Acid-Base Regulation

Foundations and mechanisms

Core concepts: what the body is regulating

Acid-base physiology is the regulation of extracellular hydrogen ion concentration, expressed as pH. Normal arterial pH is 7.35–7.45, corresponding to a very low free H⁺ concentration of approximately 40 nEq/L. Because proteins, enzymes, ion channels, and hemoglobin are pH-sensitive, even small deviations can impair cellular function. The major extracellular buffer system is the bicarbonate buffer system, described by the Henderson-Hasselbalch equation:

pH = 6.1 + log([HCO₃⁻] / 0.03 × PCO₂)

Thus, acid-base status depends on the ratio of metabolic base to respiratory acid. Normal values are HCO₃⁻ ≈ 24 mEq/L and arterial PCO₂ ≈ 40 mm Hg, giving a ratio near 20:1. The lungs regulate PCO₂ within minutes by changing alveolar ventilation; the kidneys regulate HCO₃⁻ and net acid excretion over hours to days.

Variable	Normal value	Primary regulator	Time scale
Arterial pH	7.35–7.45	Lungs + kidneys + buffers	Seconds to days
PCO2	35–45 mm Hg	Alveolar ventilation	Minutes
HCO3−	22–28 mEq/L	Renal tubular handling	Hours to days
Serum anion gap	Usually 8–12 mEq/L	Unmeasured anions	Context-dependent

Daily acid load and buffering

Metabolism generates two major acid categories. Volatile acid is CO₂, produced at approximately 15,000–20,000 mmol/day from aerobic metabolism and eliminated by the lungs. Nonvolatile acid, approximately 50–100 mEq/day, comes mainly from sulfur-containing amino acids and phosphoproteins, generating sulfuric and phosphoric acids. Nonvolatile acid cannot be exhaled and must be excreted by the kidney.

Buffers minimize pH change by accepting or donating H⁺. Important buffers include bicarbonate extracellularly, hemoglobin in red blood cells, phosphate in urine and intracellular fluid, and bone salts during chronic acid loads. Buffering is immediate but does not eliminate acid; definitive removal requires ventilation and renal excretion.

Renal bicarbonate reclamation

The kidney filters roughly 4,000–5,000 mEq/day of HCO₃⁻ because plasma HCO₃⁻ is freely filtered at the glomerulus. Nearly all filtered bicarbonate must be reabsorbed to prevent rapid metabolic acidosis. About 80–90% is reclaimed in the proximal tubule, with the remainder primarily in the thick ascending limb and collecting duct.

In the proximal tubule, H⁺ is secreted into the lumen mainly by the Na⁺/H⁺ exchanger NHE3. Luminal H⁺ combines with filtered HCO₃⁻ to form H₂CO₃, which is converted by carbonic anhydrase to CO₂ and H₂O. CO₂ diffuses into the tubular cell, where intracellular carbonic anhydrase reforms H₂CO₃, dissociating into H⁺ and HCO₃⁻. The H⁺ is recycled into the lumen, while HCO₃⁻ exits basolaterally, often with Na⁺. A key Step 1 concept is that this process reclaims filtered bicarbonate but does not create “new” bicarbonate.

Generation of new bicarbonate: titratable acid and ammonium

To replace bicarbonate consumed buffering nonvolatile acids, the kidney must generate new HCO₃⁻. This occurs when secreted H⁺ is excreted in urine bound to non-bicarbonate buffers. The two principal mechanisms are titratable acid excretion and ammonium excretion.

• Titratable acid: secreted H⁺ binds filtered phosphate, especially HPO₄²⁻, forming H₂PO₄⁻. For each H⁺ excreted this way, one new HCO₃⁻ enters blood.
• Ammonium: proximal tubular cells metabolize glutamine to produce NH₄⁺ and new HCO₃⁻. NH₄⁺ is secreted into the lumen, partly substituting for H⁺ on NHE3. In the medulla, NH₃ diffuses into the collecting duct and buffers secreted H⁺ as NH₄⁺, which is “trapped” in urine.

Ammoniagenesis is highly adaptable and increases markedly during chronic metabolic acidosis. Minimum urine pH is approximately 4.5; below this, free H⁺ excretion is limited, so urinary buffering is essential.

