2.2: Pathophysiology of Major Organ Systems

2.2 Pathophysiology of Major Organ Systems

A deep understanding of disease mechanisms is critical for overseas pharmacists preparing for the OPRA exam. This section details the cellular and systemic pathophysiology of endocrine, cardiovascular, and renal disorders, aligning these concepts with Australian practice standards.

---

1. Endocrine Disorders

Pathogenesis of Diabetes Mellitus: Type 1 vs. Type 2

Diabetes Mellitus is characterized by chronic hyperglycaemia. However, the underlying pathophysiological mechanisms differ fundamentally between the two major types.

Detailed Autoimmune Destruction in T1DM

T1DM is a chronic, T-cell-mediated autoimmune disease. A genetic susceptibility, particularly involving major histocompatibility complex (MHC) class II genes HLA-DR3 and HLA-DR4, is combined with an environmental trigger (e.g., viral infections like Coxsackievirus or dietary factors). This initiates the loss of immune tolerance, leading to the activation of auto-reactive CD4+ and CD8+ T lymphocytes, macrophages, and B-cells. These inflammatory cells infiltrate the pancreatic islets of Langerhans (a process called insulitis) and selectively destroy insulin-producing beta cells. Symptoms manifest only when approximately 80-90% of beta cells are destroyed. The absolute lack of insulin prevents glucose uptake in insulin-sensitive tissues (muscle and adipose), stimulating counter-regulatory hormones (glucagon, catecholamines, cortisol) and triggering diabetic ketoacidosis.

Progressive Pathogenesis of T2DM

T2DM is characterized by a dual defect:

1. Insulin Resistance: A state where target tissues (primarily skeletal muscle, adipose tissue, and the liver) fail to respond normally to physiological concentrations of insulin. This is strongly driven by obesity and physical inactivity, which lead to elevated circulating free fatty acids (FFAs) and inflammatory cytokines (e.g., TNF-alpha, IL-6) that impair insulin receptor substrate (IRS) signaling pathways.
2. Beta-Cell Dysfunction: Initially, the pancreas compensates by secreting more insulin (hyperinsulinaemia) to maintain normal blood glucose. Over time, chronic demand leads to beta-cell exhaustion. This is worsened by amyloid polypeptide deposition in the islets, lipotoxicity (high lipid concentrations toxic to beta cells), and glucotoxicity (chronic hyperglycaemia impairing beta-cell function), leading to a progressive decline in insulin secretion.

DKA vs. HHS Pathophysiology

• DKA Pathophysiology: Absolute insulin deficiency prevents glucose from entering tissues, causing the body to perceive starvation. Counter-regulatory hormones (glucagon, adrenaline, cortisol) rise. In adipose tissue, the absence of insulin allows hormone-sensitive lipase (HSL) to run unchecked, breaking down triglycerides into free fatty acids (FFAs). The liver takes up these FFAs and converts them to acetyl-CoA, which enters the ketogenesis pathway to produce acidic ketone bodies (acetoacetate and beta-hydroxybutyrate). Accumulation of ketones exceeds the body's buffering capacity, resulting in a high anion gap metabolic acidosis.
• HHS Pathophysiology: Patients with T2DM retain some endogenous insulin secretion. This small amount of circulating insulin is sufficient to act on adipose tissue, suppressing lipolysis and ketogenesis. However, it is insufficient to overcome insulin resistance in skeletal muscle and liver. As a result, glucose levels rise extremely high (often >33 mmol/L), causing severe osmotic diuresis, profound dehydration, and serum hyperosmolality (>320 mOsm/kg) without significant ketosis or acidosis.

Thyroid Disorders: Hypo- and Hyperthyroidism

1. Hypothyroidism (Hashimoto's Thyroiditis)

• Pathogenesis: Hashimoto's is the most common cause of hypothyroidism in iodine-sufficient countries. It is an autoimmune disease where T-cell-mediated cytotoxicity and autoantibodies (anti-thyroid peroxidase [anti-TPO] and anti-thyroglobulin) target thyroid follicular cells, leading to chronic inflammation, follicular destruction, and fibrosis. The destruction of follicular architecture prevents the synthesis of thyroid hormones (T4 and T3).
• Symptoms: Bradycardia, weight gain, cold intolerance, constipation, dry skin, lethargy, goitre, and myxedema (accumulation of glycosaminoglycans in interstitial tissues, causing non-pitting edema).
• Labs: Elevated thyroid-stimulating hormone (TSH) due to lack of negative feedback, and low free thyroxine (FT4).
• Australian Practice Context: Treatment is levothyroxine (synthetic T4). Patients must be counselled to take levothyroxine on an empty stomach, 30-60 minutes before breakfast or caffeine, to optimize absorption. Critically, it must be separated by at least 4 hours from substances that impair its absorption, such as iron supplements, calcium carbonate, antacids, and proton pump inhibitors (PPIs).

