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Gas Exchange &
Partial Pressure Gradients

Understand how oxygen and carbon dioxide move between the lungs and tissues by following the physics of partial pressure and diffusion.

Anatomy & Physiology

What Is Partial Pressure?

In a mixture of gases, each individual gas exerts its own pressure independently of the others. This individual contribution is called the partial pressure of that gas, symbolized as \( P_{\text{gas}} \).

At sea level, atmospheric pressure is approximately 760 mmHg. Oxygen makes up about 21% of air, so: \( P_{O_2} = 0.21 \times 760 \approx 160 \text{ mmHg} \).

Atmospheric Gas Composition

\( N_2 \) Nitrogen
~78.6% of air: \( P_{N_2} \approx 597 \text{ mmHg} \)
\( O_2 \) Oxygen
~20.9% of air: \( P_{O_2} \approx 159 \text{ mmHg} \)
\( CO_2 \) Carbon Dioxide
~0.04% of air: \( P_{CO_2} \approx 0.3 \text{ mmHg} \)

Why Gradients Drive Everything

Gases move by simple diffusion: from regions of higher partial pressure to regions of lower partial pressure. No energy (ATP) is required: the gradient does all the work.

High \( P \)
 Net Diffusion
Low \( P \)
 Molecules move down the pressure gradient

Dalton's Law of Partial Pressures

1
Calculate the fraction: Determine the percentage of the gas in the mixture.
2
Multiply by total pressure: \( P_{\text{gas}} = \text{fraction} \times P_{\text{total}} \).
3
Apply to physiology: Use the partial pressure to predict diffusion direction.

Henry's Law and Gas Solubility

When a gas contacts a liquid, molecules dissolve in the liquid until an equilibrium is reached. Henry's Law states that the amount of gas dissolved is proportional to its partial pressure above the liquid:

Clinical relevance: \( CO_2 \) is about 20 times more soluble in blood plasma than \( O_2 \). This means \( CO_2 \) diffuses efficiently even with a smaller pressure gradient.

External vs. Internal Respiration

External Respiration
Gas exchange at the pulmonary capillaries (lungs). \( O_2 \) moves from alveoli into blood; \( CO_2 \) moves from blood into alveoli.
Internal Respiration
Gas exchange at the systemic capillaries (tissues). \( O_2 \) moves from blood into cells; \( CO_2 \) moves from cells into blood.

External Respiration: The Lungs

Alveolus
\( P_{O_2} = 104 \) mmHg
\( P_{CO_2} = 40 \) mmHg
 Pulmonary Capillary
\( P_{O_2} = 40 \) mmHg
\( P_{CO_2} = 45 \) mmHg
\( O_2 \)
\( CO_2 \)
 After exchange:
\( P_{O_2} \approx 104 \) mmHg

Internal Respiration: The Tissues

Systemic Capillary
\( P_{O_2} = 100 \) mmHg
\( P_{CO_2} = 40 \) mmHg
 Tissue Cell
\( P_{O_2} = 40 \) mmHg
\( P_{CO_2} = 45 \) mmHg
\( O_2 \)
\( CO_2 \)
 After exchange:
\( P_{O_2} \approx 40 \) mmHg

\( O_2 \) Gradient vs. \( CO_2 \) Gradient

\( O_2 \) \( CO_2 \)
Gradient at lungs 104 → 40 mmHg (64 mmHg) 45 → 40 mmHg (5 mmHg)
Gradient at tissues 100 → 40 mmHg (60 mmHg) 45 → 40 mmHg (5 mmHg)
Solubility in plasma Low ~20x higher
Efficient exchange? Needs large gradient Small gradient suffices

Factors Affecting Diffusion Rate

Gradient Magnitude
Steeper \( \Delta P \) = faster diffusion. Exercise increases gradients.
Membrane Thickness
Thinner membranes = faster exchange. Pulmonary edema thickens the membrane and slows gas transfer.
Surface Area
More area = more exchange. Emphysema destroys alveoli and reduces surface area.
Gas Solubility
Higher solubility = easier dissolution into blood. \( CO_2 \) is highly soluble; \( O_2 \) relies on hemoglobin.

