Nephron & Glomerular Filtration

Why the nephron matters

• It cleans your entire blood volume ~36 times a day. The two kidneys receive about 1.1 L of blood per minute — roughly 20–25% of cardiac output — and filter ~180 L of plasma every 24 hours. Since you only have about 3 L of plasma, the whole supply is processed dozens of times daily. This is how nitrogenous waste (urea, creatinine, uric acid) is cleared before it reaches toxic levels.
• It is the body's master volume and electrolyte controller. By adjusting how much filtered Na+, K+, Ca2+, water, and H+ it reabsorbs or secretes, the nephron sets blood volume, blood pressure, plasma osmolality, and pH. The renin-angiotensin-aldosterone system and ADH act here, which is why the kidney is central to long-term blood-pressure regulation.
• GFR is the number that defines kidney health. Clinicians stage chronic kidney disease (CKD) almost entirely by estimated GFR. A normal eGFR is ≥90 mL/min/1.73 m²; below 60 for three months defines CKD; below 15 is kidney failure requiring dialysis or transplant. About 1 in 7 US adults has CKD, most of it driven by diabetes and hypertension damaging the glomerular barrier.
• Drug targets concentrate here. Diuretics act on specific tubule segments: loop diuretics (furosemide) block the Na-K-2Cl cotransporter in the thick ascending limb; thiazides block the Na-Cl cotransporter in the distal tubule; spironolactone blocks aldosterone in the collecting duct. SGLT2 inhibitors (empagliflozin, dapagliflozin) block glucose reabsorption in the proximal tubule and have become major heart-failure and CKD drugs.
• It is a model for size-and-charge molecular sorting. The glomerulus is one of biology's most precise passive filters: it freely passes water and small solutes, sharply excludes anything near the 66 kDa / 3.6 nm size of albumin, and uses fixed negative charge to repel anions. Understanding it underlies the design of dialysis membranes and the interpretation of proteinuria.
• Evolution scaled it for life on land. The vertebrate nephron originated as a way to dump excess water in freshwater fish; the loop of Henle — which lets mammals and birds concentrate urine above plasma osmolality — is a key adaptation for conserving water on land. A desert kangaroo rat can produce urine ~5,000 mOsm/kg, far above human maximum (~1,200 mOsm/kg).

How glomerular filtration works

Blood enters each glomerulus through a relatively wide afferent arteriole and leaves through a narrow efferent arteriole. That arrangement is the trick: putting a high-resistance vessel on the outlet traps a high hydrostatic pressure inside the glomerular capillaries — about 55 mmHg, roughly double the pressure in ordinary capillaries elsewhere in the body. This pressure is the engine that pushes fluid out of the blood and into Bowman's capsule.

The net force driving filtration is governed by the Starling forces. The 55 mmHg of glomerular capillary hydrostatic pressure (pushing fluid out) is opposed by two inward forces: the plasma colloid osmotic (oncotic) pressure of about 30 mmHg — generated because plasma proteins stay behind and pull water back — and the hydrostatic pressure in Bowman's capsule of about 15 mmHg. The net filtration pressure is therefore roughly 55 − 30 − 15 = 10 mmHg outward. Multiply that modest pressure by the enormous total surface area of ~2 million glomeruli (a high filtration coefficient, K_f) and you get a glomerular filtration rate of about 125 mL/min, or 180 L/day.

What actually crosses the barrier is sorted by size and charge. Fluid passes three layers in series: the fenestrated endothelium (capillary cells with 70–100 nm pores that block cells but pass plasma), the glomerular basement membrane (a thick collagen-IV / laminin / heparan-sulfate mesh that is the main charge barrier), and the podocyte slit diaphragm (foot processes leaving 25–40 nm slits bridged by the proteins nephrin and podocin). Water, ions, glucose, amino acids, and urea pass freely; red cells, white cells, platelets, and large proteins like albumin (66 kDa, ~3.6 nm, negatively charged) are held back.

