Tubular Reabsorption

The problem reabsorption solves

The kidney does not decide what to keep by filtering selectively. The glomerulus is a coarse sieve: it lets almost everything smaller than plasma proteins pour into the tubule, producing about 180 liters of filtrate per day at a normal glomerular filtration rate (GFR) of 125 mL/min. That filtrate contains roughly 1.5 kg of sodium, more than 150 grams of glucose, and the body's entire plasma volume many times over. If none of it were recovered, you would die of dehydration and hypoglycemia within minutes.

Tubular reabsorption is the answer. Rather than filter carefully, the kidney filters generously and then reclaims almost everything worth keeping. By the time tubular fluid reaches the end of the collecting duct, about 99% of the water and sodium and essentially all of the filtered glucose, amino acids, and bicarbonate have been pulled back into the peritubular capillaries. What remains — 1 to 2 liters of urine — is the metabolic waste plus a tightly regulated surplus of water and electrolytes. The genius of the design is that regulation happens on the output side: the body adjusts how much it reclaims, moment to moment, rather than how much it filters.

How the proximal tubule reabsorbs

Every reabsorptive step in the nephron ultimately rides on one engine: the basolateral Na+/K+-ATPase. This pump sits on the blood-facing side of each tubular cell and uses ATP to push 3 Na+ out into the interstitium and bring 2 K+ in. The result is a steep electrochemical gradient — intracellular sodium is kept very low (~10-15 mM versus ~140 mM in the lumen and blood) and the cell interior is electrically negative. Nearly all luminal sodium entry, and the solutes coupled to it, is powered by this gradient. Roughly two-thirds of the kidney's substantial oxygen consumption goes to running these pumps.

In the proximal convoluted tubule (PCT), sodium re-enters the cell from the lumen down its gradient, but it does not come alone. It is coupled to the substances the body wants to keep:

• Glucose piggybacks on sodium via SGLT2 (a high-capacity, low-affinity cotransporter in the early proximal tubule) and then SGLT1 (lower-capacity, high-affinity, downstream). Glucose then exits the cell into blood through GLUT2 and GLUT1 facilitated transporters. Under normal conditions, 100% of filtered glucose is reabsorbed and none reaches the urine.
• Amino acids use a family of sodium-dependent cotransporters; defects cause aminoacidurias such as cystinuria and Hartnup disease.
• Bicarbonate is reclaimed indirectly: the Na+/H+ exchanger (NHE3) secretes H+, which combines with filtered HCO3- to form CO2 and water (catalyzed by carbonic anhydrase), the CO2 diffuses in, and bicarbonate is regenerated inside the cell. This is why carbonic anhydrase inhibitors such as acetazolamide cause bicarbonate wasting and a metabolic acidosis.
• Phosphate uses sodium-phosphate cotransporters whose density is reduced by parathyroid hormone, which is how PTH promotes phosphate excretion.

Water follows the reabsorbed solute osmotically, much of it through aquaporin-1 channels and through the leaky tight junctions between proximal cells. Because solute and water are reabsorbed in nearly equal proportions, proximal reabsorption is essentially iso-osmotic — tubular fluid leaving the PCT has roughly the same osmolality as plasma (~300 mOsm/kg) even though its volume has fallen by two-thirds. The driving force for water and solute to move from the interstitium into the peritubular capillary is set by Starling forces: the capillary has high oncotic pressure (because protein-free filtrate was removed at the glomerulus) and low hydrostatic pressure, both favoring uptake.

The loop, distal tubule, and fine-tuning

After the proximal tubule, the loop of Henle reabsorbs about another 25% of filtered sodium. Its thick ascending limb uses the NKCC2 cotransporter (1 Na+, 1 K+, 2 Cl-) and is impermeable to water — this is what builds the hypertonic medullary interstitium that the countercurrent multiplier later exploits to concentrate urine. Loop diuretics such as furosemide block NKCC2, which is why they are the most powerful diuretics available.

