## **Core Concept**
Intrapleural pressure, also known as intrathoracic pressure, is the pressure within the pleural cavity. It is slightly negative due to the outward elastic recoil of the chest wall and the downward elastic recoil of the lungs. This negative pressure is essential for keeping the lungs inflated and for facilitating breathing.

## **Why the Correct Answer is Right**
The correct answer is $-5$ cm of H2O. At the beginning of inspiration, the intrapleural pressure becomes more negative (or less positive) to expand the lungs and draw air into them. Normally, intrapleural pressure is about $-5$ cm of H2O at the start of inspiration. This negative pressure increases (becomes more negative) during inspiration to facilitate lung expansion.

## **Why Each Wrong Option is Incorrect**
- **Option A:** $-10$ cm of H2O. This value might be seen during inspiration when the lungs are expanding, but it's not the typical value at the beginning of inspiration.
- **Option B:** $+5$ cm of H2O. This positive pressure would not be normal for intrapleural pressure as it does not reflect the negative pressure needed to maintain lung inflation.
- **Option D:** $0$ cm of H2O. This would imply that there is no negative pressure in the pleural space, which is not accurate for a normal, healthy individual.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that intrapleural pressure is normally negative, around $-5$ cm of H2O, and becomes more negative during inspiration. This negative pressure is crucial for lung expansion. Conditions that equalize intrapleural pressure with atmospheric pressure (e.g., pneumothorax) can lead to lung collapse.

## **Correct Answer:** C. -5.