitude increases

Pressure Below Sea Level

Increases by 1 atm every 33 ft (10 m) descent

HBOT (Hyperbaric Oxygen Therapy)

Oxygen therapy at greater than 1 atm in a sealed chamber

Henry’s Law (related to HBOT)

Increased PO₂ leads to increased dissolved O₂ in blood, which applies to HBOT

Indications for HBOT

Gas embolism, decompression sickness, CO poisoning, radiation necrosis, diabetic wounds, gangrene, severe anemia

Dalton’s Law

States that total pressure equals the sum of the partial pressures of gases in a mixture

Henry’s Law

States that the amount of gas dissolved in a liquid is proportional to its partial pressure at the surface

Solubility Coefficient

A constant specific to each gas, where increased temperature leads to decreased solubility

Solubility Coefficient of O₂ at 37°C

0.0244 mL/mmHg/mmH₂O

That value tells us how much oxygen will dissolve (diffuse) in a given liquid (like plasma) for every unit of pressure difference across a membrane.
It’s a solubility constant used in Fick’s Law of Diffusion — part of what determines how fast gases move across the alveolar-capillary membrane.

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⚙ Breaking down the units

Unit	Meaning
mL	the volume of gas (oxygen) that dissolves
mmHg	the driving pressure difference or gradient across the membrane
mmH₂O	the membrane thickness (water column used as a physical thickness unit)

So that number literally means:

For every 1 mmHg difference in O₂ pressure across a 1 mmH₂O-thick barrier, 0.0244 mL of O₂ will diffuse per unit area (under standard conditions).

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🧠 Why both mmHg and mmH₂O appear

They measure different parts of the equation:

• mmHg = the pressure gradient (force pushing oxygen across).

• mmH₂O = the thickness of the barrier (distance oxygen must travel).

They are not interchangeable here — one describes how hard the gas is pushed, the other how far it has to go.

It’s like saying:

“Oxygen moves 0.0244 mL for every unit of push (mmHg) across each millimeter-of-water-thick wall.”

Solubility Coefficient of CO₂ at 37°C

0.592 mL/mmHg/mmH₂O

Graham’s Law

States that the rate of diffusion is proportional to solubility and inversely proportional to the square root of molecular weight

Respiratory Exchange Ratio (R)

RER is the measured gas exchange ratio at the lungs (alveolar diffusion level).

Calculated as CO₂ out / O₂ in, typically 200/250 = 0.8

Fick’s Law

States that diffusion rate is proportional to surface area, gradient, and diffusion constant, and inversely proportional to thickness

Boyle’s Law

States that pressure is inversely proportional to volume (P ∝ 1/V) if temperature is constant

Boyle’s Law (Inspiration Example)

Chest expands, pressure decreases, and air moves into the lungs

Boyle’s Law (Expiration Example)

Chest contracts, pressure increases, and air moves out of the lungs

Charles’s Law

States that volume is proportional to absolute temperature (V ∝ T) if pressure is constant

Gay-Lussac’s Law

States that pressure is proportional to temperature (P ∝ T) if volume is constant

Pressure Gradient

Refers to the bulk gas movement (ventilation)

Diffusion Gradient

Refers to the independent movement of individual gas molecules

Pulmonary Gas Diffusion Direction for O₂

From alveoli to capillaries

Pulmonary Gas Diffusion Direction for CO₂

From capillaries to alveoli

PiO₂ of Room Air (dry)

PiO2: partial pressure of inspired oxygen (PiO₂) represents the amount of oxygen available in the air you breathe before it reaches the alveoli.

