**Question:** Oxygen comes from alveoli to blood by-

A. Inhalation
B. Diffusion
C. Blood vessels
D. Pulmonary circulation

**Core Concept:** Oxygen gas is essential for aerobic cellular respiration in our body. It needs to be transported from the air we breathe (alveoli) to the cells (blood). This process involves diffusion and occurs in the lungs, specifically in the alveoli and capillaries.

**Why the Correct Answer is Right:**

* Option B (Diffusion): Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration. In this context, oxygen molecules are more concentrated in the alveoli compared to the blood. As a result, they passively move from the alveoli to the blood, making option B the right answer.
* Option D (Pulmonary circulation): Oxygen-rich blood (from the heart) travels to the lungs through the pulmonary arteries and returns to the heart through the pulmonary veins after picking up oxygen. This is part of the larger circulation system and not directly related to the process of oxygen reaching the blood.

**Why Each Wrong Option is Incorrect:**

* Option A (Inhalation): Inhalation is the conscious act of breathing in air, whereas the question is asking about the passive process of oxygen reaching the blood.
* Option C (Blood vessels): While blood vessels are involved in the overall circulatory system, they are not directly responsible for the diffusion of oxygen from the alveoli to the blood.

**Clinical Pearl:**

The process of oxygen reaching the blood is crucial for maintaining cellular respiration in our body. It highlights the importance of the alveoli, capillaries, and the passive diffusion process (B) for ensuring oxygen reaches our blood, ensuring proper functioning of our cells and tissues.