Final answer:
The term for the increased anterior-posterior chest diameter in emphysema is 'barrel chest,' caused by the trapping of air in the damaged alveoli, leading to the rounded shape of the thoracic cavity.
Step-by-step explanation:
The increase in the anterior-posterior diameter of the chest that occurs in emphysema is commonly referred to as 'barrel chest'. It is characterized by a more rounded chest due to the effects of emphysema on the lungs. In emphysema, the walls between the alveoli break down, reducing elasticity and trapping air in the lungs. This leads to a change in the shape of the thoracic cavity. The physics behind this involves the concept of surface tension; as alveoli lose their structure and become larger due to the destroyed walls, the surface tension is reduced, impacting the pressure exerted by these alveolar sacs. In a healthy lung, the surface tension of the alveoli contributes to lung recoil during exhalation, helping to expel air. Damaged and enlarged alveoli due to emphysema lose this recoil capability, thereby trapping air and contributing to the 'barrel' shaped chest.