OPTOM 103: Systemic Pathophysiology

Sources and References

Primary textbook — Kathryn L. McCance & Sue E. Huether, Pathophysiology: The Biologic Basis for Disease in Adults and Children (Elsevier). Supplementary texts — Vinay Kumar, Abul K. Abbas, Jon C. Aster Robbins Basic Pathology; Carol Mattson Porth Porth’s Pathophysiology; Guyton & Hall Textbook of Medical Physiology; Abul K. Abbas Cellular and Molecular Immunology. Online resources — Merck Manual Professional Version; American Diabetes Association Standards of Care.

1. Homeostasis, Allostasis, and the Stress Response

Pathophysiology studies how normal physiology is deranged by disease, bridging molecular events and clinical syndromes. The central organizing concept is homeostasis: the maintenance of a narrow internal environment — core temperature, pH, osmolality, glucose, oxygen tension — through negative feedback loops that sense deviation and drive compensatory effectors. A thermostat-like set point is defended by receptors, integrators (largely the hypothalamus and brainstem), and effector organs. When a set point is persistently reset to meet a sustained challenge, the more flexible concept of allostasis applies: stability is achieved through change rather than constancy, but prolonged allostatic load drives wear and tear that underlies many chronic diseases.

Hans Selye’s general adaptation syndrome describes the sequence of alarm, resistance, and exhaustion that follows a stressor of any kind. The alarm reaction is dominated by sympathetic nervous system activation and adrenal medullary release of catecholamines, producing tachycardia, bronchodilation, glycogenolysis, and vasoconstriction in splanchnic beds. In parallel, the hypothalamic-pituitary-adrenal (HPA) axis releases corticotropin-releasing hormone, then ACTH, and finally cortisol from the adrenal cortex. Cortisol mobilizes glucose via gluconeogenesis, suppresses inflammation, and restrains immune responses. Short-term, these responses are adaptive; sustained hypercortisolism contributes to insulin resistance, visceral adiposity, muscle wasting, osteoporosis, hypertension, and impaired wound healing.

Disease results when a stressor exceeds the adaptive range or the regulatory machinery itself fails. Etiology names the cause (genetic, infectious, chemical, physical, nutritional, immunological, idiopathic); pathogenesis traces the sequence of molecular and cellular events that produce the clinical phenotype. Morphologic changes and functional derangements then give rise to signs (observable) and symptoms (reported). Risk factors modify probability without being deterministic. A key distinction separates acute processes, in which the insult and response resolve within days to weeks, from chronic processes, in which low-grade injury, incomplete repair, and maladaptive remodeling persist over years. Understanding these definitions frames every subsequent chapter, because every organ-specific disease is ultimately a story of a stressor, a compensatory response, and the point at which that response becomes pathological in its own right.

2. Cell Injury, Adaptation, and Death

Cells respond to persistent stress with reversible adaptations before they die. Hypertrophy increases cell size through protein synthesis, as in left ventricular pressure overload. Hyperplasia increases cell number in labile tissues, exemplified by endometrial proliferation under estrogen. Atrophy shrinks cell size when demand, perfusion, or trophic signals fall; disuse atrophy follows immobilization. Metaplasia substitutes one mature differentiated cell type for another, classically columnar-to-squamous change in chronic bronchitis or squamous-to-columnar Barrett esophagus from reflux. Metaplasia itself is reversible but creates a substrate for dysplasia and, eventually, neoplastic transformation.

When adaptation fails, injury ensues. The common pathways are ATP depletion, mitochondrial dysfunction with release of reactive oxygen species, loss of calcium homeostasis, membrane damage, and protein misfolding. Hypoxia from ischemia is the most frequent cause clinically; free radicals mediate reperfusion injury, radiation, and many toxins. Reversible injury shows cellular swelling and fatty change as the sodium pump falters. If the insult continues, irreversible injury follows, marked by severe mitochondrial damage, extensive membrane rupture, and lysosomal enzyme release.

