The nephron is the functional unit of the kidney, responsible for processing blood and producing urine. Each human kidney contains approximately one million structures that work continuously to maintain homeostasis. Their collective function is to filter plasma, reabsorb necessary substances like water and nutrients back into the circulation, and secrete waste products for excretion. This process ensures the body retains the perfect balance of fluid volume and electrolyte concentration.
The Renal Corpuscle
The journey of blood filtration begins in the renal corpuscle, composed of the glomerulus and the Bowman’s capsule. The glomerulus is a dense, high-pressure tuft of capillaries supplied by the afferent arteriole and drained by the efferent arteriole. This arrangement maintains the hydrostatic pressure necessary for initial filtration.
Surrounding the glomerulus is the cup-shaped Bowman’s capsule, which collects the filtered fluid. The filtration barrier is a highly selective three-layered structure designed to allow small molecules through while blocking larger ones. This barrier consists of the fenestrated capillary endothelium, the glomerular basement membrane, and the filtration slits formed by specialized epithelial cells called podocytes.
Blood pressure forces water, ions, glucose, amino acids, and nitrogenous wastes into the capsule space, forming the glomerular filtrate. The barrier prevents the passage of blood cells and large plasma proteins, such as albumin. The resulting filtrate must be extensively modified before it becomes urine.
The Proximal and Loop Segments
Following the renal corpuscle, the filtrate enters the proximal convoluted tubule (PCT), where bulk reabsorption takes place. The epithelial cells lining the PCT are adapted for high volume transport, possessing a dense brush border of microvilli that increases surface area. These cells also contain abundant mitochondria, supplying the energy required for active transport mechanisms.
The PCT reclaims the majority of the filtered material, including approximately 65% of the filtered water, sodium, and potassium. Virtually 100% of filtered glucose, amino acids, and bicarbonate are also recovered here. This reabsorption uses various cotransporters and pumps, with water passively following the osmotic gradient created by solute movement.
The fluid then flows into the U-shaped Loop of Henle, which establishes a concentration gradient in the kidney’s medulla. The descending limb is highly permeable to water but impermeable to solutes. As the filtrate descends deeper, water passively exits the tubule, concentrating the fluid within.
Conversely, the ascending limb is impermeable to water but actively transports sodium and chloride ions out of the filtrate. This salt pumping is pronounced in the thick ascending limb, contributing significantly to the high osmotic concentration of the medullary interstitial fluid. The countercurrent flow through the two limbs generates the osmotic gradient necessary for final adjustments to urine concentration.
The Distal Tubule and Collecting System
The fluid leaving the Loop of Henle enters the distal convoluted tubule (DCT), where fine-tuning of the electrolyte balance begins. The DCT is primarily responsible for the precise regulation of calcium levels, a process stimulated by parathyroid hormone. It also continues the reabsorption of sodium and chloride, contributing to the body’s salt balance.
The DCT connects to the collecting duct, a final passageway that receives fluid from multiple nephrons and extends through the medulla. This segment is the final determinant of the body’s fluid and electrolyte status, acting under the influence of several hormones. For instance, aldosterone acts on principal cells here to increase sodium reabsorption and potassium secretion.
The most precise water regulation occurs in the collecting duct, controlled by antidiuretic hormone (ADH), also known as vasopressin. When the body needs to conserve water, ADH causes the insertion of aquaporins, allowing water to exit the filtrate and be reabsorbed into the hyperosmotic medulla. Intercalated cells within the collecting duct also regulate acid-base balance by controlling the secretion of hydrogen ions and the reabsorption of bicarbonate.