Gas Exchange in Humans - Breathing and lung function

The pathway of air involves the nasal cavity, larynx, trachea, bronchi, bronchioles, and finally the alveoli where gas exchange occurs.

The trachea and bronchi are lined with ciliated cells and goblet cells. Goblet cells secrete mucus to trap pathogens, while cilia sweep the mucus towards the throat.

Inhalation (Inspiration) occurs when the external intercostal muscles contract and the diaphragm contracts (moves down/flattens). This increases thorax volume and decreases pressure, drawing air in.

Exhalation (Expiration) occurs when the internal intercostal muscles contract (during forced breathing) and the diaphragm relaxes (moves up/dome-shaped). This decreases thorax volume and increases pressure, pushing air out.

Alveoli are adapted for gas exchange by having a very large surface area, a wall only one cell thick (short diffusion distance), a moist lining to dissolve gases, and a dense network of capillaries to maintain a steep concentration gradient for $O_2$ and $CO_2$.

Gas exchange occurs via diffusion: $O_2$ moves from the alveoli into the blood, while $CO_2$ moves from the blood into the alveoli.

Inspired air contains approximately $21\%$ $O_2$ and $0.04\%$ $CO_2$, whereas expired air contains approximately $16\%$ $O_2$ and $4\%$ $CO_2$ due to aerobic respiration.

The limewater test is used to identify $CO_2$; it turns from clear to cloudy/milky when $CO_2$ is bubbled through it.

Physical activity increases the rate and depth of breathing because the body requires more energy, leading to increased $CO_2$ production from aerobic respiration ($C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$), which lowers blood $pH$ and triggers the brain to increase ventilation.