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What is the Renal Pelvis?

By Mike Calhoun
Updated Mar 03, 2024
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The renal pelvis is an important part of the kidney in humans and many animals that essentially acts as a filter for urine before it passes out of the kidney and into the bladder. It looks sort of like a cup-shaped open cavity, and it can usually expand or retract in response to urine volume. Its main role is to regulate the amount of urine flowing to keep things more or less constant and to avoid overwhelming the bladder or urinary tract. Some people have abnormally large or strangely shaped cavities, and this doesn’t usually pose a problem. The area is commonly the site of cell mutation, however, and this often is problematic. This cavity is where most kidney cancers start. Tumors and cancerous growths here can be really dangerous, since they can block the flow of urine and lead to infection, inflammation, and ultimately organ failure.

Location and Basic Anatomy

The human kidneys are two bean-shaped organs that are located near the middle of the back, immediately below the rib cage. Kidneys perform the essential function of removing waste products from the blood and regulating fluid levels. The waste and extra fluids become urine, which drains into the pelvis. This cavity collects the urine before it is passed on to the urinary bladder.

Generally, the renal pelvis is shaped like a funnel and collects urine from the thousands of nephrons found in the kidney. Each kidney has one pelvis, and each consists of a network of intersecting smooth muscle fibers. Smooth muscles are involuntary muscles, meaning that they cannot be consciously controlled. The smooth muscle fibers are bound together into sheets or bundles by reticular fibers and can be thought of as elastic fiber “nets” that are somewhat stretchable.

Looking at a cross-section of the cavity will usually reveal three distinct layers. The serosa is the outer-most layer, and is a thick layer of tissue. Just below that is the muscle layer, which is the bulk of the cavity. The muscle is what expands and contracts. This layer is lined with what is known as mucosa, which is made mostly of transitional cells that act as insulation.

Main Function

There are two main jobs that this cavity performs: first, it collects the urine produced by the kidney; then, it regulates the speed and timing with which that urine flows out into the bladder. Both are important to proper organ functioning and both contribute to a healthy urinary tract. Funneling the fluid in a controlled way puts less of a strain on the kidneys and the bladder, and also helps ensure that the each kidney is working in tandem with the other.

Abnormalities and Defects

In some individuals, the cavity becomes enlarged and protrudes from the kidney. This isn’t normal, but it isn’t usually a problem, either. The condition is called extra-renal pelvis in the medical community, which basically means that it’s sticking out. Most of the time people who have this condition don’t know it unless they are getting their kidneys imaged or have the organs exposed during surgery, since they usually work just fine and the protrusion doesn’t typically impact the cavity’s functioning.

Cancerous Growths

Most kidney cancers start in the renal pelvis, which makes the area one that medical professionals often pay particular attention to during exams. Cancers here usually are carcinomas, which basically means that they originate in smooth muscle epithelial cells. They typically start as small cell growths on the mucosa walls that spread to the renal tubes. As they grow into tumors they can block these tubes, or use them as vehicles to spread to the bladder, the stomach, and elsewhere in the digestive tract. This type of cancer is usually somewhat rare. It is often very treatable, but a lot of this depends on how early it was detected and how much it has spread.

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Discussion Comments

By GrumpyGuppy — On Oct 28, 2011

@snowywinter- Yes, it is true that smoking can increase your risks of developing renal cell cancer. Smoking can actually increase your risks of developing most kinds of cancers. There are several risk factors for developing renal cancer. However, just because it is a risk factor doesn’t mean that you will develop the cancer. You should always discuss all risk factors with your doctor, including smoking.

By SnowyWinter — On Oct 27, 2011

Is it true that smoking can contribute to one's chances of getting renal pelvis carcinoma?

By JavaGhoul — On Feb 04, 2011

Bacterial infection in the region of the renal pelvis and kidney may cause an inflammation called pyelonephritis. Symptoms may include chills and back pain.

By BigBloom — On Feb 02, 2011

The cup-like collecting region of the renal pelvis is called the "calyx." This word comes to us from Greek via Latin, and means "case of a bud," and is related to the word for "to hide."

By Renegade — On Jan 31, 2011

The most common cause of kidney cancer is renal carcinoma, which is ani infection of the blood filters in the kidney. Signs of this disease are many and may be considered strange, including brown urine, excessive hair growth in females, or enlargement of the left testicle.

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