3.1 Biochemistry and Systemic Physiology

1.2 Biochemistry and Systemic Physiology

Collagen Biosynthesis, Maturation & Scurvy Pathogenesis

Collagen is the most abundant structural protein in the human body, serving as the principal organic matrix component of dentin, cementum, alveolar bone, periodontal ligament (PDL), and oral mucosa.

Step-by-Step Collagen Synthesis Pathway

Collagen processing involves a tightly regulated sequence of intracellular and extracellular post-translational modifications:

INTRACELLULAR STEPS (Rough ER & Golgi):
1. Gene Transcription & Translation --> Preprocollagen
2. Signal Peptide Cleavage --> Procollagen alpha-chains
3. Hydroxylation of Proline & Lysine Residues (Requires Vitamin C + Fe2+)
4. Glycosylation of Hydroxylysine
5. Triple Helix Formation --> Procollagen Secretion into Extracellular Space

EXTRACELLULAR STEPS:
6. Cleavage of N- & C-Terminal Propeptides (by Procollagen Peptidases) --> Tropocollagen
7. Microfibril Alignment & Covalent Cross-linking (by Lysyl Oxidase [Requires Cu2+]) --> Mature Collagen Fiber

Clinical Correlate: Scurvy (Vitamin C Deficiency)

• Biochemical Defect: Ascorbic acid (Vitamin C) acts as a reducing agent to maintain iron in the ferrous (Fe2+) state required by prolyl hydroxylase and lysyl hydroxylase inside the rough endoplasmic reticulum.
• Pathophysiology: Without Vitamin C, unhydroxylated procollagen alpha-chains cannot form stable triple helices and undergo rapid intracellular degradation at body temperature.
• Oral & Systemic Manifestations: Defective collagen synthesis leads to capillary fragility (perifollicular hemorrhages, petechiae, ecchymoses), impaired wound healing, swollen/bleeding gingiva, and breakdown of the periodontal ligament leading to severe tooth mobility and loss.

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Mineral Metabolism: Calcium, Phosphate & Hormonal Control

Serum calcium (normal: 8.5–10.5 mg/dL) and phosphate (normal: 2.5–4.5 mg/dL) concentrations are strictly regulated by three key hormones: Parathyroid Hormone (PTH), Active Vitamin D (Calcitriol), and Calcitonin.

Exam Key Point: PTH has a phosphaturic effect (inhibits phosphate reabsorption in proximal tubule), whereas Vitamin D increases intestinal and renal absorption of BOTH calcium and phosphate.

Clinical Manifestations in Dentistry

• Hyperparathyroidism: Excessive PTH production leads to subperiosteal bone resorption. Classic radiographic triad: generalized loss of the lamina dura, "ground-glass" trabecular pattern, and osteolytic lesion known as Brown Tumors of Hyperparathyroidism (osteitis fibrosa cystica).
• Rickets (Children) / Osteomalacia (Adults): Vitamin D deficiency resulting in unmineralized osteoid matrix. Dental features: Delayed eruption, enamel hypoplasia, and wide pulp chambers with prominent interglobular dentin.

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Blood Coagulation Cascade & Anticoagulant Pharmacotherapy

Hemostasis occurs in two phases: Primary Hemostasis (vascular spasm and platelet plug formation) and Secondary Hemostasis (coagulation cascade generating a stable fibrin clot).

INTRINSIC PATHWAY (Monitored by aPTT)         EXTRINSIC PATHWAY (Monitored by PT/INR)
Surface Contact (Subendothelial Collagen)     Tissue Factor (Factor III) + Factor VIIa
          ↓                                                    ↓
Factors XII → XI → IX + VIIIa (Tenase)                         ↓
          └──────────────────────────┬─────────────────────────┘
                                     ↓
                          COMMON PATHWAY (Factor X)
                                     ↓
                      Factor Xa + Va (Prothrombinase)
                                     ↓
                    Prothrombin (II) → Thrombin (IIa)
                                     ↓
                   Fibrinogen (I) → Fibrin Monomer (Ia)
                                     ↓
                     Factor XIIIa Cross-links Fibrin Clot

Coagulation Pathways & Laboratory Assessment

1. Extrinsic Pathway: Triggered by tissue injury releasing Tissue Factor (Factor III), activating Factor VII. Assessed by Prothrombin Time (PT) and International Normalized Ratio (INR). Normal INR = 0.8–1.1.
2. Intrinsic Pathway: Triggered by subendothelial collagen contact, activating Factors XII, XI, IX, and VIII. Assessed by Activated Partial Thromboplastin Time (aPTT). Normal aPTT = 25–35 seconds.
3. Common Pathway: Factor X activation → converts Prothrombin (II) to Thrombin (IIa) → converts Fibrinogen (I) to Fibrin (Ia) → cross-linked by Factor XIIIa.

