A. Renal Blood Flow (RBF)

Overview: - Renal blood flow: ~1000 mL/min (~20-25% of cardiac output) in resting state - Delivered to only ~0.4% of body weight (kidneys ~300 g of 70 kg body) - Remarkably high perfusion: 4 mL/min per gram kidney tissue

Calculation of Effective Renal Plasma Flow (eRPF): - eRPF = RBF × (1 - hematocrit) - Normal RBF ~1000 mL/min → eRPF ~600 mL/min (at hematocrit 40%) - Clearance concept: eRPF measured via PAH (para-aminohippuric acid) clearance

Oxygen Consumption and Metabolic Rate: - Kidneys consume ~7% of total body O₂ (despite being only 0.4% of body weight) - High O₂ consumption reflects high metabolic activity (active transport in tubules) - Medulla has lower O₂ tension than cortex (metabolically vulnerable to ischemia) - Acute renal failure often results from medullary ischemia (ATN)

B. Renal Vascular Resistance

Resistances in Series:

Afferent Resistance (Ra) → Glomerulus → Efferent Resistance (Re)

Pressure Relationships: - Renal perfusion pressure (RPP) typically ~100 mmHg (mean arterial pressure minus venous pressure) - Mean glomerular hydrostatic pressure (Pgg) ~45 mmHg (intermediate between afferent and efferent pressures) - Pressure drop across afferent arteriole ~50 mmHg - Pressure drop across efferent arteriole ~30 mmHg

Resistance is proportional to vessel radius (Poiseuille’s Law): $$R = \frac{8 \eta L}{\pi r^4}$$

Where R = resistance, η = viscosity, L = length, r = radius

Clinical significance: - Afferent arteriole diameter ~20 μm; efferent ~15 μm (efferent smaller → higher resistance) - Small changes in arteriolar tone dramatically affect resistance (radius raised to 4th power) - Vasoconstriction of afferent arteriole or vasodilation of efferent arteriole → reduced GFR

C. Autoregulation of Renal Blood Flow and GFR

Definition: Maintenance of relatively constant RBF and GFR despite changes in renal perfusion pressure.

Normal Autoregulatory Range: - Renal perfusion pressure: 80-180 mmHg - RBF remains: ~600-800 mL/min (±5-10% variation) - GFR remains: ~125 mL/min (±5-10% variation)

Mechanisms of Autoregulation:

1. Myogenic Mechanism (Intrinsic)

Principle: Afferent arteriole smooth muscle responds directly to mechanical stretch.

Mechanism: - Increased renal perfusion pressure → stretching of afferent arteriole wall - Stretch activates mechanoreceptor channels (TRPM4, TRPV4 — transient receptor potential channels) - Ca²⁺ influx → smooth muscle contraction → vasoconstriction - Reduced afferent diameter → increased resistance → restores pressure gradient - Decreased perfusion pressure → opposite sequence → vasodilation

Timeline: Occurs within seconds; immediate pressure compensation

Molecular Details: - Bayliss effect: Direct mechanical response of vascular smooth muscle to pressure - Involves L-type calcium channels and calcium-activated potassium channels - Requires intact afferent arteriole smooth muscle (absent in denervated kidneys)

2. Tubuloglomerular Feedback (TGF) Mechanism (Extrinsic)

Principle: Changes in distal tubular NaCl concentration feedback to regulate afferent arteriole tone.

Anatomical Basis: - Juxtaglomerular apparatus (JGA): Specialized region where thick ascending limb (TAL) contacts its own glomerulus - Macula densa: Specialized epithelial cells in thick ascending limb; NaCl sensors - Granular cells: Renin-secreting juxtaglomerular (JG) cells in afferent arteriole wall - Extraglomerular mesangium: Supporting cells

Mechanism (High GFR scenario):

Increased GFR
    ↓
Increased fluid delivery to TAL
    ↓
Increased NaCl reabsorption in TAL via NKCC2
    ↓
Increased intracellular Na⁺ and Cl⁻ in macula densa
    ↓
Increased basolateral Na⁺/K⁺-ATPase activity → hyperpolarization of cell membrane
    ↓
Decreased K⁺ channel activity (ROMK channels close)
    ↓
Membrane depolarization via paracellular pathway
    ↓
Increased intracellular Ca²⁺
    ↓
Release of ATP (and adenosine) from macula densa apical membrane
    ↓
Activation of purinergic receptors (A₁ adenosine receptors, P2Y₁ ATP receptors) on afferent arteriole and JG cells
    ↓
Afferent arteriole VASOCONSTRICTION
    ↓
DECREASED GFR → equilibrium

