Basic Pathophysiology Concepts

Effect of Hypoxia

• In anaerobic conditions, the glucose derived from glycogen can only produce limited quantities of energy through anaerobic glycolysis.

• As a result, glycogen is quickly depleted and the intracellular pH is reduced due to the increased glycolysis resulting in accumulating lactic acid.

• The low pH may produce denaturation of intracellular proteins.

• This denaturation leads to inactivation of enzymes within the cell, including those in energy generation and protein synthesis.

• It also leads to damage of the structural proteins, possibly disturbing the cellular cytoskeleton, cellular and organelle membranes and nuclear chromatin.

• The reduction in protein synthesis leads to the detachment of ribosomes from the endoplasmic reticulum.

• If hypoxia persists, the weakening of the cellular membrane and its increased permeability, cause ‘blebs’ to form on the cell surface as well as the mitochondria, endoplasmic reticulum and whole cells appear swollen.

• If oxygen supply is restored, all disturbances are reversible, but if ischaemia persists, irreversible injury follows.

Under anaerobic conditions, glycogen-derived glucose produces limited energy, leading to rapid glycogen depletion and a drop in intracellular pH due to lactic acid accumulation. This low pH can denature proteins, inactivate essential enzymes, and damage structural components of the cell, resulting in reduced protein synthesis and ribosome detachment. Persistent hypoxia causes increased membrane permeability and cellular swelling from bleb formation. Restoration of oxygen can reverse these effects, but ongoing ischemia can lead to irreversible injury.

Irreversible Injury

• Irreversible injury leads to cellular death.

• Necrosis is a term that refers specifically to cell death in an organ or tissue thats still a part of a living person.

• It is associated with several events:

  • The mitochondria is permanently shut down due to the extensive damage to the mitochondrial membrane and the influx of cytoplasmic calcium that forms intracellular calcifications as the calcium binds phosphates forming insoluble precipitates.

  • The cell membrane is substantially damaged with the cell losing proteins, amino acids, essential enzymes, ADP and ribonucleic acids.

  • Leakage of the hydrolytic enzymes into the cytoplasm and the degradation of cytoplasmic and nuclear components (autodigestion) results from damage to the lysosomal membranes.

• After cell death cellular constituents are progressively digested by lysosomal hydrolases, but widespread leakage of cellular enzymes into the extracellular space can damage still vital neighbouring cells.

• Neutrophil leukocytes and/or macrophages collect the residual cellular debris and undergo phagocytosis, digesting the material during the acute inflammation response.

• The leakage of intracellular proteins into the peripheral circulation provides a means of detecting tissue-specific cellular injury and death in blood samples.

  • For example liver cells contains ALT (alanine aminotransferase) and AST (aspartate aminotransferase) enzymes and the increased levels of these in the circulation reflects liver cell necrosis.

  • The presence of mildly elevated enzymes isnt confirmation of irreversible cell injury as reversible can produce the same phenomenon.

• A particular form of cellular death is apoptosis, also termed programmed cell death.

  • Apoptosis is responsible for several normal physiologic processes such as the killing of foreign or abnormal cells by cytotoxic T cells or cell death in proliferating cell population.

  • It is the mechanism of death for aged cells and cells damaged beyond repair.

  • It is carried out through activation of the intracellular proteolytic enzymes caspases (cysteine-aspartic-acid-proteases) that destroy cellular DNA.

  • The activation of caspases can be extrinsic through activation of membrane-bound receptors or intrinsic through the release of proteins from the damage mitochondria.

  • During apoptosis cell membrane blebs form and the cell becomes fragmented into many apoptotic bodies surrounded by sections of cell membrane that are phagocytosed by resident macrophages.

  • It does not initiate inflammation & does not involve the neutrophils from blood.

• There are distinctive morphologic appearances in necrotic tissue, and this results in four types.

• Coagulative necrosis is characterised by the preservation of the basic structural outline of necrotic cells and tissue for several days.

  • The dead area is likely relatively soft and pale.

  • Leukocytes remove the necrotic cells through fragmentation and leukocytes, which is followed by healing.

  • The coagulative necrosis process is characteristic of hypoxic death in all tissues but the brain.

• Liquefactive necrosis is characteristic of focal bacterial infections, as they provide powerful stimuli for leukocyte accumulation.

  • They release hydrolytic enzymes into the area of inflammation and liquefaction completely digests the dead cells.

  • For example in an abscess where the tissue is destroyed and replaced by pus.

  • Hypoxic death of brain tissue results in liquefactive necrosis as theres no supportive stromal framework in nervous tissue, so it liquefies easily.

• Gangrenous necrosis is not a distinctive pattern of cell death, but the term is used in surgical practice.

  • It refers to ischaemic coagulative necrosis of a limb or part of it, and usually refers to lower limbs.

