Urine is continuously formed by nephrons and it flows into urinary bladder drop by drop through ureters. When urine collects in the pelvis of ureter, the contraction sets up in pelvis. This contraction is transmitted through rest of the ureter in the form of peristaltic wave up to trigone of the urinary bladder. Peristaltic wave usually travels at a velocity of 3 cm/second. It develops at a frequency of 1 to 5 per minute. The peristaltic wave moves the urine into the bladder. After leaving the kidney, the direction of the ureter is

initially downward and outward. Then, it turns horizontally before entering the bladder. At the entrance of ureters into urinary bladder, a valvular arrangement is present. When peristaltic wave pushes the urine towards bladder, this valve opens towards the bladder. The position of ureter and the valvular arrangement at the end of ureter prevent the back flow of urine from bladder into the ureter when the detrusor muscle contracts. Thus, urine is collected in bladder drop by drop. A reasonable volume of urine can be stored in urinary bladder without any discomfort and without much

increase in pressure inside the bladder (intravesical pressure). It is due to the adaptation of detrusor muscle. This can be explained by cystometrogram.


Cystometry is the technique used to study the relationship between intravesical pressure and volume

of urine in the bladder. Cystometrogram is the graphical registration (recording) of pressure changes in urinary bladder in relation to volume of urine collected in it.

Method of Recording Cystometrogram

A doublelumen catheter is introduced into the urinary bladder. One of the lumen is used to infuse fluid into the bladder and the other one is used to record the pressure changes by connecting it to a suitable recording instrument. First, the bladder is emptied completely. Then, a known quantity of fluid is introduced into the bladder at regular intervals. The intravesical pressure developed by the fluid is recorded continuously. A graph is obtained by plotting all the values of volume and the pressure.

This graph is the cystometrogram.

Description of Cystometrogram

Cystometrogram shows three segments. Segment I Initially, when the urinary bladder is empty, the intravesical pressure is 0. When about 100 mL of fluid is collected, the pressure rises sharply to about 10 cm H2O.

Segment II

Segment II shows the plateau, i.e. no change in intra vesical pressure. It remains at 10 cm H2O even after

introducing 300 to 400 mL of fluid. It is because of adaptation of urinary bladder by relaxation. It is in

accordance with law of Laplace.

Law of Laplace

According to this law, the pressure in a spherical organ is inversely proportional to its radius, the tone remaining constant. That is, if radius is more, the pressure is less and if radius is less the pressure is more, provided the tone remains constant.


P =


Where, P = Pressure

T = Tension

R = Radius

Accordingly in the bladder, the tension increases as the urine is filled. At the same time, the radius also

increases due to relaxation of detrusor muscle. Because of this, the pressure does not change and plateau appears in the graph. With 100 mL of urine and 10 cm H2O of intravesical pressure, the desire for micturition occurs. Desire for micturition is associated with a vague feeling in the perineum. But it can be controlled voluntarily. An additional volume of about 200 to 300 mL of urine can be collected in bladder without much increase in pressure. However, when total volume rises beyond 400 mL, the pressure starts rising sharply.

Segment III

As the pressure increases with collection of 300 to 400 mL of fluid, the contraction of detrusor muscle becomes intense, increasing the consciousness and the urge for micturition. Still, voluntary control is possible up to volume of 600 to 700 mL at which the pressure rises to about 35 to 40 cm H2O.

When the intravesical pressure rises above 40 cm water, the contraction of detrusor muscle becomes still

more intense. And, voluntary control of micturition is notpossible. Now, pain sensation develops and micturition is a must at this stage.

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