Describe why oxygen moves from the air to the blood in the alveolar capillaries and again why oxygen moves from the blood to the interstitial fluid in the body tissues. Why is the partial pressure (concentration) of oxygen so low in the interstitial fluid?

Higher partial pressure of oxygen in alveolar air than that of alveolar capillaries drives the diffusion of oxygen gas from air to the blood.

Higher partial pressure of oxygen in blood at tissue level as compared to the interstitial fluid drives diffusion of oxygen from blood to the tissue fluid.

Consumption of oxygen for cellular respiration in cells reduces the its concentration in the interstitial fluid.

Explanation:

Gaseous exchange in the body occurs through the process of diffusion wherein respiratory gases are exchanges down the concentration gradient.

Since the alveolar air has a higher partial pressure of oxygen than that of blood present in the surrounding blood capillaries, oxygen diffuses from the alveolar air to the blood making the blood oxygen-rich.

As the oxygen-rich blood reaches the body tissues, oxygen is diffused from the blood into the interstitial fluid since the later has lower oxygen concentration.

The supply of oxygen from the blood to the body tissues is required as the cells consume the oxygen to perform aerobic respiration and retrieve energy from the nutrients.