The kidneys are bean-shaped organs located on either side of the spine, beneath the rib cage. They filter waste products from the blood and maintain the body’s fluid and electrolyte balance. Millions of microscopic structures called nephrons, the functional units within each kidney, process blood to form urine.
The Renal Corpuscle
Blood filtration begins in the renal corpuscle, located in the kidney’s outer region, the renal cortex. This component consists of two primary structures. The glomerulus is a dense network of capillaries where blood plasma is filtered.
Bowman’s capsule, a cup-shaped structure, surrounds the glomerulus and collects the filtered fluid. This capsule receives the initial filtrate, which is blood plasma without large proteins or cells. Glomerular filtration occurs as blood pressure forces water and small solutes, such as glucose and amino acids, from the glomerulus capillaries into Bowman’s capsule.
This movement creates glomerular filtrate, which proceeds into the next nephron segment. The filtration barrier, composed of specialized cells, selectively allows small molecules to pass through while retaining larger components in the bloodstream.
The Renal Tubule
Extending from Bowman’s capsule, the renal tubule is a long, winding structure where filtrate composition is significantly modified. It divides into three distinct segments, each contributing uniquely to the reabsorption of useful substances and the secretion of waste. The first part, the proximal convoluted tubule (PCT), is also situated in the renal cortex. Here, a large proportion of water, glucose, amino acids, and essential ions are reabsorbed from the filtrate back into the bloodstream.
Following the PCT, the filtrate enters the loop of Henle, a U-shaped segment that extends deep into the renal medulla and loops back towards the cortex. This loop consists of a descending limb and an ascending limb, each with different permeabilities. The descending limb is permeable to water, allowing water to exit the filtrate as it moves into the increasingly salty environment of the medulla.
The ascending limb is impermeable to water but actively transports salts out of the filtrate. This differential permeability establishes and maintains a concentration gradient within the kidney’s medulla. This gradient is essential for urine concentration, ensuring the body can conserve water.
Finally, the filtrate reaches the distal convoluted tubule (DCT) in the renal cortex. This segment fine-tunes water and salt reabsorption, often influenced by hormonal signals. The DCT also secretes waste products and excess ions into the filtrate, further refining its composition.
The Collecting Duct
The collecting duct serves as the final common pathway for filtrate processed by multiple nephrons. These ducts extend through the renal medulla, receiving modified filtrate from several distal convoluted tubules. Its primary function is to regulate the final concentration and volume of urine.
Water reabsorption in the collecting duct is precisely controlled, influenced by antidiuretic hormone (ADH). When ADH levels are high, collecting ducts become more permeable to water, allowing more water to be reabsorbed, resulting in concentrated urine. Conversely, when ADH levels are low, ducts are less permeable, leading to more dilute urine.
This regulated water reabsorption ensures the body maintains proper fluid balance. The collecting ducts represent the final point where adjustments are made to the filtrate. Once fluid leaves the collecting ducts, it is considered urine and proceeds to the renal pelvis, then to the ureters for excretion.