Comprehensive Human Pathophysiology and Pharmacology Review (Weeks 1 to 9)

Cellular Adaptation, Injury, and Death

• Cellular Adaptations:

  • Atrophy: Reduction in cell size.

  • Hypertrophy: Increase in cell size.

  • Hyperplasia: Increase in the number of cells.

  • Metaplasia: Reversible transformation of one mature, differentiated cell type into another.

  • Dysplasia: Disordered cellular growth characterised by abnormal variation in size, shape, and architectural arrangement.

  • Neoplasia: Uncontrolled, autonomous new cellular growth (benign or malignant).

• Mechanisms of Cell Injury:

  • ATP Depletion: Cellular energy loss impairs plasma membrane ion pumps, leading to sodium and water accumulation and cell swelling.

  • Free Radical Production: Reactive oxygen species cause oxidative destruction of membrane lipids, structural proteins, and nuclear DNA.

  • Intracellular Calcium Influx: Loss of calcium homeostasis causes excessive intracellular calcium levels, activating destructive catabolic enzymes.

  • Injury States: Reversible injury permits structural recovery if the stressor terminates; irreversible injury culminates in cell death.

• Forms of Cell Death:

  • Apoptosis: Regulated, programmed cell death that packages cellular components into apoptotic bodies without cell lysis; induces minimal or no inflammatory response.

  • Necrosis: Unregulated cell death resulting from acute pathological injury; involves plasma membrane degradation, enzymatic leakage, and inflammatory cascade activation.

  • Pathological Patterns of Necrosis: Coagulative (preserves tissue architecture in solid organs post-ischaemia), liquefactive (rapid enzymatic dissolution into fluid; characteristic of cerebral infarction and abscesses), caseous (friable, cheese-like appearance classic to tuberculosis), fat (enzymatic digestion of adipose tissue, as in pancreatitis), fibrinoid (antigen-antibody complex deposition in vascular walls), and gangrenous (clinical classification of severe extremity necrosis, presenting in dry or wet forms).

Mechanisms and Mediators of Inflammation

• Acute Inflammatory Sequence:

  • Tissue injury or microbial presence triggers sentinel immune cells (such as mast cells) to release inflammatory chemical mediators.

  • Arteriolar vasodilation increases local blood flow, producing erythema (redness) and warmth.

  • Microvascular permeability increases, allowing protein-rich fluid to leak into interstitial tissues, producing oedema (swelling).

  • Chemotaxis directs circulating leukocytes: neutrophils arrive rapidly to initiate phagocytosis of pathogens and debris; macrophages arrive subsequently (around $24\,\text{hours}$) to perpetuate phagocytosis and cytokine secretion.

  • Clotting systems establish a fibrin mesh to limit haemorrhage and isolate the site, transitioning into tissue repair.

• Cardinal Signs of Local Inflammation:

  • Redness and heat: Secondary to microvascular vasodilation and increased hyperaemic blood flow.

  • Swelling: Secondary to increased endothelial permeability and interstitial fluid accumulation.

  • Pain: Secondary to chemical stimulation of nociceptive nerve endings by prostaglandins and bradykinin, exacerbated by mechanical tissue pressure from oedema.

  • Loss of function: Secondary to pain, structural swelling, and tissue destruction.

• Primary Inflammatory Mediators:

  • Histamine: Released from mast cells; induces rapid arteriolar vasodilation and endothelial permeability.

  • Prostaglandins: Synthesised via the cyclooxygenase (COX) pathway; induce pain, fever, and vasodilation.

  • Leukotrienes: Synthesised via the lipoxygenase pathway; increase vascular permeability and stimulate bronchial smooth muscle constriction.

  • Cytokines: Signalling proteins ($\text{IL-1}$, $\text{IL-6}$, $\text{TNF-}\alpha$) that activate immune cells, recruit leukocytes, and induce systemic acute-phase reactions.

  • Bradykinin: Component of the kinin cascade; stimulates vascular permeability and peripheral pain receptors.

  • Complement System: Plasma cascade promoting microbial opsonisation, chemotaxis, and membrane lysis.

  • Coagulation System: Fibrin matrix generation to arrest blood loss and prevent pathogen dissemination.

• Systemic Manifestations and Pyrogenesis:

  • Systemic response presents with pyrexia, lethargy, and elevated inflammatory plasma biomarkers.

  • Fever mechanism: Macrophage-derived inflammatory cytokines circulate to the preoptic area of the hypothalamus, stimulating prostaglandin $\text{E}_2$ ($\text{PGE}_2$) synthesis; this elevates the central thermal set-point, triggering peripheral vasoconstriction and shivering to preserve and generate heat.