Mechanism (Low GFR scenario - opposite): - Decreased NaCl delivery → decreased macula densa [Na⁺/Cl⁻] - Increased renin release from JG cells - Angiotensin II formation → afferent vasodilation + efferent vasoconstriction - GFR increases → equilibrium

Sensitivity: - TGF responds to changes as small as 1-2 mEq/L change in luminal [NaCl] - Gain: ~0.4 (change in GFR is 40% of the stimulus that caused it) - Acts as a “brake” on GFR changes

Time Course: - Onset: Several seconds - Peak effect: ~20-30 seconds - Duration: Minutes (oscillatory; GFR exhibits spontaneous oscillations ~0.3-0.4 Hz due to TGF feedback loops)

D. Additional Autoregulatory Mechanisms

1. Renin-Angiotensin System (RAS)

Activated by: - Decreased renal perfusion pressure (detected by juxtaglomerular cells) - Decreased distal tubular NaCl (via TGF) - Increased sympathetic nerve activity (β1-adrenergic receptors on JG cells)

Effects of Angiotensin II: - Afferent arteriole: MILD constriction - Efferent arteriole: STRONG constriction (preserves GFR during hypotension) - Mesangial contraction: Reduces glomerular surface area - Distal tubule: Increased NaCl reabsorption via ENaC and NCC - Collecting duct: Increased water reabsorption via ADH-stimulated aquaporin-2 - Net effect: Restores renal perfusion pressure and GFR; expands plasma volume

2. Prostaglandins (PGE2, PGI2)

Actions: - Afferent arteriole: VASODILATION - Mesangium: RELAXATION → increased glomerular surface area - Distal tubule: Decreased NaCl reabsorption

Triggers: - Macula densa activation (high NaCl) → COX-mediated PG synthesis - Acts as counterregulatory to TGF vasoconstriction

Clinical Significance: - NSAIDs block COX → block PG synthesis → loss of afferent vasodilation → acute renal failure in dehydrated patients - Particularly dangerous in patients already on ACEi (combined RAS blockade + loss of PG vasodilation)

3. Nitric Oxide (NO)

Actions: - Afferent arteriole: VASODILATION - Endothelial function: Maintains vascular homeostasis - Glomerular hemodynamics: Increases Pgg and GFR

Sources: - Constitutive endothelial nitric oxide synthase (eNOS) in endothelium - Inducible NOS (iNOS) in response to inflammation

Pathophysiology: - Loss of NO bioavailability → endothelial dysfunction → vasoconstriction → CKD progression - Seen in hypertension, diabetes, atherosclerosis, aging

E. Integrated Response to Hypotension and Hypertension

In Hypotension (e.g., hemorrhage, dehydration):

1. Immediate: Myogenic vasodilation of afferent arteriole (pressure drop sensed)
2. Seconds: TGF-mediated renin release (low distal delivery)
3. Minutes: RAAS activation → Ang II constricts efferent arteriole → preserves Pgg despite low systemic pressure
4. Result: GFR maintained near-normal; plasma volume expanded via sodium/water retention

In Hypertension (e.g., essential hypertension):

1. Immediate: Myogenic vasoconstriction (pressure elevation sensed)
2. Minutes: Increased GFR due to hypertension-induced afferent vasodilation (pressure wave transmitted)
3. Hours: Increased sodium and water excretion (pressure natriuresis) → blood pressure normalization
4. Result: “Self-limiting” autoregulation; kidneys adjust sodium excretion to match intake at the elevated pressure

Break-Through in Hypertension: - In severe hypertension, autoregulatory mechanisms are overwhelmed - Arteriolar necrosis occurs (acute arteriolitis/arteriolar necrosis) - Result: “Acute kidney injury” with sclerosis of remaining glomeruli - Seen in malignant hypertension, preeclampsia, scleroderma renal crisis

A. Definition and Measurement

GFR Definition: Volume of plasma filtered from glomeruli into Bowman’s space per unit time.

