1.
Now, after having thoroughly discussed the kidney in the previous sections (6 pages!), we come to the rest of the urinary system which consists of three parts:
a) two ureters
b) one bladder
c) one urethra
2.
The ureter is a long tube, consisting of smooth muscle cells, which squeezes (“push”) the urine from the kidneys to the bladder.
3.
The bladder, of course, is where the urine is stored until it becomes too full and we need to pee!
4.
And, when we pee, we do this through the urethra, which connects the bladder to the outside world, in this case, the perineum.
B. The two ureters:
1.
The two ureters push the continuous flow of urine from the kidney pelvis to the bladder.
2.
This propulsion is an active phenomenon, very similar to what happens in the small intestine, and these propagating contractions (= peristalsis) push the urine into the bladder.
3.
Normally, one does not notice these waves but they can become very painful when a patient suffers from kidney stones.
4.
These stones are formed in the kidney pelvis and can get stuck somewhere in the ureter because of their relative large size.
5.
Extremely painful! It used to require surgery to remove these stones.
6.
Nowadays most stones are disintegrated to rubble with lithioptropsy (extracorporeal shock waves) and are then lost with the urine.
1.
There are two sphincters that control the release of urine (=micturition).
2.
The inner sphincter, located at the bottom of the bladder, consists of smooth muscles, which are controlled by the autonomic nervous system. This is the main sphincter that controls micturition.
3.
The outer sphincter, located further along the urethra, consists of skeletal muscle cells and is therefore controlled by the voluntary nervous system.
4.
There are some differences however in the external sphincter between males and females.
5.
In males, the prostate gland (which releases semen during ejaculation) is located just below the bladder and between the internal and external sphincters.
6.
Therefore, if a male ejaculates (= orgasm!), the internal sphincter must be closed (to avoid the semen to disappear in the bladder) but the external sphincter must be opened so that the semen can ejaculate through the penis.
7.
Females don’t have a prostate. In their case, the urethra can be closed both by the external sphincter and by the muscles surrounding the urethra and the vagina as their urethra ends just close to the clitoris at the top of the vagina, and the outside world.
8.
In males on the other hand, after the external sphincter, the urethra runs all along the length of the penis before arriving in the outside world!
9.
Therefore, in females, the urethra is quite short, typically 5-6 cm whereas this is much longer in males as it runs through the penis (about 10-12 cm long).
10.
It is therefore easier for bacteria to invade the bladder in females than in males, hence bladder infections are more common in females.
1.
By the way, we have seen such a similar arrangement of two sphincters before! I mean, two sphincters, one involuntary and one voluntary sphincter!
2.
We saw this at the end of the colon, in the digestive system (E.6. colon, panel E) and, also, at the start of the digestive system, in the oesophagus.
3.
However, in the oesophagus, interestingly, the sequence is opposite to that in the urethra and the anal canal; a (voluntary) skeletal sphincter at the beginning of the oesophagus and a smooth (involuntary) muscle sphincter located at the end of the oesophagus, close to the stomach (remember the UES and the LES?).
4.
In other words, when things have to go in or out of the body, we have two separately controlled gateways to avoid mistakes!
1.
The muscle of the bladder (detrusor muscle) and the sphincters are controlled by nerves coming from the spinal cord and, higher up, from the brain.
2.
The major nerves are the pelvic nerves that contain both sensory nerves and motor nerves. The sensory nerves detect the amount of stretch in the bladder wall and send signals to the spinal cord. This will increase as the bladder fills up with urine.
3.
The motor nerves, from the spinal cord, can induce the smooth muscles of the bladder to contract. It is controlled by the parasympathetic nerve system.
4.
Then there is a special nerve for the external (skeletal) sphincter. This is the pudendal fibre that also comes from the spinal cord. The pudendal nerves are somatic nerves, controlled by the brain, like all skeletal muscles.
5.
Interestingly, when the bladder gradually fills, the parasympathetic nerves will increasingly initiate a micturition reflex.
6.
As the stretch receptors are activated, they send impulses to the spinal cord and activate the parasympathetic nerves to contract the bladder wall. This is automatic, a reflex, that occur in ‘waves’.
7.
After a few minutes, the micturition reflex stops and the bladder relax. You may have noticed that, when you have an urge to urinate, after some time, the urge disappears. This is normal.
8.
However, after some time, as the bladder gradually fills, these reflexes (‘urges’) will occur more and more frequently.
9.
Of course, in babies, who have not learned to control their urges, they will spontaneously urinate! (pampers!).
10.
But at a certain age, children are taught to control their bladder. In fact, what they learn is to control their external sphincters.
11.
The brain therefore does, to a degree, control how the reflex is perceived in the brain. Usually, after a few minutes, the brain will inhibit the reflex. It also keeps sending signals to the external sphincter to remain closed.
12.
But when it is time (and appropriate!) to urinate, then the brain signals to the bladder to start another micturition reflex and, at the same time, relaxes (inhibits) the external sphincter. Result: micturition!
1.
Finally, what about urine itself? We know it as a yellowish fluid that also has not such a nice smell?
2.
Urine contains 95% water and several dissolved compounds or waste:
3.
These compounds can be classified as:
a) nitrogenous (= containing nitrogen)
b) electrolytes (ions)
c) urobilinogen (breakdown of erythrocytes) (link)
4.
The main nitrogenous compounds are:
a) urea (breakdown product of amino acids)
b) creatinine (from muscle cells)
c) ammonia (breakdown product of proteins)
d) uric acid (breakdown product of nucleic acids)
5.
The yellowish colour comes from the dissolved urobilinogen. If it is pale yellow, then the urine contains a lot of water and you are well hydrated. If it becomes darker, that means there is less water in the urine and therefore in the body.
6.
Consumption of all kind of foods can also change the colour of urine, such as eating asparagus (‘greenish’) but may also indicate the presence of pathology, such as reddish, which could be caused by a bleeding somewhere ‘upstream’!
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Diagram of the kidneys, the ureters, bladder and urethra:
F.3.5. The Collecting Ducts