• Anti-Inflammatory Pharmacology:

  • Non-Steroidal Anti-Inflammatory Drugs (NSAIDs): Inhibit cyclooxygenase enzymes (COX-1 and COX-2), halting prostaglandin synthesis; this attenuates pain, pyrexia, and inflammatory hyperaemia; potential adverse effects include gastrointestinal ulceration/haemorrhage, impaired renal perfusion, and fluid retention.

  • Corticosteroids: Exogenous glucocorticoids (e.g., prednisolone) bind intracellular receptors to downregulate pro-inflammatory gene transcription and suppress multiple cytokine networks; potential adverse effects include hyperglycaemia, secondary infection risk, fluid retention, hypertension, skeletal muscle catabolism, osteopenia, and hypothalamic-pituitary-adrenal axis suppression upon prolonged systemic use.

Neurobiology and Management of Pain

• Taxonomy of Pain:

  • Nociceptive Pain: Arises from activation of intact nociceptors sensing actual or threatened non-neural tissue injury; subdivided into somatic pain (localised to integument, muscle, or skeletal structures) and visceral pain (originating in internal visceral organs, frequently vague or referred).

  • Neuropathic Pain: Caused by a primary structural lesion or disease of the somatosensory nervous system; presents with burning, lancinating, tingling, or electric sensations.

  • Nociplastic Pain: Pain arising from altered nociceptive processing without evidence of actual/threatened tissue damage or nervous system lesions.

  • Chronology: Acute pain is self-limiting and linked to healing; chronic pain persists or recurs beyond normal tissue recovery (typically exceeding $3\,\text{months}$).

• Neuroanatomy of the Nociceptive Pathway:

  • Transduction: Noxious thermal, chemical, or mechanical stimuli are converted into electrical action potentials by peripheral nociceptive free nerve endings.

  • Transmission: Action potentials travel along peripheral nerve fibres (myelinated $\text{A-}\delta$ fibres for rapid, sharp pain; unmyelinated C fibres for slower, dull aching pain) into the dorsal horn of the spinal cord, ascending the spinothalamic tract to the thalamus.

  • Perception: Conscious cortical awareness of pain; the primary somatosensory cortex discriminates anatomical location and intensity, while the anterior cingulate cortex and insula process affective unpleasantness and emotional motivation.

  • Modulation: Spinal and supraspinal pathways inhibit or amplify nociceptive signal transmission; descending inhibitory pathways from the brainstem release serotonin and noradrenaline; endogenous opioids (endorphins, enkephalins, dynorphins) activate opioid receptors to suppress presynaptic neurotransmitter release.

• Terminology of Sensory Processing:

  • Allodynia: Pain evoked by a stimulus that does not normally provoke pain (e.g., light cutaneous touch).

  • Hyperalgesia: Abnormally increased sensitivity or exaggerated pain response to a noxious stimulus.

  • Hypoalgesia: Diminished pain response to a normally noxious stimulus.

  • Pain Threshold: The minimum intensity of a stimulus at which pain is first perceived.

  • Pain Tolerance: The maximum duration or intensity of pain that an individual is willing to endure.

• Analgesic Pharmacology:

  • Paracetamol: Exerts antipyresis and analgesia primarily through central nervous system actions, lacking meaningful peripheral anti-inflammatory properties; excessive doses deplete hepatic glutathione, causing accumulation of the hepatotoxic intermediate N-acetyl-p-benzoquinone imine (NAPQI); treated acutely with the antidote N-acetylcysteine (NAC).

  • Opioid Agonists (e.g., Morphine): Act selectively on $\mu$ (mu) and $\kappa$ (kappa) opioid receptors within the central nervous system; therapeutic analgesia is coupled with dose-dependent adverse risks of respiratory depression, sedation, nausea, pruritus, and constipation; tolerance develops to analgesic, euphoric, and respiratory effects, whereas constipation and pupillary miosis demonstrate negligible tolerance.

  • Opioid Antagonists: Naloxone competitively displaces opioids from receptors to reverse life-threatening respiratory depression in acute overdose; naltrexone provides long-acting antagonism for dependence disorders; methylnaltrexone acts peripherally to alleviate opioid-induced constipation.

Immune Disorders and Hypersensitivity

• Immunological Architecture:

  • Innate Immunity: Non-specific, immediate defence mechanisms devoid of immunological memory (comprising epidermal barriers, neutrophils, macrophages, and natural killer cells).

  • Natural Killer (NK) Cells: Target abnormal cells displaying diminished MHC class I molecules or aberrant stress proteins; cytotoxic degranulation releases perforin and granzymes to induce target-cell apoptosis.

  • Adaptive Immunity: Antigen-specific protection characterised by clonal selection and long-term memory; mediated by T lymphocytes (cellular immunity) and B lymphocytes (humoral immunity).

  • Immunoglobulin Classes: IgM (first antibody generated in primary responses; mediates ABO haemolysis), IgG (predominant systemic antibody, provides long-term immunity, crosses placenta), IgA (confers mucosal defence in secretions), IgE (binds Fc receptors on mast cells and basophils; mediates allergy and helminth defence), and IgD (acts primarily as a naive B-cell antigen receptor).

