Diffusion of Gases — Revision Notes

⚡ 30-Second Revision

Definition: — Passive movement of gases down their partial pressure gradient.
Driving Force: — Partial Pressure Gradient ($Delta P$).
Fick's Law: — $V_{gas} propto \frac{A \times D \times \Delta P}{T}$

- $A$: Surface Area (Directly proportional) - $D$: Diffusion Constant ($D \propto S / \sqrt{MW}$) (Directly proportional) - $Delta P$: Partial Pressure Gradient (Directly proportional) - $T$: Membrane Thickness (Inversely proportional)

Key Values (approx. mmHg):

- Atmospheric $PO_2$: 159, $PCO_2$: 0.3 - Alveolar $PO_2$: 104, $PCO_2$: 40 - Arterial Blood $PO_2$: 95, $PCO_2$: 40 - Venous Blood $PO_2$: 40, $PCO_2$: 45 - Tissue $PO_2$: 40, $PCO_2$: 45

$CO_2$ vs. $O_2$ Diffusion: — $CO_2$ diffuses 20-25 times faster than $O_2$ due to much higher solubility, despite higher molecular weight.

2-Minute Revision

Diffusion of gases is a passive process where gas molecules move from an area of higher partial pressure to an area of lower partial pressure. This partial pressure gradient is the sole driving force.

In the lungs, oxygen moves from the alveoli (high $PO_2$) into the blood (low $PO_2$), while carbon dioxide moves from the blood (high $PCO_2$) into the alveoli (low $PCO_2$). The reverse occurs at the tissue level.

The rate of this diffusion is governed by Fick's Law, which states it's directly proportional to the surface area, the diffusion constant (influenced by solubility and molecular weight), and the partial pressure gradient, but inversely proportional to the membrane thickness.

A crucial point for NEET is that carbon dioxide diffuses significantly faster than oxygen, primarily because of its much higher solubility in biological fluids, which outweighs its slightly greater molecular weight.

Conditions like emphysema (reduced surface area) or pulmonary fibrosis (increased thickness) impair this vital process.

5-Minute Revision

Gas diffusion is the cornerstone of respiration, facilitating the exchange of $O_2$ and $CO_2$ across respiratory membranes. It's a passive process, meaning no cellular energy is expended, driven entirely by the kinetic energy of gas molecules and the resulting partial pressure gradients.

Each gas in a mixture exerts its own partial pressure, and it's the difference in this specific pressure across a membrane that dictates its movement. For instance, in the lungs, alveolar $PO_2$ (approx.

$104,\text{mmHg}$) is higher than in deoxygenated blood ($40,\text{mmHg}$), creating a $64,\text{mmHg}$ gradient that pushes $O_2$ into the blood. Conversely, blood $PCO_2$ ($45,\text{mmHg}$) is higher than alveolar $PCO_2$ ($40,\text{mmHg}$), driving $CO_2$ out.

Fick's Law of Diffusion quantifies this rate: $V_{gas} propto \frac{A \times D \times \Delta P}{T}$. This means a larger surface area (A), a steeper partial pressure gradient ($Delta P$), and a higher diffusion constant (D, which depends on solubility and molecular weight) all increase diffusion. Conversely, a thicker membrane (T) reduces it. The respiratory membrane is optimized for this with a vast surface area (70-100 $m^2$) and extreme thinness (0.2-0.5 $mu m$).

A key NEET concept is the comparison of $O_2$ and $CO_2$ diffusion. Despite $CO_2$ having a higher molecular weight than $O_2$, it diffuses 20-25 times faster. This is because $CO_2$ is significantly more soluble (20-25 times) in water and plasma than $O_2$.

This higher solubility dramatically increases its diffusion constant, overriding the molecular weight effect. Understanding these factors and the typical partial pressure values at different physiological locations is essential for solving both conceptual and application-based questions in NEET.

Prelims Revision Notes

Diffusion of Gases: NEET Quick Facts

1. Definition & Driving Force:

Diffusion: — Passive movement of gas molecules from higher partial pressure to lower partial pressure.
Driving Force: — Partial pressure gradient ($Delta P$). No ATP required.

2. Fick's Law of Diffusion (Biological Context):

Rate of diffusion ($V_{gas}$) is:

* Directly proportional to: * Surface Area ($A$) of the respiratory membrane (e.g., alveolar surface). * Diffusion Constant ($D$) of the gas (depends on solubility and molecular weight). * Partial Pressure Gradient ($Delta P$) of the gas across the membrane. * Inversely proportional to: * Thickness ($T$) of the respiratory membrane.

Formula: $V_{gas} \propto \frac{A \times D \times \Delta P}{T}$

3. Factors Affecting Diffusion Constant ($D$):

$D \propto \frac{Solubility}{\sqrt{Molecular Weight}}$
Solubility: — Higher solubility = faster diffusion. $CO_2$ is 20-25 times more soluble than $O_2$ in water/plasma.
Molecular Weight: — Lower molecular weight = faster diffusion (Graham's Law). $O_2$ ($MW approx 32$) is lighter than $CO_2$ ($MW approx 44$).

4. Relative Diffusion of $O_2$ and $CO_2$:

$CO_2$ diffuses 20-25 times faster than $O_2$ — across the alveolar membrane.
Reason: — The much higher solubility of $CO_2$ (20-25x) in biological fluids outweighs its slightly higher molecular weight, making its diffusion constant significantly larger.

5. Partial Pressure Values (Approximate, in mmHg):

Location	$PO_2$	$PCO_2$
Atmospheric Air	159	0.3
Alveolar Air	104	40
Oxygenated Blood	95	40
Deoxygenated Blood	40	45
Tissues	40	45

6. Gas Exchange Sites & Gradients:

Alveoli to Blood:

* $O_2$: $104 \to 40$ (Gradient $64,\text{mmHg}$) - $O_2$ moves into blood. * $CO_2$: $45 \to 40$ (Gradient $5,\text{mmHg}$) - $CO_2$ moves into alveoli.

Blood to Tissues:

* $O_2$: $95 \to 40$ (Gradient $55,\text{mmHg}$) - $O_2$ moves into tissues. * $CO_2$: $45 \to 40$ (Gradient $5,\text{mmHg}$) - $CO_2$ moves into blood.

7. Physiological Adaptations for Efficient Diffusion:

Large Surface Area: — Millions of alveoli (70-100 $m^2$).
Thin Membrane: — Alveolar-capillary membrane (0.2-0.5 $mu m$).
Rich Blood Supply: — Ensures steep partial pressure gradients are maintained.

8. Clinical Relevance (Impact on Diffusion):

Emphysema: — Decreases surface area ($A$) $o$ reduced diffusion.
Pulmonary Fibrosis/Edema: — Increases membrane thickness ($T$) $o$ reduced diffusion.
High Altitude: — Decreases atmospheric $PO_2$ $o$ reduced $Delta P$ for $O_2$ $o$ reduced diffusion.

Vyyuha Quick Recall

To remember the factors that *increase* the rate of gas diffusion (Fick's Law), think of 'STAGS':

Solubility (High)
Thin membrane (Low Thickness)
Area (Large Surface Area)
Gradient (Steep Partial Pressure Gradient)
Small molecular weight (Low Molecular Weight, though solubility often dominates for $CO_2$)