How the bladder works
It is important to understand how the bladder works, because there are many different types of incontinence, which may have quite different causes.
Anatomy of the bladder
The bladder consists of a flexible sac of muscle (the detrusor muscle). Urine is produced in the kidneys and passes into the bladder through the ureters. It is then stored in the bladder until it is released. During storage the urine is retained in the bladder by a ring of muscles at the bottom of the bladder, called the urethral sphincter, which squeezes shut.
The bladder neck, the area where the bladder and urethra meet, is partly supported in its position by the pelvic floor muscles which form a sling in the pelvis, helping to support the bladder, vagina and rectum.
The pelvic floor helps to hold the urethra in position on the underside of the pelvic bone. In this position, the raised pressure in the abdomen caused when you cough or sneeze is transmitted to the urethra, as well as the bladder, and has an equal effect. This is known as the pressure transmission theory and forms the basis of our understanding of continence. The pressure transmission theory is also the principle on which most surgical operations are based.
Bladder function is highly complex. It requires coordination from several parts of the brain and involves both involuntary and voluntary activity. This can be illustrated by looking in more detail at the urethral sphincter. This is made up of two parts each with a different function.
The inner sphincter is made up of involuntary muscle; the brain operates this without conscious thought. It maintains a constant steady pressure, squeezing the urethra closed. It is helped by the lining of the urethra which is folded inwards many times so that when compressed it will give a watertight seal.
The outer sphincter is made up of muscle that it is under more voluntary control and it is this, along with the pelvic floor, that can be consciously squeezed when trying to prevent leakage. It is capable of very strong contractions but only for a short period of time. The muscle can be fatigued, which is why a sneezing fit may cause leakage only after the third or fourth sneeze.
Development of bladder control
A newborn baby will empty his or her bladder about once an hour under reflex control, which means that the bladder empties automatically when it feels full. This involves only the bladder and the nerves running between the bladder and the spinal cord – at this stage the brain is not involved. The sensory nerves are stimulated by the filling of the bladder. These nerves in turn are connected to the motor nerves, which cause the bladder to contract.
At the same time the urethra relaxes, allowing urine to pass from the bladder to the outside. The bladder fills and then empties; it is not yet used for storing urine.
As the baby gets older (around the age of two years), the brain develops and starts intercepting the messages from the sensory nerves. The brain can then suppress the impulse to make the bladder muscle contract and stop the reflex emptying of the bladder. The working bladder capacity will then increase and the bladder develops into a storage organ. Through potty training we learn what is acceptable behaviour and start to use the parts of our brain connected with bladder control.
Higher brain functions may also affect the bladder, for example, wanting to urinate when you hear running water.
What is ‘normal’ bladder functioning?
Bladder function can be thought of in two phases: filling and storage of urine, and emptying (voiding).
- In filling, the urethra is squeezed shut while the bladder itself is relaxed, expanding as it fills with urine.
- In voiding, the urethra relaxes just before a contraction of the detrusor muscle in the bladder wall.
The urine is then pushed through the urethra to the outside.
How often you pass urine depends on how much urine is produced as well as how much urine the bladder will hold. If you drink 1.5 litres a day and your bladder normally holds 400 millilitres (ml), then you will empty your bladder approximately four times that day. A bladder that holds only 100 ml results in passing urine 15 times. If you drink twice as much, you will need to empty your bladder twice as frequently.
Normal frequency of voiding is up to seven times a day or not more than every two hours. In young women, the bladder normally holds 400 to 600 ml and is usually emptied when holding 250 to 400 ml. As people age, their bladder capacity tends to decrease, leading to increased frequency of micturition (voiding), especially at night.
How may problems arise?
If the bladder neck and urethral sphincters are damaged (which may happen during childbirth), they will not be as effective at sealing the urine inside the bladder. The bladder neck may also move downwards if the structures that support it are weakened, and this will add to the problem. Again this may result from childbirth, but straining may also be a cause (for example, with constipation or a chronic smoker’s cough).
The bladder itself may be unstable or overactive. This is commonly referred to as the overactive bladder syndrome or complex, which includes the symptoms of urgency, frequency plus or minus nocturia, and urgency incontinence. The symptoms are often investigated by urodynamics and a diagnosis may be made on the urodynamics of detrusor ovaractivity (this is known as urodynamic detrusor overactivity).
It is not known exactly what causes this, but it may be linked to loss of normal control of the bladder-emptying reflex, or nerve damage from childbirth or previous incontinence surgery. Anything that interferes with the parts of the brain involved in modifying bladder activity can affect bladder function – for example, a stroke or a spinal injury may interrupt the connection between the higher parts of the brain and the bottom of the spinal cord, resulting in a return to the reflex voiding pattern of a baby, incomplete emptying or loss of control.
Any kind of mass pressing on the bladder – for example, fibroids or a rectum full of faeces because of constipation – can cause problems. These problems are all looked at in more detail later.
KEY POINTS
- Normal bladder control is highly complex
- Bladder control is learned during early life
- How often you pass urine depends on how much you drink and the capacity of your bladder
- Continence relies on normal positioning of the bladder neck, normal nerve control of the bladder, and normal coordination and mental state (people who are unconscious or demented cannot control their
