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

A. Diffusion
B. Oxygen-binding to hemoglobin
C. Active transport
D. Shunting

**Correct Answer:** A. Diffusion

**Core Concept:** Oxygen (O2) is essential for aerobic cellular respiration, a process that produces energy in the form of adenosine triphosphate (ATP). Oxygen enters the body through the respiratory system, mainly the lungs, and binds to hemoglobin in red blood cells (RBCs).

**Why the Correct Answer is Right:** Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. In this context, oxygen moves from the alveoli (air sacs) of the lungs, where the concentration is high (due to thin walls and high surface area), to the blood within the alveoli, where the concentration is low. Oxygen diffuses across the thin alveolar walls and binds to hemoglobin in RBCs, transporting it throughout the body to the tissues where it is required.

**Why Each Wrong Option is Incorrect:**

B. Oxygen-binding to hemoglobin: Although hemoglobin plays a crucial role in oxygen transport, its function is not directly related to the diffusion process from alveoli to blood. Hemoglobin binds to oxygen, but the mechanism of oxygen transport primarily relies on diffusion.

C. Active transport: This refers to a cellular process that actively transports molecules against their concentration gradient using energy from ATP. Unlike diffusion, active transport does not explain the passive movement of oxygen from high to low concentration.

D. Shunting: This term refers to the bypassing of certain parts of the circulatory system, often due to disease. It does not describe the mechanism of oxygen transport from alveoli to blood.

**Clinical Pearl:** Understanding the process of oxygen diffusion helps medical students and practitioners appreciate the role of the respiratory system in supplying oxygen to the body. This concept is essential for understanding diseases affecting oxygen transport, such as pulmonary hypertension, which can impair oxygen diffusion and affect overall oxygen saturation levels in the blood.