Question

In humans, respiration occurs in the following steps. Arrange these steps in the correct order.
A. Diffusion of $O_2$ and $CO_2$ between blood and tissues
B. Diffusion of $O_2$ and $CO_2$ across alveolar membrane
C. Pulmonary ventilation by which atmospheric air is drawn in and $CO_2$ rich alveolar air is released out
D. Cellular respiration
E. Transport of gases by the blood
Choose the correct answer from the options given below:

• A. A, B, C, D, E
• B. E, A, C, D, B
• C. C, B, E, A, D
• D. C, A, B, E, D

Hint:

Logic Tip: Follow the path of an oxygen molecule: It goes into the lungs (C), crosses into the blood (B), rides the blood stream (E), crosses into the muscle (A), and is finally "burned" for energy (D).

Answer 1

The Correct Option is C

Step 1 : Understanding the Question:
The question requires us to arrange the physiological steps of respiration in the correct order as they occur in the human body. This encompasses the entire journey of oxygen from the outside air into the cells for energy production, and the return of carbon dioxide.
Step 2 : Key Formulas and Approach:
Respiration is a multi-stage process involving:
1. Mechanical movement of air (Ventilation).
2. External exchange at the lungs.
3. Internal transport via the circulatory system.
4. Internal exchange at the tissues.
5. Metabolic consumption (Cellular respiration).
Step 3 : Detailed Explanation:

Step 1: Pulmonary Ventilation (C): The process begins with breathing, where atmospheric air is taken into the lungs ($O_2$ rich) and $CO_2$ rich air is expelled from the alveoli. This is the physical act of ventilation.

Step 2: External Respiration (B): Once air is in the alveoli, oxygen must enter the blood. This occurs through the diffusion of $O_2$ and $CO_2$ across the thin alveolar membrane separating the air sacs from the capillaries.

Step 3: Gas Transport (E): The blood then serves as the medium for the transport of gases throughout the body, carrying oxygenated blood toward tissues and deoxygenated blood back to the lungs.

Step 4: Internal Respiration (A): When blood reaches the capillary beds of various organs, the second exchange happens: Diffusion of $O_2$ and $CO_2$ between blood and tissues occurs down the partial pressure gradient.

Step 5: Cellular Respiration (D): Finally, within the cells, oxygen is used in metabolic pathways (like the Krebs cycle) to oxidize glucose and produce ATP, with $CO_2$ being produced as a byproduct.

Step 4 : Final Answer:
Tracing the path of oxygen from the atmosphere to the mitochondria yields the sequence C-B-E-A-D. This logical progression of gas exchange and transport is correctly listed in the third choice.