The kidneys are a pair of bean-shaped organs that play a central role in maintaining the body’s internal balance. They are responsible for filtering blood, removing waste products, and regulating fluid and electrolyte levels. This complex work is performed by millions of microscopic structures known as nephrons.
The Nephron Defined
A nephron is the basic structural and functional unit of the kidney, acting as the primary filtering component. Each kidney contains over a million nephrons and consists of two main parts: the renal corpuscle and the renal tubule. The renal corpuscle includes a network of tiny blood vessels called the glomerulus, enclosed within a cup-shaped structure known as Bowman’s capsule. Extending from Bowman’s capsule, the renal tubule is a long, convoluted tube divided into several segments: the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule. The distal convoluted tubule then connects to a collecting duct.
Precise Location Within the Kidney
The kidney is organized into two primary regions: the outer renal cortex and the inner renal medulla. The renal cortex is the outer layer. The renal medulla is the innermost part.
The majority of the nephron’s components are situated within the renal cortex. The renal corpuscle, including the glomerulus and Bowman’s capsule, is found exclusively in the renal cortex, where initial blood filtration takes place. The proximal and distal convoluted tubules are also located within the renal cortex.
While most of the nephron resides in the cortex, the loop of Henle extends into the renal medulla. Nephrons are categorized into two types based on their location and loop length.
Cortical nephrons, making up approximately 85% of all nephrons, have their renal corpuscles located higher in the renal cortex and possess short loops of Henle that extend only a small distance into the outer renal medulla. In contrast, juxtamedullary nephrons, about 15% of nephrons, have their renal corpuscles situated closer to the boundary between the cortex and medulla. These juxtamedullary nephrons are distinguished by their long loops of Henle, which reach deep into the renal medulla.
The collecting ducts, which receive filtrate from multiple nephrons, begin in the cortex and then pass through the medulla.
How Location Facilitates Function
The specific arrangement of nephron components within the kidney’s cortex and medulla is fundamental to their function in filtering blood and concentrating urine. Initial filtration occurs in the renal corpuscle, located in the cortex. Here, the glomerulus filters large volumes of blood plasma, allowing water, ions, and small molecules to pass through while retaining larger substances like proteins and blood cells. This placement in the outer cortex ensures efficient bulk filtration.
Following filtration, the filtrate moves into the renal tubules. While parts of the proximal and distal tubules in the cortex are involved in reabsorbing essential nutrients and some water, the loop of Henle’s extension into the medulla plays a role in water reabsorption and urine concentration. The renal medulla maintains a high concentration of solutes, creating an osmotic gradient that progressively increases with depth. As the filtrate flows through the descending limb of the loop of Henle into this salty medulla, water is drawn out by osmosis, concentrating the filtrate.
The ascending limb of the loop of Henle, largely impermeable to water, actively transports salts out into the medullary interstitium, further contributing to the osmotic gradient without additional water reabsorption. This countercurrent multiplier system, established by the loops of Henle and supported by associated blood vessels called vasa recta, helps create the concentrated environment in the medulla. The collecting ducts, as they pass through this concentrated medulla, allow for final adjustments in water reabsorption, producing urine that can be more concentrated than blood plasma, thus conserving body water. This spatial separation of initial filtration in the cortex and precise water and solute regulation in the medulla supports efficient waste removal and fluid balance.