The nephron is the fundamental structural and functional unit of the kidney. Each human kidney contains approximately one million nephrons, which process blood to form urine. Nephrons maintain the body’s internal balance by removing waste products and excess substances from the blood, ensuring the body’s fluid environment remains stable.
The Nephron’s Blueprint
The nephron is composed of two main segments: the renal corpuscle and the renal tubule. The renal corpuscle, located in the renal cortex, initiates filtration. It consists of the glomerulus, a network of tiny blood vessels, encased within Bowman’s capsule.
Extending from the renal corpuscle is the renal tubule. This tubule includes the proximal convoluted tubule (PCT), located in the renal cortex; the loop of Henle; and the distal convoluted tubule (DCT). The loop of Henle descends into the renal medulla before looping back up. The DCT is also found in the renal cortex. This tubular system connects to collecting ducts, which gather processed fluid from multiple nephrons.
The Initial Cleanse: Glomerular Filtration
Glomerular filtration is the first step in urine formation, occurring within the renal corpuscle. Blood enters the glomerulus under pressure, forcing water and small dissolved substances from the blood into Bowman’s capsule.
This filtration is selective. A specialized barrier, comprising glomerular capillary endothelial cells, the glomerular basement membrane, and Bowman’s capsule podocytes, permits passage of water, ions, glucose, amino acids, and waste products like urea. Larger components, such as blood cells and large proteins, are retained in the bloodstream. The fluid collected in Bowman’s capsule, called glomerular filtrate, is similar to blood plasma but lacks large proteins. This filtrate then moves into the renal tubule for further processing.
Reclaiming Essentials: Tubular Reabsorption
Following initial filtration, the glomerular filtrate contains many substances the body still needs, such as water, glucose, amino acids, and various ions. Tubular reabsorption selectively moves these essential substances from the filtrate back into the bloodstream. This reclamation occurs along different segments of the renal tubule, ensuring the body conserves vital nutrients and maintains proper balance.
Much reabsorption takes place in the proximal convoluted tubule (PCT). Here, nearly all filtered glucose and amino acids are reabsorbed, along with a large proportion of water, sodium, potassium, and chloride ions. This reabsorption involves active transport for many solutes and osmosis for water movement, driven by the osmotic gradient created by solute reabsorption. PCT cells have numerous microvilli, increasing their surface area to maximize reabsorptive capacity.
As the filtrate moves into the loop of Henle, further reabsorption occurs, particularly of water and certain ions. The descending limb is highly permeable to water, allowing it to move out of the filtrate and back into the surrounding tissue, thereby concentrating the filtrate. The ascending limb is impermeable to water but actively reabsorbs sodium, potassium, and chloride ions, contributing to an osmotic gradient in the renal medulla. This gradient is important for the kidney’s ability to concentrate urine later.
In the distal convoluted tubule (DCT), water and ion reabsorption continues, precisely regulated by the body’s needs. For instance, calcium reabsorption in the DCT is influenced by hormones. This fine-tuning ensures the body retains appropriate levels of water and electrolytes, adapting to varying physiological conditions.
Fine-Tuning Waste: Tubular Secretion
While reabsorption brings necessary substances back into the blood, tubular secretion performs an opposite, important function. This process involves the active transport of additional waste products and excess ions directly from the blood into the filtrate within the renal tubule. It acts as a secondary mechanism for removing substances not efficiently filtered in the glomerulus or needing expulsion from the body.
Substances commonly secreted include hydrogen ions, transported into the filtrate to help regulate the body’s acid-base balance. Excess potassium ions are also secreted in the distal convoluted tubule and collecting duct to maintain proper electrolyte levels. Certain drugs, metabolic byproducts like creatinine, and some toxins are actively secreted from the peritubular capillaries into the tubular fluid. This targeted removal ensures unwanted substances are eliminated in the urine, complementing the initial filtration process.
The Final Product: Urine Formation and Regulation
The fluid’s journey through the nephron culminates in urine formation, integrating glomerular filtration, tubular reabsorption, and tubular secretion. The initial filtrate undergoes modification as it passes through the renal tubule, with essential substances reclaimed and additional wastes added. Urine’s final concentration and volume are precisely adjusted as fluid flows through the collecting ducts.
Collecting ducts play a key role in determining urine concentration, influenced by hydration. For example, antidiuretic hormone (ADH) increases collecting duct permeability to water, leading to concentrated urine when dehydrated, conserving water. When well-hydrated, less ADH is released, making ducts less permeable, resulting in dilute urine.
Beyond fluid balance, the nephron is important for maintaining electrolyte levels and blood pH. Hormones like aldosterone, acting on the distal convoluted tubule and collecting duct, regulate sodium reabsorption and potassium secretion, influencing blood volume and pressure. Hydrogen ion secretion and bicarbonate ion reabsorption throughout the tubule maintain blood pH within a healthy range. These coordinated processes ensure the body’s internal environment remains stable.