Human Physiology - Gas Exchange

Ventilation is the process of bringing fresh air into the lungs and removing stale air to maintain a concentration gradient of $O_2$ and $CO_2$.

Gas exchange is the passive process of $O_2$ diffusing from the alveoli into the blood and $CO_2$ diffusing from the blood into the alveoli across the respiratory membrane.

Cell respiration is the metabolic process within the mitochondria where $O_2$ is consumed to produce $ATP$, with $CO_2$ as a byproduct.

The alveoli are adapted for gas exchange: they have a large surface area, a thin epithelial layer (one cell thick), and are surrounded by a dense network of capillaries.

Type I pneumocytes are extremely thin alveolar cells adapted for gas exchange, while Type II pneumocytes secrete a surfactant (a phospholipid) that reduces surface tension and prevents the alveoli from collapsing.

Inspiration (inhalation) occurs when the external intercostal muscles and diaphragm contract, increasing thoracic volume and decreasing pressure ($P_{internal} < P_{atmospheric}$).

Expiration (exhalation) occurs when the internal intercostal muscles and abdominal muscles contract (during forced breathing), or simply when the diaphragm relaxes, decreasing volume and increasing pressure ($P_{internal} > P_{atmospheric}$).

Emphysema is a chronic lung disease where the alveolar walls are destroyed, reducing the surface area for gas exchange and decreasing the elasticity of the lungs.