**Question:** Transport of carbon monoxide (CO) is diffusion limited because

A. Lack of a carrier protein
B. Low solubility in blood
C. High reactivity with hemoglobin
D. Small size of CO molecule

**Core Concept:**
Carbon monoxide (CO) is a gas produced by the incomplete combustion of carbon-containing fuels. It binds to the heme group of hemoglobin (Hb) and methemoglobin (MetHb), leading to reduced oxygen-carrying capacity and potentially significant clinical consequences. The transport of CO is influenced by its diffusion properties and interactions with proteins.

**Why the Correct Answer is Right:**
CO is diffusion limited primarily due to its low solubility in blood. The solubility of a gas in a liquid depends on factors such as temperature, pressure, and the polarity of the solvent. Blood, composed primarily of water, has limited solubility for gases like CO. This reduced solubility limits the rate at which CO can diffuse from the site of production to the tissues where it's needed for binding to hemoglobin and methemoglobin, resulting in reduced oxygen delivery and sequestration.

**Why Each Wrong Option is Incorrect:**

**A.** Lack of a carrier protein: While CO can bind to plasma proteins like albumin, this option is incorrect because the correct answer focuses on the diffusion limitation, not protein-bound CO.

**B.** Small size of CO molecule: Although CO has a small molecular size, its low solubility in blood is the primary factor contributing to the diffusion limitation, not the size of the molecule itself.

**C.** High reactivity with hemoglobin: CO reacts with hemoglobin, forming carboxyhemoglobin (COHb). However, the correct answer is about diffusion limitation, not the interaction between CO and hemoglobin.

**Clinical Pearl:**
Understanding the diffusion limitation of CO transport is crucial for interpreting COHb levels in clinical practice. Elevated COHb levels should raise suspicion for CO poisoning, which can cause neurological symptoms, cyanosis, and, in severe cases, death due to hypoxia. Management includes oxygen therapy, removing the source of CO exposure, and possibly desaturation therapy (e.g., hyperbaric oxygen therapy) for severe cases.