• Mechanisms of Hypersensitivity:

  • Type I (Immediate / IgE-Mediated): Initial allergen exposure drives B-cell class-switching to IgE, which sensitises mast cells; secondary exposure cross-links cell-bound IgE, provoking immediate mast cell degranulation of histamine, leukotrienes, and prostaglandins; severe systemic manifestations constitute anaphylaxis (airway bronchoconstriction, laryngeal oedema, widespread vasodilation, and distributive shock), requiring immediate intramuscular adrenaline.

  • Type II (Antibody-Mediated Cytotoxic): Circulating IgG or IgM antibodies bind to intrinsic or extrinsic cell-surface antigens; activates complement-mediated cell lysis or antibody-dependent cellular cytotoxicity; exemplified by acute ABO-incompatible haemolytic transfusion reactions.

  • Type III (Immune Complex-Mediated): Soluble antigen-antibody complexes form in the circulation and deposit in microvascular beds; activates complement and attracts neutrophils, inducing localised vasculitis and tissue damage (characteristic of systemic lupus erythematosus).

  • Type IV (Cell-Mediated / Delayed-Type): T helper ($\text{CD4}^+$) or cytotoxic ($\text{CD8}^+$) lymphocytes recognize antigens presented by antigen-presenting cells, releasing cytokines that recruit and activate macrophages over $24\text{--}72\,\text{hours}$ (e.g., contact dermatitis, tuberculin test).

• Autoimmunity and Immunodeficiency:

  • Autoimmune Disease: Breakdown of central or peripheral self-tolerance causing immune attack against host tissues (e.g., systemic lupus erythematosus driven by antinuclear antibodies; inflammatory bowel disease involving dysregulated gut mucosal inflammation).

  • Immunodeficiency: Primary forms result from inherited genetic defects; secondary forms result from external factors, pharmacological immunosuppression, or viral destruction; human immunodeficiency virus (HIV) selectively infects and depletes $\text{CD4}^+$ T lymphocytes, predisposing to opportunistic infections.

  • Allograft Rejection: Recipient T lymphocytes and antibodies recognize foreign donor human leukocyte antigens (MHC); requires immunosuppressive prophylaxis (e.g., calcineurin inhibitors like tacrolimus, antimetabolites, and corticosteroids).

Autonomic Neuropharmacology and Hypertension

• Autonomic Nervous System Organization:

  • Somatic Nervous System: Single motor neuron directly innervates voluntary skeletal muscle.

  • Autonomic Nervous System: Two-neuron chain governing involuntary physiological functions; sympathetic division coordinates fight-or-flight energy expenditure, whereas parasympathetic division directs rest-and-digest conservation.

  • Ganglionic Neurotransmission: Both sympathetic and parasympathetic preganglionic neurons release acetylcholine (ACh) onto ionotropic nicotinic receptors located at the autonomic ganglia.

  • Postganglionic Transmission: Parasympathetic postganglionic fibres release ACh onto muscarinic receptors (M1 to M5); sympathetic postganglionic fibres predominantly release noradrenaline onto adrenergic receptors ($\alpha_1$, $\alpha_2$, $\beta_1$, $\beta_2$); sympathetic fibres to most sweat glands release ACh onto muscarinic receptors; the adrenal medulla directly secretes adrenaline and noradrenaline into systemic circulation upon preganglionic cholinergic stimulation.

• Receptor Functions and Transmitters:

  • $\alpha_1$ Adrenergic: Mediates vascular smooth muscle contraction, inducing systemic vasoconstriction and increasing blood pressure.

  • $\alpha_2$ Adrenergic: Located presynaptically; provides negative feedback inhibition to suppress further noradrenaline exocytosis.

  • $\beta_1$ Adrenergic: Located in cardiac tissue (increases chronotropy, inotropy, dromotropy) and renal juxtaglomerular cells (stimulates renin secretion).

  • $\beta_2$ Adrenergic: Mediates smooth muscle relaxation, causing bronchodilation and selective vasodilation.

  • Signal Termination: Acetylcholine is rapidly cleared via hydrolysis by synaptic acetylcholinesterase; noradrenaline is predominantly removed via presynaptic neuronal reuptake transporters, followed by intracellular enzymatic catabolism via monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT).

• Autonomic Pharmacology Classes:

  • Sympathomimetics: Activate adrenergic receptors directly or indirectly (e.g., salbutamol stimulates $\beta_2$ to induce bronchodilation; adrenaline activates $\alpha$ and $\beta$ receptors).

  • Sympatholytics: Antagonise adrenergic signalling (e.g., metoprolol selectively blocks $\beta_1$ receptors to reduce heart rate and myocardial contractility).

