Physiology of normal breathing vs mechanical ventilation

Our lungs have 23 generations of airway branches. Surface area increases as we travel deeper within the lung. The functional unit of the lungs are the alveoli, responsible for oxygenation and ventilation, which together have a surface area of around 70 m².

For healthy individuals, total lung capacity (TLC, after maximal inspiration) is 6–8 liters. Residual volume (RV, after maximal expiration) is 2–2.5L. The difference between the two is called the vital capacity (VC), 4–6L. Functional residual capacity (FRC, after normal expiration) is 3–4L, which decreases when lying down, during anesthesia, etc. In short, everything that causes atelectasis. Anatomical dead space (Vd) is around 100–150ml, does not contribute to ventilation and is about 30% of tidal volume (Vd/Vt = 0.3). This obviously increases when we add stuff that does not contribute to ventilation, like an endotracheal tube, ventilator tubing, etc. Alveolar minute ventilation is thus around 5L/min.

When we breathe, our respiratory muscles (mainly the diaphragm and intercostal muscles) pull our thoracic cavity outward. This causes a negative intrathoracic (or pleural) pressure, which results in the…