The ability of the kidney to execute its functions depends entirely on the nephron, which is the microscopic, tube-like structure representing the functional unit of the organ. Each human kidney contains approximately one million nephrons, and their collective action ensures that the blood is constantly cleansed and balanced. The nephron’s intricate design allows it to process blood plasma through a three-step mechanism involving filtration, reabsorption, and secretion, ultimately producing urine. This process is crucial for regulating the concentrations of electrolytes, maintaining the body’s pH, and controlling blood pressure.
Essential Structural Components
The nephron begins with the renal corpuscle, which is composed of a dense tuft of capillaries called the glomerulus encased within a double-walled structure known as Bowman’s capsule. This corpuscle is where the initial blood filtering takes place, situated entirely within the outer region of the kidney, the renal cortex. The filtrate then enters the renal tubule, a long, convoluted tube with distinct segments.
The first segment is the proximal convoluted tubule (PCT), which remains coiled within the renal cortex near the corpuscle. Following the PCT, the tubule descends into the renal medulla, forming the U-shaped loop of Henle. The loop of Henle consists of a descending limb and an ascending limb, and its depth in the medulla varies depending on the nephron type.
Nephrons with short loops, known as cortical nephrons, primarily reside in the cortex, while juxtamedullary nephrons have long loops that extend deep into the medulla, playing a unique role in concentrating urine. After the loop of Henle, the tubule returns to the cortex as the distal convoluted tubule (DCT). Finally, the DCT connects to a collecting duct, which receives fluid from multiple nephrons and extends back down into the medulla to drain the final urine into the renal pelvis.
The Initial Filtration Step
Glomerular filtration, the first functional stage of the nephron, occurs exclusively within the renal corpuscle. Blood is brought to the glomerulus by the afferent arteriole, where the capillary network forms a high-pressure filter. This filter, the glomerular filtration barrier, is a specialized, three-layered structure that separates blood from the inside of Bowman’s capsule.
The three layers include the fenestrated capillary endothelium, the glomerular basement membrane (GBM), and specialized epithelial cells called podocytes, which form filtration slits. Filtration is primarily based on size exclusion, allowing water, ions, glucose, amino acids, and small waste molecules like urea to pass freely into the capsule to form the filtrate. Substances with a molecular weight less than 7,000 Daltons are filtered.
The barrier effectively retains large molecules, such as blood cells and most plasma proteins like albumin, which are too big to pass through the filtration slits. The GBM and the podocytes also carry a net negative charge, which actively repels negatively charged plasma proteins, providing an additional layer of selectivity. The efficiency of this process is measured by the Glomerular Filtration Rate (GFR), the volume of fluid filtered from the blood into the capsule per unit time, typically averaging around 125 milliliters per minute.
Adjusting the Filtrate
Once the filtrate is formed, its composition is modified as it travels through the renal tubule, determining the final makeup of the urine. This adjustment involves two concurrent processes: reabsorption, which moves needed substances from the tubule back into the blood, and secretion, which moves additional waste products from the blood into the tubule. The proximal convoluted tubule (PCT) is the major site of reabsorption, reclaiming about 65% of the filtered water, sodium, and other solutes.
The PCT recovers virtually 100% of organic nutrients like glucose and amino acids, utilizing a dense brush border lining to maximize surface area for this transport. This reabsorption is often active, requiring energy, which is why the PCT cells are rich in mitochondria. The loop of Henle plays a role; its descending limb is permeable to water, which is passively reabsorbed as the filtrate descends into the salty medullary environment.
The ascending limb is impermeable to water but actively transports electrolytes (sodium, potassium, and chloride) out of the tubule, diluting the filtrate while maintaining the high osmolarity of the medulla. Secretion, the other mechanism, occurs throughout the tubules, actively moving substances from the surrounding peritubular capillaries into the tubular fluid. These substances include:
- Hydrogen ions.
- Potassium.
- Various organic acids and bases.
- Certain drug metabolites.
This combined action of reabsorption and secretion is efficient, resulting in the recovery of approximately 99% of the filtered water and valuable solutes before the fluid exits the nephron.
Maintaining Fluid and Chemical Balance
The final adjustments to the filtrate are made in the distal convoluted tubule (DCT) and the collecting ducts, fine-tuning the body’s overall balance. The DCT and collecting ducts are responsible for making the urine concentrated or dilute, regulated by the hormone vasopressin (antidiuretic hormone or ADH). When ADH is present, it increases the permeability of the collecting ducts to water, allowing water to be reabsorbed back into the body to conserve fluid and produce concentrated urine.
The nephron is also involved in managing blood pressure through the Renin-Angiotensin-Aldosterone System (RAAS). Specialized cells in the nephron release the enzyme renin in response to a drop in blood pressure or blood volume. This initiates a cascade that leads to the release of aldosterone. Aldosterone acts on the DCT and collecting ducts to increase the reabsorption of sodium, which is followed by water, increasing blood volume and raising blood pressure.
The nephron is also the primary regulator of the body’s acid-base balance, maintaining the blood’s pH. This is achieved by the selective secretion of hydrogen ions (H+) and the reabsorption of bicarbonate ions (HCO3-) in the PCT and collecting ducts. When the blood becomes too acidic, the nephron increases H+ secretion into the urine and enhances bicarbonate reabsorption back into the blood, effectively buffering the plasma and restoring the correct pH.