THE kidneys are the organs that filter off the waste products that collect in our blood, and concentrate them into the urine. We are not concerned here with how they do it; that we must leave to Physiology. Each kidney is a fleshy object that is composed of thousands and thousands of tiny tubes which secrete the urine. This passes from the kidney into a long tube known as the ureter, which leads the urine down into the bladder.
The wall of the ureter, like that of the intestine, is composed of two thicknesses of muscle tissue which by their contraction and relaxation propel the urine down into the bladder. When the ureter enters the bladder it pierces its muscular wall very obliquely so that it has an oblique course inside the bladder wall for about half an inch. While this ingenious arrangement allows urine to flow freely down the ureter into the bladder, it effectively prevents urine from pass-
ing in the opposite direction if large quantities should collect under pressure in the bladder for any reason, thus stopping the development of a back pressure upon the kidneys which might damage them. Sometimes when there is long-continued obstruction to the outflow of urine from the bladder, the bladder becomes stretched so much that this mechanism breaks down and the kidneys are inevitably seriously damaged.
The bladder is the receiver in which urine is stored until a suitable opportunity for voiding it offers itself. The ureters deliver urine from the kidneys at the rate of a drop about every two seconds, so we can readily see that if it were not for the bladder we should spend our whole day passing water continuously. The wall of the bladder is composed of muscular tissue, so that it can contract and relax at will. The circular muscle is collected into a thick ring or sphincter which surrounds the channel which carries urine away from the bladder and is known as the urethra. When the bladder muscle contracts in the act of passing water this sphincter is relaxed so that urine can escape, but at all other times it is tightly closed.
During the time that passes after the bladder is evacuated urine steadily collects inside it from the ureter. The muscle of the bladder allows itself to become progressively relaxed so that the pressure inside the bladder remains more or less at a constant level. Eventually, however, so much urine collects that it is difficult for the bladder to relax any further, so the pressure suddenly rises. The nerves in the bladder then send messages to the spinal cord complaining of the increase in pressure and asking for the bladder to be emptied. These impulses reach consciousness and we can either empty the bladder if we wish to or, if the opportunity does not nrise, we can consciously relax the bladder still further and so reduce the pressure and the demand for evacuation.
This conscious control over the bladder is only learnt by experience and education, and its lack in infants explains why they pass their water at regular intervals. It is only when they become older and acquire this conscious control that they become ‘clean.’ It is never wise, therefore, to scold or punish a child for being ‘dirty,’ because it is not his fault. It is only that this conscious control has not yet developed or has been retarded by some special circumstances.
When water is passed the bladder walls contract and the sphincter closes, and at the same time the muscles of the abdomen contract under the control of the brain so that their
effect is added to that of the bladder. The urethra is simply the tube which carries the urine away from the bladder. In the male it is long and opens at the end of the penis, but in the female it is very short. It is owing to the greater length of the canal in the male and the presence of a gland called the prostate which surrounds it in its upper part that it is so much more frequently diseased in men than in women.