<prompt> The first urge to void occurs when the bladder contains ___ ml of urineimage_questionimage_questionimage_questionimage_question  <response>: The first urge to void is felt at a bladder volume of about 150 ml, and marked sense of fullness at about 400 ml. Cystometrogram The curve shows An initial slight rise in pressure when the first increments in the volume are produced Ia; A long, nearly flat segment as fuher increments are produced Ib A sudden, sharp rise in pressure as the micturition reflex is triggered II 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. P 2TR In the case of the bladder, the tension increases as the organ fill but does the radius. Therefore, the pressure increase is slight until the organ is relatively full. Ref: Ganong;s Review of Medical Physiology 26th edition Pgno: 682 