Biology - Gas Exchange in Humans

The human respiratory system includes the lungs, trachea, bronchi, bronchioles, and alveoli, where the exchange of $O_2$ and $CO_2$ occurs via diffusion.

Efficient gas exchange surfaces (alveoli) have a large surface area, are only one cell thick to ensure a short diffusion distance, and are surrounded by a dense network of capillaries to maintain a steep concentration gradient.

Inhalation is an active process: external intercostal muscles contract, the diaphragm contracts and flattens, thoracic volume increases, and internal pressure decreases below atmospheric pressure.

Exhalation is largely passive: external intercostal muscles and the diaphragm relax, thoracic volume decreases, and internal pressure increases, forcing air out.

Inhaled air contains approximately $21\% O_2$ and $0.04\% CO_2$, while exhaled air contains approximately $16\% O_2$ and $4\% CO_2$.

Physical activity increases the rate and depth of breathing to supply more $O_2$ for aerobic respiration and to remove the excess $CO_2$ produced by contracting muscles.

Goblet cells produce mucus to trap pathogens and dust, while ciliated cells use hair-like structures called cilia to sweep the mucus away from the lungs toward the throat.