Cell death occurs by two principal modes. Necrosis is accidental, pathological death of groups of cells with membrane rupture, enzymatic digestion, and a strong inflammatory response. Subtypes include coagulative necrosis (ischemic infarct of most solid organs), liquefactive necrosis (brain, abscesses), caseous necrosis (tuberculous granulomas), fat necrosis (pancreatitis), and fibrinoid necrosis (immune vasculitis). Apoptosis, by contrast, is programmed, energy-dependent cell death in which caspases dismantle the cell into membrane-bound bodies that are phagocytosed without inflammation. It is triggered through an intrinsic (mitochondrial) pathway controlled by the Bcl-2 family or an extrinsic pathway via death receptors such as Fas. Apoptosis prunes excess cells during development, eliminates damaged or infected cells, and maintains epithelial turnover. Other regulated deaths — necroptosis, pyroptosis, ferroptosis, autophagy — blur the old binary. Pathological accumulations such as lipofuscin (wear-and-tear pigment), hemosiderin (iron overload), and amyloid (misfolded protein) reflect chronic sublethal stress. Recognizing which mode of injury and death predominates in a lesion is central to both diagnosis and rational therapy.

3. Inflammation and the Acute Response

Inflammation is the vascular and cellular reaction of vascularized tissue to injury. Its cardinal signs — rubor, tumor, calor, dolor, and functio laesa — were described by Celsus and Virchow and still summarize the physiology. The purpose is to deliver effector cells and molecules to the injured site, wall off the insult, clear debris and pathogens, and set the stage for repair. Acute inflammation develops within minutes to hours and typically resolves within days; chronic inflammation lasts weeks to years and is dominated by mononuclear cells and ongoing tissue remodeling.

The acute vascular phase begins with transient vasoconstriction followed by histamine- and nitric-oxide-driven vasodilation, increased blood flow, and increased vascular permeability. Endothelial contraction and injury allow plasma proteins to leak, producing a protein-rich exudate that dilutes toxins and delivers antibodies, fibrin, and complement. The cellular phase recruits neutrophils that marginate along the vessel wall, roll via selectins, firmly adhere via integrins binding ICAM-1, and then transmigrate through the endothelium along chemokine gradients. Once in the tissue, neutrophils phagocytose opsonized particles and destroy them using reactive oxygen species generated by NADPH oxidase, myeloperoxidase-derived hypochlorite, and lysosomal proteases.

Chemical mediators orchestrate the sequence. Vasoactive amines (histamine, serotonin), arachidonic acid metabolites (prostaglandins, leukotrienes, lipoxins), cytokines (TNF, IL-1, IL-6, chemokines), plasma protein cascades (complement, coagulation, kinin), and neuropeptides each contribute. Prostaglandins produce pain and fever; leukotrienes amplify bronchoconstriction and vascular leak. Pyrogens — predominantly IL-1 and IL-6 — raise the hypothalamic set point via prostaglandin E2, producing fever, which enhances leukocyte function and inhibits microbial replication.

Outcomes depend on injury and host factors. Complete resolution restores normal architecture; scarring follows when parenchymal loss is extensive. Abscess formation walls off pyogenic organisms. Chronic inflammation arises from persistent infection, prolonged toxic exposure, or autoimmunity and features macrophages, lymphocytes, angiogenesis, and fibrosis. Granulomatous inflammation is a specialized chronic pattern of epithelioid macrophages with giant cells that contains agents the immune system cannot easily clear — mycobacteria, fungi, foreign bodies, sarcoidosis. Healing proceeds through hemostasis, inflammation, proliferation (granulation tissue, angiogenesis, fibroblast activity), and remodeling; excessive or deficient repair produces keloids, contractures, or dehiscence.

4. Immunity — Innate and Adaptive

Host defense is organized into innate and adaptive arms that collaborate continuously. The innate system provides rapid, stereotyped, germline-encoded protection. Its physical barriers are skin, mucus, ciliated epithelium, and acidic secretions; its cellular effectors include neutrophils, monocytes/macrophages, dendritic cells, mast cells, natural killer cells, and innate lymphoid cells; and its soluble mediators include the complement cascade, acute-phase proteins, defensins, and interferons. Pattern recognition receptors such as Toll-like receptors and NOD-like receptors detect conserved pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs), triggering cytokine release and the inflammasome. Complement activation through classical, lectin, or alternative pathways opsonizes pathogens with C3b, assembles the C5b-9 membrane attack complex, and liberates anaphylatoxins C3a and C5a.