The raw filtrate is essentially protein-free plasma — far too much to excrete. The renal tubule then reclaims it. The proximal convoluted tubule (PCT) reabsorbs ~65% of the filtered water and Na+, all the glucose (via SGLT2 then SGLT1), all amino acids, and most bicarbonate, powered by the basolateral Na+/K+ ATPase. The loop of Henle reabsorbs another ~25% and, by countercurrent multiplication, builds a steep osmotic gradient in the kidney medulla. The distal convoluted tubule and collecting duct then make the final adjustments under hormonal control — aldosterone tunes Na+/K+, and ADH controls water via aquaporin-2 — leaving only ~1.5 L of urine.

The numbers: Starling forces and clearance

Net filtration pressure (NFP):
  NFP = P_GC  −  (π_GC  +  P_BS)
      = 55 mmHg − (30 mmHg + 15 mmHg)
      ≈ 10 mmHg   (outward)

  P_GC  = glomerular capillary hydrostatic pressure  (~55 mmHg)
  π_GC  = glomerular capillary oncotic pressure       (~30 mmHg)
  P_BS  = Bowman's space hydrostatic pressure          (~15 mmHg)

Glomerular filtration rate:
  GFR = Kf × NFP
      ≈ 12.5 mL/min/mmHg × 10 mmHg
      ≈ 125 mL/min  ≈ 180 L/day

Renal clearance of a substance X:
  C_X = (U_X × V) / P_X
      U_X = urine concentration of X
      V   = urine flow rate (mL/min)
      P_X = plasma concentration of X

  Inulin (freely filtered, not reabsorbed/secreted): C_inulin = GFR
  Creatinine (mostly filtered): C_creatinine ≈ GFR (clinical estimate)
  PAH (filtered + fully secreted): C_PAH ≈ renal plasma flow (~625 mL/min)

Filtration fraction:
  FF = GFR / RPF ≈ 125 / 625 ≈ 0.20  (about 20% of plasma is filtered)

Worked example: tracking 100 mL of plasma

Follow 100 mL of plasma as it enters a glomerulus. About 20 mL (the filtration fraction of ~0.2) is filtered into Bowman's capsule; the other 80 mL continues out the efferent arteriole, now with concentrated proteins (which is why oncotic pressure rises along the capillary and self-limits filtration). All of the albumin and red cells stay in that 80 mL.

Of the 20 mL of filtrate, the PCT immediately reabsorbs ~13 mL (65%) along with all the glucose and amino acids and most of the Na+ and bicarbonate. The loop of Henle takes back ~5 mL (about 25% of the original), so only ~2 mL reaches the distal tubule. The collecting duct, under ADH, reabsorbs most of that last 2 mL — typically leaving on the order of 0.1–0.2 mL to become urine. Scaled up to the whole organ, 125 mL/min filtered becomes ~1 mL/min of urine.

Now a clinical twist. Suppose a patient's plasma glucose is very high (uncontrolled diabetes, say 400 mg/dL). The filtered glucose load exceeds the PCT's transport maximum (T_m ≈ 375 mg/min, reached at a plasma glucose around 200 mg/dL, the renal threshold). SGLT2 transporters saturate, glucose spills into the urine (glucosuria), and because it is osmotically active it drags water with it — producing the classic polyuria and thirst of diabetes. This same physiology is exploited deliberately by SGLT2-inhibitor drugs.

Filtration barrier: the three layers compared

Layer	Structure	Pore / gap size	What it blocks	Key molecules
Fenestrated endothelium	Capillary lining cells with open pores	~70–100 nm fenestrae	Blood cells, platelets	PV-1, glycocalyx (charge)
Glomerular basement membrane (GBM)	Thick fused basal lamina	~3–8 nm effective mesh	Large/medium proteins; main charge barrier	Type IV collagen, laminin, heparan sulfate
Podocyte slit diaphragm	Interdigitating foot processes with a bridging zipper	~25–40 nm slits	Albumin-sized proteins (final sieve)	Nephrin (NPHS1), podocin (NPHS2)

