Exchange of Gases — Revision Notes

⚡ 30-Second Revision

Driving Force: — Partial pressure gradient (diffusion).
Sites: — Alveoli (external respiration) & Tissues (internal respiration).
Respiratory Membrane: — Alveolar epithelium, fused basement membrane, capillary endothelium (0.2-0.5 \mu m thick).
Partial Pressures (PO\textsubscript{2}}/PCO\textsubscript{2} in mmHg):

- Atmospheric: 159/0.3 - Alveolar: 104/40 - Deoxygenated Blood: 40/45 - Oxygenated Blood: 95/40 - Tissues: <40/>45

Factors Affecting Diffusion (Fick's Law): — Rate $\propto \frac{A \times D \times \Delta P}{T}$

- A: Surface Area (direct) - D: Diffusion Coefficient (direct, includes solubility) - $\Delta P$: Partial Pressure Gradient (direct) - T: Thickness of membrane (inverse)

Solubility: — CO\textsubscript{2} is 20-25 times more soluble than O\textsubscript{2} in blood.

2-Minute Revision

Gas exchange is the passive movement of O\textsubscript{2} and CO\textsubscript{2} driven by partial pressure gradients. It occurs in the lungs (external respiration) and at the tissues (internal respiration).

In the lungs, O\textsubscript{2} moves from alveoli (PO\textsubscript{2}=104 mmHg) into deoxygenated blood (PO\textsubscript{2}=40 mmHg), while CO\textsubscript{2} moves from blood (PCO\textsubscript{2}=45 mmHg) into alveoli (PCO\textsubscript{2}=40 mmHg).

At the tissues, O\textsubscript{2} moves from oxygenated blood (PO\textsubscript{2}=95 mmHg) into cells (PO\textsubscript{2}<40 mmHg), and CO\textsubscript{2} moves from cells (PCO\textsubscript{2}>45 mmHg) into blood (PCO\textsubscript{2}=40 mmHg).

The respiratory membrane, a thin three-layered structure, facilitates this. The rate of diffusion is directly proportional to surface area, diffusion coefficient (solubility), and partial pressure gradient, but inversely proportional to membrane thickness.

Crucially, CO\textsubscript{2} is 20-25 times more soluble than O\textsubscript{2}, compensating for its smaller partial pressure gradient and ensuring efficient removal.

5-Minute Revision

The exchange of gases is a vital physiological process ensuring oxygen supply to cells and carbon dioxide removal. It's entirely driven by the principle of simple diffusion, where gases move from a region of higher partial pressure to one of lower partial pressure. This occurs at two main sites: the lungs (external respiration) and the body tissues (internal respiration).

External Respiration (Lungs): Inhaled air, rich in O\textsubscript{2}, reaches the alveoli. The partial pressure of O\textsubscript{2} (PO\textsubscript{2}) in alveolar air is about 104 mmHg, while in the deoxygenated blood arriving from the body, it's only 40 mmHg.

This steep gradient drives O\textsubscript{2} into the blood. Simultaneously, deoxygenated blood has a PCO\textsubscript{2} of 45 mmHg, higher than the alveolar PCO\textsubscript{2} of 40 mmHg, causing CO\textsubscript{2} to diffuse into the alveoli for exhalation.

The blood leaving the lungs is oxygenated, with PO\textsubscript{2} around 95 mmHg and PCO\textsubscript{2} around 40 mmHg.

Internal Respiration (Tissues): Oxygenated blood reaches the systemic capillaries. Tissue cells, constantly consuming O\textsubscript{2}, have a very low PO\textsubscript{2} (<40 mmHg). This gradient drives O\textsubscript{2} from the blood (PO\textsubscript{2}=95 mmHg) into the cells.

Concurrently, cells produce CO\textsubscript{2}, leading to a high PCO\textsubscript{2} (>45 mmHg) in tissues, which drives CO\textsubscript{2} into the blood (PCO\textsubscript{2}=40 mmHg). The blood leaving the tissues is deoxygenated, returning to the lungs.