Normal GFR: - Adult: 120-130 mL/min (~100 mL/min/1.73 m² when normalized to body surface area) - Slightly lower in females (~110 mL/min) and elderly - Declines with age (~10 mL/min/decade after age 30)

Clinical GFR Measurement:

Ideal Filtration Marker Criteria: - Freely filtered at glomerulus (no restriction by size/charge) - Not reabsorbed by tubules - Not secreted by tubules - Not metabolized by kidney - Does not affect GFR - Measured accurately and inexpensively

Gold Standard: Inulin Clearance - Polysaccharide; meets all criteria - Clearance = (Uinulin × Vurine) / Pinulin - Rarely used clinically (requires infusion; expensive)

Clinical Surrogates: - Creatinine clearance: Overestimates GFR (~20%) due to tubular secretion - Estimated GFR (eGFR) from serum creatinine: KDIGO 2021 equation - eGFR = 142 × (Scr/0.7)^(-0.9) × (0.9)^age [if female; add 1.018 multiplier] - Where Scr = serum creatinine (mg/dL) - More accurate than MDRD equation; accounts for non-creatinine factors - Cystatin C: Less affected by muscle mass; emerging gold standard - eGFR from combined Scr and Cystatin C: Increasingly recommended (more accurate than either alone)

B. Determinants of GFR (Starling Forces)

GFR is determined by the net ultrafiltration pressure (NFP) and the glomerular filtration coefficient:

GFR = K_f × NFP

Where: - Kf = filtration coefficient (surface area × permeability) - NFP = net ultrafiltration pressure

Net Ultrafiltration Pressure:

NFP = (P_gg − P_bc) − (π_gg − π_bc)

Where: - Pgg = glomerular hydrostatic pressure (~45 mmHg; variable along capillary) - Pbc = Bowman’s capsule hydrostatic pressure (~10 mmHg) - πgg = glomerular plasma oncotic pressure (~25 mmHg at afferent end; ~35 mmHg at efferent end) - πbc = Bowman’s capsule oncotic pressure (~0-2 mmHg; ultrafiltrate is protein-free)

At Afferent End: - NFP = (45 - 10) - (25 - 0) = +10 mmHg - Net filtration pressure is positive → active filtration

At Efferent End (Filtration Equilibrium): - NFP = (45 - 10) - (35 - 0) = 0 mmHg (approximately) - Net filtration pressure approaches zero → filtration slows/stops - Plasma oncotic pressure has risen due to protein concentration (water loss)

C. Factors Affecting GFR

Increase GFR: 1. Increased Pgg: Afferent dilation, efferent constriction, increased blood pressure 2. Decreased πgg: Lower plasma protein (unusual; requires severe hypoproteinemia) 3. Decreased Pbc: Ureteral obstruction relief 4. Increased Kf: Glomerular vasodilation, increased surface area

Decrease GFR: 1. Decreased Pgg: Afferent constriction, efferent dilation, decreased blood pressure 2. Increased πgg: Higher plasma protein (dehydration, hemoconcentration) 3. Increased Pbc: Urinary obstruction (hydronephrosis) 4. Decreased Kf: Glomerular sclerosis, proteinuria → loss of slit diaphragm selectivity

Clinical Triggers for GFR Reduction: - Volume depletion: RAAS activation → efferent constriction preserves GFR - Sepsis: Decrease in systemic vascular resistance → hypotension → afferent constriction → GFR drop - NSAIDs: Block prostaglandin-mediated afferent vasodilation → GFR drop - ACE inhibitors: Block Ang II-mediated efferent constriction → GFR drop (especially if hypotensive) - Hepatic cirrhosis: Splanchnic vasodilation → renal vasoconstriction → prerenal azotemia

A. Proximal Convoluted Tubule (PCT)

Reabsorption in PCT: - ~65% of filtered water - ~65% of filtered sodium and chloride - ~99-100% of glucose (SGLT2, SGLT1 cotransporters) - ~99-100% of amino acids - ~60% of potassium (paracellular) - ~90% of HCO3- (H+ secretion in exchange)

Transport Mechanisms:

Primary Active Transport (ATP-dependent): - Basolateral Na⁺/K⁺-ATPase: Pumps 3 Na⁺ out, 2 K⁺ in; creates low intracellular [Na⁺] - Provides driving force for secondary active transport

Secondary Active Transport (coupled to Na⁺ gradient): - SGLT2 (sodium-glucose cotransporter): Apical; high Km (15 mM); reabsorbs ~90% of glucose - SGLT1: Apical; lower Km; reabsorbs remaining ~10% of glucose - PEPT1/PEPT2: Peptide-proton cotransporters; reabsorb di- and tripeptides - Amino acid transporters: Apical IMINO transporters and others - Na⁺/H⁺ exchanger (NHE3): Apical; secretes H⁺, reabsorbs Na⁺; critical for HCO3- reabsorption

