Final answer:
Lungs remain slightly inflated after quiet expiration due to the negative pressure within the thoracic cavity and the surfactant in the alveoli reducing surface tension, which prevents lung collapse.
Step-by-step explanation:
Lungs remain inflated at the end of quiet expiration due to the negative pressure within the thoracic cavity that keeps the airways open. This is because of the balancing act between the elastic recoil of the lungs wanting to reduce volume and the outward pull of the chest wall. Additionally, the presence of surfactant in the alveoli reduces surface tension, preventing the alveoli from collapsing and assisting in keeping the alveoli open, allowing the lungs to remain partially inflated even after passive exhalation.
During inhalation, the contraction of the diaphragm and intercostal muscles expands the thoracic cavity, creating a negative pressure that draws air in. As these muscles relax during exhalation, the elasticity of the lung tissue causes it to recoil, but the presence of surfactant and the residual negative pressure prevent the lungs from fully collapsing. This mechanism ensures that a base level of air remains in the lungs, maintaining them in a slightly inflated state even after the exhalation.