The urinary tract subsumes all organs and parts of organs that serve to collect and drain urine. All organs of the (draining) urinary tract are lined with the anatomically identical mucosa, the urothelium. Urinary tract infections can therefore spread to all organs of the urinary tract.
What are the urinary tract?
Schematic diagram showing the anatomy and structure of the urinary bladder. Click to enlarge. The beginning of the urinary tract is formed by the renal calices, which receive the secondary urine formed in the renal tubules and drain it into the renal pelvises. The secondary urine (urine) is formed by resorption of the primary urine and admixtures of certain secretions in the renal tubules. The renal pelvises act as the first collection point of the urine, so to speak. The two ureters, formed as hollow muscular organs, which connect the two renal pelvises with the urinary bladder, receive the urine and transport it into the bladder. This process occurs involuntarily through regular peristaltic contractions of the ureters. In the urinary bladder, the urine is first collected and, if it is full enough, a feeling of urgency to urinate is triggered. The urine can then be disposed of externally into the environment via the urethra. Unlike the involuntary drainage of urine from the renal pelvises to the urinary bladder, urination through the urethra is subject to volition.
Anatomy and structure
The renal calices and the renal pelvises are lined with the mucous membrane characteristic of organs of the urinary tract, the urothelium. Similarly, the ureters, which receive urine from the renal pelvises and carry it away to the urinary bladder, are also lined with urothelium. The two ureters consist of muscular tubes about 30 cm long and about 7 mm in diameter. The ureters are surrounded by a layer of smooth muscle cells that respond to signals from the autonomic nervous system and are not subject to will. Externally, the ureters are encased in a layer of connective tissue. At the point of entry into the urinary bladder, the ureters run a short distance inside the bladder wall. The urinary bladder represents a hollow organ that serves to collect and temporarily store urine. The lamina propria, a layer of connective tissue and collagen fibers, gives the bladder its strength. Emptying takes place – voluntarily – via the urethra. At the junction of the urethra and the bladder are two sphincters, one of which is vegetatively controlled by smooth muscle.
Function and tasks
The renal calices collect the secondary urine that continuously drips from the tubules into the renal calices and pass it on to the renal pelvises. The renal pelvises serve as the first intermediate storage site for the secondary urine. At the entrance to the renal pelvises, the ureters receive the urine and transport it further into the urinary bladder. The anatomical design of the ureters as muscular tubes is necessary in order to be able to drain accumulated secondary urine from the renal pelvises, even in a recumbent position, and if necessary against gravity, into the urinary bladder. The muscular tubes, which consist of smooth muscle, can perform their tasks via peristalsis, a dynamic and reflex contraction of the ureter. The unconscious contractions always run from the exit of the renal pelvis to the entrance of the urinary bladder and virtually force the urine from the renal pelvis into the urinary bladder. The entrance of the ureters into the urinary bladder is comparable to a check valve. It ensures that urine can only pass in one direction. Backflow (reflux) into the ureters or even into the renal pelvis is normally excluded. The urinary bladder performs the function of a urine collection container and can store up to a maximum of 1.5 l (man) and up to 0.9 l (woman) of urine. Urinary urgency usually occurs at a filling level of 300 ml to 500 ml. The voiding process can usually be controlled voluntarily.
Diseases and complaints
The most common disease of an organ of the urinary tract is cystitis or urinary tract infection, which is more common in women than in men because of the much shorter urethra. The inflammation, caused by bacteria, can spread to ureters and even renal pelvises, causing painful renal pelvic inflammation. Urinary stones can cause another problem.If urinary stones form in the renal pelvis, the body first tries to transport the stones to the bladder via the ureter. In most cases, the stones get stuck in the entrance area of the ureter, which stimulates the ureter to peristaltic contractions in order to transport the stone further. These unconscious contractions, which cannot be controlled voluntarily, result in severe pain and are known as renal colic. Hereditary malformations of the ureters are also known, especially at the entrance to the urinary bladder. Because all organs of the urinary tract are lined with the same, identically constructed, mucosa, urothelial carcinomas can form in all organs of the urinary tract and, if diagnosed early, can be removed by minimally invasive surgery and then subjected to chemotherapy.
