Mechanism of urine formation
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Mechanism of Urine Formation
Urine formation is the result of three sequential, interrelated renal processes: glomerular filtration, tubular reabsorption, and tubular secretion. The net result is summarized as:
Urinary Excretion = Glomerular Filtration - Tubular Reabsorption + Tubular Secretion
(Guyton and Hall Textbook of Medical Physiology)
1. Glomerular Filtration
What is filtered
Urine formation begins when a large volume of fluid - virtually protein-free - is filtered from the glomerular capillaries into Bowman's capsule. Most plasma substances (water, electrolytes, glucose, amino acids, urea, creatinine) pass freely; plasma proteins and substances bound to them are retained. The filtrate in Bowman's capsule therefore has almost the same concentration as plasma.
Driving force - Starling forces
Glomerular filtration is governed by the Starling equation:
GFR = K_f [(P_GC - P_BS) - π_GC]
| Force | Value | Effect on filtration |
|---|---|---|
| P_GC - glomerular capillary hydrostatic pressure | ~45 mm Hg | Favors filtration |
| P_BS - Bowman's space hydrostatic pressure | ~10 mm Hg | Opposes filtration |
| π_GC - glomerular capillary oncotic pressure | ~28 mm Hg (rises along capillary) | Opposes filtration |
| Net ultrafiltration pressure | ~10 mm Hg | Favors filtration |
- K_f (filtration coefficient) for glomerular capillaries is 100-fold that of systemic capillaries because of their enormous surface area and high intrinsic water permeability - this allows 180 L of filtrate to be produced every day.
- The oncotic pressure progressively rises along the capillary length as fluid is filtered out; eventually it reaches filtration equilibrium and filtration stops.
- (Costanzo Physiology 7th Edition)
GFR in numbers
- Normal GFR: ~125 mL/min = ~180 L/day of filtrate formed
- Only ~1.5 L is excreted as urine - meaning ~99% is reabsorbed
2. Tubular Reabsorption
As the filtrate flows through the nephron tubules, essential substances are recovered back into the peritubular capillary blood. Reabsorption is quantitatively large and highly selective - unlike filtration, which is nonselective.
Segment-by-segment reabsorption
Proximal Convoluted Tubule (PCT)
- Reabsorbs ~65% of filtered Na+, water, and Cl-; 100% of glucose and amino acids; most bicarbonate
- The luminal surface has an extensive brush border (increases surface area), and cells are packed with mitochondria to power active transport
- Mechanism: Na+-K+-ATPase on the basolateral side maintains low intracellular Na+; this drives Na+ entry from the lumen via:
- Co-transport with glucose and amino acids (SGLT in early PCT)
- Counter-transport with H+ (NHE - reabsorbs Na+, secretes H+, removing HCO3-)
- Passive Cl- diffusion in the late PCT (higher luminal Cl- concentration drives it)
- Water follows by osmosis through aquaporin channels
- (Guyton and Hall Textbook of Medical Physiology)
Loop of Henle - Countercurrent Multiplier
This is responsible for producing the hyperosmotic renal medullary interstitium that allows urine concentration.
| Segment | Water permeability | Solute transport |
|---|---|---|
| Descending thin limb | High (water exits by osmosis) | Low active transport |
| Ascending thin limb | Low | Passive NaCl efflux |
| Thick ascending limb (TAL) | Impermeable to water | Active Na-K-2Cl (NKCC2) cotransport OUT |
The thick ascending limb actively pumps NaCl into the interstitium but water cannot follow (impermeable). This creates a 200 mOsm/L gradient at each horizontal level. As new fluid enters from the proximal tubule, the concentrated fluid from the descending limb flows up the ascending limb - and the process repeats, multiplying the concentration gradient. The end result is an interstitial osmolarity gradient from ~300 mOsm/L at the cortex to ~1200-1400 mOsm/L deep in the medulla.
