The intravesical pressure rises abruptly when the bladder volume is about

The relation between intravesical pressure and volume can be studied by inseing a catheter and emptying the bladder, then recording the pressure while the bladder is filled with 50-mL increments of water or air(cystometry). A plot of intravesical pressure against the volume of fluid in the bladder is called a cystometrogram). The curve shows an initial slight rise in pressure when the first increments in volume are produced; along, nearly flat segment as fuher increments are produced; and a sudden, sharp rise in pressure as the micturition reflex is triggered. These three components are sometimes called segments Ia, Ib, and II. The first urge to void is felt at a bladder volume of about 150 mL, and a marked sense of fullness about 400 mL. The flatness of segment Ib is a manifestation of the law of Laplace. This law states that the pressure in a spherical viscus is equal to twice the wall tension divided by the radius. In the case of the bladder, the tension increases as the organ fill, but so does the radius. Therefore, the pressure increase is slight until the organ is relatively full. Ref: Ganong&;s review of medical physiology,23rd edition, page: 661