The adaptive system is slower to deploy but antigen-specific and endowed with memory. B lymphocytes mature in bone marrow and produce antibodies that neutralize toxins, opsonize microbes, activate complement, and recruit effector cells. The five isotypes — IgM, IgG, IgA, IgE, IgD — are tuned for different compartments and pathogens: IgM dominates primary responses, IgG mediates secondary responses and crosses placenta, IgA guards mucosae, and IgE drives allergic and antiparasitic responses. T lymphocytes develop in the thymus and recognize peptide antigens presented on MHC molecules. CD4 helper T cells orchestrate responses through subset-specific cytokine profiles: Th1 drives cellular immunity against intracellular microbes, Th2 mobilizes eosinophils and IgE against helminths and allergens, Th17 recruits neutrophils to extracellular pathogens and fungi, and regulatory T cells enforce tolerance. CD8 cytotoxic T cells kill virus-infected and tumor cells via perforin and granzymes.

Disorders of immunity fall into four Gell-and-Coombs hypersensitivities. Type I is immediate IgE-mediated allergy — anaphylaxis, asthma, urticaria. Type II is antibody-dependent cytotoxicity — autoimmune hemolytic anemia, myasthenia gravis. Type III involves immune complex deposition — serum sickness, lupus nephritis. Type IV is delayed, T-cell-mediated — contact dermatitis, tuberculin reaction, graft rejection. Autoimmunity arises when central or peripheral tolerance breaks down; immunodeficiency may be primary (SCID, common variable immunodeficiency) or secondary (HIV, chemotherapy, malnutrition). Understanding which arm is deranged dictates whether the clinical problem is infection, allergy, autoimmunity, or cancer immune escape.

5. Blood — Cells, Anemia, and Coagulation

All blood cells arise from pluripotent hematopoietic stem cells in the bone marrow under the control of cytokines and growth factors including erythropoietin (from renal peritubular cells), thrombopoietin (liver), and various colony-stimulating factors. Erythrocytes are biconcave anucleate cells packed with hemoglobin; they circulate for roughly 120 days before splenic macrophages clear senescent cells. Leukocytes include granulocytes (neutrophils, eosinophils, basophils), monocytes, and lymphocytes. Platelets are cytoplasmic fragments of megakaryocytes that survive about 10 days.

Anemia is a reduction in red cell mass or hemoglobin concentration. Clinical features — pallor, fatigue, dyspnea, tachycardia — reflect impaired oxygen delivery. Classification by mean corpuscular volume is practical. Microcytic anemias include iron deficiency (the most common globally, from blood loss or malabsorption), thalassemias (inherited reductions in globin chain synthesis), anemia of chronic disease (iron sequestered by hepcidin), and sideroblastic anemias. Macrocytic anemias include megaloblastic disease from B12 or folate deficiency, in which impaired DNA synthesis produces hypersegmented neutrophils and ineffective erythropoiesis, and non-megaloblastic causes such as alcoholism and liver disease. Normocytic anemias include acute blood loss, early anemia of chronic disease, hemolysis, and marrow failure. Hemolytic anemias are subdivided into intrinsic defects (sickle cell disease from a single β-chain substitution, hereditary spherocytosis, G6PD deficiency) and extrinsic causes (autoimmune hemolysis, mechanical destruction, infections).

White cell disorders include leukopenias (most importantly neutropenia predisposing to infection) and leukocytoses (reactive in infection, neoplastic in leukemia). The leukemias are malignant clonal proliferations classified by lineage (myeloid vs lymphoid) and tempo (acute vs chronic). Lymphomas are solid tumors of lymphoid tissue divided into Hodgkin and non-Hodgkin categories.