Collecting duct acid-base cells

The collecting duct performs final acid-base regulation through specialized intercalated cells. Alpha-intercalated cells secrete H⁺ via apical H⁺-ATPase and H⁺/K⁺-ATPase, reabsorbing HCO₃⁻ basolaterally via a Cl⁻/HCO₃⁻ exchanger. They are activated in acidosis and also link acid-base balance to potassium: increased H⁺/K⁺-ATPase activity can promote K⁺ reabsorption. Beta-intercalated cells perform the opposite function, secreting HCO₃⁻ through apical pendrin, a Cl⁻/HCO₃⁻ exchanger, and are important in alkalosis.

Classification of primary acid-base disorders

Primary disorder	Initial change	Expected compensation	Core mechanism
Metabolic acidosis	↓ HCO3−	↓ PCO2	Hyperventilation, renal acid excretion if kidneys intact
Metabolic alkalosis	↑ HCO3−	↑ PCO2	Hypoventilation limited by hypoxemia
Respiratory acidosis	↑ PCO2	↑ HCO3−	Renal bicarbonate retention and acid excretion
Respiratory alkalosis	↓ PCO2	↓ HCO3−	Renal bicarbonate excretion

For Step 1, always identify the primary disturbance, then determine whether compensation is appropriate. Compensation reduces pH change but usually does not fully normalize pH; a normal pH with abnormal PCO₂ and HCO₃⁻ suggests a mixed disorder.

Clinical assessment and investigations

Clinical presentation: recognizing acid-base disorders

Acid-base disorders often present nonspecifically; the key is linking symptoms to changes in pH, ventilation, and renal bicarbonate handling. Normal arterial values are approximately pH 7.35–7.45, PaCO₂ 35–45 mm Hg, and serum HCO₃⁻ 22–28 mEq/L. Acidemia depresses myocardial contractility, predisposes to arrhythmias, and causes hyperkalemia by extracellular K⁺ shifting. Alkalemia increases neuromuscular excitability, decreases ionized calcium binding, and may cause paresthesias, tetany, or seizures.

• Metabolic acidosis: deep, rapid Kussmaul respirations from respiratory compensation; seen in diabetic ketoacidosis, lactic acidosis, renal failure, diarrhea, and renal tubular acidosis.
• Metabolic alkalosis: hypoventilation is limited by hypoxemia; common clues include vomiting, nasogastric suction, loop/thiazide diuretics, volume contraction, and hypokalemia.
• Respiratory acidosis: hypoventilation with CO₂ retention; causes include COPD, CNS depression, neuromuscular weakness, and airway obstruction.
• Respiratory alkalosis: hyperventilation; classic causes include anxiety, pain, pregnancy, high altitude, pulmonary embolism, sepsis, and early salicylate toxicity.

Core investigations and stepwise interpretation

The principal tests are an arterial blood gas or venous blood gas, serum electrolytes, glucose, BUN/creatinine, lactate, ketones, and urinalysis. Venous pH is usually about 0.03 lower than arterial pH and is often adequate for screening, but PaCO₂ and oxygenation require arterial sampling when precision is needed.

1. Determine pH: pH <7.35 = acidemia; pH >7.45 = alkalemia.
2. Identify the primary process: if pH and PaCO₂ move in opposite directions, the primary disorder is respiratory; if pH and HCO₃⁻ move in the same direction, it is metabolic.
3. Check compensation: compensation never fully normalizes pH; a normal pH with abnormal PaCO₂ and HCO₃⁻ often indicates a mixed disorder.
4. Calculate the anion gap: AG = Na⁺ − (Cl⁻ + HCO₃⁻); normal is approximately 8–12 mEq/L.

Disorder	Expected compensation	High-yield interpretation
Metabolic acidosis	Winter formula: expected PaCO2 = 1.5 × HCO3− + 8 ± 2	Higher PaCO2 = concomitant respiratory acidosis; lower PaCO2 = respiratory alkalosis.
Metabolic alkalosis	Expected PaCO2 ≈ 0.7 × HCO3− + 20 ± 5	Hypoventilatory compensation is limited; marked hypercapnia suggests primary lung disease.
Acute respiratory acidosis	HCO3− rises 1 mEq/L per 10 mm Hg PaCO2 increase	Renal compensation is minimal initially.
Chronic respiratory acidosis	HCO3− rises 3.5–4 mEq/L per 10 mm Hg PaCO2 increase	Requires renal adaptation over ~3–5 days.
Acute respiratory alkalosis	HCO3− falls 2 mEq/L per 10 mm Hg PaCO2 decrease	Immediate buffering predominates.
Chronic respiratory alkalosis	HCO3− falls 4–5 mEq/L per 10 mm Hg PaCO2 decrease	Renal bicarbonate excretion increases.