Facilitated Diffusion: - GLUT1: Basolateral; allows glucose exit from cell - Aquaporin-1: Apical and basolateral; constitutive water channel (not regulated)

Paracellular Transport: - Tight junctions (claudins): Allow paracellular cation movement - Driving forces: Osmotic (water loss) and electrical (lumen-positive potential) - Reabsorbs ~60% of K⁺, Mg²⁺, Ca²⁺

Organic Anion and Cation Secretion:

Basolateral Entry: - Organic anion transporters (OAT 1, 3): Antiporter; α-ketoglutarate in, organic anion out - Organic cation transporters (OCT 1, 2): Antiporter; H⁺ in, organic cation out

Apical Exit: - Organic anion transporters (OAT 4): Urate, prostaglandins, others - Organic cation transporters (OCT 2): Creatinine, cimetidine, others

Secreted Substances (beyond glomerular filtration): - Penicillin, probenecid, salicylate (organic anion secretion) - Creatinine, cimetidine, morphine (organic cation secretion) - Hydrogen ions (HCO3- reabsorption) - Ammonium (NH4+; ammonia metabolism)

Clinical Applications: - Creatinine clearance >> inulin clearance → significant tubular secretion (20% of creatinine clearance is secretion) - Probenecid blocks secretion → reduces urate excretion (prevents gout) by blocking OAT1 - Penicillin competed for secretion → increased plasma levels when probenecid is co-administered

B. Loop of Henle (Descending and Ascending Limbs)

Descending Thin Limb (DTL): - Permeable to water; impermeable to NaCl - ~25% of filtered water reabsorbed passively - Osmolarity increases as fluid descends (following osmotic gradient)

Ascending Thin Limb (ATL): - Impermeable to water; permeable to NaCl - ~25% of filtered NaCl reabsorbed passively - NaCl reabsorption dilutes tubular fluid

Thick Ascending Limb (TAL):

Transport Mechanisms: - Na-K-2Cl cotransporter (NKCC2) — apical: Active transport; symporter - Driven by Na⁺ gradient created by basolateral Na⁺/K⁺-ATPase - Reabsorbs 1 Na⁺ + 1 K⁺ + 2 Cl⁻ in one turnover - K⁺ recycling via apical ROMK: K⁺ secreted into lumen; recycled to drive NKCC2 - Cl⁻ exit via basolateral ClC-Kb: Exits cell via chloride channel - Na⁺ exit via basolateral Na⁺/K⁺-ATPase: Extrudes Na⁺ in exchange for K⁺

Characteristics: - Reabsorbs ~25% of filtered NaCl (major site of salt reabsorption) - Impermeable to water → obligate dilution despite osmolyte removal - Generates positive charge in lumen (K⁺ recycling creates positive potential) → drives paracellular cation reabsorption (Ca²⁺, Mg²⁺) - Creates osmotic gradient → medullary hypertonicity (up to 1200 mOsm/kg in papilla)

Clinical Significance: - Loop diuretics (furosemide, torsemide): Block NKCC2 → massive polyuria, hypokalemia, dehydration - Bartter syndrome: Genetic defects in NKCC2, ROMK, or CLC-Kb → hypokalemic metabolic alkalosis, polyuria - Gitelman syndrome: (DCT) similar but with hypomagnesemia

C. Distal Convoluted Tubule (DCT)

Early DCT (DCT1) — NaCl Reabsorption:

Transport: - Na-Cl cotransporter (NCC) — apical: Thiazide-sensitive - Reabsorbs remaining filtered NaCl (~5-10%) - Water impermeable segment (continues dilution)

Late DCT (DCT2) and Collecting Duct Interface — Selective Reabsorption:

Principal Cell Transport: - Epithelial Na⁺ channel (ENaC) — apical: Aldosterone-regulated - Reabsorbs ~2-3% of filtered Na⁺ (fine-tuning) - Driven by Na⁺ gradient and negative lumen potential - Creates electrical potential → drives K⁺ secretion

Intercalated Cell Transport: - Type A: H⁺-ATPase; HCO3- reabsorption; urine acidification - Type B: HCO3- secretion (alternative path for base excretion)

D. Collecting Duct (CD)

Principal Cells:

Sodium Reabsorption: - ENaC on apical membrane (aldosterone-regulated) - Reabsorbs final 1-2% of filtered Na⁺ - Water reabsorption via aquaporin-2 (ADH-regulated) - K⁺ secretion driven by negative lumen potential and aldosterone

Water Permeability Regulation: - ADH (vasopressin) mechanism: 1. ADH binds V2 receptor on basolateral membrane 2. Activates Gs protein → ↑ cAMP 3. PKA activation → phosphorylates aquaporin-2 4. Aquaporin-2 translocates from intracellular vesicles to apical membrane 5. Water permeability increases from <0.1% to >80% of filtrate 6. Urine becomes concentrated (osmolarity up to 1200 mOsm/kg)

• Without ADH:
  • Aquaporin-2 internalized
  • Water impermeability maintained
  • Dilute urine produced (~50 mOsm/kg)

Intercalated Cells:

Acid-Base Handling: - Type A: H⁺-ATPase (apical) → H⁺ secretion, HCO3- reabsorption (aciduria) - Type B: HCO3- secretion via AE1 (apical anion exchanger) and H⁺-ATPase (basolateral)

Regulation: - pH: Acidemia → ↑ Type A; ↑ H⁺ secretion - K⁺ status: Hypokalemia → ↑ Type A intercalated cells, ↑ ammonia synthesis

E. Summary of Reabsorption and Secretion Pathways

A. Countercurrent Multiplier (TAL)

Concept: The thick ascending limb (TAL) actively pumps NaCl into interstitium, creating osmotic gradient without water reabsorption (water-impermeable).

Mechanism:

Step 1 — TAL NaCl Reabsorption: - NKCC2 pumps NaCl out → interstitium [Na⁺ Cl⁻] increases - TAL is water-impermeable → dilution (not osmolarity increase, but relative removal of solutes) - Tubular fluid becomes hypotonic (~100 mOsm/kg) - Interstitium becomes hypertonic (~1200 mOsm/kg in papilla)

Step 2 — DTL Water Movement: - DTL is water-permeable; follows osmotic gradient - Water moves OUT of tubule into hypertonic interstitium - Tubular fluid osmolarity increases (becomes more concentrated)

Step 3 — ATL Passive NaCl Movement: - ATL is NaCl-permeable; follows concentration gradient - NaCl moves OUT of tubule into hypotonic medullary interstitium - Tubular fluid becomes dilute

Step 4 — Recycling: - Vasa recta countercurrent exchanger removes excess solute + water - Maintains medullary gradient

Net Effect: - Medullary osmolarity gradient: ~300 mOsm/kg (cortex) → 500 mOsm/kg (outer medulla) → 1200 mOsm/kg (papilla) - Collecting duct can produce urine up to 1200 mOsm/kg (equilibrates with papillary interstitium when ADH present)

B. Countercurrent Exchanger (Vasa Recta)

Function: Prevents dissipation of medullary osmotic gradient created by countercurrent multiplier.

Mechanism:

Descending Vasa Recta: - Receives blood from efferent arteriole (~300 mOsm/kg plasma oncotic pressure) - Descends into medulla; osmolarity of interstitium increases (300 → 1200 mOsm/kg) - Plasma is hypotonic relative to interstitium → water moves OUT, solutes move IN - Blood osmolarity increases as it descends (becomes more concentrated)

Ascending Vasa Recta: - Osmolarity is high (~1200 mOsm/kg) - As it ascends, interstitial osmolarity DECREASES (1200 → 300 mOsm/kg) - Blood is hypertonic relative to interstitium → water moves IN, solutes move OUT - Blood osmolarity decreases as it ascends (becomes more dilute) - Returns to cortex at ~300 mOsm/kg (similar to entry osmolarity)

Net Result: - Solutes deposited in medulla are recirculated but not removed entirely - Gradient is maintained despite continuous blood flow - Efficiency: ~95% of solutes deposited by TAL are retained; 99% of water is excreted

Clinical Loss of Vasa Recta: - Chronic pyelonephritis: Destruction of medulla → loss of vasa recta → polyuria despite normal ADH - Papillary necrosis: Loss of vasa recta and collecting duct tips → inability to concentrate urine

A. RAAS (Renin-Angiotensin-Aldosterone System)

Activation:

Juxtaglomerular Cell Triggers: 1. Decreased renal perfusion pressure (primary sensor) 2. Decreased distal tubular NaCl (via TGF mechanism) 3. Increased sympathetic nerve activity (β1-adrenergic receptors)