Anticoagulation Pharmacology & Dental Management

• Warfarin (Coumadin): Inhibits Vitamin K epoxide reductase (VKORC1), preventing hepatic synthesis of functional Factors II, VII, IX, X, and Proteins C and S. Monitored via PT/INR.
  • Dental Guideline: Minor oral surgery (extractions, periodontal surgery) can be performed safely when INR is <= 3.0 to 3.5. Warfarin should NOT be routinely discontinued prior to dental procedures due to high risk of thromboembolism. Local hemostatic agents (gelfoam, oxidized cellulose, sutures, tranexamic acid rinse) are utilized.
• Direct Oral Anticoagulants (DOACs): Direct Factor Xa inhibitors (Rivaroxaban, Apixaban) and Direct Thrombin inhibitors (Dabigatran). Do not require routine INR monitoring.
• Heparin: Enhances Antithrombin III activity, inactivating Thrombin and Factor Xa. Monitored via aPTT. Reversed by Protamine Sulfate.
• Inherited Coagulopathies:
  • Hemophilia A: Deficiency of Factor VIII (X-linked recessive; prolonged aPTT, normal PT/INR & bleeding time).
  • Hemophilia B (Christmas Disease): Deficiency of Factor IX (X-linked recessive; prolonged aPTT).
  • Von Willebrand Disease (vWD): Deficiency/defect in vWF (Autosomal dominant). Impairs platelet adhesion (prolongs Bleeding Time) and destabilizes Factor VIII (prolongs aPTT).

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Cardiovascular Physiology & Dental Considerations

Blood Pressure Regulation & Cardiac Mechanics

• Cardiac Output (CO) = Stroke Volume (SV) × Heart Rate (HR). Stroke volume is governed by Preload (end-diastolic volume), Afterload (systemic vascular resistance), and Contractility.
• Short-Term BP Control: Baroreceptor Reflex. Baroreceptors in the carotid sinus (sensory via CN IX) and aortic arch (sensory via CN X) detect stretch changes and rapidly alter autonomic outflow.
• Long-Term BP Control: Renin-Angiotensin-Aldosterone System (RAAS). Decreased renal perfusion triggers Juxtaglomerular (JG) cells to release Renin → converts Angiotensinogen to Angiotensin I → converted by endothelial ACE (lung) to Angiotensin II (potent vasoconstrictor and stimulator of Aldosterone release from adrenal cortex).

Heart Failure (HF) & Cardiac Epinephrine Limits

• Left-Sided Heart Failure: Inability of left ventricle to pump blood, causing backup into pulmonary circulation. Manifests as exertional dyspnea, orthopnea, paroxysmal nocturnal dyspnea, pulmonary edema, and bibasilar crackles.
• Right-Sided Heart Failure: Inability of right ventricle to pump blood to lungs, causing systemic venous congestion. Manifests as peripheral pitting edema, jugular venous distension (JVD), and hepatomegaly. Most common cause is left-sided HF.
• Dental Management Guidelines:
  • Position HF patients semi-supine or upright to prevent pulmonary congestion and orthopnea.
  • Cardiac Epinephrine Dose Limit: For patients with significant cardiovascular disease or hypertension, limit exogenous epinephrine to 0.04 mg per appointment (maximum 2 cartridges of 1:100,000 epinephrine or 4 cartridges of 1:200,000 epinephrine).

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Renal Physiology & Fluid/Electrolyte Balance

Each kidney contains ~1 million functional units called nephrons responsible for filtration, reabsorption, and secretion.

Nephron Segment Functional Summary:
Proximal Convoluted Tubule (PCT) --> Reabsorbs 65-70% Na+, H2O, HCO3-, 100% Glucose/Amino Acids
Loop of Henle (Thick Ascending) --> NKCC2 Transporter (Reabsorbs Na+/K+/2Cl-); Site of Loop Diuretics
Distal Convoluted Tubule (DCT)  --> NCC Transporter (Na+/Cl-); PTH increases Ca2+ reabsorption; Thiazides
Collecting Duct                  --> Principal Cells (Na+/H2O reabsorption via Aldosterone & ADH)
                                    Intercalated Cells (H+ secretion & Acid-Base balance)

1. Proximal Convoluted Tubule (PCT): Bulk reabsorption zone. Reabsorbs ~65–70% of filtered Na+ and water, along with 100% of filtered glucose and amino acids (via Na+-coupled secondary active transport).
2. Loop of Henle: Creates hyperosmotic medullary gradient. The thick ascending limb actively reabsorbs Na+, K+, and 2Cl- via the NKCC2 cotransporter (inhibited by Loop Diuretics like Furosemide). Impermeable to water.
3. Distal Convoluted Tubule (DCT): Reabsorbs Na+ and Cl- via the NCC cotransporter (inhibited by Thiazide Diuretics). Site of active Ca2+ reabsorption stimulated by PTH.
4. Collecting Duct: Target for hormonal control:
   • Aldosterone: Acts on Principal Cells to upregulate luminal ENaC channels and basolateral Na+/K+-ATPase, promoting Na+ and water reabsorption and K+/H+ secretion.
   • Antidiuretic Hormone (ADH / Vasopressin): Binds V2 receptors on Principal Cells to insert Aquaporin-2 water channels, producing concentrated urine.