What causes oxygen to enter pulmonary capillaries from alveoli and to enter tissue cells from systemic capillaries?

Explanation

To understand what causes oxygen to enter pulmonary capillaries from alveoli and to enter tissue cells from systemic capillaries, we need to focus on the concept of partial pressure and diffusion. Here’s a step-by-step explanation:

### Step 1: Understand Partial Pressure
- **Definition**: Partial pressure is the pressure exerted by a specific gas in a mixture of gases. It is denoted as P (for example, PO2 for the partial pressure of oxygen).
- **Importance**: The movement of gases, including oxygen, is influenced by their partial pressures in different locations.

### Step 2: Recognize the Role of Diffusion
- **Diffusion**: This is the process by which molecules move from an area of higher concentration (or higher partial pressure) to an area of lower concentration (or lower partial pressure).
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