Why percentage of oxygen in alveolus less than in inspired air?

The percentage of oxygen in the alveolus is less than in inspired air because as we breathe in, oxygen from the inspired air diffuses into the bloodstream in the lungs. This oxygen is then transported to the body's tissues and cells for use in cellular respiration. As a result, the oxygen content in the alveolus decreases as it is continuously being taken up by the bloodstream, leading to a lower percentage of oxygen in the alveolus compared to the inspired air.

The percentage of oxygen in the alveoli is less than in the inspired air due to the following reasons:

- Diffusion: Oxygen diffuses from the alveoli into the capillary blood, reducing the concentration of oxygen in the alveoli.
- Solubility: Oxygen is less soluble in water than other gases, such as nitrogen and carbon dioxide. As a result, a greater proportion of nitrogen and carbon dioxide remains in the alveoli than oxygen.
- Ventilation-perfusion inequality: Not all alveoli are ventilated (receive air) and perfused (have blood flowing through them) equally. Some alveoli are poorly ventilated or perfused, which can lead to a lower concentration of oxygen in those alveoli.
- Shunt: A shunt is a connection between the pulmonary arteries and pulmonary veins that bypasses the alveoli. This allows some deoxygenated blood to bypass the alveoli and enter the systemic circulation, reducing the overall oxygen concentration in the alveoli.

Typically, the percentage of oxygen in the inspired air is around 21%, while the percentage of oxygen in the alveoli is around 14-15%.