How to Predict Gas Movement

1
Identify the two compartments: Which two regions are separated by a membrane (e.g., alveolus vs. capillary)?
2
Look up or calculate \( P_{\text{gas}} \) in each compartment: Use Dalton's Law if needed.
3
Determine the gradient direction: Gas flows from higher \( P \) to lower \( P \).
4
Calculate gradient magnitude: \( \Delta P = P_{\text{high}} - P_{\text{low}} \). Larger = faster diffusion.
5
Consider modifiers: Membrane thickness, surface area, and gas solubility all affect rate.

Example 1: \( O_2 \) at the Lungs

Problem: Alveolar \( P_{O_2} = 104 \) mmHg. Blood arriving at the pulmonary capillary has \( P_{O_2} = 40 \) mmHg. In which direction does \( O_2 \) move, and what is the gradient?

1
Compartments: Alveolus (air side) and pulmonary capillary (blood side).
2
Partial pressures: Alveolus = 104 mmHg, Capillary = 40 mmHg.
3
Direction: \( O_2 \) moves from alveolus → capillary (high to low).
4
Gradient: \( \Delta P = 104 - 40 = \) 64 mmHg.

Example 2: \( CO_2 \) at the Lungs

Problem: Blood entering pulmonary capillaries has \( P_{CO_2} = 45 \) mmHg. Alveolar \( P_{CO_2} = 40 \) mmHg. What is the gradient, and is it sufficient?

1
Partial pressures: Blood = 45 mmHg, Alveolus = 40 mmHg.
2
Direction: \( CO_2 \) moves from blood → alveolus (high to low).
3
Gradient: \( \Delta P = 45 - 40 = \) 5 mmHg. Much smaller than \( O_2 \)'s gradient.

Example 3: High Altitude Challenge

Problem: At 3,000 m elevation, atmospheric pressure drops to ~523 mmHg. What is the new alveolar \( P_{O_2} \), and how does this affect the gradient?

1
Inspired \( P_{O_2} \): \( 0.21 \times 523 \approx 110 \) mmHg (vs. 160 at sea level).
2
Alveolar \( P_{O_2} \): After humidification and \( CO_2 \) mixing, drops to ~67 mmHg (vs. 104 at sea level).
3
New gradient: \( 67 - 40 = \) 27 mmHg (vs. 64 at sea level).

Common Mistakes

✕ Confusing Total and Partial Pressure
Remember: each gas has its own partial pressure. Only the individual \( P_{\text{gas}} \) determines that gas's diffusion, not the total pressure of the mixture.
✕ Ignoring Solubility for \( CO_2 \)
Students often predict \( CO_2 \) won't diffuse well because its gradient is small. High solubility makes the small gradient (5 mmHg) perfectly adequate.
✕ Reversing Gradient Direction
Gases always move from high \( P \) to low \( P \). At the lungs, \( O_2 \) enters blood and \( CO_2 \) leaves. At tissues, the reverse occurs.

The Complete Gas Exchange Circuit

Location \( P_{O_2} \) (mmHg) \( P_{CO_2} \) (mmHg)
Atmosphere (inspired air) ~160 ~0.3
Alveolar air 104 40
Deoxygenated blood (entering lungs) 40 45
Oxygenated blood (leaving lungs) 100 40
Tissue cells \( < 40 \) \( > 45 \)

Question 1 of 5

According to Dalton's Law, if the total atmospheric pressure is 760 mmHg and oxygen makes up 21% of air, what is the approximate partial pressure of \( O_2 \)?

Question 2 of 5

During external respiration, in which direction does \( O_2 \) move?

Question 3 of 5

The \( CO_2 \) gradient at the lungs is only about 5 mmHg, yet \( CO_2 \) is eliminated efficiently. What is the primary reason?

Question 4 of 5

A patient has pulmonary edema (fluid accumulation in the lungs). Which factor of gas exchange is most directly impaired?

Question 5 of 5

At high altitude (3,000 m), the \( O_2 \) diffusion gradient at the lungs drops from ~64 mmHg to ~27 mmHg. Which physiological compensation occurs first?