The bladder is a hollow muscular organ, capable of increasing and decreasing in size, similar to a balloon. It sits in the pelvis, underneath the uterus when the woman is not pregnant. In pregnancy, as the uterus grows, the bladder sits in front of the uterus. In late pregnancy, if the bladder becomes very full, it can distend out of the pelvis and be seen as a bulge above the pubic line in the lower abdomen. This is something your caregiver may look for in labour if you are having difficulty passing urine.
Image 2-19 shows the bladder before pregnancy.
Image 2-20 shows the bladder in late pregnancy.
The hollow bladder has 3 openings that lead to tubes. The 2 tubes coming into the bladder from the top are the '
ureters'. They feed the urine into the bladder from the kidneys. The tube at the base of the bladder is the
urethra. This tube releases the urine from the bladder when you 'micturate' or pass urine. The urethra passes through the pelvic floor muscle and in women, is about 3.5 cm in length
At the point where the urethra meets the bladder, there is what is known as the 'urethral sphincter'. This is a circular muscle surrounding the urethra that acts as a valve, contracting and relaxing to control urination. It is the relaxing of this sphincter and the relaxing of the pelvic floor muscle that allows you to pass urine.
As the bladder fills with urine, the pressure increases inside the bladder. When the pressure becomes greater than the pressure of the urethral sphincter (usually when you have about 300-500 mls of urine stored, 500-700mls for men!) you start to feel the urge to empty your bladder. When you are ready (hopefully), you then consciously relax your pelvic floor muscle and internal sphincter to pass urine.
Having a higher pressure in the bladder can also happen when the baby presses on it from the neighbouring uterus.