Final answer:
Option b is the correct answer. The flow-type plethysmograph differs from the constant volume plethysmograph mainly in how lung volumes are measured: the former measuring air flow and the latter estimating volume changes based on pressure alterations in an airtight chamber.
Step-by-step explanation:
The flow-type plethysmograph and the constant volume (variable-pressure) plethysmograph are two methods used to measure respiratory volumes and lung function. The flow-type plethysmograph measures lung volume indirectly by assessing the flow of air into and out of the lungs, while the constant volume plethysmograph, based on Boyle's law, measures changes in lung volume directly by monitoring pressure changes within an airtight chamber. The key difference between the two is that the constant volume plethysmograph measures changes in thoracic gas volume (b), which is not direct measurement but rather an estimation based on pressure changes, whereas the flow-type plethysmograph typically measures the flow rate of air, which can be used to determine lung volumes.