Factors Affecting Diffusion: The rate of diffusion is governed by Fick's Law: Rate $\propto \frac{A \times D \times \Delta P}{T}$.

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Partial Pressure Gradient ($\Delta P$): — The larger the difference, the faster the diffusion.
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Surface Area (A): — A larger surface area (e.g., healthy alveoli) increases diffusion.
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Thickness of Membrane (T): — A thinner membrane (e.g., healthy respiratory membrane) increases diffusion. Conditions like pulmonary edema or fibrosis increase thickness, reducing diffusion.
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Solubility of Gases (part of D): — CO\textsubscript{2} is 20-25 times more soluble in blood than O\textsubscript{2}. This is crucial because despite a smaller partial pressure gradient for CO\textsubscript{2} (5 mmHg vs. 64 mmHg for O\textsubscript{2} in lungs), its high solubility ensures efficient removal.

Example: If a person moves to high altitude, the atmospheric PO\textsubscript{2} decreases. This reduces the alveolar PO\textsubscript{2} and thus the partial pressure gradient for O\textsubscript{2}, making it harder for O\textsubscript{2} to diffuse into the blood.

Prelims Revision Notes

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Definition: — Exchange of O\textsubscript{2} and CO\textsubscript{2} across membranes, driven by partial pressure gradients.
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Sites:

* External Respiration: Alveoli $\leftrightarrow$ Pulmonary capillaries (Lungs). * Internal Respiration: Systemic capillaries $\leftrightarrow$ Tissue cells (Body).

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Respiratory Membrane: — Extremely thin (0.2-0.5 \mu m) barrier in lungs. Composed of alveolar squamous epithelium, fused basement membrane, and capillary endothelium.
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Partial Pressure Values (in mmHg):

* Atmospheric Air: PO\textsubscript{2} = 159, PCO\textsubscript{2} = 0.3 * Alveolar Air: PO\textsubscript{2} = 104, PCO\textsubscript{2} = 40 * Deoxygenated Blood (Pulmonary Artery): PO\textsubscript{2} = 40, PCO\textsubscript{2} = 45 * Oxygenated Blood (Pulmonary Vein/Systemic Artery): PO\textsubscript{2} = 95, PCO\textsubscript{2} = 40 * Tissue Cells: PO\textsubscript{2} < 40, PCO\textsubscript{2} > 45

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Direction of Gas Movement (Always High P to Low P):

* Lungs: O\textsubscript{2} (Alveoli $\to$ Blood), CO\textsubscript{2} (Blood $\to$ Alveoli) * Tissues: O\textsubscript{2} (Blood $\to$ Tissues), CO\textsubscript{2} (Tissues $\to$ Blood)

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Factors Affecting Diffusion Rate (Fick's Law):

* Directly Proportional: Surface Area (A), Diffusion Coefficient (D), Partial Pressure Gradient ($\Delta P$). * Inversely Proportional: Thickness of Diffusion Membrane (T).

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Diffusion Coefficient (D): — Proportional to solubility and inversely proportional to $\sqrt{\text{molecular weight}}$.
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Solubility of Gases: — CO\textsubscript{2} is 20-25 times more soluble in blood than O\textsubscript{2}. This explains why CO\textsubscript{2} diffuses much faster despite a smaller partial pressure gradient.
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Clinical Correlations:

* Pulmonary Edema/Fibrosis: Increases membrane thickness (T), decreases diffusion. * Emphysema: Decreases surface area (A), decreases diffusion. * High Altitude: Decreases atmospheric PO\textsubscript{2}, reduces $\Delta P$ for O\textsubscript{2} in lungs, decreases O\textsubscript{2} diffusion.

Vyyuha Quick Recall

To remember the factors affecting gas diffusion rate, think of 'SToP D':

Surface Area (larger = faster)
Thickness (thinner = faster)
oP — (partial Pressure) Gradient (steeper = faster)
Diffusion Coefficient (higher = faster, remember CO\textsubscript{2} is more soluble!)