The renal medulla is the innermost region of the kidney, located beneath the renal cortex. It is composed of cone-shaped structures called renal pyramids, which house the loops of Henle, collecting ducts, and associated blood vessels.[8] The renal medulla plays a critical role in concentrating urine by creating a high osmotic gradient, allowing the reabsorption of water and solutes. This process helps maintain the body’s fluid and electrolyte balance and ensures the excretion of waste products through the urine.[1]
Structure
The renal medulla is organized into distinct cone-shaped structures and associated components that work together to perform critical kidney functions.[6]
Let’s take a look at it’s structure in detail:
Renal Pyramids
The renal medulla consists of renal pyramids, which are triangular or cone-shaped structures.
Each pyramid has:
- Base: Positioned toward the renal cortex.
- Apex (Renal Papilla): Points toward the renal pelvis, where urine is collected.
Renal Columns
- Columns of Bertin are extensions of the renal cortex that project into the medulla between adjacent pyramids.[3]
- These columns provide structural support and house blood vessels that supply the medulla.
Nephron Components
The medulla contains parts of the nephron that are critical for urine concentration:
- Loop of Henle: Extends into the medulla, creating a countercurrent mechanism for water and solute reabsorption.
- Collecting Ducts: Run through the medulla, concentrating urine and transporting it to the renal pelvis.
Blood Supply
The medulla is supplied by:
- Vasa Recta: Specialized capillaries that run parallel to the loops of Henle, maintaining the medullary osmotic gradient.
- Interlobar Arteries and Veins: Run within the renal columns.[4]
Location
The renal medulla is located beneath the renal cortex in the inner region of the kidney. It consists of cone-shaped renal pyramids, with their bases facing the cortex and their apices (renal papillae) pointing toward the renal pelvis.[5] The medulla is surrounded by renal columns, which extend from the cortex and separate the pyramids. This central location enables the medulla to perform its role in urine concentration and transport.
Function
- Urine Concentration: Creates a high osmotic gradient through the loops of Henle and vasa recta to concentrate urine.
- Water Reabsorption: Facilitates water reabsorption in the collecting ducts, regulated by antidiuretic hormone (ADH).
- Electrolyte Regulation: Maintains sodium, potassium, chloride, and urea balance during filtration and reabsorption.
- Acid-Base Balance: Regulates pH by secreting hydrogen ions and reabsorbing bicarbonate.
- Waste Excretion: Concentrates metabolic waste products for elimination through the renal pelvis.
Clinical significance
The renal medulla is crucial for maintaining the body’s fluid and electrolyte balance, and its dysfunction can lead to significant clinical conditions.[7] Impaired function of the medulla, as seen in diseases like chronic kidney disease or diabetes insipidus, can result in an inability to concentrate urine, leading to dehydration or excessive water loss. Damage to the medullary structures, such as the loops of Henle or collecting ducts, can disrupt sodium and water balance, contributing to conditions like hyponatremia or hypernatremia. Additionally, ischemia or reduced blood flow to the medulla, commonly associated with renal artery stenosis, can impair its function and exacerbate kidney failure.[2] Disorders like medullary sponge kidney or nephrocalcinosis can also arise from abnormalities in the medulla, affecting its ability to excrete waste effectively.