Hemostasis proceeds in three phases. Primary hemostasis forms a platelet plug: vessel injury exposes collagen, von Willebrand factor bridges platelets to the subendothelium via GpIb, platelets activate and release ADP and thromboxane A2, and GpIIb/IIIa mediates aggregation. Secondary hemostasis amplifies thrombin generation through the coagulation cascade, converging on fibrin formation; the extrinsic (tissue factor, VII), intrinsic (XII, XI, IX, VIII), and common (X, V, II, I) limbs are monitored by PT/INR and aPTT. Fibrinolysis by plasmin remodels the clot. Bleeding disorders include thrombocytopenia, von Willebrand disease, hemophilias A and B, and vitamin K deficiency. Thrombotic disorders (factor V Leiden, antiphospholipid syndrome) and disseminated intravascular coagulation occupy the opposite pole, where simultaneous clotting and consumption produce bleeding.

6. The Cardiovascular System I — Physiology and Heart Failure

The heart is a dual pump whose output equals heart rate times stroke volume. Stroke volume depends on three variables: preload (end-diastolic volume, governed by venous return and the Frank-Starling relation), afterload (the resistance the ventricle must overcome, largely aortic pressure), and contractility (the intrinsic force generation at any given preload, modulated by sympathetic tone and calcium handling). The cardiac cycle alternates systole and diastole, with pressure-volume loops that graphically display stroke work. Blood pressure equals cardiac output times systemic vascular resistance; it is defended by baroreceptor reflexes, the renin-angiotensin-aldosterone system (RAAS), and antidiuretic hormone. Hypertension — primary in most cases, secondary in a minority from renovascular, endocrine, or renal parenchymal causes — drives atherosclerosis, ventricular hypertrophy, stroke, and kidney disease.

Heart failure is the inability of the heart to deliver output sufficient for metabolic demand at normal filling pressures. It is classified by ejection fraction into heart failure with reduced ejection fraction (HFrEF, typically from myocardial infarction, dilated cardiomyopathy, or chronic volume overload) and heart failure with preserved ejection fraction (HFpEF, a disorder of diastolic filling often associated with hypertension, aging, obesity, and diabetes). Left-sided failure produces pulmonary congestion with dyspnea, orthopnea, paroxysmal nocturnal dyspnea, and crackles; right-sided failure produces systemic venous congestion with jugular distension, hepatomegaly, ascites, and peripheral edema. High-output failure (thyrotoxicosis, anemia, beriberi, AV fistula) is uncommon.

Compensatory mechanisms activated by falling cardiac output initially maintain perfusion but ultimately become maladaptive. Sympathetic activation raises heart rate and contractility at the cost of increased oxygen demand. RAAS retains sodium and water, raising preload but worsening congestion. Ventricular remodeling — eccentric dilation in volume overload, concentric hypertrophy in pressure overload — alters geometry and efficiency and promotes arrhythmia. Neurohormonal overdrive (angiotensin II, aldosterone, endothelin, vasopressin) directly injures myocardium, which is why modern therapy blocks each of these pathways with ACE inhibitors or ARBs, aldosterone antagonists, β-blockers, and newer agents (ARNIs, SGLT2 inhibitors). Natriuretic peptides (BNP) oppose RAAS and serve as diagnostic biomarkers. Cardiomyopathies — dilated, hypertrophic, restrictive — describe structural substrates. Valvular lesions (stenosis, regurgitation) and congenital shunts likewise converge on the pressure-volume language of cardiac mechanics.

7. The Cardiovascular System II — Ischemia and Arrhythmias

Ischemic heart disease results from an imbalance between myocardial oxygen supply and demand, usually due to atherosclerotic coronary artery disease. Atherogenesis begins with endothelial dysfunction provoked by hypertension, smoking, LDL cholesterol, and diabetes. LDL infiltrates the intima, is oxidized, and is taken up by macrophages via scavenger receptors to form foam cells. A fatty streak enlarges into a fibrofatty plaque with a necrotic lipid core, a fibrous cap, and a surrounding inflammatory infiltrate. Stable plaques progressively narrow the lumen; vulnerable plaques with thin caps and abundant macrophages rupture, exposing thrombogenic material and precipitating acute thrombosis.

