Exchange of gases Questions in English

(A) The partial pressure of oxygen $(PO_2)$ varies in different parts of the respiratory system to facilitate diffusion:
$1$. In atmospheric air, the $PO_2$ is approximately $159\, mm\, Hg$.
$2$. In oxygenated blood (systemic arteries), the $PO_2$ is approximately $95\, mm\, Hg$.
$3$. In tissues, where oxygen is consumed for cellular respiration, the $PO_2$ is low, approximately $40\, mm\, Hg$.
Therefore, the correct matching is: $P-I, Q-III, R-II$.

(A) The exchange of gases like $O_2$ and $CO_2$ at the alveolar membrane and between blood and tissues occurs primarily by the process of simple diffusion.
Simple diffusion is a passive process driven by the partial pressure gradients of the gases.
Since gases move from a region of higher partial pressure to a region of lower partial pressure without the expenditure of energy,it is classified as simple diffusion.

(D) The rate of diffusion is influenced by several factors as described by Fick's Law of Diffusion.
$1$. Solubility of gases: Higher solubility of a gas in the diffusion membrane increases the rate of diffusion.
$2$. Thickness of the diffusion membrane: The rate of diffusion is inversely proportional to the thickness of the membrane. $A$ thinner membrane allows for faster diffusion.
$3$. Pressure gradient: The difference in partial pressure between the two sides of the membrane is a primary driver.
$4$. Temperature: Generally,an increase in temperature increases the kinetic energy of molecules,thereby increasing the rate of diffusion.
Therefore,all the given factors affect the rate of diffusion.

(B) In the human respiratory system,the partial pressure of carbon dioxide $(pCO_2)$ in deoxygenated blood that reaches the alveoli is approximately $45\, mm\, Hg$.
After the exchange of gases in the alveoli,the blood becomes oxygenated and the $pCO_2$ decreases to approximately $40\, mm\, Hg$.
Therefore,at point $P$ (deoxygenated blood),$pCO_2 = 45\, mm\, Hg$ and at point $Q$ (oxygenated blood),$pCO_2 = 40\, mm\, Hg$.

(C) The exchange of gases ($O_2$ and $CO_2$) occurs by simple diffusion based on pressure gradients.
$1$. For $O_2$,the partial pressure is higher in the alveoli $(104 \ mmHg)$ compared to the deoxygenated blood $(40 \ mmHg)$,creating a gradient from lungs to tissues.
$2$. For $CO_2$,the partial pressure is higher in the tissues $(45 \ mmHg)$ compared to the blood $(40 \ mmHg)$ and alveoli $(40 \ mmHg)$,creating a gradient from tissues to lungs.
$3$. Therefore,the concentration gradient for $CO_2$ is from tissues to lungs,and for $O_2$ is from lungs to tissues.

(A) The solubility of gases is a critical factor in the exchange of gases across the alveolar membrane.
According to the principles of gas exchange in the human respiratory system,the solubility of $CO_2$ is $20-25$ times higher than that of $O_2$.
This high solubility allows $CO_2$ to diffuse much more rapidly across the diffusion membrane compared to $O_2$ for the same partial pressure gradient.
Therefore,the correct option is $A$.

(D) The diffusion membrane is composed of three major layers:
$1$. The thin squamous epithelium of the alveoli.
$2$. The endothelium of the alveolar capillaries.
$3$. The basement substance (basement membrane) in between them.
As $O_2$ moves from the alveoli into the blood,it first crosses the alveolar epithelium,then the basement membrane,and finally the capillary endothelium. Therefore,the correct sequence is: Epithelium of alveoli $\rightarrow$ Basement membrane $\rightarrow$ Endothelium of capillaries.

(D) The diffusion membrane is composed of three major layers: the thin squamous epithelium of alveoli,the endothelium of alveolar capillaries,and the basement substance in between them.
These layers are extremely thin,and the total thickness of the diffusion membrane is much less than a millimeter $(< 1 \text{ mm})$.
This minimal thickness is crucial for the efficient diffusion of gases ($O_2$ and $CO_2$) across the membrane.