Anion gap metabolic acidosis: differential and thresholds

An elevated anion gap reflects accumulation of unmeasured anions. Correct the anion gap for hypoalbuminemia because albumin is a major unmeasured anion: add approximately 2.5 mEq/L to the AG for each 1 g/dL decrease in albumin below 4 g/dL. A useful Step 1 mnemonic is GOLD MARK: glycols, oxoproline, L-lactate, D-lactate, methanol, aspirin, renal failure, ketoacidosis.

The delta gap helps detect mixed metabolic disorders: compare the increase in AG above 12 with the decrease in HCO₃⁻ below 24. If ΔAG > ΔHCO₃⁻, concurrent metabolic alkalosis is likely; if ΔAG < ΔHCO₃⁻, concurrent normal-gap metabolic acidosis is likely.

Normal anion gap metabolic acidosis and urine studies

Normal-gap, hyperchloremic metabolic acidosis usually results from GI bicarbonate loss or impaired renal acid excretion. The urine anion gap estimates urinary NH₄⁺ excretion: UAG = urine Na⁺ + urine K⁺ − urine Cl⁻. A negative UAG suggests appropriate NH₄Cl excretion, typical of diarrhea. A positive UAG suggests impaired renal acid secretion, as in renal tubular acidosis.

Renal tubular acidosis	Defect	Urine pH	Potassium	Classic associations
Type 1 distal	Impaired H+ secretion by α-intercalated cells	>5.5	Low	Nephrolithiasis, autoimmune disease, amphotericin B
Type 2 proximal	Impaired HCO3− reabsorption	Initially >5.5, later <5.5	Low	Fanconi syndrome, acetazolamide
Type 4	Hypoaldosteronism or aldosterone resistance	Variable, often <5.5	High	Diabetic nephropathy, ACE inhibitors, spironolactone

Metabolic alkalosis: urine chloride classification

Urine chloride distinguishes volume-responsive from volume-resistant metabolic alkalosis. A urine Cl⁻ <10–20 mEq/L suggests saline-responsive alkalosis, typically vomiting or remote diuretic use, where kidneys avidly retain chloride. A urine Cl⁻ >20 mEq/L suggests saline-resistant alkalosis, such as current diuretic use, mineralocorticoid excess, Bartter syndrome, or Gitelman syndrome. Hypokalemia perpetuates alkalosis by increasing proximal HCO₃⁻ reabsorption and stimulating H⁺ secretion by α-intercalated cells.

Toxicology and osmolar gap

When high-gap acidosis is unexplained, calculate the serum osmolar gap: measured osmolality − calculated osmolality, where calculated osmolality ≈ 2Na⁺ + glucose/18 + BUN/2.8 + ethanol/3.7. A gap >10–15 mOsm/kg suggests unmeasured osmoles such as methanol or ethylene glycol. Early salicylate toxicity classically causes respiratory alkalosis; later, organic acid accumulation produces a mixed respiratory alkalosis plus anion-gap metabolic acidosis.

Management, pharmacology and procedures

General approach to acid-base disorders

Management begins with identifying the primary disorder, estimating expected compensation, and treating the underlying cause rather than “normalizing” the pH alone. Normal arterial pH is 7.35–7.45; severe acidemia, especially pH <7.10, depresses myocardial contractility, predisposes to arrhythmias, and decreases catecholamine responsiveness. Severe alkalemia, especially pH >7.55, increases neuromuscular excitability, lowers ionized calcium, and shifts the oxyhemoglobin dissociation curve left.

• Immediate assessment: airway, breathing, circulation; obtain arterial or venous blood gas, serum electrolytes, glucose, lactate, creatinine, ketones, and toxicology when indicated.
• Anion gap: AG = Na⁺ − (Cl⁻ + HCO₃⁻); normal is approximately 8–12 mEq/L without potassium. Correct for hypoalbuminemia: add 2.5 mEq/L to AG for each 1 g/dL albumin below 4.
• Ventilatory compensation: in metabolic acidosis, expected PCO₂ by Winter formula = 1.5 × HCO₃⁻ + 8 ± 2. A higher PCO₂ suggests concomitant respiratory acidosis.

