2.2 Renal, Endocrine & Acid-Base Physiology

2.2 Renal, Endocrine & Acid-Base Physiology

Quick Answer: The kidneys filter plasma at the glomerulus (GFR), then reabsorb and secrete along the tubule to control volume, K⁺, and H⁺/HCO₃⁻. PTH, vitamin D (calcitriol), and calcitonin set extracellular calcium—directly relevant to bone, teeth, and hyperparathyroid oral findings. Thyroid and adrenal hormones control metabolic rate and stress response. ABG interpretation starts with pH, then decides whether the primary problem is respiratory (PaCO₂) or metabolic (HCO₃⁻).

AFK applied biomedical items expect you to connect organ physiology to clinical patterns: chronic kidney disease and bleeding/drug clearance, hyperparathyroidism and giant-cell lesions of bone, Addisonian crisis risk, and respiratory vs metabolic acid–base shifts during hypoventilation or anxiety-driven hyperventilation.

Renal Filtration and Tubular Function

Glomerular Filtration Rate (GFR)

GFR is the volume of fluid filtered from glomerular capillaries into Bowman’s space per unit time (normal adult order ~90–120 mL/min/1.73 m², declining with age and disease). Creatinine-based eGFR estimates this clinically.

Filtration is governed by Starling forces:

• Glomerular capillary hydrostatic pressure favors filtration
• Bowman space hydrostatic pressure opposes filtration
• Plasma oncotic pressure opposes filtration
• Net filtration pressure = forces out − forces in

Autoregulation keeps RBF/GFR relatively stable across a MAP range via afferent arteriolar myogenic tone and tubuloglomerular feedback (macula densa sensing NaCl delivery).

Nephron Segments—What Each Does (Exam Level)

Clearance concepts (high yield):

• Substance freely filtered and neither reabsorbed nor secreted (classically inulin) → clearance = GFR
• PAH at low levels ≈ renal plasma flow (filtered + secreted)
• Glucose: completely reabsorbed until transport maximum (Tm) exceeded → glucosuria (diabetes)

Volume, Sodium, and Potassium

• Effective circulating volume drives RAAS and sympathetic renal effects
• Aldosterone promotes Na⁺ retention and K⁺ (and H⁺) excretion—excess → hypokalemia/metabolic alkalosis tendency; deficiency → hyperkalemia/metabolic acidosis tendency
• ADH (vasopressin) inserts aquaporins in collecting duct; high ADH → concentrated urine; low ADH (or nephrogenic resistance) → dilute large-volume urine

Dental relevance of renal disease:

• Reduced drug clearance (many antimicrobials, NSAIDs risk) — dose/adjust or avoid nephrotoxins
• Platelet dysfunction in uremia → bleeding risk even with normal count
• Hypertension and fluid overload → cardiovascular risk in the chair
• Secondary hyperparathyroidism of CKD → bone disease and oral radiographic changes

Acid–Base Physiology and ABG Basics

Core definitions

Blood pH is tightly held near 7.35–7.45. Acid–base status is described by the equilibrium:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

• Respiratory component: PaCO₂ (ventilation)
• Metabolic component: HCO₃⁻ (renal generation/reabsorption and buffers)

Primary disorders (pattern recognition)

Interpretation algorithm for AFK-level stems:

1. Look at pH: acidosis (<7.35) or alkalosis (>7.45)
2. Look at PaCO₂ and HCO₃⁻: which moved in the direction that explains the pH?
3. Check the other value for compensation (partial vs full—full compensation returns pH toward normal but primary process remains)
4. For metabolic acidosis, think anion-gap vs non-gap conceptually (lactic/keto/toxins vs diarrhea/RTA), even if full electrolyte math is not required

Buffers: bicarbonate, phosphate, and proteins (including Hb) blunt free [H⁺] changes. Lungs adjust CO₂ in minutes; kidneys adjust H⁺ secretion and HCO₃⁻ generation over hours to days.

