Nephron

The nephron is the microscopic structural and functional unit of the vertebrate kidney responsible for blood ultrafiltration, selective tubular reabsorption, active solute secretion, acid–base balance, and systemic osmoregulation. Each adult human kidney contains approximately 1.0 to 1.3 million nephrons, which cannot be regenerated once lost after birth. Every nephron consists of two continuous anatomical divisions: the renal corpuscle (comprising the tuft of fenestrated glomerular capillaries encased within Bowman's capsule in the renal cortex) and a segmented, single-layered epithelial renal tubule (sequentially divided into the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and the connecting tubule draining into the collecting duct system).

Mammalian nephrons are classified into two populations based on the cortical depth of their glomeruli and the length of their loops of Henle: cortical nephrons (~85% of human nephrons), whose glomeruli reside in the outer and mid-cortex with short loops of Henle that penetrate only to the outer renal medulla, and juxtamedullary nephrons (~15% of human nephrons), whose glomeruli lie near the corticomedullary border and project long loops of Henle accompanied by hairpin vasa recta capillaries deep into the inner renal papilla. Together, the ~2 million nephrons receive ~20–25% of resting cardiac output (~1,100–1,200 mL/min of renal blood flow, or ~625 mL/min of renal plasma flow) and generate roughly 180 liters of glomerular filtrate per day, reabsorbing >99% of the filtered water and electrolytes to excrete 1.0–2.0 L/day of final urine.

The Three-Layer Glomerular Filtration Barrier

Urine formation begins at the renal corpuscle, where hydrostatic pressure forces a protein-free ultrafiltrate of blood plasma across a three-layered size- and charge-selective barrier into Bowman's space:

1. Fenestrated Glomerular Endothelium: Capillary endothelial cells perforated by open transcellular fenestrations (70–100 nm in diameter) coated with a negatively charged luminal glycocalyx that excludes blood cells and platelets while permitting rapid fluid flux.
2. Glomerular Basement Membrane (GBM): A fused, 300–350 nm thick acellular matrix synthesized jointly by endothelial cells and podocytes, composed of cross-linked type IV collagen networks (α3α4α5 chains), laminin-521, nidogen, and negatively charged heparan sulfate proteoglycans (agrin and perlecan). The polyanionic heparan sulfate chains electrostatically repel anionic plasma proteins such as serum albumin (molecular mass ~66.5 kDa, effective Stokes–Einstein radius ~3.6 nm, net charge −15 at physiological pH 7.4).
3. Visceral Epithelial Podocytes and the Slit Diaphragm: Specialized stellate cells whose interdigitating foot processes (pedicels) wrap the outer aspect of the capillaries, bridged across 30–40 nm filtration slits by a zipper-like slit diaphragm composed of nephrin, NEPH1, and podocin anchored to the actin cytoskeleton.

Consequently, solutes with an effective molecular radius <1.8 nm (inulin, glucose, amino acids, urea, Na⁺, K⁺, Cl⁻, HCO₃⁻) have a filtration sieving coefficient of 1.0 (freely filtered), whereas molecules >4.2 nm are excluded.

Starling Forces and Glomerular Filtration Rate (GFR)

The glomerular filtration rate (GFR)—averaging 125 mL/min (180 L/day) in a healthy 70-kg adult—is governed by the balance of transcapillary Starling hydrostatic and colloid osmotic (oncotic) pressures across the glomerular tuft:

GFR = K_f × (P_GC − P_BS − π_GC + π_BS)

where K_f is the ultrafiltration coefficient (product of hydraulic conductivity and surface area, ~12.5 mL·min⁻¹·mmHg⁻¹, roughly 100-fold higher than in skeletal muscle capillaries), P_GC is glomerular capillary hydrostatic pressure (~55–60 mmHg, maintained high along the tuft by the muscular efferent arteriole positioned in series downstream of the glomerulus), P_BS is Bowman's space hydrostatic pressure (~15 mmHg), π_GC is glomerular capillary oncotic pressure (rising from ~25 mmHg at the afferent end to ~35 mmHg at the efferent end as ~20% of plasma water is filtered out, giving a filtration fraction FF = GFR / RPF ≈ 0.20), and π_BS is Bowman's space oncotic pressure (≈0 mmHg in health). This yields a net filtration pressure (NFP) of ~10–15 mmHg. GFR is kept nearly constant across systemic mean arterial pressures of 80 to 180 mmHg via two intrinsic autoregulatory mechanisms: the myogenic reflex of the afferent arteriole (stretch-activated cation channels) and tubuloglomerular feedback (TGF) mediated by the macula densa.