Metabolic acidosis

Treatment depends on whether the acidosis is anion gap or normal anion gap. High-anion-gap causes include lactic acidosis, ketoacidosis, renal failure, and toxins such as methanol, ethylene glycol, and salicylates. Normal-anion-gap acidosis commonly reflects gastrointestinal bicarbonate loss, renal tubular acidosis, or carbonic anhydrase inhibitor use.

Intervention	Mechanism and indications	High-yield complications
Sodium bicarbonate	Provides extracellular HCO3− buffer. One ampule of 8.4% NaHCO3 contains 50 mEq in 50 mL. Consider in severe acidemia, commonly pH <7.1, or bicarbonate loss states. In diabetic ketoacidosis, guidelines generally reserve bicarbonate for pH <6.9.	Hypernatremia, volume overload, hypokalemia, reduced ionized calcium, CO2 generation with possible paradoxical intracellular/CNS acidosis.
Insulin and fluids in DKA	Insulin suppresses lipolysis and ketogenesis; isotonic fluids restore perfusion and renal acid excretion. Potassium must be monitored because insulin shifts K+ intracellularly.	If serum K+ is <3.3 mEq/L, insulin is delayed while potassium is replaced to avoid arrhythmia.
Hemodialysis	Removes acids/toxins and corrects electrolytes. Classic urgent indications are remembered as AEIOU: Acidosis, Electrolytes, Intoxications, Overload, Uremia. Consider for refractory acidemia, often pH <7.1, renal failure, or toxic alcohols.	Hypotension, disequilibrium syndrome, access complications, rapid electrolyte shifts.

Evidence note: the BICAR-ICU trial found no overall mortality benefit from bicarbonate in severe metabolic acidemia, but suggested benefit in the subgroup with acute kidney injury; Step 1 emphasis remains mechanism and appropriate indications rather than trial memorization.

Metabolic alkalosis

Metabolic alkalosis is maintained by volume depletion, chloride depletion, hypokalemia, mineralocorticoid excess, or impaired renal bicarbonate excretion. Urine chloride helps classify it: urine Cl⁻ <10–20 mEq/L suggests chloride-responsive alkalosis, whereas >20 mEq/L suggests chloride-resistant causes such as hyperaldosteronism or ongoing diuretic effect.

Type	Examples	Management principle
Chloride-responsive	Vomiting, nasogastric suction, remote diuretic use	Give 0.9% saline and KCl. Chloride permits renal HCO3− excretion via pendrin in collecting duct intercalated cells.
Chloride-resistant	Primary hyperaldosteronism, Cushing syndrome, Liddle syndrome	Treat mineralocorticoid pathway: adrenal surgery when appropriate, spironolactone/eplerenone for aldosterone excess, or amiloride for ENaC-mediated Liddle syndrome.
Diuretic-induced/volume overloaded	Loop or thiazide use, heart failure	Acetazolamide 250–500 mg PO/IV inhibits proximal tubule carbonic anhydrase, causing bicarbonaturia and metabolic acidosis.

Acetazolamide also causes alkaline urine, hypokalemia, renal stones, and sulfonamide-type reactions. It is useful conceptually because it demonstrates how proximal bicarbonate reabsorption depends on carbonic anhydrase.

Respiratory acid-base disorders

Respiratory disorders are managed by changing alveolar ventilation. Respiratory acidosis results from hypoventilation with elevated PCO₂; respiratory alkalosis results from hyperventilation with reduced PCO₂. Acute compensation is limited because renal H⁺ secretion and ammoniagenesis require hours to days.

• Respiratory acidosis: treat airway obstruction, CNS depression, neuromuscular weakness, or lung disease. Support may include supplemental oxygen, noninvasive ventilation, or intubation with mechanical ventilation. Opioid-induced hypoventilation is treated with naloxone 0.04–0.4 mg IV titrated upward; duration is about 30–90 minutes, so recurrent respiratory depression can occur with long-acting opioids.
• Respiratory alkalosis: treat pain, anxiety, pregnancy-related hyperventilation, sepsis, pulmonary embolism, or salicylate toxicity. Salicylates classically cause early respiratory alkalosis followed by anion-gap metabolic acidosis.
• Salicylate toxicity: urinary alkalinization with IV bicarbonate targets urine pH 7.5–8.0, trapping salicylate in ionized form. Hemodialysis is indicated for severe toxicity, renal failure, pulmonary edema, altered mental status, or very high levels.

