Nephron Segments and Their Primary Functions
Learn how the nephron's segments work sequentially to filter blood, recover vital substances, and precisely regulate the body's water and solute balance.
The nephron is the microscopic functional unit of the kidney, with each kidney containing approximately one million. These units are responsible for producing urine and maintaining the body’s fluid and electrolyte balance. The complex process of transforming blood into urine involves filtering the blood, reabsorbing necessary substances, and secreting waste materials.
The Renal Corpuscle
Urine formation begins in the renal corpuscle, the nephron’s initial filtering component. This structure is composed of two parts: the glomerulus, a dense network of capillaries, and Bowman’s capsule. The pressure within the glomerular capillaries forces water, ions, small molecules, and waste products out of the bloodstream.
This filtered fluid, known as filtrate, is collected by Bowman’s capsule. The filtration process is selective based on size; large elements like red blood cells and proteins are retained in the blood. The resulting plasma-like filtrate then moves to the subsequent segments of the nephron for processing.
The Proximal Convoluted Tubule
After its formation in the renal corpuscle, the filtrate enters the proximal convoluted tubule (PCT). This segment is where the majority of reabsorption occurs, reclaiming substances and returning them to the blood. The cells lining the PCT feature a dense brush border of microvilli that increases their surface area for transport and are packed with mitochondria to supply energy for active transport.
The PCT reabsorbs approximately two-thirds of the water and sodium that was initially filtered. It also reclaims nearly all of the glucose and amino acids. This bulk reabsorption is a non-regulated process, occurring consistently regardless of the body’s hydration status. The PCT also performs secretion, actively transporting certain waste products and drugs from the blood into the tubular fluid.
The Loop of Henle
From the PCT, the filtrate flows into the Loop of Henle, a U-shaped structure that dips from the renal cortex into the medulla. This segment is responsible for creating a concentration gradient in the surrounding interstitial fluid. The Loop of Henle consists of a descending limb and an ascending limb, each with different permeability characteristics.
The descending limb is highly permeable to water but not salts. As filtrate travels down this limb, water moves out via osmosis, concentrating the filtrate. In contrast, the ascending limb is impermeable to water but actively transports salts out of the tubule. This countercurrent mechanism, with fluid flowing in opposite directions, establishes the salty environment of the medulla.
The Distal Tubule and Collecting System
The final segment of the nephron is the distal convoluted tubule (DCT), which drains into the collecting duct system. This is where the fine-tuning of the filtrate occurs, a process regulated by hormones to meet the body’s specific needs. Unlike the bulk reabsorption in the PCT, transport here is adjusted to maintain precise control over electrolyte and water balance.
Two hormones regulate the functions of this final section. Aldosterone acts on the DCT to increase the reabsorption of sodium, which can affect water retention and blood pressure. Antidiuretic hormone (ADH) influences the collecting duct, increasing its permeability to water. When ADH is present, water is drawn from the filtrate into the salty medullary interstitium, producing concentrated urine. Without ADH, the collecting duct remains impermeable to water, resulting in dilute urine.