  • Parasympathomimetics: Agonise muscarinic receptors to stimulate secretions and smooth muscle contraction (e.g., bethanechol stimulates bladder detrusor muscle).

  • Antimuscarinics: Competitively block muscarinic acetylcholine receptors (e.g., atropine blocks cardiac muscarinic receptors to treat bradycardia; ipratropium causes bronchodilation).

• Hypertension:

  • Definition: Sustained clinic blood pressure exceeding diagnostic thresholds (commonly $\ge 140/90\,\text{mmHg}$ in office practice).

  • Haemodynamic Determinants: Determined by $\text{Blood Pressure} = \text{Cardiac Output} \times \text{Systemic Vascular Resistance}$.

  • Classification: Primary hypertension develops from multifactorial genetic, dietary, and neurohumoral factors; secondary hypertension arises from identifiable underlying pathologies (e.g., renovascular disease, primary aldosteronism, endocrine tumours).

  • Complications: Chronic barotrauma accelerates endothelial injury, leading to ischaemic stroke, intracerebral haemorrhage, coronary atherosclerosis, left ventricular hypertrophy, chronic kidney disease, and hypertensive retinopathy.

• Atherosclerosis Pathogenesis:

  • Arteriosclerosis: Generalised broad term for thickening, loss of elasticity, and calcific hardening of arterial walls.

  • Atherosclerosis Sequence: Endothelial dysfunction secondary to vascular stress or smoking \rightarrow accumulation and oxidation of low-density lipoprotein (LDL) particles in the tunica intima \rightarrow recruitment of circulating monocytes that differentiate into macrophages \rightarrow uncontrolled macrophage phagocytosis of oxidised LDL, transforming them into lipid-laden foam cells \rightarrow coalescence into a fatty streak \rightarrow vascular smooth muscle proliferation and extracellular collagen deposition creating a fibrous plaque with a necrotic lipid core \rightarrow luminal stenosis or unstable fibrous cap rupture with acute occlusive thrombosis.

Coronary Heart Disease, Acute Coronary Syndromes, and Heart Failure

• Coronary Heart Disease (CHD) Spectrum:

  • Myocardial Ischaemia: Disparity between coronary myocardial oxygen delivery and metabolic demand, generating cellular hypoxia; silent ischaemia occurs without overt pain symptoms, often in patients with diabetic autonomic neuropathy.

  • Stable Angina: Predictable, transient substernal chest discomfort evoked by physical exertion or emotional stress; relieved promptly by rest or sublingual glyceryl trinitrate (GTN); characterized by intact plaque without acute thrombosis or cell necrosis.

  • Acute Coronary Syndromes (ACS): Continuum triggered by acute atherosclerotic plaque disruption (erosion or rupture) with overlying platelet aggregation and thrombosis.

  • Unstable Angina: Disrupted plaque with non-occlusive thrombus causing crescendo, new-onset, or rest pain; demonstrates no acute rise in serum cardiac biomarkers.

  • Non-ST-Segment Elevation Myocardial Infarction (NSTEMI): Incomplete or intermittent coronary occlusion causing subendocardial myocardial necrosis; troponin concentrations are elevated; ECG demonstrates ST depression, T-wave inversion, or non-specific findings.

  • ST-Segment Elevation Myocardial Infarction (STEMI): Complete, persistent occlusive coronary thrombosis producing transmural myocardial necrosis; troponin concentrations are elevated; 12-lead ECG displays diagnostic ST-segment elevations or new left bundle branch block equivalent, requiring immediate mechanical reperfusion via percutaneous coronary intervention (PCI) or emergency fibrinolysis.

  • Complications of Infarction: Lethal ventricular arrhythmias (ventricular tachycardia, ventricular fibrillation), acute pulmonary oedema, cardiogenic shock, post-infarction pericarditis (pleuritic chest pain worsened by inspiration and supine positioning), ventricular wall or papillary muscle rupture, and thromboembolism.

• Heart Failure (HF) Pathophysiology:

  • Definition: Inability of the cardiac ventricles to pump sufficient blood to meet peripheral metabolic requirements, or doing so only under abnormally high intracardiac filling pressures.

  • Left-Sided Heart Failure: Left ventricular dysfunction causes retrograde blood accumulation into pulmonary veins and pulmonary capillary beds; features include dyspnoea, orthopnoea, paroxysmal nocturnal dyspnoea, tachypnoea, bibasal inspiratory crackles, and cardiogenic pulmonary oedema.

  • Right-Sided Heart Failure: Right ventricular failure leads to venous pooling in the systemic venous circulation; features include elevated jugular venous pressure (JVP), bilateral peripheral dependent oedema, ascites, hepatomegaly, and congestive liver dysfunction; most commonly precipitated by left heart failure or chronic pulmonary disease.

