The renal medulla is the inner part of the kidney. The medulla helps regulate the concentration of urine by filtering out water, salts, and acid.

The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDKD) notes that kidneys maintain the balance of salts and minerals, including calcium, sodium, potassium, and phosphorus, in the body. This enables the tissues, nerves, and muscles of the body to function properly.

The three main internal regions of the kidney include the:

  • renal cortex
  • renal medulla
  • renal pelvis or hilum, where ureters, nerves, and blood vessels enter and leave the organ

Read on to learn more about the anatomy, function, and medical conditions of the kidney medulla.

The medulla is the inner part of the kidney.

It consists of cone-shaped renal pyramids that contain the blood vessels and tubular structures of nephrons. The NIDDKD notes that nephrons are the filtering units of the kidney and include a glomerulus — a specially modified blood vessel that filters blood — and tubule.

The blood vessels and tubular structures within the medulla include the:

  • medullary collecting ducts
  • loops of Henle
  • vasa recta
  • interstitium

There are approximately eight pyramids in each kidney.

The renal medulla also consists of the renal columns, which are the spaces between the renal pyramids, and the renal papillae.

The renal papillae refer to the tips of the renal pyramids, which collect the remainder of the filtrate that consists of waste and solutions. The papillae then deliver the filtrate, or urine, into minor calyces, which connect to the ureters to deliver it to the bladder.

Where is it in relation to the cortex?

The renal cortex sits on the outside of the medulla. The cortex makes erythropoietin, which the body uses to manufacture red blood cells.

According to an older 2012 article, the main function of the medulla is to regulate the concentration of urine.

As fluid passes through the medulla, the loop of Henle and the collecting ducts absorb water, salts, and acids.

The loop of Henle filters water and other substances, such as sodium chloride, from the urine. The collecting tubules also reabsorb water from the urine.

The filtered fluid then travels to the part of the kidney called the renal pelvis.

Various medical conditions may affect the kidney medulla and the function of the kidney.

These include the below.

Medullary sponge kidney

This is a birth abnormality, also known as Cacchi-Ricci disease.

Small cysts form in the medulla tubules and form a sponge-like appearance. The cysts prevent the urine from flowing freely through the tubules.

People do not typically develop symptoms until they reach their teenage years or their 20s. Symptoms include:

  • painful or burning urination
  • back pain
  • lower abdominal pain
  • groin pain
  • urine that is cloudy or dark
  • blood in the urine, or hematuria
  • foul-smelling urine
  • vomiting

Medullary cystic kidney

This rare, inherited condition causes fluid-filled sacs called cysts to form in the kidneys. Scarring can also affect the tubules.

People may experience the following symptoms:

  • polyuria, which means a person passes atypically large amounts of urine
  • excessive thirst
  • weakness
  • pallor, or a lack of typical color in a person’s face
  • incontinence, particularly during the night

Pyelonephritis

Pyelonephritis is a bacterial infection that causes inflammation of the kidneys. It is a complication of a urinary tract infection.

The infection and inflammation of the renal pelvis can damage the medulla.

Pyelonephritis can cause the following symptoms:

  • fever
  • flank pain
  • nausea
  • vomiting

The symptoms will usually develop within a few hours.

Renal damage due to medications

Kidney damage can occur due to medications such as nonsteroidal anti-inflammatory drugs (NSAIDs) and acetaminophen.

According to a 2022 article, NSAIDs inhibit prostaglandins, a type of lipid. They can improve blood flow in the kidneys.

When these lipids become inhibited, they can build up in the renal medullar. This can cause:

  • papillary necrosis, a condition in the kidneys that causes the renal papillae to die
  • chronic interstitial nephritis, when the tubules become inflamed
  • chronic tubular interstitial nephritis, which refers to the inflammation of the tubules of the kidneys and the interstitial tissue that surrounds them

A 2020 systematic review and meta-analysis notes that although acetaminophen does not cause many side effects compared with other painkillers, it can significantly increase the risk of developing kidney damage.

Deterioration of kidney function due to sickle cell disease

Sickle cell disease refers to a group of inherited conditions that affect a person’s red blood cells.

A 2016 article notes that red blood cells tend to sickle in the medulla. This is because the medulla contains high levels of acid, salt, glucose, and other substances. It also has low levels of oxygen.

Sickle cell disease can affect the structures and function of the kidney and can lead to many conditions. These conditions can include acute kidney injury, an increased risk of urinary tract infections, chronic kidney disease, and the presence of blood in the urine.

It can also increase the risk of renal medullary carcinoma.

Renal medullary carcinoma

Renal medullary carcinoma is a rare type of cancer.

It usually affects those of African American descent who have sickle cell disease or carry its trait.

The most common symptom is blood in the urine. Other symptoms include:

  • pain around the kidney area
  • a mass in the abdomen on the right side
  • unintended weight loss
  • fevers
  • night sweats

The kidney medulla is the inner part of the kidney. Its main function is to control the concentration of urine.

The loop of Henle and the collecting ducts absorb water, salts, and acids as fluid passes through the medulla.

Many conditions can damage the medulla, including medullary sponge kidney, medullary cystic kidney, kidney infection, and renal medullary carcinoma.

Some medications, such as NSAIDs and acetaminophen, can also damage the medulla.