Which of the following statements is true regarding the sheath fluid in a flow cytometer?

The statement that sheath fluid provides a constant pressure for sample alignment is true. In flow cytometry, sheath fluid plays a crucial role in hydrodynamic focusing, which helps to align the cells in a single-file stream as they pass through the laser detection area. This constant pressure ensures that the cells are positioned appropriately without aggregation, allowing for accurate measurements of their characteristics.

Maintaining a constant pressure with sheath fluid is essential for reproducibility and reliability in flow cytometry results, as fluctuations in flow rate could lead to inconsistent sample presentation and thus variable data outputs. This alignment is foundational for the effective functioning of the flow cytometer and is vital for accurate analysis.

This function of the sheath fluid distinguishes it from other options, which do not accurately represent the role of sheath fluid in the flow cytometry process. For instance, the pressure dynamics are more nuanced than just varying with sample flow rate or being exclusively for staining purposes. Moreover, sheath fluid does not heat the cells prior to analysis; instead, it primarily serves to keep the cells in the correct orientation and flow for measurement.