Clinical syndromes form a spectrum. Stable angina is transient chest pain on exertion relieved by rest, reflecting fixed stenosis. Unstable angina is new, worsening, or rest pain from non-occlusive plaque disruption without myocardial necrosis. Non-ST elevation myocardial infarction (NSTEMI) involves subendocardial necrosis with elevated troponin but without ST elevation on ECG. ST-elevation myocardial infarction (STEMI) reflects complete occlusion of an epicardial coronary artery with transmural necrosis; early reperfusion (PCI, thrombolysis) limits infarct size. Sequelae include contractile dysfunction, papillary muscle rupture, ventricular septal rupture, aneurysm, mural thrombus, and arrhythmia. Chronic ischemic cardiomyopathy is a leading cause of heart failure.

Arrhythmias arise when impulse formation (automaticity), conduction, or both are deranged. The normal pacemaker is the sinoatrial node; impulses traverse the atria, the AV node (where conduction slows), the bundle of His, and the Purkinje system. Bradyarrhythmias include sinus node dysfunction and AV block (first-, second-, third-degree). Tachyarrhythmias are divided into supraventricular (atrial fibrillation, atrial flutter, AVNRT, AVRT) and ventricular (VT, VF). Atrial fibrillation — the most common sustained arrhythmia — is characterized by disorganized atrial activity, irregular ventricular response, loss of atrial kick, and a high risk of thromboembolic stroke that warrants anticoagulation based on CHA2DS2-VASc scoring. Ventricular fibrillation is the terminal rhythm of sudden cardiac death and requires immediate defibrillation. Long-QT syndromes, Brugada syndrome, and catecholaminergic polymorphic VT are inherited channelopathies predisposing to malignant ventricular arrhythmias. Management uses rate control, rhythm control with antiarrhythmics or ablation, and device therapy (pacemakers, ICDs).

8. The Respiratory System

The lungs perform gas exchange, acid-base regulation, and host defense. Ventilation moves air between atmosphere and alveoli; perfusion delivers mixed venous blood through the pulmonary capillaries; diffusion moves oxygen and carbon dioxide across the alveolar-capillary membrane down partial pressure gradients. Efficient gas exchange requires matching of ventilation and perfusion (V/Q). V/Q mismatch is the commonest cause of hypoxemia: high V/Q areas waste ventilation (dead space), low V/Q areas waste perfusion, and complete shunt (V/Q = 0) does not correct with supplemental oxygen. Other mechanisms of hypoxemia are hypoventilation, diffusion impairment, and low inspired PO2 at altitude.

Pulmonary function testing divides disease into obstructive and restrictive patterns. Obstructive disease is characterized by a reduced FEV1/FVC ratio due to expiratory airflow limitation and includes asthma, chronic obstructive pulmonary disease (COPD), bronchiectasis, and cystic fibrosis. Asthma is a chronic inflammatory disorder with reversible bronchoconstriction, airway hyperresponsiveness, mucus plugging, and remodeling, often driven by Th2 cytokines and IgE. COPD encompasses chronic bronchitis (mucus hypersecretion and small-airway inflammation) and emphysema (permanent enlargement of distal airspaces with alveolar wall destruction), driven chiefly by cigarette smoke and, in a minority, α1-antitrypsin deficiency. Loss of elastic recoil and small airway collapse produce air trapping, hyperinflation, and dyspnea.

Restrictive disease is characterized by reduced lung volumes with preserved FEV1/FVC, from either parenchymal disease (idiopathic pulmonary fibrosis, sarcoidosis, pneumoconioses) or extrapulmonary causes (kyphoscoliosis, neuromuscular weakness, obesity, pleural disease). Interstitial fibrosis impairs diffusion and reduces compliance. Acute respiratory distress syndrome (ARDS) is a noncardiogenic pulmonary edema produced by diffuse alveolar damage following sepsis, trauma, aspiration, or pneumonia, with refractory hypoxemia and bilateral infiltrates.

Pulmonary vascular disease includes pulmonary embolism — most often from deep venous thrombosis — which produces acute dyspnea, hypoxemia, and, if large, right heart strain. Pulmonary hypertension arises from left heart disease, lung disease, chronic thromboembolism, or primary vasculopathy, and culminates in right ventricular failure (cor pulmonale). Respiratory failure is defined by arterial blood gases: type I (hypoxemic, PaO2 < 60) reflects gas-exchange failure; type II (hypercapnic, PaCO2 > 45) reflects pump failure of the ventilatory apparatus.

