The nephron is the microscopic, functional unit of the kidney, acting as the body’s primary blood-cleaning and fluid-balancing system. Each human kidney contains approximately one million nephrons, working continuously to filter the entire blood volume many times each day. Their collective function is to maintain homeostasis by removing metabolic waste products while regulating the body’s water, salt, and acid-base levels.
Structure and Types of Nephrons
The nephron begins with the renal corpuscle, a spherical structure located in the renal cortex. The corpuscle consists of the glomerulus, a dense tuft of capillaries, encased by Bowman’s capsule. Fluid filtered from the blood then enters the renal tubule, a long tube divided into distinct segments:
- The proximal convoluted tubule.
- The U-shaped loop of Henle.
- The distal convoluted tubule.
The collecting duct receives fluid from multiple nephrons.
Nephrons are classified based on their anatomical position within the kidney. Cortical nephrons represent the majority (about 85%) and are situated almost entirely within the renal cortex. Juxtamedullary nephrons make up the remaining 15% and are distinguished by their long loops of Henle that extend deep into the renal medulla. This deep penetration establishes the high osmotic gradient necessary for producing highly concentrated urine when the body needs to conserve water.
The Three Stages of Blood Filtration
Cleansing the blood and forming urine involves three stages within the nephron. The first stage, glomerular filtration, is a passive, size-selective process driven by hydrostatic pressure. Blood pressure forces water and small solutes (glucose, salts, and waste molecules) out of the glomerulus capillaries and into Bowman’s capsule. The filtration membrane prevents large components like blood cells and most plasma proteins from passing into the tubule.
The second stage, tubular reabsorption, selectively recovers substances the body needs to retain from the filtered fluid, now called filtrate. Approximately two-thirds of the water, sodium, and potassium, along with nearly all glucose and amino acids, are immediately reabsorbed into the bloodstream from the proximal convoluted tubule. Specialized transport proteins move these essential materials back into the peritubular capillaries surrounding the nephron. Reabsorption continues along the loop of Henle and the distal convoluted tubule, adjusting the filtrate’s volume and concentration.
The final stage is tubular secretion, an active process that transports specific substances directly from the blood into the remaining tubule fluid for excretion. This mechanism eliminates materials that were not effectively filtered in the glomerulus due to their size or charge. Waste products, drugs, and excess ions like hydrogen and potassium are actively pumped into the distal convoluted tubule and collecting duct. Secretion works alongside filtration and reabsorption to fine-tune the final composition of the urine.
Systemic Regulation and Hormonal Control
Nephron function is controlled by systemic signals for fluid and electrolyte balance. One major regulator of water is Antidiuretic Hormone (ADH), or vasopressin, released in response to high blood osmolarity, indicating dehydration. ADH acts on the collecting ducts, causing them to insert water channels called aquaporins into their membranes. This action increases the permeability of the duct walls, allowing water to be reabsorbed back into the blood and producing a smaller volume of highly concentrated urine.
The Renin-Angiotensin-Aldosterone System (RAAS) is the primary mechanism for regulating blood pressure and sodium balance. When blood pressure drops, the kidney releases the enzyme renin, which initiates a cascade that culminates in the release of aldosterone from the adrenal glands. Aldosterone targets the distal convoluted tubule and collecting duct, promoting the reabsorption of sodium ions back into the blood. Since water follows sodium by osmosis, this increases fluid retention and helps restore blood volume and pressure.
The nephron maintains the body’s acid-base balance, or pH, through the handling of hydrogen and bicarbonate ions. The renal tubules adjust the pH of the blood by either secreting excess hydrogen ions into the filtrate or reabsorbing bicarbonate, a crucial buffer, back into the circulation. This regulatory capacity ensures the blood pH remains within the narrow, healthy range.
Clinical Relevance of Nephron Function
Damage to the nephrons directly impairs the kidney’s ability to function. Chronic Kidney Disease (CKD) is characterized by a progressive, long-term decline in nephron function, which is often irreversible. Conditions like diabetes and high blood pressure are common causes of CKD, as they damage the capillary network of the glomeruli over time. The loss of functional nephrons necessitates compensatory hyperfiltration in the remaining healthy units, which can accelerate their failure.
Acute Kidney Injury (AKI) represents a sudden, rapid decrease in kidney function, caused by severe dehydration, toxic drug exposure, or obstruction. AKI demonstrates the immediate systemic effects of filtration failure. Both conditions are diagnosed using specific markers that reflect nephron health and performance.
The Glomerular Filtration Rate (GFR) measures kidney function, estimating the volume of fluid filtered per minute by all functioning nephrons. GFR is estimated (eGFR) using blood levels of creatinine, a waste product of muscle metabolism that nephrons normally clear from the blood. Elevated creatinine levels and the presence of albumin (protein) in the urine (albuminuria) indicate that the nephron’s filtration barrier is compromised.