It's time to bone up on everything you never wanted to know about the bowel and bladder so you can understand what you're up dealing with. Read carefully!
Two donut-shaped muscles called “sphincters” control the flow of waste from the body. The bladder sphincter controls urine, and the anal sphincter controls bowel movements. When the child contracts a sphincter, the donut hole closes up, keeping urine and/or waste inside the body. When he relaxes a sphincter, it opens and waste flows out through the hole.
When the bowel and bladder are overly full, they expel waste automatically. If the pressure on the sphincter muscles becomes too great, they collapse, and the result is an accident.
Normal children eventually learn to control their sphincter muscles, but they can only control them to a certain point. When the bladder or bowel become too full, they expel waste, and the pressure on the sphincters increases until they give way. If a youngster is not on the potty at the moment, she will have an accident.
Toddler boys mature more slowly and finish potty training a few months later than girls, on average. Without any training or help, most children achieve nighttime bowel control first. This happens on its own, as a result of physical maturity. Daytime bowel control usually comes next, perhaps because most youngsters have at least one bowel movement at a predictable time each day so parents can help them get to the bathroom at that time to start teaching them. The contractions of the intestine signaling the start of a bowel movement are often pretty obvious, causing children to grunt and strain, so it is easier for parents and children alike to recognize when a bowel movement is starting.
During bowel movements both sphincters relax, so children urinate and defecate at the same time. By focusing on bowel training, children often become bladder trained at the same time.