  • Systolic Heart Failure (HFrEF): Impaired ventricular contraction resulting in a reduced left ventricular ejection fraction ($\text{LVEF} < 40\%$).

  • Diastolic Heart Failure (HFpEF): Impaired ventricular relaxation and structural stiffening resulting in elevated filling pressures despite a preserved ejection fraction ($\text{LVEF} \ge 50\%$).

  • Neurohumoral Vicious Cycle: Diminished cardiac output reduces renal perfusion, triggering excessive compensatory activation of the renin-angiotensin-aldosterone system (RAAS) and sympathetic nervous system; secondary vasoconstriction and fluid retention exacerbate cardiac afterload and preload, driving pathological ventricular remodelling.

• Arrhythmias:

  • Atrial Fibrillation (AF): Disorganised, chaotic atrial electrical depolarisation producing an irregularly irregular ventricular response; elevates stroke risk secondary to left atrial appendage thrombus formation.

  • Ventricular Tachycardia (VT) and Ventricular Fibrillation (VF): Fast, aberrant ventricular depolarisations (VT) or chaotic, non-contractile ventricular twitching (VF); cause pulseless collapse and cardiac arrest requiring immediate cardiopulmonary resuscitation (CPR) and electrical defibrillation.

  • Asystole and Pulseless Electrical Activity (PEA): Non-shockable cardiac arrest states treated with continuous CPR and identification of reversible physiological causes.

Acute and Chronic Respiratory Disorders

• Ventilatory Mechanics and Gas Exchange:

  • Ventilation refers to mechanical airflow into and out of the pulmonary system; gas exchange refers to alveolar-capillary oxygen uptake and carbon dioxide diffusion.

  • Hypoxaemia indicates diminished partial pressure of oxygen in arterial blood; hypoxia indicates inadequate cellular oxygen delivery to peripheral tissues.

  • Obstructive Lung Pathology: Characterised by increased resistance and airflow limitation predominantly during expiration.

  • Restrictive Lung Pathology: Characterised by reduced lung compliance, diminished thoracic expansion, and restricted total lung capacity.

• Asthma:

  • Pathophysiology: Chronic inflammatory disease of the conducting airways driven by immune hyperreactivity to external triggers; characterized by reversible bronchial smooth muscle constriction, submucosal inflammatory oedema, and goblet-cell mucus hypersecretion.

  • Spirometric Hallmarks: Demonstrates disproportionate reduction in forced expiratory volume in $1\,\text{second}$ ($\text{FEV}_1$) relative to forced vital capacity ($\text{FVC}$), yielding a decreased $\text{FEV}_1/\text{FVC}$ ratio; airway reversibility is confirmed when post-bronchodilator spirometry demonstrates an increase in $\text{FEV}_1$ or $\text{FVC}$ exceeding $10\%$ of predicted values.

  • Clinical Manifestations: Recurrent expiratory wheeze, dyspnoea, dry or productive nocturnal cough, and chest tightness; a silent chest denotes severe, life-threatening air trapping with impending respiratory arrest.

• Chronic Obstructive Pulmonary Disease (COPD):

  • Pathophysiology: Progressive, largely irreversible airflow limitation secondary to chronic noxious inhalation (primarily tobacco smoke); combines emphysema and chronic bronchitis.

  • Emphysema: Permanent structural destruction of interalveolar septa and extracellular elastin, eliminating elastic recoil and causing expiratory dynamic airway collapse, progressive air trapping, and alveolar hyperinflation.

  • Chronic Bronchitis: Chronic bronchial inflammation, goblet cell hyperplasia, and excessive mucus production; clinically defined as a chronic productive cough occurring for at least $3\,\text{months}$ per year in each of $2$ consecutive years.

  • Diagnostic Criterion: Confirmed on post-bronchodilator spirometry by a fixed $\text{FEV}_1/\text{FVC}$ ratio below $0.70$.

  • Cor Pulmonale: Chronic alveolar hypoxaemia induces widespread hypoxic pulmonary vasoconstriction; vascular smooth muscle remodelling establishes secondary pulmonary arterial hypertension, forcing the right ventricle to contract against elevated afterload, resulting in right ventricular hypertrophy and eventual right heart failure.

  • Oxygen Therapy Considerations: In chronic hypercapnic respiratory failure, high-flow unregulated oxygen can worsen carbon dioxide retention; titration to a precise target saturation of $88\text{--}92\%$ is maintained.

• Acute Respiratory Distress Syndrome (ARDS):

  • Pathophysiology: Catastrophic systemic inflammation (e.g., sepsis, severe trauma, aspiration) causes widespread damage to the pulmonary capillary endothelium and alveolar epithelium; microvascular hyperpermeability fills alveolar spaces with proteinaceous exudate and cellular debris; surfactant inactivation causes diffuse microatelectasis and profound intrapulmonary shunting, causing non-cardiogenic pulmonary oedema, stiff lungs, and severe hypoxaemia refractory to standard oxygen delivery.