Renin Synthesis and Release:

Juxtaglomerular Cells: - Synthesize prorenin (inactive precursor) - Store renin in secretory granules - Release renin in response to above triggers

Renin Half-life: ~15 minutes (inactive after)

Angiotensinogen → Angiotensin I: - Liver produces angiotensinogen - Renin cleaves decapeptide → angiotensin I

Angiotensin I → Angiotensin II: - Angiotensin-converting enzyme (ACE) in lungs (capillary endothelium) - Cleaves dipeptide → angiotensin II (octapeptide) - ACE also degrades bradykinin (reduces vasodilation)

Angiotensin II Effects:

Renal Effects: - Afferent arteriole: MILD constriction (pressure-maintaining) - Efferent arteriole: STRONG constriction (GFR-preserving in hypotension) - Mesangial contraction: Reduces glomerular surface area - Proximal tubule: Increased NaCl reabsorption via enhanced Na⁺/H⁺ exchange - Collecting duct (direct): Enhanced water reabsorption - Renin release inhibition (negative feedback)

Systemic Effects: - Increased vasomotor tone (vasoconstriction) - Increased aldosterone secretion - Enhanced ADH secretion (osmolarity-independent) - Increased sympathetic activation (CNS and peripheral) - Thirst stimulation

Angiotensin II at Different Blood Pressures: - At low BP: Efferent >> afferent constriction → maintains Pgg despite low systemic pressure → preserves GFR - At normal BP: Preferential efferent constriction still occurs (fine-tuning salt/water) - At high BP: Loss of preferential efferent effect in some tissues; systemic vasoconstriction dominates

B. Aldosterone

Source: Zona glomerulosa of adrenal cortex

Triggers: 1. Angiotensin II (primary) 2. Hyperkalemia (potent direct effect on zona glomerulosa) 3. ACTH (minor; increases sensitivity to Ang II)

Mechanism of Action:

Mineralocorticoid Receptor (MR) in Principal Cell: 1. Aldosterone crosses cell membrane 2. Binds cytoplasmic mineralocorticoid receptor (MR) 3. MR-aldosterone complex translocates to nucleus 4. Binds to hormone response elements (HREs) on DNA 5. Increases transcription of: - ENaC: Increases apical sodium channel expression - Na⁺/K⁺-ATPase: Increases basolateral pump - Serum and glucocorticoid-regulated kinase (SGK1): Phosphorylates proteins; prevents ENaC degradation

Effects on Collecting Duct: - Increased Na⁺ reabsorption (via ENaC) - Increased K⁺ secretion (electrical consequence of Na⁺ reabsorption) - Increased H⁺ secretion (via Type A intercalated cells)

Time Course: - Onset: 30 minutes to 1 hour (gene transcription) - Peak: 4-6 hours - Duration: 1-3 days (mRNA stability)

Regulation of Aldosterone: - Suppressed by: Hypernatremia (inhibits; rare stimulus), plasma volume expansion, natriuretic peptides, dopamine - Stimulated by: Hypokalemia (independent of Ang II), ACTH, Ang II

C. ADH (Antidiuretic Hormone, Vasopressin)

Source: Posterior pituitary (released from magnocellular neurons of supraoptic and paraventricular nuclei)

Primary Stimulus: - Plasma osmolarity: Osmoreceptors in hypothalamus (subfornical organ, organum vasculosum) - Threshold: ~280-290 mOsm/kg - Linear increase: +1% ADH for every 1 mOsm/kg increase above threshold

Secondary Stimuli (Non-osmotic): 1. Hypovolemia: Baroreceptor activation; ~10% volume loss threshold 2. Hypotension: Powerful stimulus (>50 mmHg drop) 3. Nausea/vomiting: Chemoreceptor activation 4. Stress: Psychological, physical trauma, pain 5. Hypoxemia: <60 mmHg O₂ tension

Mechanism of Action in Collecting Duct:

V2 Receptor (Basolateral Membrane): 1. ADH binds V2 receptor (AVPR2 gene) 2. Activates Gs → ↑ adenylyl cyclase → ↑ cAMP 3. cAMP activates PKA (protein kinase A) 4. PKA phosphorylates aquaporin-2 (AQP2) 5. Phosphorylated AQP2 translocates from intracellular vesicles to apical membrane 6. Water permeability increases dramatically