2. Hyperthyroidism (Graves' Disease)

• Pathogenesis: Graves' disease is caused by type II hypersensitivity, where autoantibodies called Thyroid Stimulating Immunoglobulins (TSI) bind to and activate the TSH receptor on thyroid follicular cells. This bypasses the normal negative feedback loop, stimulating continuous thyroid hormone synthesis and release.
• Symptoms: Tachycardia (atrial fibrillation risk), weight loss despite increased appetite, heat intolerance, fine tremor, anxiety, diarrhoea, exophthalmos (due to orbital fibroblast activation and swelling), and pretibial myxedema.
• Labs: Suppressed TSH due to excessive negative feedback, and elevated FT4/FT3.
• Australian Practice Context: Antithyroid drugs (carbimazole, propylthiouracil [PTU]) are first-line. Carbimazole is a prodrug of methimazole and is preferred due to its once-daily dosing. However, PTU is preferred in the first trimester of pregnancy (due to carbimazole's association with fetal abnormalities) and in thyroid storm (because PTU additionally inhibits the peripheral conversion of T4 to active T3 by blocking 5'-deiodinase).
• Agranulocytosis Warning: Both drugs can cause life-threatening bone marrow suppression (agranulocytosis). Patients must be strictly counselled to stop the medication and seek an immediate full blood count if they develop symptoms of infection, such as a sore throat, fever, or mouth ulcers.

---

2. Cardiovascular Pathology

Pathogenesis of Atherosclerosis

Atherosclerosis is a chronic inflammatory disease of the arterial wall that underlies coronary artery disease, stroke, and peripheral arterial disease.

1. Endothelial Injury: Caused by physical or chemical stressors (hypertension, smoking, dyslipidaemia, hyperglycaemia). The injured endothelium becomes permeable and expresses adhesion molecules (VCAM-1, ICAM-1).
2. Lipid Accumulation: Low-Density Lipoprotein (LDL) particles enter the subendothelial space (intima) and become oxidized (oxLDL) by free radicals.
3. Monocyte Recruitment and Foam Cell Formation: Adhesion molecules recruit monocytes, which migrate into the intima and differentiate into macrophages. Macrophages express scavenger receptors that engulf oxLDL without negative feedback control. The macrophages become engorged with cholesterol, transforming into "foam cells."
4. Fatty Streak to Fibrous Plaque: Foam cells die via apoptosis, releasing a lipid-rich necrotic core. Smooth muscle cells migrate from the media to the intima, proliferate, and secrete collagen and extracellular matrix, forming a fibrous cap over the necrotic core.
5. Plaque Rupture and Thrombosis:
   • Stable Plaque: Thick fibrous cap, small necrotic core. Causes stable angina by narrowing the lumen.
   • Unstable Plaque: Thin fibrous cap, large necrotic core, and high macrophage density. Inflammatory cells secrete matrix metalloproteinases (MMPs) that degrade collagen in the fibrous cap.
   • When the cap ruptures, the highly thrombogenic necrotic core is exposed to circulating blood. This triggers platelet activation, adhesion, aggregation, and the coagulation cascade, forming an occlusive thrombus that precipitates an acute myocardial infarction (AMI) or ischemic stroke.

• Statin Pleiotropy: Statins (HMG-CoA reductase inhibitors) do more than lower LDL; they stabilize plaques by increasing collagen content, reducing vascular inflammation, and restoring endothelial nitric oxide production.

Heart Failure Compensatory Mechanisms

Heart failure (HF) occurs when the heart cannot pump sufficient blood to meet metabolic demands. When cardiac output falls, the body activates neural and hormonal pathways to maintain perfusion:

1. Sympathetic Nervous System (SNS) Activation:
   • A drop in stroke volume is sensed by arterial baroreceptors, reducing their inhibitory signaling to the medulla.
   • Sympathetic outflow increases, releasing adrenaline and noradrenaline.
   • This stimulates cardiac beta-1 receptors to increase heart rate (chronotropy) and contractility (inotropy) to maintain cardiac output. It also stimulates alpha-1 receptors on arterioles, causing vasoconstriction to maintain systemic blood pressure (increasing afterload).
   • Pathological Remodeling: Chronic catecholamine exposure causes myocardial hypertrophy, apoptosis, and downregulation of beta-1 receptors, worsening pump function over time.
2. Renin-Angiotensin-Aldosterone System (RAAS) Activation:
   • Reduced renal blood flow and sympathetic stimulation of juxtaglomerular cells trigger renin release.
   • Renin cleaves angiotensinogen to Angiotensin I, which is converted to Angiotensin II by Angiotensin-Converting Enzyme (ACE) in the pulmonary vasculature.
   • Angiotensin II is a potent vasoconstrictor (increasing afterload) and stimulates the adrenal cortex to release aldosterone, which increases sodium and water reabsorption in the collecting duct (increasing preload).
   • Pathological Remodeling: Angiotensin II and aldosterone directly stimulate myocardial fibroblasts, promoting extracellular matrix deposition (fibrosis) and hypertrophy, which increases ventricular stiffness.
3. Natriuretic Peptides (ANP and BNP):
   • Released by the atria (ANP) and ventricles (BNP) in response to increased wall stretch.
   • They act as a counter-regulatory system, promoting vasodilation, natriuresis, and diuresis, and directly inhibiting renin and aldosterone release.
   • ARNI Therapy: Sacubitril/valsartan combines a neprilysin inhibitor with an ARB. By blocking neprilysin, it prevents the degradation of BNP, enhancing its beneficial vasodilatory and diuretic effects, while blocking the harmful effects of Angiotensin II.