• Infectious and Malignant Pulmonary Conditions:

  • Upper Respiratory Tract Infections: Primarily viral (e.g., rhinovirus); acute pharyngitis secondary to Group A beta-haemolytic Streptococcus carries the complication of acute rheumatic fever.

  • Pneumonia: Microbial proliferation in the pulmonary parenchyma triggering alveolar consolidation; lobar pneumonia exhibits uniform consolidation of a pulmonary lobe; bronchopneumonia exhibits patchy peribronchial inflammatory infiltrates; typical community-acquired bacterial pathogen is Streptococcus pneumoniae; hospital-acquired pneumonia develops $\ge 48\,\text{hours}$ post-admission.

  • Lung Malignancy: Classified into Non-Small-Cell Lung Cancer (NSCLC: including adenocarcinoma, squamous cell carcinoma, and large cell carcinoma) and Small-Cell Lung Cancer (SCLC: aggressive neuroendocrine malignancy exhibiting rapid metastases and strong association with smoking).

Pleural, Infectious, and Vascular Pulmonary Pathologies

• Pleural and Alveolar Fluid Collections:

  • Pulmonary Oedema: Fluid accumulation inside the lung parenchyma and alveoli; frequently cardiogenic secondary to elevated hydrostatic pressure in left ventricular failure, producing hypoxaemia and pink, frothy sputum.

  • Pleural Effusion: Fluid accumulation in the pleural space between the visceral and parietal pleura; compresses adjacent lung tissue, blunts costophrenic angles on imaging, and produces dyspnoea, stony dullness to percussion, and decreased breath sounds.

  • Empyema: Collection of gross purulent exudate (pus) within the pleural cavity, secondary to complicated bacterial pneumonia or intrathoracic contamination; requires drainage and antimicrobials.

• Pneumothorax:

  • Pathophysiology: Entry of atmospheric air into the pleural space, abolishing the physiological negative intrapleural pressure, precipitating pulmonary collapse.

  • Aetiology: Spontaneous pneumothorax arises from rupture of subpleural apical bullae/blebs; traumatic pneumothorax results from blunt or penetrating thoracic trauma or iatrogenic instrumentation.

  • Tension Pneumothorax: A mechanical one-way valve defect allows air to enter the pleural space during inspiration but prevents egress during expiration; progressive intrapleural air accumulation raises intrathoracic pressure, collapsing the ipsilateral lung, displacing the mediastinum and trachea contralaterally, and compressing the vena cava; this obstructs venous return, causing obstructive shock, systemic hypotension, and electromechanical arrest; requires immediate emergency needle decompression prior to diagnostic imaging.

• Atelectasis and Bronchiectasis:

  • Atelectasis: Incomplete expansion or collapse of lung parenchyma; driven by bronchial mucus plugging, compressive pleural collections, or prolonged hypoventilation after surgery; promotes ventilation-perfusion ($\text{V/Q}$) mismatch.

  • Bronchiectasis: Permanent, irreversible dilation and destruction of bronchial walls secondary to recurrent infection and impaired clearance; manifests as a chronic cough with copious foul-smelling sputum, recurrent infections, and haemoptysis.

• Tuberculosis (TB):

  • Pathophysiology: Inhalation of airborne droplet nuclei containing Mycobacterium tuberculosis; bacilli are engulfed by alveolar macrophages; intact cellular immunity forms granulomatous tubercles (caseating granulomas) to contain the pathogen (latent TB infection); failure of immune control permits active multiplication, tissue cavitation, and dissemination.

  • Clinical Manifestations: Fevers, drenching night sweats, chronic productive cough, anorexia, unexplained weight loss, and haemoptysis.

  • Pharmacology: Requires multi-drug regimens over multiple months to prevent bacterial resistance; standard primary therapy comprises rifampicin, isoniazid, pyrazinamide, and ethambutol.

Endocrine Regulations and Disorders

• Endocrine Principles and Receptor Dynamics:

  • Hormone Transport and Binding: Water-soluble hormones (peptides, catecholamines) circulate unbound in plasma and interact with cell-surface membrane receptors, generating intracellular second messengers; lipid-soluble hormones (steroid hormones, thyroid hormones) circulate predominantly bound to plasma carrier proteins and diffuse across cell membranes to bind intracellular receptors, modulating gene transcription.

  • Feedback Regulation: Negative feedback systems suppress initial endocrine release once target physiological levels are achieved; positive feedback systems amplify hormone secretion until a definitive physiological termination point occurs (e.g., oxytocin secretion during parturition).

• Posterior Pituitary / Fluid Balance Disorders:

  • Syndrome of Inappropriate Antidiuretic Hormone (SIADH): Pathological hypersecretion of ADH (vasopressin); induces excessive renal water reabsorption via collecting duct aquaporins; results in systemic hypervolaemia, concentrated urine, and dilutional hyponatraemia; rapid overcorrection of chronic hyponatraemia poses the risk of central pontine myelinolysis / osmotic demyelination syndrome.