The distal convoluted tubule reabsorbs sodium chloride via the NCC cotransporter, the target of thiazide diuretics, and reabsorbs calcium under PTH control. Finally, the collecting duct handles the last 3-5% of sodium through the epithelial sodium channel (ENaC) in principal cells, regulated by aldosterone, and adjusts water permeability through aquaporin-2 channels inserted into the membrane in response to antidiuretic hormone (ADH/vasopressin). It is here, on this small final fraction, that the body exerts precise control over salt and water balance — which is why a tiny percentage change in distal reabsorption can swing daily urine output by liters.

Tubular reabsorption vs tubular secretion

Reabsorption is one of three processes that shape urine; it is easy to confuse with secretion, which moves substances in the opposite direction. Net excretion of any substance is: filtered − reabsorbed + secreted.

Feature	Tubular reabsorption	Tubular secretion
Direction	Lumen (filtrate) → peritubular blood	Peritubular blood / cell → lumen
Purpose	Reclaim water, glucose, Na+, HCO3-, amino acids	Eliminate H+, K+, drugs, organic acids/bases
Main site	Proximal tubule (bulk), distal nephron (fine)	Proximal tubule (drugs), distal nephron (K+, H+)
Example transporters	SGLT2, NHE3, NKCC2, NCC, ENaC, aquaporins	OAT/OCT for drugs, H+-ATPase, ROMK for K+
Effect of saturation	Excess solute spills into urine (e.g. glucosuria)	Drug accumulates in blood, prolonging half-life

A key clinical handle is that glucose is freely filtered, normally fully reabsorbed, and not secreted. That makes its renal handling a clean illustration of a transport maximum. As plasma glucose rises, the filtered load rises linearly; below the threshold of about 180-200 mg/dL every molecule is reabsorbed. Once the SGLT carriers saturate at their Tm of roughly 375 mg/min, the excess is excreted — glucosuria — and that osmotically active sugar drags water with it, producing the osmotic diuresis, polyuria, and thirst of uncontrolled diabetes.

Clinical correlations and disease

• Diabetes mellitus. Chronic hyperglycemia exceeds the glucose threshold; glucosuria and osmotic diuresis follow. Persistent hyperfiltration and proximal sodium-glucose reabsorption also blunt tubuloglomerular feedback, contributing to diabetic nephropathy.
• SGLT2 inhibitors. Dapagliflozin, empagliflozin, and canagliflozin deliberately lower the glucose threshold to dump glucose, sodium, and water in the urine — lowering blood glucose insulin-independently and, importantly, protecting the heart and kidney.
• Fanconi syndrome. Generalized proximal tubule dysfunction causes wasting of glucose, amino acids, phosphate, bicarbonate, and uric acid all at once — a window onto how much the PCT normally reclaims. Causes include cystinosis, Wilson disease, multiple myeloma, and tenofovir or ifosfamide toxicity.
• Renal tubular acidosis. Type 2 (proximal) RTA is impaired bicarbonate reabsorption; type 1 (distal) is impaired H+ secretion. Both disturb acid-base balance.
• Diuretic pharmacology. Each diuretic class is named by the reabsorptive transporter it blocks: acetazolamide (proximal carbonic anhydrase), furosemide (NKCC2), thiazides (NCC), spironolactone and amiloride (aldosterone/ENaC). Knowing where each acts predicts both efficacy and electrolyte side effects.
• Acute and chronic kidney injury. Because the Na+/K+-ATPase pumps consume so much oxygen, the proximal tubule and the medullary thick ascending limb are exquisitely vulnerable to ischemia — the basis of acute tubular necrosis. As nephrons are lost in chronic kidney disease, surviving tubules cannot reclaim the filtered load efficiently, and reabsorptive capacity falls in step with GFR.

This article is educational and is not medical advice. For diagnosis or treatment, consult a qualified clinician.