Long-term management of renal tubular acidosis

Renal tubular acidoses demonstrate segment-specific renal physiology and are frequently tested. Therapy prevents chronic acid retention, bone buffering, nephrolithiasis, and growth impairment in children.

• Type 1 distal RTA: impaired distal H⁺ secretion; urine pH remains >5.5. Treat with alkali, often potassium citrate or bicarbonate 1–2 mEq/kg/day. Potassium citrate is useful when nephrolithiasis occurs because citrate complexes calcium.
• Type 2 proximal RTA: impaired proximal HCO₃⁻ reabsorption. Requires larger alkali doses, often 10–15 mEq/kg/day, plus potassium supplementation because bicarbonaturia increases distal sodium delivery and K⁺ wasting.
• Type 4 RTA: hypoaldosteronism or aldosterone resistance causing hyperkalemic normal-anion-gap acidosis. Treat hyperkalemia, stop offending drugs, consider loop/thiazide diuretics, sodium bicarbonate, or fludrocortisone 0.1 mg/day when mineralocorticoid deficiency is appropriate.

Follow-up for significant acid-base disorders includes serial pH or bicarbonate, electrolytes including potassium and chloride, renal function, urine pH or urine chloride when relevant, and reassessment of the underlying cause. The most testable complications of therapy are potassium shifts, sodium/volume overload from bicarbonate, and overcorrection producing the opposite acid-base disorder.

Exam controversies and advanced synthesis

High-yield “viva” synthesis: do not label an acid-base disorder until compensation is tested

For USMLE Step 1, the most common pitfall is to identify the primary disorder from pH and then stop. A physiologic response should be predictable; if measured compensation is outside the expected range, a mixed disorder is present. Normal arterial values are approximately pH 7.35–7.45, PaCO₂ 35–45 mm Hg, and HCO₃⁻ 22–28 mEq/L. The Henderson-Hasselbalch relationship is clinically summarized as: pH is proportional to HCO₃⁻/PaCO₂. The kidney regulates the metabolic component by reclaiming filtered bicarbonate, generating new bicarbonate via ammoniagenesis, and excreting titratable acid; the lungs regulate PaCO₂ within minutes by changing alveolar ventilation.

Primary disorder	Expected compensation	Classic pitfall
Metabolic acidosis	Winter formula: expected PaCO2 = 1.5 × HCO3− + 8 ± 2	Higher PaCO2 = concurrent respiratory acidosis; lower PaCO2 = concurrent respiratory alkalosis
Metabolic alkalosis	Expected PaCO2 ≈ 0.7 × HCO3− + 20 ± 5	Respiratory compensation is limited by hypoxemia; PaCO2 rarely rises above 55–60 mm Hg without lung disease
Acute respiratory acidosis	HCO3− rises by ~1 mEq/L per 10 mm Hg PaCO2 increase	Renal compensation has not had time to occur
Chronic respiratory acidosis	HCO3− rises by ~3.5–4 mEq/L per 10 mm Hg PaCO2 increase	Requires renal adaptation over ~3–5 days
Acute respiratory alkalosis	HCO3− falls by ~2 mEq/L per 10 mm Hg PaCO2 decrease	Can occur rapidly in pain, anxiety, sepsis, pregnancy, or high altitude
Chronic respiratory alkalosis	HCO3− falls by ~4–5 mEq/L per 10 mm Hg PaCO2 decrease	Renal bicarbonate wasting creates a low serum HCO3−

Anion gap controversies and hidden mixed disorders

The serum anion gap is calculated as Na⁺ − (Cl⁻ + HCO₃⁻); normal is roughly 8–12 mEq/L when potassium is excluded. Albumin is the major unmeasured anion, so hypoalbuminemia can mask a high-gap acidosis. Correct the anion gap by adding approximately 2.5 mEq/L for every 1 g/dL decrease in albumin below 4 g/dL. This correction is a frequent exam discriminator in critically ill or cirrhotic patients.

