During spontaneous breathing, the fall in intrapleural pressure that draws air into the lungs during inspiration also draws blood into the major thoracic vessels. This phenomenon increases which of the following?

a. Systemic vascular resistance
b. Right ventricular afterload
c. Right ventricular preload
d. Pulmonary capillary resistance

ANS: C
During spontaneous breathing, the fall in intrapleural pressure that draws air into the lungs during inspiration also draws blood into the major thoracic vessels and heart. With this increased return of blood to the right side of the heart and the stretching and enlargement of the right-side heart volume, the right ventricular preload increases, resulting in an increased right ventricular stroke volume (i.e., Frank-Starling mechanism).