(B) The partial pressures of respiratory gases at different sites are as follows:
$1$. Alveoli: $PO_2 = 104 \ mmHg$ and $PCO_2 = 40 \ mmHg$.
$2$. Deoxygenated blood: $PO_2 = 40 \ mmHg$ and $PCO_2 = 45 \ mmHg$.
$3$. Atmospheric air: $PO_2 = 159 \ mmHg$ and $PCO_2 = 0.3 \ mmHg$.
$4$. Tissue: $PO_2 = 40 \ mmHg$ and $PCO_2 = 45 \ mmHg$.
Matching the given values:
- $A$ (Alveoli) matches with $iii$ $(104 \ mmHg \ PO_2)$.
- $B$ (Deoxygenated blood) matches with $iv$ $(40 \ mmHg \ PO_2)$.
- $C$ (Atmospheric air) matches with $i$ $(0.3 \ mmHg \ PCO_2)$.
- $D$ (Tissue) matches with $ii$ $(45 \ mmHg \ PCO_2)$.
Therefore,the correct match is $A-iii, B-iv, C-i, D-ii$.

(D) The rate of diffusion of gases across the respiratory membrane is governed by Fick's Law of Diffusion.
Factors affecting this rate include:
$1$. Solubility of the gas: Gases with higher solubility (like $CO_2$) diffuse more rapidly than those with lower solubility (like $O_2$).
$2$. Partial pressure gradient: The difference in partial pressure between the alveoli and the blood drives the diffusion.
$3$. Thickness of the respiratory membrane: The thinner the membrane, the faster the diffusion rate.
Since all these factors influence the rate of diffusion, the correct option is $D$.

(A) The solubility of gases is a critical factor in the diffusion process across the respiratory membrane.
According to Fick's law of diffusion,the rate of diffusion is directly proportional to the solubility of the gas.
Experimental data shows that the solubility of $CO_2$ is $20-25$ times higher than that of $O_2$.
This high solubility allows $CO_2$ to diffuse much more rapidly across the respiratory membrane compared to $O_2$,even when the partial pressure gradient is smaller.

(D) The diffusion membrane is the structure through which gas exchange occurs in the lungs. It consists of three major layers:
$1$. The thin squamous epithelium of the alveoli.
$2$. The endothelium of the alveolar capillaries.
$3$. The basement substance (basement membrane) in between them.
Oxygen diffuses from the alveoli into the blood,and carbon dioxide diffuses from the blood into the alveoli through this membrane. The total thickness of this membrane is much less than a millimeter. Therefore,all the given statements are correct.

(D) Statement $I$ is incorrect because the respiratory membrane is composed of three layers: the squamous epithelium of the alveoli,the endothelium of the alveolar capillaries,and the basement substance in between them. Furthermore,these layers rest on a thin basement membrane,not a thick one.
Statement $II$ is incorrect because the diffusion of respiratory gases occurs from a region of higher partial pressure to a region of lower partial pressure,following the concentration gradient.

(C) In the process of external respiration,the partial pressure of carbon dioxide $(pCO_2)$ in atmospheric (inspired) air is approximately $0.3 \ mm \ Hg$.
This low partial pressure allows for the diffusion of $CO_2$ from the blood (where $pCO_2$ is $45 \ mm \ Hg$) into the alveoli (where $pCO_2$ is $40 \ mm \ Hg$) and subsequently out of the body through expiration.

(B) The exchange of gases between the blood and the tissues occurs through the systemic capillaries.
Oxygenated blood reaches the tissues via systemic arteries,which branch into a network of capillaries.
Since the partial pressure of oxygen $(pO_2)$ is lower in the tissue cells compared to the blood in the capillaries,oxygen dissociates from oxyhemoglobin and diffuses into the tissues.
Therefore,the maximum exchange of oxygen from the blood to the cells occurs in the capillaries surrounding the tissue cells.