The delta ratio helps detect mixed metabolic disorders in high anion gap metabolic acidosis: ΔAG/ΔHCO₃⁻ = (AG − 12)/(24 − measured HCO₃⁻). A ratio <1 suggests an additional normal-anion-gap metabolic acidosis; a ratio >2 suggests concurrent metabolic alkalosis or chronic respiratory acidosis with renal bicarbonate retention. The classic Step 1 examples are diabetic ketoacidosis plus diarrhea, and vomiting plus lactic acidosis.

Urine indices: useful, but easy to overinterpret

In normal-anion-gap metabolic acidosis, urine studies distinguish gastrointestinal bicarbonate loss from renal tubular acidosis. The urine anion gap is Na⁺ + K⁺ − Cl⁻. It is a surrogate for urinary NH₄⁺ excretion because NH₄⁺ is excreted largely with Cl⁻. A negative urine anion gap suggests appropriate renal ammonium excretion, as in diarrhea; a positive urine anion gap suggests impaired renal acid excretion, as in distal or type 4 renal tubular acidosis. However, this surrogate fails when unusual urinary anions are present, such as ketoanions, hippurate from toluene, or large amounts of penicillin-derived anions. The urine osmolar gap is a more direct ammonium surrogate but is less commonly tested on Step 1.

Guidelines and trial-level correlations relevant to acid-base physiology

Although Step 1 rarely asks management algorithms, modern controversies illustrate mechanisms. In diabetic ketoacidosis, major guidelines generally avoid bicarbonate unless arterial pH is <6.9, because insulin and volume restore ketoacid metabolism and regenerate bicarbonate. If used in severe acidemia, a commonly cited adult regimen is 100 mmol sodium bicarbonate in 400 mL sterile water with 20 mEq KCl over 2 hours, repeated until pH exceeds 7.0. The physiologic concern is that bicarbonate generates CO₂, which diffuses into cells and cerebrospinal fluid faster than HCO₃⁻, potentially worsening intracellular acidosis.

For chronic kidney disease with metabolic acidosis, Kidney Disease: Improving Global Outcomes (KDIGO) guidance has historically supported oral alkali when serum HCO₃⁻ is <22 mEq/L, aiming to maintain the value in the normal range. Typical sodium bicarbonate dosing is 650 mg orally two to three times daily; each 650 mg tablet provides about 7.7 mEq bicarbonate. The rationale is reduced bone buffering, less muscle catabolism, and possibly slower nephron injury from endothelin and complement activation. Trials have been mixed: smaller studies suggested slower estimated GFR decline, whereas larger contemporary trials have shown less consistent kidney-outcome benefit and more concern for sodium load, edema, and hypertension.

Fluid choice also links directly to acid-base chemistry. Normal saline contains 154 mEq/L sodium and 154 mEq/L chloride, producing hyperchloremic normal-anion-gap metabolic acidosis by lowering the strong ion difference and increasing renal chloride delivery. Balanced crystalloids such as lactated Ringer solution contain lower chloride and metabolizable anions. In the SMART trial and SALT-ED trial, balanced crystalloids modestly reduced major adverse kidney events compared with saline in large pragmatic cohorts; the absolute differences were small but biologically consistent with chloride-mediated renal vasoconstriction and acidosis. Step 1 relevance: chloride load can cause non-anion-gap acidosis without implying bicarbonate loss.

Pharmacology pitfalls

• Acetazolamide inhibits carbonic anhydrase in the proximal tubule, causing bicarbonaturia and normal-anion-gap metabolic acidosis; it is used for glaucoma, altitude sickness prophylaxis, and sometimes metabolic alkalosis. Sulfonamide allergy is a caution.
• Loop and thiazide diuretics cause contraction alkalosis, increased distal Na⁺ delivery, increased aldosterone-mediated H⁺/K⁺ secretion, hypokalemia, and metabolic alkalosis.
• Mineralocorticoid excess causes hypertension, hypokalemia, and metabolic alkalosis; type 4 renal tubular acidosis is the opposite physiology: hypoaldosteronism or aldosterone resistance causing hyperkalemia and non-anion-gap metabolic acidosis.

The unifying exam principle is that acid-base disorders are not lists to memorize: they are predictable consequences of mass balance, electroneutrality, buffer chemistry, ventilation, and nephron segment transport.