Proximal Convoluted Tubule: Isotonic Bulk Reabsorption

Lined by cuboidal epithelial cells possessing a dense apical microvillar brush border and abundant mitochondria, the proximal convoluted tubule (PCT) reabsorbs 65–70% of filtered Na⁺, Cl⁻, K⁺, and water, ~85% of filtered bicarbonate (HCO₃⁻), and virtually 100% of filtered D-glucose and amino acids. The thermodynamic engine for all proximal transport is the basolateral Na⁺/K⁺-ATPase transmembrane pump, which extrudes 3 Na⁺ in exchange for 2 K⁺, maintaining intracellular [Na⁺] low (~12–15 mM) and intracellular potential negative (−70 mV).

Filtered D-glucose (~800 mmol/day) is reclaimed via secondary active symport: in the early S1/S2 segments, the low-affinity, high-capacity SGLT2 cotransporter (1 Na⁺ : 1 glucose; target of gliflozin antidiabetic drugs) reabsorbs ~90% of filtered glucose, while in the late S3 segment, the high-affinity SGLT1 cotransporter (2 Na⁺ : 1 glucose) scavenges the remaining ~10%, before glucose exits basolaterally via GLUT2/GLUT1 facilitated diffusion. The transport maximum (T_m) for glucose averages ~375 mg/min; when plasma glucose exceeds the renal threshold (~180–200 mg/dL in diabetes mellitus), transporters saturate and glucosuria ensues. Filtered HCO₃⁻ (~4,320 mEq/day) is reclaimed via the apical Na⁺/H⁺ exchanger 3 (NHE3) coupled to brush-border carbonic anhydrase IV, which converts luminal H₂CO₃ into CO₂ and H₂O for diffusion into the cell and rehydration by cytosolic carbonic anhydrase II (governed by the Henderson-Hasselbalch equation). Water follows solute reabsorption osmotically via constitutive aquaporin-1 (AQP1) channels and paracellular claudin-2 pores, keeping tubular fluid isotonic (~290–300 mOsm/kg) throughout the PCT.

Loop of Henle and Countercurrent Multiplication

To enable terrestrial mammals to excrete hyperosmotic urine during dehydration, the loop of Henle of juxtamedullary nephrons functions as a countercurrent multiplier that establishes a vertical corticomedullary interstitial osmotic gradient rising from 300 mOsm/kg in the cortex to 1,200 mOsm/kg at the human inner medullary papilla (and up to 6,000–9,000 mOsm/kg in desert rodents like the kangaroo rat). This multiplier relies on opposing transport properties of its two limbs:

1. Thin Descending Limb: Highly permeable to water via constitutive AQP1 channels, but virtually impermeable to NaCl and urea. As filtrate descends into the hypertonic medulla, water exits osmotically into the interstitium, concentrating the luminal fluid up to 1,200 mOsm/kg at the hairpin turn.
2. Thick Ascending Limb (TAL): Completely impermeable to water (lacking aquaporins) while actively pumping solute out of the lumen via the apical NKCC2 (Na⁺-K⁺-2Cl⁻) cotransporter (targeted by loop diuretics such as furosemide and bumetanide). Apical ROMK channels recycle K⁺ back into the lumen, generating a lumen-positive electrical potential (+8 to +10 mV) that drives paracellular reabsorption of divalent cations (Ca²⁺ and Mg²⁺) through claudin-16/19 tight junctions. This active NaCl extrusion without water creates a ~200 mOsm/kg single-effect osmotic gradient at every horizontal level, which is multiplied axially by countercurrent flow while diluting the fluid exiting the TAL to ~100 mOsm/kg (earning the TAL the title "diluting segment").

Inner medullary hyperosmolality is doubled by urea recycling (contributing ~500–600 mOsm/kg of the 1,200 mOsm/kg papillary osmolality via vasopressin-activated UT-A1/UT-A3 urea transporters in the inner medullary collecting duct) and preserved against washout by passive countercurrent exchange in the hairpin vasa recta capillaries.

At the junction of the TAL and the distal convoluted tubule (DCT), specialized plaque cells called the macula densa monitor luminal NaCl delivery via NKCC2 as part of the juxtaglomerular apparatus (JGA). If GFR rises and excessive NaCl reaches the macula densa, macula densa cells release ATP/adenosine to constrict the adjacent afferent arteriole via A₁ receptors (tubuloglomerular feedback) and inhibit renin secretion from granular juxtaglomerular cells. Conversely, low NaCl delivery or reduced renal perfusion pressure stimulates renin release, activating the renin–angiotensin–aldosterone system (RAAS).