  • Diabetes Insipidus (DI): Failure of ADH synthesis/secretion (central DI) or renal tubular resistance to ADH (nephrogenic DI); kidneys cannot conserve free water, resulting in persistent excretion of massive volumes of dilute urine, severe unquenchable thirst (polydipsia), systemic dehydration, and hypernatraemia.

• Adrenocortical Pathologies:

  • Adrenal Cortex Zonation: Zona glomerulosa secretes mineralocorticoids (aldosterone, regulated by RAAS); zona fasciculata secretes glucocorticoids (cortisol, regulated by ACTH); zona reticularis secretes adrenal androgens.

  • Cushing Syndrome: Clinical state of prolonged glucocorticoid excess (exogenous therapy, ACTH-secreting pituitary tumours, or adrenal neoplasia); manifests with central adiposity, moon facies, buffalo hump, skeletal muscle proteolysis, dermal atrophy, purplish cutaneous striae, impaired glucose tolerance, and secondary hypertension.

  • Addison Disease (Primary Adrenal Insufficiency): Autoimmune or destructive failure of the adrenal cortex, causing deficient secretion of cortisol and aldosterone; manifestations include chronic fatigue, anorexia, weight loss, salt craving, hypotension, hyponatraemia, hyperkalaemia, and cutaneous hyperpigmentation from elevated proopiomelanocortin/ACTH levels.

  • Acute Adrenal Crisis: Emergency decompensation precipitated by severe stress, trauma, or abrupt cessation of long-term steroid therapy in an adrenal-insufficient state; manifests with severe hypotension, cardiovascular collapse, vomiting, dehydration, and hypoglycaemia; treated immediately with intravenous hydrocortisone and fluid resuscitation.

  • Hyperaldosteronism: Excessive autonomous aldosterone production (Conn syndrome or adrenal hyperplasia); stimulates excessive renal sodium retention and potassium/hydrogen ion excretion, causing hypertension and hypokalaemia.

• Thyroid and Parathyroid Pathophysiology:

  • Hyperthyroidism: Excessive circulating triiodothyronine ($\text{T}_3$) and thyroxine ($\text{T}_4$) inducing a hypermetabolic state; most commonly driven by Graves disease (autoantibodies stimulate the TSH receptor); features include weight loss, heat intolerance, diaphoresis, tremors, tachycardia, diarrhoea, suppressed TSH, and elevated free $\text{T}_4$; life-threatening thyrotoxic crisis (thyroid storm) causes hyperthermia, extreme tachycardia, agitation, and congestive heart failure.

  • Hypothyroidism: Inadequate thyroid hormone production; most commonly driven by Hashimoto thyroiditis (autoimmune lymphocytic destruction); features include reduced metabolic rate, fatigue, cold intolerance, weight gain, constipation, dry skin, bradycardia, elevated TSH, and decreased free $\text{T}_4$; extreme severe decompensation produces myxoedema coma, characterized by hypothermia, hypoventilation, bradycardia, and lethargy.

  • Parathyroid Hormone (PTH) and Calcium Balance: PTH elevates serum ionized calcium via bone resorption, renal tubular calcium reabsorption, and activation of vitamin D ($1,25\text{-}(\text{OH})_2\text{D}_3$) to increase enteral absorption.

  • Hyperparathyroidism: Unregulated PTH secretion causing persistent hypercalcaemia; associated with nephrolithiasis, osteopenia/pathological fractures, abdominal pain, and neurocognitive disturbances.

  • Hypoparathyroidism: Inadequate PTH secretion (often following neck/thyroid surgery); leads to hypocalcaemia and hyperphosphataemia; induces neuromuscular irritability, perioral/digital paraesthesia, carpopedal spasm, tetany, and electrocardiographic QT interval prolongation.

Diabetes Mellitus: Pathophysiology, Complications, and Therapeutics

• Classification and Pathogenesis:

  • Normal Endocrine Pancreas: Islets of Langerhans contain $\beta$ cells that secrete anabolic insulin (promoting glucose entry into tissues and glycogen synthesis) and $\alpha$ cells that secrete catabolic glucagon (stimulating hepatic glycogenolysis and gluconeogenesis).

  • Type 1 Diabetes Mellitus: Autoimmune-mediated selective destruction of pancreatic $\beta$ cells; results in absolute insulin deficiency; patients require lifelong exogenous insulin replacement to prevent ketoacidosis.

  • Type 2 Diabetes Mellitus: Peripheral tissue resistance to insulin combined with progressive, compensatory $\beta$-cell secretory exhaustion; results in relative insulin deficiency, progressive hyperglycaemia, and microvascular/macrovascular tissue injury.

