How does the urinary system work?

Once urine is produced in the kidneys and collected by the calyces and renal pelvis, it flows down the ureters into the bladder. The ureters are very thin tubes, about 5 mm in diameter, which have peristaltic movements (like those of the intestine) that allow the urine to move more easily. Once it reaches the bladder, urine is stored in the bladder until it is expelled during urination.

 The functioning of the bladder is very complex, as it depends on three different types of nerves, namely the hypogastric nerve belonging to the sympathetic nervous system, the pelvic nerve belonging to the parasympathetic and the pudendal nerve belonging to the voluntary or somatic nervous system. The sympathetic and parasympathetic nervous systems belong to the autonomic nervous system, which, as the name suggests, is a nervous system that is not under the voluntary control of the brain. It is not the aim of this article to explain the complex micturition reflex in detail, I will only point out that all these nerves start from neuron nuclei in the spinal cord (T11-L2 for the hypogastric nerve, S2-S4 for the pelvic nerve and for the pudendal nerve), which in turn are regulated by higher brain structures. So one can imagine that when there are problems in the spinal column (a herniated disc for example) or there is a neurological disease (such as Parkinson's disease, Alzheimer's disease, multiple sclerosis or stroke, among many others) the micturition reflex can be affected. Under normal conditions, the coordinated work of these three nervous systems is essential for both micturition and continence to occur properly. Thus, during urination, the detrusor muscle of the bladder must contract thanks to the nerve impulse it receives from parasympathetic fibres, while the bladder neck (or internal sphincter) and the external sphincter relax, the former governed by sympathetic fibres and the latter by somatic fibres of the pudendal nerve. If this coordination does not occur properly, dyssynergic micturition or uncoordinated micturition will occur, with contractions of one or both sphincters during voiding, incomplete bladder emptying, or both. Outside of micturition, the detrusor muscle is at rest because there is a high sympathetic tone and low parasympathetic tone which allows its fibres to relax and give capacity to the bladder. At the same time, the bladder neck is contracted thanks to sympathetic tone and the external sphincter is contracted thanks to the action of the pudendal nerve. This situation allows us to be continent and not lose urine while the bladder is filling.

 Having seen this, we can understand that any obstruction to the outflow of urine, whether of anatomical origin (hypertrophy of the prostate, cicatricial or congenital stenosis of the urethra) or of functional origin by which the sphincters do not relax properly during urination (uncoordinated urination, dyssynergia of neurological cause), can promote urine infections, dyssynergia of neurological cause), can promote urine infections, as can any disease that causes a weakness of the bladder muscle that does not allow the bladder to expel urine correctly, even if there is no obstruction.