Tubule segments and what each reabsorbs

Segment	~% of filtrate reabsorbed	Main job	Key transporters / hormones	Diuretic target?
Proximal convoluted tubule (PCT)	~65% of water & Na+	Bulk reabsorption: glucose, amino acids, HCO₃⁻	SGLT2, Na+/K+ ATPase, NHE3	Acetazolamide (carbonic anhydrase)
Descending loop of Henle	~15% of water	Water out (permeable), solutes stay	Aquaporin-1	Osmotic diuretics (mannitol)
Thick ascending limb	~25% of Na+ (water impermeable)	Builds medullary gradient	NKCC2 (Na-K-2Cl)	Loop diuretics (furosemide)
Distal convoluted tubule (DCT)	~5% of Na+	Fine Na+ / Ca2+ tuning	NCC (Na-Cl), PTH-controlled Ca2+	Thiazides
Collecting duct	final ~3–5% (variable)	Water & Na+/K+/H+ fine-tuning	Aquaporin-2 (ADH), ENaC (aldosterone)	Amiloride, spironolactone

Diseases and real examples

• Diabetic nephropathy is the leading cause of kidney failure worldwide. Chronic high glucose thickens the GBM and injures podocytes; the earliest sign is microalbuminuria (30–300 mg/day) as the charge barrier degrades, progressing to overt proteinuria and falling GFR. SGLT2 inhibitors slow this decline by reducing intraglomerular pressure.
• Nephrotic syndrome is defined by heavy proteinuria (>3.5 g/day), low blood albumin, and edema — the result of a leaky filtration barrier. Minimal change disease (common in children) shows podocyte foot-process effacement under electron microscopy. Genetic forms come from mutations in nephrin (NPHS1, congenital nephrotic syndrome) or podocin (NPHS2).
• Glomerulonephritis covers inflammatory glomerular damage. In IgA nephropathy (the most common glomerulonephritis worldwide), immune complexes deposit in the mesangium; in post-streptococcal glomerulonephritis, immune complexes inflame the barrier days to weeks after a strep infection, producing hematuria, "cola-colored" urine, and a sharp GFR drop.
• Acute kidney injury (AKI) from shock or hemorrhage is pre-renal: blood pressure falls, net filtration pressure collapses toward zero, and GFR plummets — initially reversible if perfusion is restored, but prolonged ischemia kills tubule cells (acute tubular necrosis).
• Diabetes insipidus is failure of the ADH–aquaporin-2 axis: central DI (no ADH) or nephrogenic DI (unresponsive collecting duct) produces up to 20 L/day of dilute urine because water reabsorption in the collecting duct fails. It is distinct from diabetes mellitus despite the shared name.
• The kangaroo rat and the loop of Henle. Desert rodents have exceptionally long loops of Henle, building a medullary gradient steep enough to concentrate urine to ~5,000 mOsm/kg — letting them survive without drinking, on metabolic water alone. Humans top out near 1,200 mOsm/kg; beavers, with short loops, manage only ~600.

Common misconceptions

• "The kidney decides what to filter." Glomerular filtration is essentially non-selective among small molecules — it filters water, glucose, amino acids, urea, and ions indiscriminately based on size and charge. The selectivity comes afterward, in tubular reabsorption and secretion. The kidney's strategy is "filter everything small, then take back what you need."
• "Filtration uses ATP / is active transport." Glomerular filtration is a passive, pressure-driven process — no ATP is spent at the barrier itself. The energy comes from the heart generating blood pressure. ATP is consumed downstream, in the tubule, where the Na+/K+ ATPase powers reabsorption.
• "Urine is just filtered blood." Urine is filtered plasma that has been radically modified: over 99% of the water and nearly all useful solutes are reabsorbed, and some substances (K+, H+, drugs, creatinine) are actively secreted into the tubule. Final urine bears little resemblance to the initial filtrate.
• "More blood pressure always means more urine." Autoregulation keeps GFR nearly constant across a mean arterial pressure of ~80–180 mmHg via the myogenic response and tubuloglomerular feedback. Within that range, raising blood pressure barely changes GFR. (Pressure-natriuresis does increase urine somewhat, but it is a smaller effect than the naive intuition suggests.)
• "The glomerulus filters out toxins specifically." The glomerulus has no chemical recognition of "toxins." Urea and creatinine are cleared simply because they are small and not reabsorbed. Many actual toxins and drugs are eliminated mainly by active tubular secretion, not glomerular filtration.
• "A trace of protein in urine is always disease." A small amount of protein (mostly Tamm-Horsfall protein secreted by the tubule, plus <30 mg/day of albumin) is normal. Transient proteinuria can follow heavy exercise or fever. Persistent albuminuria above 30 mg/day, however, is an early marker of glomerular damage.