In the early DCT, the thiazide-sensitive Na⁺-Cl⁻ cotransporter (NCC) reabsorbs ~5–8% of filtered NaCl while parathyroid hormone (PTH) regulates transcellular Ca²⁺ reabsorption via apical TRPV5 channels. Final adjustments occur in the collecting duct, which contains two cell types:

• Principal Cells: Express apical epithelial Na⁺ channels (ENaC) and ROMK K⁺ channels. Adrenal zona glomerulosa steroid hormone aldosterone binds cytosolic mineralocorticoid receptors to upregulate ENaC and basolateral Na⁺/K⁺-ATPase transcription, increasing Na⁺ reabsorption and lumen-negative driving force for K⁺ secretion. Posterior pituitary peptide hormone arginine vasopressin (AVP / antidiuretic hormone, ADH) binds basolateral V₂ G_s-coupled receptors, activating adenylyl cyclase and PKA to trigger exocytic insertion of aquaporin-2 (AQP2) water channel vesicles into the apical membrane, allowing water to equilibrate osmotically with the 1,200 mOsm/kg medullary interstitium (with water exiting basolaterally via constitutive AQP3 and AQP4).
• α- and β-Intercalated Cells: Regulate systemic acid–base balance; α-intercalated cells secrete H⁺ into the urine via apical vacuolar H⁺-ATPase and H⁺/K⁺-ATPase while generating new HCO₃⁻ during acidosis, whereas β-intercalated cells secrete HCO₃⁻ via the apical Cl⁻/HCO₃⁻ exchanger pendrin during alkalosis.

Why does constriction of the efferent arteriole initially increase GFR, whereas constriction of the afferent arteriole always decreases GFR?

The glomerular capillaries are uniquely interposed between two resistance vessels: the upstream afferent arteriole and the downstream efferent arteriole. Constriction of the afferent arteriole reduces blood flow into the glomerulus and lowers glomerular hydrostatic pressure (P_GC), lowering GFR. In contrast, moderate constriction of the efferent arteriole (for example, by angiotensin II, which preferentially constricts efferent arterioles) impedes outflow from the glomerulus, backing up pressure to raise P_GC and increase (or preserve) GFR despite reduced total renal blood flow.

Why do loop diuretics like furosemide cause both hypokalemia and urinary calcium wasting, whereas thiazide diuretics reduce urinary calcium excretion?

Furosemide inhibits the NKCC2 transporter in the thick ascending limb, abolishing the lumen-positive electrical potential (+10 mV) that normally drives paracellular Ca²⁺ and Mg²⁺ reabsorption, thereby increasing urinary Ca²⁺ excretion. In contrast, thiazide diuretics inhibit NCC in the distal convoluted tubule; the resulting intracellular Na⁺ depletion enhances basolateral Na⁺/Ca²⁺ exchange (NCX1) and apical TRPV5 Ca²⁺ entry, increasing renal Ca²⁺ reabsorption. Both classes increase Na⁺ and fluid delivery to collecting duct principal cells, accelerating ENaC-driven Na⁺ uptake and lumen-negative K⁺ wasting (hypokalemia).

What happens to urine volume and osmolality in central or nephrogenic diabetes insipidus?

In the absence of circulating arginine vasopressin (central diabetes insipidus) or due to loss-of-function mutations in the V₂ receptor or AQP2 gene (nephrogenic diabetes insipidus, or lithium toxicity), collecting duct principal cells fail to insert AQP2 water channels into their apical membranes. Because the collecting duct remains impermeable to water despite an intact medullary osmotic gradient, the hypotonic fluid exiting the distal convoluted tubule cannot be concentrated, resulting in the excretion of 10–20 L/day of dilute urine (osmolality 50–100 mOsm/kg) and hypernatremia if water intake is restricted.

1. Boron, W. F., & Boulpaep, E. L. (2016). Medical Physiology (3rd ed.). Elsevier. https://doi.org/10.1016/C2011-0-06165-3
2. Greka, A., & Mundel, P. (2012). "Cell biology and pathology of podocytes." Annual Review of Physiology, 74, 299–323. https://doi.org/10.1146/annurev-physiol-020911-153238
3. Sands, J. M., & Layton, H. E. (2014). "Advances in understanding the urine-concentrating mechanism." Annual Review of Physiology, 76, 387–409. https://doi.org/10.1146/annurev-physiol-021113-170350
4. Knepper, M. A., Kwon, T. H., & Nielsen, S. (2015). "Molecular physiology of water balance." New England Journal of Medicine, 372(14), 1349–1358. https://doi.org/10.1056/NEJMra1404726