The kidneys are two organs that continuously filter the entire blood supply multiple times a day to manage the body’s internal environment. This process removes waste products and regulates the body’s fluid volume and the balance of essential minerals, known as electrolytes. To accomplish this task, the kidney is organized into distinct zones, each playing a specialized role in transforming blood plasma into urine.
The Renal Cortex (The Outer Region)
The outermost layer of the kidney is the renal cortex, sitting immediately beneath a protective fibrous layer called the renal capsule. This region has a granular appearance due to the dense concentration of the nephrons’ initial filtering components. The cortex is where the first step of blood processing, known as ultrafiltration, occurs.
The main structures within the cortex are the renal corpuscles, which include the glomerulus—a network of tiny blood vessels—and the surrounding Bowman’s capsule. Blood flows into the glomerulus, and pressure forces water and small dissolved solutes out of the blood, forming a preliminary filtrate. The filtrate then moves through the proximal and distal convoluted tubules, which are confined primarily to the cortical region.
The Renal Medulla (The Middle Region)
The renal medulla is the middle layer, situated directly beneath the cortex, responsible for fine-tuning the concentration of the forming urine. This inner region is characterized by several cone-shaped masses of tissue called renal pyramids. Their striped appearance comes from the parallel arrangement of the nephron segments that extend down into this layer.
The parts of the nephron found here include the Loop of Henle and the collecting ducts. The Loop of Henle descends deep into the medulla, creating an environment highly concentrated with salts and urea. This hypertonic environment draws water out of the filtrate as it passes through the descending limb and the collecting ducts. By reabsorbing water and necessary solutes back into the bloodstream, the medulla ensures the body retains fluid while concentrating waste products for excretion.
The Renal Pelvis (The Inner Collecting System)
The renal pelvis is the innermost, funnel-shaped region of the kidney, acting as the central collection point for the final product—urine. The tips of the renal pyramids, called renal papillae, drain the concentrated urine into small cup-like structures known as minor calyces. Several minor calyces then merge to form larger channels called major calyces.
These major calyces converge to form the wide basin of the renal pelvis. The function of this system is to gather the urine from the functional tissue and ensure its transport out of the kidney. The renal pelvis narrows significantly as it exits the kidney, transitioning into the ureter, which carries the urine down to the bladder for storage. The walls contain smooth muscle, which generates wave-like contractions to propel the urine forward and facilitate efficient drainage.