When the bladder is adequately distended it assumes an oval shape and usually contains about 0.5 litres of urine. The long diameter measures around 12cm, and is directed upwards and forwards. The bladder presents a postero-superior, an antero-inferior and tow lateral surfaces, a fundus and a summit. The postero-superior surface is covered by peritoneum and is separated from the rectum by the rectovescial excavation. The antero-inferior surface has no peritoneum on it and rests against the pubic bones, above which it is in contact with the back of the anterior abdominal wall. The lower parts of the lateral surfaces are also devoid of peritoneum and are in contact with the lateral walls of the pelvis. The fundus does not alter much in position and exhibits a narrow triangular area which is bounded below the prostate, above by the rectovescial fold of peritoneum, and laterally by the ductus deferentes. They are frequently in contact with each other above the prostate, which leads to the lower part of the triangle being obliterated. The line of reflection of the peritoneum from the rectum to the bladder undergoes little or no change when the bladder is distended and is situated about 10 cm from the anus. The summit is directed upward and forward above the attachment point of the middle umblical ligament. The peritoneum which follows the ligament forms a pouch of varying depth between the summit of the bladder and the anterior abdominal wall. A distended bladder can cause a person to be unable to urinate, even if they have a full bladder. A distended bladder can be caused by an enlarged prostate which blocks off the urethra and makes urinating difficult, if not impossible. A distended bladder can also be caused by infection and various prescription and over-the-counter medicines.