How is urine produced?

There are normally a pair of kidneys at the back of the abdomen in the area of the loins. Within the kidneys part of the blood plasma is filtered from the capillaries into fine tubules. Much of the fluid is selectively reabsorbed as it passes through these tubules. When the body is short of fluid almost all the fluid is reabsorbed and when there is an excess of fluid the urine volume is increased. The tubules connect to the ureters there is one from each kidney which pass the urine down to be collected in the bladder.

The bladder is a bag of muscle (detrusor muscle) that can stretch without contracting as urine is collected. There are sensors in the bladder wall that tell the nervous system how far the bladder is being stretched as it fills. A normal bladder can hold about half a litre (1 pint). The bladder can empty the urine through the outlet tube called the urethra (Figure 2.2). The urethra is surrounded by muscle which contracts to keep the bladder exit closed but can relax at the appropriate times.

The pelvic floor is composed of muscle and ligaments strung across the pelvis like a hammock. The urethra, vagina and rectum pass through the pelvic floor. Muscles are considered to be involuntary (smooth) or voluntary (striped)). Smooth muscle contracts slowly and we cannot influence when it works. The muscle of the bowel is smooth you cannot stop it rumbling. Striped muscles can usually be controlled voluntarily and they contract quickly muscles in the limbs are examples. Both smooth and striped muscle play a part in bladder function. Most of the time, the smooth muscle of the bladder remains relaxed whilst the muscles around the urethra keep it closed. These muscles have a voluntary element so that it is possible to stop the flow in mid-stream. Additional closure pressure can be exerted by the pelvic floor muscle. Most of the time, you will not be aware of the bladder and its control, which is largely achieved by reflex action. When the bladder becomes distended, the messages from the bladder sensors reach the higher centres of the brain. During micturition (voiding of urine), the reverse occurs. The abdominal wall muscles can be used to speed voiding or initiate the flow.

During a cough or sneeze there is an automatic reflex which results in the pelvic floor muscles contracting. As the muscles contract the pelvic floor rises and this helps to squeeze the urethra keeping it closed.


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This is the personal website of David A Viniker MD FRCOG, retired Consultant Obstetrician and Gynaecologist - Specialist Interests - Reproductive Medicine including Infertility, PCOS, PMS, Menopause and HRT.
I do hope that you find the answers to your women's health questions in the patient information and medical advice provided.

I do hope that you find the answers to your women's health questions in the patient information and medical advice provided.

The aim of this web site is to provide a general guide and it is not intended as a substitute for a consultation with an appropriate specialist in respect of individual care and treatment.

David Viniker retired from active clinical practice in 2012.
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