NephronEdit

The nephron is the kidney’s fundamental unit of structure and function, translating blood filtration into the formation of urine while preserving the body’s essential fluids and electrolytes. In a healthy adult, each kidney contains approximately one million nephrons, all working together to regulate fluid balance, waste elimination, and acid-base homeostasis. The nephron’s design features two broad parts: the renal corpuscle, where blood filtration begins, and the renal tubule, where filtrate is processed through reabsorption and secretion to yield urine. The core components are the glomerulus and Bowman's capsule in the renal corpuscle, followed by the proximal tubule, loop of Henle, distal tubule, and collecting duct in the tubular system. For a detailed map of these structures, see glomerulus and Bowman's capsule within the broader context of the kidney.

From a physiological perspective, the nephron acts as a highly selective filter and a sophisticated reclaimer. Blood enters the glomerulus under pressure, and the filtration barrier—composed of fenestrated capillary endothelium, a specialized basement membrane, and podocyte foot processes—allows water and small solutes to pass while restricting larger proteins. The filtrate then enters Bowman's capsule and moves into the tubular system, where most of the water and many solutes are reabsorbed back into the bloodstream. The proximal tubule is the main site of reabsorption, followed by the loop of Henle, which creates osmotic gradients essential for concentrating urine, and the distal tubule and collecting duct, where fine-tuning of water and electrolyte balance occurs under hormonal control. See glomerulus and proximal tubule for the early stages of this process, and loop of Henle, distal tubule, and collecting duct for later adjustments.

Anatomy and structure

Renal corpuscle: The glomerulus sits inside Bowman's capsule, forming the site of plasma filtration. The glomerular filtration rate (glomerular filtration rate) reflects how much filtrate the nephrons generate per unit time and is a primary index of kidney function. The filtration barrier’s integrity is essential; damage can lead to proteinuria or reduced filtration capacity. Cross-links: glomerulus, Bowman's capsule, GFR.
Renal tubule: The tubular system extends from Bowman's capsule through the proximal tubule, loop of Henle, distal tubule, and collecting duct. Each segment harbors specialized transporters and channels that move ions, water, and solutes, shaping the final urine. Cross-links: proximal tubule, loop of Henle, distal tubule, collecting duct.
Supporting vessels: Surrounding capillary networks—the peritubular capillaries and, in parts of the loop of Henle, the vasa recta—reclaim reabsorbed substances and help maintain local and systemic fluid balance. Cross-links: peritubular capillaries, vasa recta.

Physiology of filtration, reabsorption, and secretion

Filtration: Blood pressure forces plasma through the filtration barrier, producing filtrate that is typically devoid of large proteins. The quality and rate of filtration are central to renal health and are reflected in the eGFR, a clinically useful proxy for GFR. Cross-links: GFR, Bowman's capsule.
Reabsorption: The proximal tubule reclaims the majority of filtered water, sodium, glucose, and other solutes. Transcellular and paracellular transport mechanisms, aided by transport proteins, drive this recovery. Water movement is tightly coupled to solute reabsorption via aquaporins and osmotic gradients.
Secretion: The distal parts of the nephron secrete certain wastes and excess ions into the tubular fluid, aiding in precise acid-base and electrolyte management. This process complements filtration and reabsorption to finalize urine composition.
Regulation: Hormonal and autonomic signals influence nephron function. The renin-angiotensin system modulates glomerular pressure and sodium handling; aldosterone increases sodium reabsorption and potassium excretion in the distal tubule; antidiuretic hormone (ADH) adjusts water reabsorption in the collecting duct; natriuretic peptides oppose sodium and water reabsorption when volume is high. These regulatory systems help the body respond to changes in blood pressure, fluid intake, and electrolyte status. See renin-angiotensin system, aldosterone, ADH, and natriuretic peptides.

Regulation and control

Autoregulation: The kidney maintains a relatively stable GFR across a range of blood pressures through mechanisms that adjust the tone of the afferent arteriole, protecting the filtration apparatus from injury due to blood pressure swings. This self-regulating feature helps preserve nephron function under varying conditions.
Hormonal signaling: The interplay of aldosterone, ADH, and the renin-angiotensin system, along with counter-regulatory signals, shapes how much fluid and which electrolytes are retained or excreted. In clinical terms, this regulation underpins approaches to treating conditions like hypertension and edema by targeting specific transport processes in the nephron. See aldosterone, ADH, and renin-angiotensin system.

Development and evolution

Development: Nephrons form during embryogenesis through interactions between the metanephric mesenchyme and the ureteric bud, leading to the mature architecture of the renal corpuscle and tubular segments. The process establishes the functional units that will be refined throughout life.
Evolution: Across vertebrates, nephrons exhibit variations in length, segment composition, and looping architecture that reflect adaptations to different environments and excretory demands. See metanephros for embryology and renal physiology for functional comparisons.

Clinical significance

Chronic kidney disease (CKD): Progressive nephron loss reduces overall filtration capacity and can culminate in end-stage renal disease without intervention. Early detection often relies on markers like eGFR and protein levels in urine. Cross-links: kidney disease, GFR.
Acute kidney injury (AKI): Sudden impairment of nephron function can arise from ischemia, toxins, or severe systemic illness, requiring prompt management to prevent long-term damage.
Nephrolithiasis and urinary tract issues: Stones can form from mineral deposits along the urinary tract, occasionally impacting urine flow and renal function. See nephrolithiasis for stone formation and management.
Therapies: When nephron loss is extensive, dialysis and kidney transplantation provide essential treatment options. See dialysis and kidney transplant for broader discussions of therapy and outcomes.

Historical notes

The concept of a functional kidney unit and the key role of filtration were clarified over centuries of anatomical and physiological study, with experimental work converging on the essential idea that a microscopic structure—later named the nephron—operates as the kidney’s workhorse. The naming of Bowman's capsule honors Sir William Bowman for his early work on the renal corpuscle, while the glomerulus reflects observations by early anatomists like Malpighi.