9. The Endocrine System and Diabetes

The endocrine system uses hormones released into blood to regulate metabolism, growth, reproduction, and stress responses over seconds to months. Hormones act through cell surface receptors (peptides, catecholamines) or intracellular receptors (steroids, thyroid hormone). Most axes use negative feedback: the hypothalamus releases releasing hormones that drive the anterior pituitary, which releases trophic hormones that act on end organs, whose products suppress hypothalamic and pituitary output. The posterior pituitary stores hypothalamic ADH and oxytocin.

Pituitary disorders include hyperfunction from adenomas (prolactinoma causing galactorrhea and hypogonadism; acromegaly from growth hormone excess; Cushing disease from ACTH excess) and hypofunction from tumors, infarction (Sheehan syndrome), or infiltration. Diabetes insipidus reflects ADH deficiency (central) or renal resistance (nephrogenic), producing dilute polyuria; SIADH produces hyponatremia and concentrated urine. Thyroid disease encompasses hyperthyroidism (Graves disease with TSH receptor antibodies, toxic nodules) producing weight loss, tremor, palpitations, heat intolerance, and ophthalmopathy, and hypothyroidism (Hashimoto thyroiditis, iodine deficiency, iatrogenic) producing fatigue, cold intolerance, weight gain, bradycardia, and myxedema. TSH is the single most sensitive screening test.

Adrenal disorders include Cushing syndrome (cortisol excess from exogenous steroids, pituitary or adrenal tumors, ectopic ACTH) with central obesity, striae, hyperglycemia, hypertension, muscle wasting, and osteoporosis; Addison disease (primary adrenal insufficiency, usually autoimmune) with fatigue, hypotension, hyperpigmentation, and electrolyte derangement; and primary hyperaldosteronism (Conn syndrome) with hypertension and hypokalemia. Pheochromocytomas of the adrenal medulla produce episodic catecholamine surges.

Diabetes mellitus is a chronic hyperglycemic disorder driven by defects in insulin secretion, action, or both. Type 1 (5–10% of cases) results from autoimmune destruction of pancreatic β-cells with absolute insulin deficiency, often presenting in youth with ketoacidosis. Type 2 (the large majority) features peripheral insulin resistance and progressive β-cell failure in the setting of obesity, inactivity, and genetic susceptibility. Gestational diabetes develops in pregnancy. Diagnosis (per ADA) uses fasting glucose ≥ 126 mg/dL, 2-hour OGTT ≥ 200, HbA1c ≥ 6.5%, or random glucose ≥ 200 with symptoms. Acute complications include diabetic ketoacidosis (type 1, insulin deficiency with ketogenesis) and hyperosmolar hyperglycemic state (type 2, profound dehydration without ketosis). Chronic complications reflect microvascular disease (retinopathy, nephropathy, neuropathy) and macrovascular disease (coronary, cerebrovascular, peripheral artery). Tight glycemic control, blood pressure control, lipid management, and lifestyle modification all reduce complication rates.

10. The Gastrointestinal System

The gut processes food through mechanical breakdown, enzymatic digestion, selective absorption, and expulsion of waste. Salivary amylase and lingual lipase begin digestion; gastric acid and pepsin continue protein breakdown and sterilize ingested material; pancreatic enzymes and bile emulsify and hydrolyze nutrients; the small intestine’s villous epithelium absorbs nutrients; the colon recovers water and electrolytes and houses the microbiome. Motility is coordinated by enteric neurons, interstitial cells of Cajal, and extrinsic autonomic innervation.

Esophageal disorders include reflux disease (GERD) from lower esophageal sphincter incompetence, producing heartburn, erosive esophagitis, and, over years, metaplastic Barrett esophagus and adenocarcinoma risk. Achalasia features failure of LES relaxation. Gastric disease centers on the balance between mucosal protection (mucus, bicarbonate, prostaglandins, blood flow) and aggressive factors (acid, pepsin, Helicobacter pylori, NSAIDs, alcohol). Gastritis and peptic ulcer disease (duodenal and gastric ulcers) result when this balance fails; complications include bleeding, perforation, and gastric cancer. Zollinger-Ellison syndrome reflects gastrin excess from a neuroendocrine tumor.

