Each kidney comprises several structures including the renal capsule that surrounds the organ, an outer layer called the renal cortex, and an inner part called a medulla. A network of blood vessels pass through the kidneys, and waste fluids are normally collected by the medullary pyramids, of which there are typically eight in each organ. Urine usually passes through here into the renal papilla on the way to the renal pelvis and then the ureter. Collecting ducts generally converge into a single channel for the waste fluids.
The major and minor calyces are the parts of the kidney where urine passes out of the renal papilla. Muscle movements called peristalsis are typically generated by pacemaker cells in the calyces to move urine out of the kidney. These structures are usually located at the tip of the renal papilla. The toxins, hormones, salts, and water that are normally filtered out of the body in urine typically pass through the papilla before they are excreted.
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Some research has found that adult kidney stem cells may be found in the renal papilla. While the cells in the kidney typically do not divide often, the organ can regenerate to repair damage. The studies showed that cells marked with a substance were generally not found during a repair stage following kidney damage. Cells in that section were not damaged, but developed sphere-like structures often characteristic of stem cells in an active state, leading some researchers to believe they may be stored in the renal papilla.
Various types of kidney disease can occur in the renal papilla. If parts of the structure die, the condition is called necrosis, and the results can be total kidney failure. Symptoms of tissue death in this area often include back pain, bloody or cloudy urine, as well as frequent or painful urination. Other problems such as chills and fever may occur as well. Diabetes, kidney infections, and sickle cell anemia are some causes of renal papilla necrosis; people with these conditions are often advised by doctors to be tested for signs of tissue death.
Many chemicals in the environment are potentially damaging to the kidney. Effects brought on by these substances are often focused on the papilla, causing cells in this area to die off. The renal papilla is just one of several structures essential for normal functioning of the kidney, which is usually the size of a typical computer mouse.