• Caseous necrosis is a distinctive form of necrosis typically found in tissues affected by tuberculosis infection.

  • At the microscopic level, caseous necrosis is found in the centre of tuberculous granulomas.

  • The necrosis can involve larger areas of affected tissue, completely destroying the tissue architecture.

• For example in pulmonary tuberculosis, removal of the necrotic material via coughing, leaves characteristic cavities behind.

Irreversible injury results in necrosis, the death of cells in living tissues, characterised by significant mitochondrial damage, membrane destruction, and enzyme leakage that can harm neighbouring cells. Cell death is marked by the accumulation of hydrolytic enzymes, which leads to auto-digestion, and the release of cellular components into the bloodstream that can signal specific tissue damage, such as elevated ALT and AST levels in liver injury. In contrast, apoptosis, or programmed cell death, occurs without inflammation and involves the systematic breakdown of cells via caspases. Necrosis presents in various forms: coagulative, where cell outlines are preserved; liquefactive, common in bacterial infections; gangrenous, associated with ischemic limb death; and caseous, typically linked to tuberculosis, where affected tissue architecture is destroyed, leaving cavities post-excretion.

Inflammation

• Inflammation is a protective response to local injury intended to eliminate the initial cause of cell injury and the necrotic cells and tissues that result from it.

• Most infections, by causing tissue injury, trigger inflammation, but inflammation follows tissue injury caused by any noxious stimulus.

• The inflammatory response is tightly linked to tissue repair processes and although its usually helpful it has considerable potential for harm.

• It is divided into two basic forms, acute and chronic.

• Acute inflammation is of a short duration, lasting from a few minutes to a few days, while chronic inflammation is of a longer duration lasting from weeks to years.

Inflammation is a protective response to tissue injury, initiated to remove the cause of cell damage and necrotic tissue. It is triggered by infections and various harmful stimuli, and while generally beneficial, it can also be harmful. Inflammation is classified into two forms: acute inflammation, lasting minutes to days, and chronic inflammation, which can persist for weeks to years.

Acute

• It is the immediate, early response to tissue injury, delivering leukocytes to the injury site, to help clear invading bacteria and degrade the necrotic tissue from the injury.

• Leukocytes release enzymes, chemical mediators and reactive oxygen species while fighting invading agents that can prolong inflammation and damage tissue.

• It has three major components:

  • The vasodilation of arterioles via mediators like prostaglandin E2, histamine, serotonin and nitric oxide to increase local blood flow, which generates redness and warmth.

  • An increase in blood vessel permeability permits plasma proteins and fluid to leave the circulation.

    • This is caused by a combination of endothelial cell contraction via mediators, loosening of intercellular junctions, direct endothelial injury and endothelial injury by leukocyte-derived enzymes & ROS.

  • Leukocytes emigrate from the microcirculation and accumulate in the injury focus, subsequently undergoing phagocytosis of injurious agents and necrotic material.

• In the earliest stage, the vasodilation and increased blood flow raise intravascular hydrostatic pressure, increasing the filtration of fluid from the capillaries.

  • This forms the fluid transudate that is an ultrafiltrate of plasma containing little protein.

• Transudation is followed by the increasing vascular permeability, that permits the flow of protein rich fluid called exudate that eventually contains inflammatory cells.

• Accumulation of excess fluid in the interstitial space called oedema is caused by transudation and exudation.

• The process of stasis occurs in which vascular leakage causes the blood to become more concentrated increasing blood viscosity and slowing circulation.

• Soon leukocytes begin to leave the blood vessels & move to the site of invasion or injury, through a process primarily occurring in the venules.

• This occurs through a 4 step process.

• Margination & Rolling - The vasodilation and stasis causes the leukocytes to settle in the vessel periphery rather then the central column. The leukocytes tumble on the endothelial surface, transiently sticking.

• Adhesion & Emigration - The leukocytes firmly adhere to the exposed adhesion molecules on the endothelial cells. They then crawl between the cells and through the basement membrane, into the extravascular space.

• Chemotaxis & Activation - The leukocytes follow a chemical gradient of various inflammatory mediators, moving toward the site of injury. The leukocytes extend pseudopods that anchor them to the extracellular matrix and pull the remainder of their cell body in order to move.

  • Both exogenous and endogenous substances can act as chemotactic agents for leukocytes, including soluble bacterial products, products of activated leukocytes & macrophages and activated complement components.

• Phagocytosis - It is the major benefit of the accumulation of leukocytes and consists of three steps.

  • Recognition & attachment of the particle to the leukocyte is facilitated by coating it in the serum proteins, opsonins that bind to receptors on the leukocyte.

  • Binding of the opsonised particles triggers engulfment. The pseudopods are extended around the particle, forming a phagocytic vacuole. The vacuole membrane fuses to the lysosome membrane, resulting in lysosomal contents being discharged into the phagolysosome.