159 mmHg

PiO₂ after Humidification

149 mmHg (after subtracting 47 mmHg for water vapor)

Normal Alveolar PaO₂

~100 mmHg

Alveolar Gas Equation

PAO₂ = (PB – PH₂O)FiO₂ – (PaCO₂ / RQ)

Average RQ Value

~0.8

Alveolar–Arterial (A–a) Gradient

The difference between alveolar partial pressure of O₂ (PAO₂) and arterial partial pressure of O₂ (PaO₂)

Normal Diffusion Time Across A–C Membrane

0.25 seconds (compared to a blood transit time of 0.75 seconds)

Effect of Exercise on Diffusion

Blood velocity increases, but healthy lungs still achieve equilibrium

Interstitial Lung Disease (ILD) Impact on Diffusion

Causes a thickened alveolar-capillary (A–C) membrane, impairing diffusion

Emphysema Impact on Diffusion

Causes a decreased surface area, impairing diffusion

Perfusion-Limited Gas Exchange

Limited by blood flow (e.g., as measured by an NO test)

Diffusion-Limited Gas Exchange

Limited by membrane transfer (e.g., as measured by a CO test)

Gas Concentration & Temperature

“When gas temperature increases, kinetic energy increases and molecules move faster, spreading farther apart.”

Explanation:

• Temperature affects the kinetic energy of gas molecules.

• Higher temperature = molecules move faster and farther apart, so gas becomes less dense (lower concentration per volume).

• Lower temperature = molecules slow down and pack closer together, so gas becomes denser (higher concentration per volume).

In respiratory context:

• Gas delivered to a patient is often warmed and humidified to reach BTPS conditions (Body Temperature, Pressure, Saturated = 37°C, 760 mmHg, 100% humidity).

• Warm gases expand slightly — this affects how we measure and standardize lung volumes.

mmH2O

Definition:

• mmH₂O = millimeters of water column

• It measures how much pressure is needed to move or support a column of water a certain height in millimeters.

• It’s a very small unit of pressure, much smaller than mmHg (millimeters of mercury).

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From your presentation:

“Pressures in the respiratory system are often measured in cmH₂O or mmH₂O because they are much smaller than atmospheric or barometric pressures.”

PaO2 vs SpO2

PaO₂ vs. SpO₂ — the key difference

Feature	PaO₂	SpO₂
Full term	Partial pressure of oxygen in arterial blood	Peripheral capillary oxygen saturation
What it measures	The amount of O₂ dissolved in the plasma (measured in mmHg)	The percentage of hemoglobin saturated with oxygen (%)
How it’s measured	Arterial blood gas (ABG) test — drawn from an artery	Pulse oximeter (non-invasive, finger clip or sensor)
Units	mmHg	%
Normal value	~80–100 mmHg	~95–100%
What it reflects	Oxygen pressure driving diffusion into tissues	Oxygen binding on hemoglobin
Relationship	SpO₂ is estimated from PaO₂ using the Oxyhemoglobin Dissociation Curve
Example	PaO₂ = 90 mmHg → SpO₂ ≈ 97%	SpO₂ = 97% → PaO₂ is probably ~90 mmHg

Respiratory Quotient R/Q

Respiratory Quotient (RQ): The ratio of CO₂ produced to O₂ consumed during metabolism.
Typical value: 0.8 (from 200 mL CO₂ ÷ 250 mL O₂).

Calculation includes:

• VCO₂ = volume of CO₂ exhaled (out)

• VO₂ = volume of O₂ consumed (in)

Typical Values

At rest, in a normal adult on a mixed diet:

• About 250 mL of O₂ is consumed per minute.

• About 200 mL of CO₂ is produced per minute.

Why It Matters

• RQ reflects the metabolic fuel being used:

Fuel Source	CO₂ Produced / O₂ Used	RQ Value
Carbohydrates	Equal amounts	1.0
Fats	Less CO₂ per O₂	0.7
Proteins	Intermediate	0.8

• So when RQ = 0.8, it means the body is burning a mixed diet (normal metabolism).

In Respiratory Physiology:

• The RQ is used in the alveolar gas equation to estimate alveolar oxygen:

If RQ decreases (e.g., on a high-fat diet), PAO₂ will slightly decrease for the same CO₂ level.

Absolute and Relative Humidity

• Absolute Humidity
  The actual mass of water vapor in a given volume of gas (mg H₂O/L). At body temperature (37°C): 44 mg/L = 47 mmHg PH₂O

• Relative humidity = the ratio of actual water vapor in air to the maximum possible at that temperature.