Final answer:
After anesthesia induction, lungs are kept at functional residual capacity due to the balance between the chest wall's outward elasticity and the inward elastic recoil of the lung tissue, combined with the negative intrapleural pressure and the effects of surfactant.
Step-by-step explanation:
The forces that keep the lungs at functional residual capacity (FRC) after anesthesia induction involve the balance between the inward elastance of the lung tissue and the opposing outward force from the chest wall. FRC is the sum of the expiratory reserve volume (ERV) and the residual volume (RV), representing the amount of air that remains in the lungs after a tidal expiration. During general anesthesia, the muscles of respiration are affected, which can impact the maintenance of FRC. The diaphragm and intercostal muscles normally facilitate the lungs' expansion and contraction; however, under anesthesia, they do not function effectively. This is why endotracheal intubation is often employed to secure the airway and ensure that the lungs can still be ventilated despite the relaxed respiratory muscles. After surgery and the removal of the endotracheal tube, it takes time for the respiratory muscles to regain control of respiration and maintain the FRC.
Physical factors affecting ventilation such as surface tension within the alveoli, contributed by surfactant, and the negative intrapleural pressure are crucial for keeping the lungs inflated to the FRC level during anesthesia. The negative intrapleural pressure is created by a slight balance in favor of the outward pull from the elasticity of the chest wall against the inward pull of the elastic lungs and the surface tension of alveolar fluid. Even after anesthesia, these passive forces remain constant, ensuring that the lungs do not fully deflate.