The nephron is the microscopic structural and functional unit of the human kidney. Each mature human kidney contains roughly one million of these tiny, intricate filtering units. Their primary task is to regulate water and soluble substances in the body by filtering the blood, reabsorbing what the body needs, and excreting the rest as urine.
Every single nephron is composed of two main components: a renal corpuscle, where fluid is filtered out of the blood, and a renal tubule, where the filtered fluid is modified into urine. Blood flows into the kidney via the renal artery, which branches into smaller arterioles that feed each nephron. Understanding this microscopic setup is essential for midwives, as normal fetal and maternal kidney function depends entirely on these tiny structures managing fluid balance and blood pressure.
The renal corpuscle is the initial blood-filtering component of the nephron. It consists of two primary structures: the glomerulus and Bowman's capsule.
The wall facing the capillaries features specialized cells called podocytes. These cells have foot-like extensions called pedicels that interlock to form tiny slits. Together with the capillary endothelium and basement membrane, these filtration slits ensure that large proteins and blood cells stay in the bloodstream while water and small molecules pass into the tubule.
After fluid enters Bowman's capsule, it flows immediately into the proximal convoluted tubule (PCT). Microscopically, the PCT is lined with a simple cuboidal epithelium featuring a dense brush border made of numerous microvilli. These microvilli drastically increase the surface area available for reabsorption.
Midwifery students should note that about 65 to 70 percent of all water, sodium, and chloride filtered by the glomerulus is reabsorbed right here in the PCT. In addition, virtually all filtered glucose and amino acids are actively pumped back out of the tubule and into the surrounding peritubular capillaries. Mitochondria crowd these cells to supply the large amount of ATP required for this active transport.
The Loop of Henle dips down from the cortex into the renal medulla and back up. It has two main limbs with very different microscopic properties:
This countercurrent arrangement creates a high salt concentration deep in the medulla, which is crucial for the kidney's ability to produce concentrated urine and prevent dehydration.
Leaving the Loop of Henle, the fluid enters the distal convoluted tubule (DCT). Unlike the PCT, the DCT has fewer microvilli and is specialized for fine-tuning the electrolyte and acid-base balance of the blood. Reabsorption and secretion here are heavily controlled by hormones, particularly aldosterone, which stimulates sodium reabsorption and potassium secretion.
Multiple distal tubules then empty their contents into the cortical collecting duct. This duct travels back down through the medulla. The cells here respond to antidiuretic hormone (ADH). When ADH is present, the collecting duct inserts aquaporin water channels into its membranes, allowing water to leave and producing concentrated urine. When ADH is absent, large volumes of dilute urine are excreted.
Disease processes often target specific microscopic parts of the nephron, altering both structure and clinical presentation. For instance, in glomerulonephritis, an immune response damages the glomerular basement membrane and podocytes. This microscopic breakdown allows red blood cells and large proteins to leak into the urine, presenting clinically as hematuria and protein-rich edema—common findings monitored in antenatal care for pre-eclampsia.
In acute tubular necrosis (ATN), ischemia or toxins cause the simple cuboidal epithelial cells of the proximal tubule to die and slough off, blocking the tubular lumen and halting urine output. Recognizing how microscopic damage translates to macroscopic signs like low urine output or abnormal urinalysis results helps midwives identify early kidney complications in pregnant women.