**Question:** Partial pressure of CO2 is lowest in:

A. Tissues
B. Capillary blood
C. Alveoli
D. Venous blood

**Core Concept:**
The partial pressure of carbon dioxide (CO2) is an important parameter that reflects the balance between carbon dioxide production, elimination, and buffering capacity in the body. In the context of the question, we are considering the partial pressure of CO2 in different compartments within the respiratory and circulatory system.

**Why the Correct Answer is Right:**
The correct answer, option C (Alveoli), highlights the primary site of CO2 elimination from the body. In the respiratory system, alveoli are the air sacs where gas exchange occurs, including the elimination of CO2 from the blood into the alveoli. The partial pressure of CO2 is lowest in alveoli as it is the site of CO2 elimination.

**Why Each Wrong Option is Incorrect:**

A. Tissues: Partial pressure of CO2 is relatively high in tissues due to its production by cellular respiration and limited elimination pathways.

B. Capillary blood: Although CO2 is present in capillary blood, the partial pressure is not significantly lower than in the surrounding tissues due to the continuous exchange of gases between blood and tissue spaces.

D. Venous blood: Venous blood contains a higher partial pressure of CO2 than capillary blood due to the continuous removal of CO2 from venous blood by the respiratory system and tissues.

**Clinical Pearl:**

The partial pressure of CO2 is important to understand as it can provide clues about respiratory function and gas exchange in the body. A low partial pressure of CO2 in capillary blood or venous blood may indicate impaired gas exchange, while a low partial pressure in alveolar or arterial blood can help identify respiratory acidosis (low pH) or respiratory alkalosis (high pH), respectively. Understanding these relationships and interpreting blood gas analysis results is crucial for diagnosing and managing respiratory disorders.