Water Reabsorption: - With ADH: Aquaporin-2 in apical membrane; aquaporin-3/4 in basolateral membrane - Water reabsorption: ~80% of collecting duct fluid - Urine osmolarity: ~1000-1200 mOsm/kg - Urine volume: ~0.5-1 L/day - Plasma osmolarity: 280-290 mOsm/kg (maintained at set-point)

• Without ADH: No aquaporin-2 in apical membrane
  • Water reabsorption: <10% of collecting duct fluid
  • Urine osmolarity: ~50 mOsm/kg (maximally dilute)
  • Urine volume: up to 20 L/day
  • Plasma osmolarity rises unless high water intake

Time Course: - Onset: Minutes (receptor binding and cAMP generation) - Peak: 10-20 minutes - Duration: 15-20 minutes (metabolized; T½ ~10 minutes)

Regulation of ADH: - Suppressed by: Hypoosmolarity (osmoreceptor inhibition), hypervolemia, alcohol, nicotine - Stimulated by: Hyperosmolarity, hypovolemia, hypotension, nausea

D. Atrial Natriuretic Peptide (ANP) and Natriuretic Peptide System

Source: - ANP: Atrial myocytes (released in response to atrial stretch from volume expansion) - BNP: Ventricular myocytes (heart failure, volume overload) - CNP: Vascular endothelium (local vascular effects)

Mechanism of Action:

Natriuretic Peptide Receptor-A (NPR-A) — guanylate cyclase-linked: 1. ANP binds NPR-A (coupled to guanylate cyclase) 2. Increases cGMP 3. cGMP-dependent protein kinase (PKG) activation 4. Phosphorylates and inhibits: - TRPC6 channels (reduced podocyte contraction) - Phosphodiesterase-5 (increased cGMP) - Ion channels in collecting duct principal cells (reduces ENaC activity)

Effects on the Kidney:

Glomerular Hemodynamics: - Afferent arteriole: VASODILATION (increases GFR) - Efferent arteriole: VASOCONSTRICTION (modest; partially offsets afferent dilation) - Mesangium: RELAXATION (increases glomerular surface area) - Net effect: Increased GFR (↑ Kf, ↑ Pgg)

Tubular Effects: - Collecting duct principal cells: Inhibited ENaC → decreased Na⁺ reabsorption → increased Na⁺ excretion - Inhibits renin release (opposes RAAS) - Inhibits aldosterone secretion (opposes volume expansion)

ADH Suppression: - Hypervolemia reduces osmolarity perception → ADH suppression - ANP directly inhibits ADH release (hypothalamic neurons)

Physiologic Effects: - ↑ GFR - ↓ Proximal tubule reabsorption (direct effect) - ↓ Collecting duct reabsorption (ENaC inhibition) - ↓ Aldosterone secretion - Net: Marked natriuresis and diuresis (fluid volume reduction)

Time Course: - Onset: Minutes (receptor binding, cGMP generation) - Duration: 15-20 minutes (C-type natriuretic peptide receptor-C; clearance receptor; C-type ANP receptor degrades ANP)

Clinical Correlates: - Heart failure: Markedly elevated BNP (>400 pg/mL diagnostic; indicates ventricular strain) - Acute decompensated heart failure: ANP + BNP infusion considered therapeutic (nesiritide) - Nephrotic syndrome: ANP resistance; despite volume expansion, ANP levels paradoxically low (abnormal clearance or blunted response)

A. Overview of Acid-Base Balance

pH Maintenance: The kidneys maintain plasma pH in a narrow range (7.35-7.45) by: 1. Reabsorbing filtered HCO3- (virtually 100% reabsorbed under normal conditions) 2. Excreting excess acid as titrable acid and ammonia 3. Producing new HCO3- to replace buffered acid

Net Acid Excretion (NAE): NAE = TA + (NH4+ excretion) − (HCO3- excretion)

Where: - TA = titrable acid (H+ buffered by urinary phosphate, other weak bases) - Normal NAE = 0.5-1.0 mEq/kg/day (equals daily acid load)

B. Proximal Tubule Acid-Base Handling

HCO3- Reabsorption (85-90% of filtered load):