Small bowel disorders include malabsorption syndromes (celiac disease — an autoimmune gluten-triggered villous atrophy; tropical sprue; Whipple disease; pancreatic insufficiency; bile salt deficiency), obstruction from adhesions or hernia, and ischemia. Inflammatory bowel disease comprises Crohn disease (transmural, skip lesions, any segment of the GI tract, granulomas, fistulas) and ulcerative colitis (mucosal, continuous, colon only, with risk of toxic megacolon and colorectal cancer). Irritable bowel syndrome is a functional disorder of brain-gut interaction without structural pathology. Infectious diarrheas are categorized as secretory (cholera, enterotoxigenic E. coli) or inflammatory (shigellosis, Campylobacter, C. difficile). Appendicitis, diverticulitis, and colorectal cancer complete the colonic picture.

The liver performs synthesis (albumin, clotting factors, complement), detoxification (drugs, ammonia to urea), bile production, and metabolic regulation. Hepatocellular injury elevates ALT and AST; cholestasis elevates alkaline phosphatase and bilirubin. Viral hepatitis (A, B, C, D, E), alcoholic liver disease, and nonalcoholic fatty liver disease are the main causes of chronic liver injury. Cirrhosis — end-stage scarring with regenerative nodules — produces portal hypertension (varices, ascites, splenomegaly, caput medusae), hepatic encephalopathy (hyperammonemia), coagulopathy, hypoalbuminemia, and increased risk of hepatocellular carcinoma. Gallstones, cholecystitis, and pancreatitis (gallstones and alcohol are the usual triggers) round out the hepatobiliary and pancreatic territory.

11. The Renal System

Each kidney contains roughly a million nephrons, each consisting of a glomerulus and a tubule. The glomerulus filters plasma across a three-layered barrier (fenestrated endothelium, basement membrane, podocyte slit diaphragms) that restricts cells and large proteins. Glomerular filtration rate (GFR) depends on hydrostatic and oncotic pressures and is tightly autoregulated through myogenic and tubuloglomerular feedback mechanisms. The proximal tubule reabsorbs the bulk of filtered sodium, glucose, amino acids, bicarbonate, and water; the loop of Henle establishes the medullary osmotic gradient; the distal convoluted tubule fine-tunes sodium, calcium, and magnesium; and the collecting duct performs final water handling under ADH control and potassium/hydrogen secretion under aldosterone.

Beyond filtration and reabsorption, the kidneys regulate acid-base balance by reabsorbing filtered bicarbonate and generating new bicarbonate through ammonium and titratable acid excretion. Simple acid-base analysis distinguishes metabolic acidosis (high or normal anion gap), metabolic alkalosis, respiratory acidosis, and respiratory alkalosis, with compensation by the opposite system. The kidneys also produce erythropoietin, activate vitamin D (1-α-hydroxylation), and secrete renin that triggers the RAAS cascade for blood pressure control.

Glomerular disease presents as nephritic or nephrotic syndromes. Nephritic disease (post-infectious glomerulonephritis, IgA nephropathy, rapidly progressive GN with crescents, lupus nephritis) features hematuria, red cell casts, mild proteinuria, hypertension, and azotemia. Nephrotic syndrome (minimal change disease, focal segmental glomerulosclerosis, membranous nephropathy, diabetic nephropathy) features heavy proteinuria (> 3.5 g/day), hypoalbuminemia, edema, and hyperlipidemia. Tubulointerstitial disease includes acute tubular necrosis from ischemia or toxins, interstitial nephritis from drugs and infection, and pyelonephritis from ascending urinary tract infection.