  • Gestational Diabetes Mellitus: Placental steroid and peptide hormones induce maternal insulin resistance; occurs when maternal $\beta$-cell capacity cannot compensate for the pregnancy-related metabolic demand.

• Diagnostic Criteria for Diabetes:

  • Glycated haemoglobin ($\text{HbA}_{1c}$) $\ge 6.5\%$ ($48\,\text{mmol/mol}$).

  • Fasting plasma glucose $\ge 7.0\,\text{mmol/L}$.

  • Two-hour oral glucose tolerance test (OGTT) plasma glucose $\ge 11.1\,\text{mmol/L}$.

  • Random plasma glucose $\ge 11.1\,\text{mmol/L}$ in the presence of classic hyperglycaemic symptoms.

  • Classical triad: Polyuria (osmotic diuresis from glycosuria exceeding renal threshold), polydipsia (compensatory fluid replacement for dehydration), and polyphagia (inability of insulin-dependent cells to take up glucose).

• Acute Metabolic Complications:

  • Hypoglycaemia: Defined clinically as blood glucose level $< 4.0\,\text{mmol/L}$; driven by mismatched insulin/secretagogue administration, fasting, or exertion; symptoms include autonomic sympathetic activation (sweating, palpitations, tremors) and neuroglycopenia (confusion, drowsiness, seizures, coma); awake patients receive $15\,\text{g}$ of rapid-acting oral carbohydrates; unconscious patients receive parenteral glucagon or intravenous dextrose.

  • Diabetic Ketoacidosis (DKA): Characterised by the triad of hyperglycaemia, ketonaemia/ketonuria, and high-anion-gap metabolic acidosis; profound insulin deficiency stimulates unopposed lipolysis into free fatty acids, converted by the liver into acetoacetic acid and $\beta$-hydroxybutyrate; manifests with nausea, vomiting, abdominal pain, dehydration, fruity acetone breath, and rapid, deep Kussmaul respirations; managed with intravenous fluid resuscitation, continuous intravenous insulin infusion, and electrolyte correction (specifically potassium).

  • Hyperosmolar Hyperglycaemic State (HHS): Characteristic of Type 2 diabetes; relative presence of minimal insulin suppresses lipolysis and ketogenesis, while extreme hyperglycaemia drives massive osmotic diuresis; results in severe dehydration, plasma hyperosmolality, and neurological impairment without marked ketoacidosis.

• Chronic Complications of Sustained Hyperglycaemia:

  • Microvascular Angiopathy: Hyperglycaemia-induced basement membrane thickening and capillary endothelial damage; produces diabetic retinopathy (microaneurysms, neovascularisation, blindness), diabetic nephropathy (glomerular hyperfiltration, microalbuminuria, end-stage kidney failure), and diabetic neuropathy (distal symmetrical polyneuropathy, sensory loss, autonomic instability).

  • Macrovascular Disease: Accelerated large-vessel atherosclerosis; substantially increases risk of coronary artery disease, myocardial infarction, ischaemic stroke, and peripheral arterial disease.

  • Diabetic Foot Disease: Combined neuropathy (loss of protective pain sensation) and peripheral arterial disease (impaired local blood flow) promote unnoticed skin breakdown, infectious ulceration, osteomyelitis, and lower limb amputation.

• Antidiabetic Pharmacology:

  • Insulin Preparations:

  • Rapid-acting analogues: Aspart, lispro, glulisine (rapid onset, short duration; administered for mealtime prandial control).

  • Short-acting insulin: Regular/soluble human insulin (used intravenously in acute ketoacidosis and prandially).

  • Intermediate-acting insulin: Isophane/NPH (provides intermediate basal coverage, displaying an intermediate peak).

  • Long-acting analogues: Glargine, detemir, degludec (provide steady, peakless basal coverage over $24\,\text{hours}$).

  • Non-Insulin Glucose-Lowering Agents:

  • Biguanides (Metformin): Decreases hepatic gluconeogenesis and enhances peripheral insulin sensitivity; does not provoke clinical hypoglycaemia when used as monotherapy.

  • Sulfonylureas (e.g., Gliclazide): Closes ATP-sensitive potassium channels on $\beta$ cells, stimulating endogenous insulin exocytosis; risks include hypoglycaemia and weight gain.

  • SGLT2 Inhibitors (e.g., Empagliflozin): Inhibits sodium-glucose cotransporter 2 in the renal proximal convoluted tubule, promoting urinary glucose elimination; carries risks of mycotic genital infections and euglycaemic DKA.

  • GLP-1 Receptor Agonists (e.g., Semaglutide): Incretin mimetics that increase glucose-dependent insulin secretion, suppress glucagon release, and delay gastric emptying.

  • DPP-4 Inhibitors: Prolong the biological half-life of endogenous incretin hormones by inhibiting dipeptidyl peptidase-4.