Medulla means middle, so in anatomy, medullary pyramids are areas in the middle of an organ that are shaped like a pyramid. The medulla oblongata pyramid of the brain is one such area. Most often, however, when medical professionals discuss the medullary pyramids, they are referring to the renal pyramids of the kidneys. The renal medullary pyramids are an important element in the production of urine, which is how the body rids itself of toxins and wastes that have been filtered from the blood by the kidneys.
Each human kidney has between seven and 15 medullary pyramids. They consist primarily of tubes through which the urine produced in the outer portion of the kidney travels. The end of the pyramid extends into a cup-shaped calyx where the urine collects, then is passed to bladder. Interlobar arteries, located in the space between the pyramids, carry blood to the inner and outer layers of the kidneys through a network of capillaries and arteries.
Nephrons are miniature structures within the medullary pyramids and the cortex, or outer layer, of the kidneys. Each kidney contains a million or more nephrons, which use a system of small tubes to filter the blood received. Nephrons receive liquids through a wide opening called the renal corpuscle, which consists of an area named the Bowman's capsule and the glomerulus, a ball-shaped construction. Elements of the renal tubule include the distal and proximal tubules; these carry urine to collecting tubules, which in turn pass the urine to the bladder through the ureter.
Not all liquids that the body takes in are expelled. Large quantities are reabsorbed passively when the pressure within the proximal tubules forces the fluids to return to capillaries, where the liquids are then carried throughout the body in the blood. These fluids also return sugar, salt and other nutrients that the body needs.
One abnormality that can occur in the medullary pyramids is a condition known as Medullary Sponge Kidney (MSK). The condition is defined as congenital, meaning that patients are born with the condition. Patients with MSK cysts that form in the collecting tubules; the condition may affect as few as one pyramid in one kidney, or it may affect all pyramids in both. Most cases of MSK are discovered during an examination for kidney stones or infection, as the condition itself seldom produces symptoms. Researchers have found only a few rare cases where the condition is present in both a parent and child, and MSK is not considered to be an inherited abnormality.