Acute kidney injury is defined by an abrupt rise in serum creatinine or fall in urine output and is categorized as prerenal (volume depletion, heart failure — a response to underperfusion with preserved tubular function), intrinsic (ATN, glomerulonephritis, interstitial nephritis), or postrenal (obstruction). Chronic kidney disease is classified by GFR and albuminuria; diabetes and hypertension are the dominant causes worldwide. As nephron mass falls, the remnant kidney retains solute, water, phosphate, and hydrogen ions and fails to produce erythropoietin and activated vitamin D, producing uremia, anemia, metabolic acidosis, hyperkalemia, hyperphosphatemia, hypocalcemia, secondary hyperparathyroidism, and renal osteodystrophy. End-stage renal disease requires dialysis or transplantation.

12. Neoplasia — Cancer Cell Biology and Metastasis

Neoplasia is autonomous, excessive, and coordinated proliferation of cells that persists after the inciting stimulus is withdrawn. Tumors are classified as benign (localized, encapsulated, well-differentiated, slow-growing, non-metastatic) or malignant (invasive, poorly differentiated, rapidly growing, metastatic). Nomenclature uses the suffix -oma for benign lesions, -carcinoma for epithelial malignancies, -sarcoma for mesenchymal malignancies, and special terms for leukemias, lymphomas, teratomas, and blastomas. Grading describes histologic differentiation; staging describes anatomic extent, most often with the TNM system (tumor, node, metastasis).

Carcinogenesis is a multistep process in which successive genetic and epigenetic alterations confer the “hallmarks of cancer” defined by Hanahan and Weinberg: sustained proliferative signaling, evasion of growth suppressors, resistance to apoptosis, replicative immortality through telomerase reactivation, induction of angiogenesis, activation of invasion and metastasis, reprogrammed metabolism (the Warburg effect), and evasion of immune destruction. Four classes of genes are disturbed. Proto-oncogenes encode growth-promoting proteins that become oncogenes when mutated, amplified, or translocated (examples: RAS, MYC, HER2, BCR-ABL). Tumor suppressor genes normally restrain proliferation and must typically be inactivated on both alleles (“two-hit” hypothesis of Knudson) — TP53, RB, APC, BRCA1/2. DNA repair genes (MSH2, MLH1) maintain genomic stability, and their loss produces microsatellite instability and hypermutation. Apoptosis regulators (BCL-2, BAX) tip the balance between cell death and survival.

Carcinogens include chemicals (tobacco, aflatoxin, asbestos, benzene), radiation (ultraviolet and ionizing), and oncogenic viruses (HPV in cervical cancer, HBV/HCV in hepatocellular carcinoma, EBV in lymphomas and nasopharyngeal carcinoma, HTLV-1, Kaposi sarcoma herpesvirus). Chronic inflammation is itself a powerful promoter of malignancy (colitis and colon cancer, H. pylori and gastric adenocarcinoma and MALT lymphoma, viral hepatitis and liver cancer).

Invasion and metastasis — the chief causes of cancer mortality — proceed in steps: loss of E-cadherin and other cell-cell adhesions, breakdown of basement membrane by matrix metalloproteinases, migration through stroma, intravasation into vessels, survival in the circulation, arrest in a capillary bed, extravasation, and colonization of the new site. Lymphatic spread tends to involve carcinomas, hematogenous spread to involve sarcomas, though overlap is extensive. Preferred metastatic sites reflect anatomy and the “seed and soil” compatibility of tumor and host tissue.

Clinical effects of cancer include local mass effects, cachexia driven by TNF and other cytokines, paraneoplastic syndromes (endocrine: SIADH from small cell lung cancer, hypercalcemia from PTHrP in squamous carcinomas; hematologic; neurologic), pain, and immunosuppression. Diagnosis integrates imaging, biopsy with histology, immunohistochemistry, and molecular profiling; serum tumor markers support monitoring. Therapy combines surgery, radiation, cytotoxic chemotherapy, targeted agents directed against specific oncogenic drivers, hormonal therapy, and immunotherapy (checkpoint inhibitors, CAR-T cells). Early detection through screening (cervical cytology and HPV testing, mammography, colonoscopy, low-dose chest CT in selected smokers) remains among the most effective tools for reducing cancer mortality, because the earlier a tumor is detected the better the chance that curative local therapy can precede the metastatic cascade that ultimately defines malignancy.