Mechanism: 1. H+ secretion via Na⁺/H⁺ exchanger (NHE3) in apical membrane 2. H+ + HCO3- → H2CO3 (in tubular lumen) 3. Carbonic anhydrase (CA IV) on brush border → H2O + CO2 (rapid) 4. CO2 diffuses into proximal tubule cell 5. Inside cell: CO2 + H2O → H2CO3 → H+ + HCO3- 6. HCO3- exits via basolateral HCO3-/Cl- exchanger (AE2) 7. H+ recycled via NHE3 apical pump

Regulation: - Increased by: Alkalemia, volume depletion (Ang II), hypocapnia - Decreased by: Acidemia, volume expansion, hypercapnia

Phosphate Buffering in PCT: - H+ secretion also titrates urinary phosphate (HPO4²⁻ → H2PO4⁻) - Phosphate becomes “unavailable” for new base formation (buffered H+ cannot be reclaimed)

C. Loop of Henle and Distal Tubule

No direct HCO3- reabsorption in these segments (water-impermeable or diluting segments limit HCO3- reabsorption).

TAL and DCT: Participate in establishing tubular fluid composition but not major acid-base sites.

D. Collecting Duct Acid-Base Handling

Type A (α) Intercalated Cells — H+ Secretion and HCO3- Reabsorption:

Mechanism: 1. Basolateral Na⁺/HCO3- cotransporter (NBC1) brings HCO3- into cell (net removal from lumen concept) 2. Apical H+-ATPase (vacuolar-type ATPase; V-ATPase) secretes H+ into lumen 3. H+ + HCO3- → H2CO3 → H2O + CO2 (in lumen; non-specific) 4. H+ also titrates ammonia: H+ + NH3 → NH4+ (trapped in lumen)

Function: - Reabsorbs HCO3- not reabsorbed by proximal tubule - Produces acidic urine (pH can drop to 4.5-5.0) - Excretes “new HCO3-” generated from ammonia metabolism

Regulation: - Increased by: Acidemia, hypokalemia - Decreased by: Alkalemia, hyperkalemia

Type B (β) Intercalated Cells — HCO3- Secretion:

Mechanism: 1. Apical anion exchanger (AE1) secretes HCO3- into lumen in exchange for Cl⁻ 2. Basolateral H⁺-ATPase extrudes H+ (not shown; alternative model under debate)

Function: - Excretes excess HCO3- (when alkalemia present) - Produces alkaline urine (pH ~8-9)

Regulation: - Increased by: Alkalemia, hyperkalemia - Decreased by: Acidemia, hypokalemia

E. Ammonia Metabolism and Ammoniagenesis

Ammonia Source: - Glutamine (primary source): Proximal tubule glutaminase converts glutamine → glutamate + NH3 - Glycine: Minor source - Histidine: Minor source

Regulation of Ammonia Production: - Enhanced by: Acidemia (increased glutaminase expression; chronic adaptation) - Suppressed by: Alkalemia

Ammonia Fate:

In Proximal Tubule: 1. Glutaminase: Glutamine → glutamine + NH3 2. NH3 crosses into tubular lumen (passive diffusion; lipid-soluble) 3. In lumen (low pH): NH3 + H+ → NH4+ (trapped; cannot diffuse back)

In Distal Tubule/Collecting Duct: 1. Type A intercalated cells: H+ secretion traps NH3 as NH4+ 2. NH4+ excreted in urine (cannot cross cell membrane)

Net Acid Excretion via Ammonia: - Normal: 0.5-1.0 mEq/kg/day - In acidemia: Can increase to 3-4 mEq/kg/day (chronic adaptation; takes 4-7 days to fully develop) - Important for chronic respiratory or metabolic acidosis compensation

F. Titrable Acid (TA) Excretion

Definition: H+ buffered by urinary weak acids (phosphate, sulfate, other organic acids).

Phosphate Buffering: - Filtered phosphate: HPO4²⁻ (main species at physiologic pH) - In urine: H+ + HPO4²⁻ → H2PO4⁻ (titrable acid) - Amount excreted depends on: - Urine pH (lower pH → more H2PO4⁻ formation) - Urinary phosphate concentration (dietary intake)

Sulfate Buffering: - Sulfate from methionine/cysteine metabolism - H+ + SO4²⁻ → HSO4- (weak buffering)

Other Organic Acids: - Lactate, β-hydroxybutyrate (ketones), creatinine, urate - Contribute to titrable acid load

TA Excretion Under Different Conditions: - Normal: 20-40 mEq/day - Acidemia: Increases to 50-100 mEq/day (via increased H+ secretion, lower urine pH) - Alkalemia: Decreases (H+ secretion suppressed)