The renal pelvises are part of the urinary tract. They catch urine from the kidneys and form the transition to the ureters. Urine flows through them into the urinary bladder.
What is the renal pelvis?
The renal pelvis (pelvis renalis) is a round to funnel-shaped sac that connects the kidney and bladder. It is the collection point for the final urine, which contains waste products from the blood: The kidneys filter pollutants, breakdown products of medications, and other end products of metabolism from the blood, purifying it. The final urine flows through tubular projections, the renal calices, into the renal pelvis. This tapers into the ureter, through which the urine flows into the bladder. There, the urine collects until it is excreted through the urethra. Experts refer to the system of renal pelvis and renal calices that delivers urine to the ureters as the renal pelvic caliceal system (NBKS). The renal pelvises are part of the kidneys, but together with ureters, bladder and urethra, they form the urinary tract.
Anatomy and structure
The renal pelvises lie within the kidneys, surrounded by the renal medulla. Toward the bladder, they narrow to form the ureters. Toward the renal medulla, they widen in a funnel shape to form the so-called renal calices, although this caliceal system can vary in appearance: In the ampullary type, about eight to ten short renal calices branch from the renal pelvises. In the dendritic type, on the other hand, the renal pelvis first merges into two larger calices, which in turn divide into eight to ten shorter calices. This gives the structure a tree-like appearance. The renal calices enclose the renal papillae – the tips of the pyramid-shaped renal medulla. There are collecting tubes, into each of which about ten renal tubules open. Each of these tubules merges into a capsule that encloses a tangle of vessels called the glomerulus. The encapsulated vascular tangle is also called the renal corpuscle. Together, the renal tubules and renal corpuscles form the so-called nephrons. About 1.4 million of these small filters are located in the renal cortex, which surrounds the renal medulla and thus forms the outermost part of the kidney.
Function and tasks
The renal pelvis serves as a collection container for the final urine produced by the kidney tissue. The muscle cords in the renal pelvis and renal calices contract rhythmically for this purpose: As a result, the urine is continuously forced out of the calyces and fed into the ureter. From there, the urine flows into the bladder. When the bladder is full, it signals this in the form of an urge to urinate. Emptying can be consciously controlled. Urine is produced by the nephrons. To do this, the entire volume of blood flows through the kidneys around 300 times a day – the equivalent of almost 1,500 liters. The nephrons retain proteins and blood cells, while water, glucose, minerals and dissolved end products and waste products flow into the urinary tubules. More than 99 percent of the so-called primary urine, including mainly glucose and minerals, passes through the wall of the tubules back into the blood. This process is called reabsorption and prevents the body from drying out and losing all vital salts and minerals. The remainder of the filtrate, called final urine, consists mainly of decomposition substances and water. It passes through the urinary tubules into the renal medulla and from there through the renal papillae into the renal calices and the renal pelvis. This can hold about six to ten milliliters of urine and is part of the body’s own sewage treatment plant. In total, the body produces up to two liters of final urine per day.
Diseases and ailments
The most common disease of the renal pelvis is renal pelvic inflammatory disease. It usually develops from a bacterial infection of the bladder: the bacteria multiply and ascend through the ureter into the renal pelvis. Symptoms include flank pain and fever, and many patients also complain of difficulty urinating. Kidney stones, diabetes, insufficient fluid intake and malformations of the urinary tract increase the risk. Complaints can also be triggered by renal pelvic stones. They occur when substances that are normally dissolved in the urine – such as calcium or uric acid – are present in excessive quantities. These substances can then crystallize. Stones are often the result of an unhealthy diet. In particular, those who eat too much protein, drink too little or consume too much coffee and alcohol increase their risk. The majority of stones pass away on their own with the urine. However, if there is severe pain, the stone may be too large and stuck.Doctors can then remove it with the help of medication or smash it with sound waves. Very rarely, stones in the ureters can cause the renal pelvis to rupture (fornix rupture). Urine can then leak into the surrounding tissue, which is why a specialist should treat the tear immediately. Also extremely rare is renal pelvic carcinoma: This type of cancer occurs mainly in men over 50 years of age. Smoking is considered an additional risk factor. Renal pelvis cancer can be easily diagnosed by ultrasound or CT scan, but symptoms such as blood in the urine or kidney pain usually appear late.
