The kidney is the body’s sophisticated filtration system. This complex work is performed by millions of microscopic structures called nephrons, which are the functional units of the kidney. The nephron’s overall purpose is to process blood plasma by removing metabolic waste products while recovering water and essential solutes. This three-step process—filtration, reabsorption, and secretion—ensures that the body maintains a precise balance of fluids, electrolytes, and pH.
The Filtration Barrier: Glomerulus and Bowman’s Capsule
The journey of blood plasma begins at the renal corpuscle, a structure composed of the glomerulus and Bowman’s capsule. The glomerulus is a specialized, coiled capillary network that receives blood from the afferent arteriole and is drained by an efferent arteriole. This arrangement helps maintain the high hydrostatic pressure necessary for filtration to occur.
This pressure forces fluid across the filtration barrier, a highly selective three-layer structure designed to separate small molecules from large ones. The first layer is the fenestrated endothelium of the capillaries, which prevents blood cells from passing through. Next is the thick glomerular basement membrane, a non-cellular layer that acts as a physical and electrical barrier, repelling negatively charged plasma proteins like albumin.
The final layer consists of specialized epithelial cells called podocytes, which wrap around the capillaries. These cells possess foot-like processes, or pedicels, that interdigitate to form tiny filtration slits bridged by a slit diaphragm. This structure restricts the passage of intermediate and large-sized solutes. The resulting fluid, known as the glomerular filtrate, is essentially protein-free plasma containing water, electrolytes, glucose, amino acids, and waste products.
Bulk Reabsorption in the Proximal Convoluted Tubule
Once the filtrate enters Bowman’s capsule, it flows directly into the Proximal Convoluted Tubule (PCT), where the goal is to reclaim useful substances. The cells lining the PCT are equipped with a dense brush border and numerous mitochondria, reflecting their high capacity for active transport and bulk reabsorption. This reabsorption is largely unregulated and obligatory, occurring regardless of the body’s immediate needs.
The PCT reclaims approximately 65 to 70 percent of the filtered water, sodium, and potassium, and 85 to 90 percent of bicarbonate. Almost 100 percent of filtered glucose and amino acids are reabsorbed here to prevent their loss in the urine. The driving force for this movement is the active pumping of sodium ions out of the tubule cells into the surrounding interstitial fluid by the Na+/K+ ATPase pump.
This sodium gradient powers co-transporters that move glucose and amino acids from the filtrate back into the cell, a mechanism known as secondary active transport. As solutes move out, water passively follows the osmotic gradient, a process called isosmotic reabsorption. The PCT also serves as a site for secretion, actively adding certain organic acids, bases, and drugs from the blood into the tubular fluid for excretion.
Establishing the Gradient: The Loop of Henle
The remaining filtrate then enters the U-shaped Loop of Henle, which establishes an osmotic gradient in the kidney’s medulla. This gradient is necessary for the eventual concentration of urine and is achieved through countercurrent multiplication. The loop consists of two distinct segments with contrasting permeabilities that work in tandem.
The descending limb extends deep into the medulla and is highly permeable to water but impermeable to solutes. As the filtrate descends toward the salty, hypertonic environment of the medulla, water is drawn out of the tubule into the interstitium by osmosis. This water loss concentrates the filtrate as it reaches the bottom of the loop.
The ascending limb, conversely, is impermeable to water, yet it actively transports solutes, primarily sodium and chloride ions, out of the filtrate. This active removal of salt without water contributes to the hyperosmolarity of the medullary interstitium, reinforcing the gradient. By the time the filtrate reaches the Distal Convoluted Tubule, it is hypoosmotic, or dilute, because salt has been removed while water was retained in the loop.
Fine-Tuning and Excretion: Distal Tubule and Collecting Duct
The final composition of urine is determined in the Distal Convoluted Tubule (DCT) and the Collecting Duct (CD), where reabsorption and secretion are tightly regulated by hormones. The DCT continues to recover about 10 to 15 percent of water and is the main site where parathyroid hormone controls the reabsorption of calcium. This segment fine-tunes the body’s fluid and electrolyte status based on current physiological needs.
The collecting duct system is sensitive to hormonal control, influencing the final volume and concentration of the urine. Antidiuretic hormone (ADH), also known as vasopressin, acts on the CD cells to insert water channels called aquaporins into their membranes. When ADH levels are high, water flows rapidly out of the tubule and is reabsorbed, resulting in a small volume of concentrated urine.
Another hormone, Aldosterone, targets the principal cells in the DCT and CD to manage sodium and potassium balance. Aldosterone increases the reabsorption of sodium back into the blood while promoting the secretion of potassium into the tubular fluid for excretion. The fluid that leaves the collecting duct has been fully processed, representing the final urine that will be excreted from the body.