---

3. Renal Impairment

Acute Kidney Injury (AKI): Etiology and Pathophysiology

AKI is an abrupt decline in glomerular filtration rate. Under the KDIGO criteria, it is defined by a rise in serum creatinine (greater than or equal to 26.5 micromol/L within 48 hours or greater than or equal to 1.5 times baseline within 7 days) or a decline in urine output (less than 0.5 mL/kg/hour for greater than or equal to 6 hours).

• Prerenal AKI (Hypoperfusion): The kidneys are structurally intact, but blood flow is reduced (dehydration, hemorrhage, sepsis).
  • The "Triple Whammy" Interaction: A common clinical trap in Australia. It occurs when a patient is concurrently prescribed a diuretic, an ACE inhibitor or ARB, and an NSAID.
    1. Diuretics reduce intravascular volume, decreasing renal perfusion.
    2. NSAIDs block prostaglandins (PGE2 and PGI2), which normally dilate the afferent arteriole. This results in afferent vasoconstriction, reducing blood entering the glomerulus.
    3. ACEIs/ARBs block Angiotensin II, which normally constricts the efferent arteriole to maintain filtration pressure. This causes efferent vasodilation, allowing blood to leave the glomerulus too easily.
    • Together, these drugs cause a catastrophic drop in intraglomerular hydrostatic pressure, precipitating acute prerenal kidney injury.
• Intrinsic AKI (Parenchymal Damage):
  • Acute Tubular Necrosis (ATN): Most common intrinsic cause, triggered by prolonged renal ischemia or nephrotoxic drugs. Classic offenders include aminoglycosides (e.g., gentamicin), iodinated contrast media, cisplatin, and amphotericin B. The tubular cells slough off and obstruct the lumen, causing a decline in GFR.
  • Acute Interstitial Nephritis (AIN): An immunologically mediated hypersensitivity reaction in the renal interstitium. It is commonly drug-induced (e.g., penicillins, NSAIDs, proton pump inhibitors). Patients may present with eosinophiluria, rash, and fever.
• Postrenal AKI (Obstruction): Caused by blockages in the urinary tract (benign prostatic hyperplasia [BPH], kidney stones, tumors).

Chronic Kidney Disease (CKD) Pathophysiology and Complications

CKD is defined as GFR less than 60 mL/min/1.73m^2 or markers of kidney damage (e.g., albuminuria) for greater than or equal to 3 months. As nephrons are progressively lost, the remaining nephrons hyperfilter to compensate, which eventually leads to glomerular sclerosis and progressive renal decline. This leads to systemic complications:

1. Hyperkalaemia: Impaired potassium excretion in the collecting duct, which can cause life-threatening cardiac arrhythmias (characterized by peaked T waves on an ECG).
2. Metabolic Acidosis: Reduced ability of the kidneys to excrete hydrogen ions and regenerate bicarbonate.
3. Mineral and Bone Disorder (CKD-MBD):
   • Phosphate Retention: Reduced GFR leads to phosphate retention (hyperphosphataemia). Free phosphate complexes with ionized calcium, lowering serum calcium levels.
   • Calcitriol Deficiency: Loss of functional renal mass impairs the enzyme 1-alpha-hydroxylase in the proximal tubule. This prevents the conversion of vitamin D to its active form, calcitriol (1,25-dihydroxyvitamin D), which reduces intestinal calcium absorption.
   • Secondary Hyperparathyroidism: The combination of low serum calcium, high phosphate, and low calcitriol stimulates the parathyroid glands to release parathyroid hormone (PTH). High PTH levels cause bone resorption (renal osteodystrophy) to restore calcium balance, leading to weak bones and vascular calcification.
4. Anaemia: Renal peritubular capillary fibroblasts produce erythropoietin (EPO) in response to hypoxia. As these cells are replaced by fibrous tissue in CKD, EPO production declines, causing a normocytic, normochromic anaemia.

Renal Dosing Calculations: Cockcroft-Gault vs. eGFR

• Cockcroft-Gault Equation: CrCl (mL/min) = [ (140 - Age) x Weight (kg) ] / [ 0.814 x Serum Creatinine (micromol/L) ] x (0.85 if female)
• eGFR (CKD-EPI): Reported directly by Australian pathology labs, this is standardized to a standard Body Surface Area (1.73 m^2) and is intended for staging CKD.
• Clinical Distinction: For drug dosing (e.g., direct oral anticoagulants [DOACs], gabapentin, digoxin, aminoglycosides), the Australian Medicines Handbook (AMH) and renal guidelines recommend calculating CrCl using the Cockcroft-Gault equation. Relying on eGFR can lead to overdosing in patients with small body mass or underdosing in obese patients, because eGFR does not adjust for the patient's actual weight.