(Guyton and Hall Textbook of Medical Physiology)
Distal Convoluted Tubule (DCT)
- Fluid leaving the loop of Henle enters the DCT at only ~100-140 mOsm/L (dilute)
- The early DCT, like the TAL, actively reabsorbs NaCl (via NCC cotransporter) but is impermeable to water - further diluting tubular fluid
- Aldosterone (acting on principal cells) increases Na+ reabsorption and K+ secretion in the late DCT and collecting duct
- PTH promotes Ca2+ reabsorption here
Collecting Duct (Cortical and Medullary)
- The final arbiter of urine concentration
- ADH (vasopressin) inserts aquaporin-2 (AQP2) channels into the luminal membrane, making the duct water-permeable
- With high ADH: water is osmotically drawn out into the hyperosmotic medullary interstitium → concentrated urine (~1200 mOsm/L)
- Without ADH: water stays in the tubule → dilute urine (~50-100 mOsm/L)
- Urea recycling in the inner medullary collecting duct also contributes significantly to medullary hyperosmolarity
3. Tubular Secretion
Secretion moves substances from peritubular capillary blood into the tubular lumen - this adds to what the filtrate already contains.
Key secreted substances:
| Substance | Primary site | Significance |
|---|---|---|
| H+ | PCT, intercalated cells of collecting duct | Acid-base balance |
| K+ | Principal cells of collecting duct | Regulated by aldosterone; major route of K+ excretion |
| NH4+/NH3 | PCT and collecting duct | Buffer in acidosis |
| Organic acids & bases | PCT | Drug/toxin elimination (e.g., PAH, penicillin, uric acid) |
| Creatinine | Small amount in PCT | Near-complete excretion (mainly filtered) |
Secretion is especially important for K+ and H+ - their urinary excretion rates can actually exceed their filtered loads, proving net secretion has occurred. (Costanzo Physiology 7th Edition)
Summary Table: Fate of Key Filtered Substances
| Substance | Filtered/day | Excreted/day | % Reabsorbed | Key mechanism |
|---|---|---|---|---|
| Water | 180 L | 1.5 L | 99% | Osmosis (AQP), ADH control |
| Na+ | 25,200 mEq | 100 mEq | 99.6% | Na-K-ATPase, aldosterone |
| Glucose | 180 g | 0 g | 100% | SGLT1/2 (PCT) |
| HCO3- | ~4320 mEq | ~2 mEq | ~99.9% | Linked to H+ secretion |
| Urea | 56 g | 28 g | ~50% | Passive (PCT, inner medulla) |
| Creatinine | ~1.8 g | ~1.8 g | ~0% | Marker of GFR |
| K+ | ~800 mEq | ~100 mEq | ~87% | Active reabsorption + secretion |
Key Regulatory Hormones
| Hormone | Site of action | Effect |
|---|---|---|
| ADH (vasopressin) | Collecting duct | ↑ water reabsorption (AQP2 insertion) |
| Aldosterone | Late DCT, collecting duct | ↑ Na+ reabsorption, ↑ K+ secretion |
| ANP/BNP | Glomerulus, collecting duct | ↑ GFR, ↓ Na+ reabsorption |
| PTH | DCT | ↑ Ca2+ reabsorption, ↓ phosphate reabsorption |
| Angiotensin II | PCT, afferent/efferent arterioles | ↑ Na+ reabsorption, vasoconstriction |
Why Filter 180 L and Reabsorb 99%?
This seemingly wasteful strategy has two key advantages:
- Rapid clearance of metabolic waste products (urea, creatinine, uric acid) that are poorly reabsorbed - a high GFR ensures they are quickly removed
- Precise regulation - fine control over excretion is achieved by adjusting reabsorption, even for substances filtered in large amounts
About 10% of the body's resting energy goes to powering this reabsorption machinery.
(Guyton and Hall Textbook of Medical Physiology, p. 330)
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