Schematic of the model. Top: a view of the cell from above. The contractile vacuole (darker circle) is treated as a cylinder sitting inside of a cylindrical cell (lighter circle). Middle: a side view of a cell. The contractile vacuole has a diameter, DV, and a height, H, which is equal to the height of the cell. Cytoplasmic pressure, denoted PC, squeezes water out of the contractile vacuole through a pore. The pressure inside the vacuole is assumed to be equal to the cytoplasmic pressure. As water exits through the pore, the diameter of the contractile vacuole begins to shrink, whereas the height of the contractile vacuole remains constant. Inset: a side view of the pore with diameter DP and width LP. Bottom: an equation describing the reduction in the contractile vacuole cross-sectional area, A, over time, t, as water leaves the vacuole through the pore. The contractile vacuole area is described as a function of the initial vacuole area A0, the area of the pore AP, the pressure difference across the pore (PC PO), the viscosity of water , the length of the pore LP, and the height of the contractile vacuole, H.