Dental scenarios:

• Anxiety hyperventilation → acute respiratory alkalosis (paresthesias, lightheadedness)—coach slow breathing, rebreathing only with appropriate clinical judgment
• Airway obstruction / oversedation → respiratory acidosis and hypoxemia—open airway, support ventilation, O₂
• Chronic kidney disease → tendency toward metabolic acidosis and impaired compensation

Calcium, PTH, and Vitamin D (High Dental Relevance)

Extracellular Ca²⁺ is critical for neuromuscular excitability, clotting, and hard-tissue mineral. About half of serum calcium is ionized (active); the rest is protein-bound (mainly albumin) or complexed.

Hormonal control triad

Vitamin D pathway (exam sequence): skin (UV) / diet → cholecalciferol → liver 25-hydroxylation → kidney 1α-hydroxylation → active calcitriol. CKD impairs the last step → hypocalcemia drive and secondary hyperparathyroidism.

Clinical–oral links AFK loves

• Primary hyperparathyroidism (excess PTH): hypercalcemia, bone resorption, "brown tumors" (osteitis fibrosa cystica / giant-cell lesions of bone), loss of lamina dura, ground-glass bone patterns historically described—connects endocrine physiology to oral pathology imaging
• Hypoparathyroidism: hypocalcemia → neuromuscular irritability (tetany, Chvostek/Trousseau), enamel/dental developmental defects if onset early
• Vitamin D deficiency / rickets / osteomalacia: impaired mineralization of osteoid; in children, rachitic dental/craniofacial growth effects
• CKD–mineral bone disorder: low calcitriol, high phosphate, secondary high PTH → renal osteodystrophy

Calcium matters for local anesthesia and muscle function too: severe hypocalcemia increases excitability; hypercalcemia can cause polyuria, stones, groans, psychiatric overtones—the classic mnemonic still organizes systemic features.

Thyroid Physiology

• Hypothalamus TRH → pituitary TSH → thyroid T4/T3
• T3 is more active; peripheral conversion of T4 → T3 occurs in tissues
• Thyroid hormones ↑ basal metabolic rate, heart rate/contractility sensitivity to catecholamines, and bone turnover

Uncontrolled thyroid disease is a medical consult issue before elective invasive dentistry—physiology explains why: cardiac sensitivity and metabolic fragility.

Adrenal Physiology

Cortex (steroid hormones)

Cortisol is under CRH–ACTH control (diurnal rhythm; stress increases ACTH). Chronic exogenous glucocorticoids suppress the HPA axis—abrupt withdrawal or surgical stress without coverage can precipitate adrenal crisis (hypotension, hypoglycemia, shock). Know the physiology; specific steroid-cover protocols appear in medical-emergency/pharmacology teaching.

Medulla

Chromaffin cells release epinephrine and norepinephrine into blood—amplifying fight-or-flight: ↑ HR, contractility, bronchodilation (β₂), and glucose mobilization. This is the same catecholamine physiology that links anxiety, vasoconstrictors in LA, and emergency "epinephrine" use.

Addison disease (primary adrenal insufficiency): low cortisol ± aldosterone → hypotension, hyperpigmentation (high ACTH/MSH activity), hyponatremia, hyperkalemia risk—poor stress reserve in the dental setting if undiagnosed/undertreated.

Cushing syndrome (excess cortisol): central obesity, hypertension, hyperglycemia, impaired healing and infection risk—wound and periodontal considerations.

Integrating Renal–Endocrine Axes for AFK

Trace these feedback loops until they are automatic:

1. Low MAP → renin → Ang II → aldosterone + vasoconstriction → volume and SVR restore MAP
2. Low ionized Ca²⁺ → PTH ↑ → bone/kidney/vitamin D activation → Ca²⁺ restored
3. High PaCO₂ → respiratory acidosis → renal HCO₃⁻ retention over time
4. Stress → ACTH and cortisol + adrenal medullary catecholamines → mobilize energy and support BP

When a stem mentions CKD, hyperparathyroidism, thyroid disease, or steroid use, name the broken limb of the loop first—then predict the laboratory and oral findings. That approach scores higher than memorizing disconnected fact lists.