The component parts of the urinary tract, responsible for the passage and control of urine from the kidney to the outside of the body, are the ureters, the bladder and the urethra.
From kidney to bladder
The ureters are muscular tubes 25cm long which conduct urine from the kidney to the bladder. Each has an inner lining of a waterproof layer of transitional epithelium, found only in the conducting passages of the urinary system. The cell walls are much thicker than most and the junctions betweenare particularly tight, thus preventing urine leaking between them into the body . Moreover, the cell layers of this epithelium can be stretched by the volume of urine without losing the property of preventing leakage. The muscle of the ureter is arranged in spiral bundles each interwined with another outside the epithelial lining, known as the urothelium. The ureters function by propelling boluses (small packages) of urine from the kidney by a process of rhythmic muscular contraction called peristalsis. This is not controlled by any nerves; instead, electrical excitation is transferred from one smooth muscle cell to the next in the ureter.
Although nerves do not cause the ureter to propel urine, there is, however, a rich nerve supply which is responsible for the colicky pain felt when a stone passes down the ureter from the kidney. This pain is typically felt in the loin, from where it can radiate in the direction of the groin.
Each ureter enters the bladder at an angle, through its muscular wall, and opens as an inverted U at a place called the trigone, a triangular area at the base of the bladder. The angle of the connection makes it function as a valve. When the bladder contracts in order to void urine, the valve mechanism prevents urine from flowing back up to the ureters.
The structure of the bladder wall has similarities to that of the ureter. There is urothelium which has the same layers of waterproof. As with the ureter, the outer connective tissue layer contains vessels and lymphatics. The muscles surrounding the bladder are in a loose arrangement like the woven strands of a basket; individual fibres sweep round the bladder and pass from the outer layer to the inner layers and back again. Elastic fibres are interspersed with these muscular strips. This musculature is known as the detrusor muscle.
The structure of the male and female urethra is different. In men it is a tube 20 cm long. The first part of it passes through the prostate gland. If this gland hypertrophies – as it often does in old age – it can cause difficulties in urinating.
The female urethra is much shorter and wider than its male counterpart. This makes the female much more susceptible to urinary tract infections.
The bladder stores urine, until the time comes when it is appropriate to urinate. At rest continence is ensured because the muscle of the bladder is relaxed and the sphincters around the urethra are contracted shut. The bladder and its sphincters get their impulses from nerves. In fact there is a two-tier system of nervous control. At a basic level there is a local reflex arc which causes the bladder to empty automatically when full. Nerve cells that monitor the stretch of the bladder travel to the spinal cord. To initiate urination they stimulate nerve cells that cause the bladder to contract and the bladder sphincters to relax. In newborn children urination is dependent only on this reflex arc. As the child grows older, higher centres in the brain begin to have an influence on this reflex arc also. It then becomes possible to contract a sphincter voluntary, so urinating can be postponed until an appropriate moment. This conscious effort to hold back urination works only until the bladder is really full, when the reflex can no longer be inhibited.
Under normal circumstances about 1 to 1.5 litres of urine is produced every day. The total amount de- pends on how high the fluid intake has been and how much water is lost by sweating, by breathing or in the digestive system. All kinds of substances are dissolved in the urine. The most important source of waste products is the metabolism of protein and the breakdown of old cells. This results in considerable amounts of urea, the amino acid creatine and uric acid, all of which are filtered out of theby the kidneys and incorporated in the urine. To maintain the level of electrolytes in the body at an optimal level, any excess has to be excreted. This is also an important task of the kidneys; therefore sodium chloride (salt), potassium, calcium, phosphate and magnesium salts can be found in varying amounts in the urine.
Vitamins,and drugs (whether converted or detoxified by the liver or not) are also constituents of the urine. During the first half of pregnancy relatively large amounts of a special hormone, human chorionic gonadotrophin, are excreted. An accurate pregnancy test is based on detecting this hormone in only a few drops of morning urine.
Examination of the urine reflects the way in which the body is able to perform some vital functions. It can therefore be an important clue in diagnosing several disorders – not only those of the kidneys and the urinary system, but also diseases of other systems such as diabetes mellitus.
On investigating the urine, attention is paid to various qualities. Normal urine is a clear yellow solution. When it looks cloudy, this may be the result of an infection. A red discoloration can be caused by food (beetroot), certain laxatives or it may be due to the presence of blood in the urine. If the latter is the case, a more thorough investigation is necessary to look for the cause of the bleeding, for example a stone in the urinary system (that has damaged the epithelial linings) or an infection.
Other abnormal substances in the urine, which can be detected only by using special tests, are glucose (sugar) and proteins. Normally the kidneys reabsorb all the sugar that is filtered, and thus sugar should be absent from the urine. In diabetes however the sugar level in the blood gets so high that it is not possible for the kidneys to reabsorb all the filtered sugar. As sugar passes into the urine, a large amount of water is attracted also; this is why diabetics urinate so much. And because sugar provides a good growing medium for bacteria, it is obvious that infections are more liable to develop.
Normally urine in the kidneys and bladder is sterile; bacteria are present only in the end part of the urethra. This means that voided urine always is contaminated to some extent.
In the case of a urinary infection however, examination of urine under microscope shows that there is a great excess of bacteria (and that there are white blood cells present as a sign of an inflammation).