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Why Do Bladders Act Small?

The bladder is a muscular sac at the bottom of the pelvis. The two tubes (ureters) that carry the urine from each kidney enter on the same side behind and below the bladder. The tube (urethra) that the pee comes out of is in the middle at the bottom of the bladder. There is a sphincter muscle wrapped around the first part of this tube, and this is meant to be tight all the time and only to relax when we want the pee to come out. The sphincter muscle is the reason we don’t leak all the time.

The pelvis of a child is small, and the bones of the pelvis are rigid. The pelvis forms a boney funnel, and there is very little room at the bottom of the pelvis. The bowel comes down the left side of the abdomen and is cheek-to-jowl with the bladder at the bottom of the pelvis. The rectum is right alongside the bladder. The bladder and bowel are in a very intimate relationship; a full bladder pushes on the rectum, and a full rectum pushes on the bladder.

Pregnant women understand about pressure on the bladder. The baby in their womb causes pressure on the bladder, and as the baby grows and takes up progressively more space in the pelvis, the mother needs to pee more often during the day and needs to get up at night to pee.

The pressure of poop causes a similar situation in the pelvis of a small child. The most common reason why a bladder does not hold enough urine is the effect of poop pressing on the bladder.

I teach parents that it is not a question of whether the poop is pressing on the bladder, but rather, in their child, how much of an impact this makes on the size of the bladder.

Some children who come to my office for the first visit have constipation that is already under treatment or that has been treated in the past. Some children have longstanding constipation that has not been recognized. However, most of the children who come to my office do not fit with the diagnostic criteria for constipation. These children might not have constipation per se, but neither do they have what I call “bladder-friendly bowel health.” Improving the bowel health is necessary to cure bedwetting.