  • Killing in the case of living microorganisms and the degradation of ingested material occurs. For bacterial and other microbial organsims, reactive oxygen species generated by a respiratory burst and hydrolytic enzymes are responsible.

Acute inflammation is the body's immediate and early response to tissue injury, which aims to eliminate invading bacteria and clear necrotic tissue through the recruitment of leukocytes. This process involves three major components: vasodilation of arterioles, which increases local blood flow and causes redness and warmth; enhanced blood vessel permeability that allows plasma proteins and fluids to exit the circulation, leading to the accumulation of excess fluid or edema; and the migration of leukocytes from the bloodstream to the injury site, where they perform phagocytosis to engulf and degrade harmful agents. Initially, the increase in vascular permeability leads to the formation of transudate and exudate, and as leukocytes arrive through a sequence of margination, adhesion, and chemotaxis, they activate and extend pseudopods to facilitate the engulfment and destruction of pathogens.

Chronic Inflammation

• It is characterised by:

  • Infiltration with mononuclear cells, including macrophages, lymphocytes and plasma cells, which are activated B lymphocytes.

  • Tissue destruction, largely induced by the inflammatory cells attempting to remove the ‘enemy’.

  • Tissue repair involving new blood vessel formation and fibrosis.

• It can be considered as inflammation of prolonged duration, in which active inflammation, tissue injury and healing proceed simultaneously.

• It may follow acute inflammation if the acute response cannot be successfully resolved, for some reason.

• It generally arises in four settings:

  • Persistent infection, most characteristically by selected microorganisms including mycobacteria (cause of tuberculosis), Treponema pallidum (cause of syphilis) and certain fungi. These organisms replicate slowly & slowly induce tissue damage, evoking chronic lingering inflammation.

  • Prolonged exposure to potentially toxic agents such as non-degradable exogenous material can induce a chronic inflammatory response in the lungs in the form of silicosis, berylliosis and coal workers pneumoconiosis. End result of these may be fibrosis of the lungs with reduce lung compliance.

  • Autoimmune diseases, where an individual develops an immune response to self-antigens and tissues for unknown reasons. The antigens causing the response are structural components of normal tissue, inflammation is chronic and persistent with substantial tissue damage.

  • Allergic conditions, where the immune system responds to environmental antigens, called allergens, triggering a ongoing inflammatory process that persists as long as there is exposure to the allergen.

• Some chronic inflammations develop for no clear reason, & such conditions are only labelled as chronic inflammatory.

  • Granulomatous inflammation is a distinctive pattern of chronic inflammation dominated by macrophages & encountered in relatively few pathologic states such as tuberculosis or retained foreign bodies.

  • This type of inflammation is characterised by the presence of granulomas which are compact micro-modules composed of layers of lymphocytes, histiocytes , epithelioid cells, giant cells and fibroblasts.

  • Granulomatous inflammation is activated when the invading agent cannot be efficiently destroyed and removed but instead it is sealed off and blocked so further invasion and tissue destruction cant occur.

  • However, normal tissue can be damaged or destroyed and in some cases more harm is done by the reaction to the agent then by the agent itself.

Chronic inflammation is characterised by the infiltration of mononuclear cells such as macrophages, lymphocytes, and plasma cells, leading to tissue destruction as these cells attempt to eliminate the cause of damage. This prolonged inflammatory response can occur simultaneously with tissue repair, often following unresolved acute inflammation. Common triggers include persistent infections by slow-replicating microorganisms, exposure to non-degradable toxic agents resulting in conditions like silicosis, autoimmune diseases where the immune system targets self-antigens, and allergic reactions to environmental allergens. Granulomatous inflammation, a specific type of chronic inflammation, features granulomas formed when the body seals off agents that cannot be effectively removed, potentially causing more damage than the agents themselves.

Clinical Significance of Inflammation

• The five classic local signs of acute inflammation are heat, redness, swelling, pain and loss of function of the affected body part.

• Systemic effects of inflammation are fever, loss of appetite, headache, muscle aches, joint aches and malaise.

• TNF-𝜶 and IL-1 work via local prostaglandins produced in the hypothalamus to increase temperature set point and antipyretic drugs reduce fever by blocking prostaglandin synthesis.

• The acute phase proteins released from the liver is reaction to inflammation are:

  • C reactive protein, participates in opsonisation & phagocytosis.

  • Fibrinogen, blood clotting protein which also binds to erythrocytes making them stickier.

  • Hepcidin, reduces iron available to pathogens by decreasing intestinal iron absorption and preventing the release of iron from macrophages.

  • Ferritin, storage form iron increases in blood to limit the amount of free iron available to pathogens.

• Markers are inflammation blood tests used clinically to assess a patient for presence/absence of an active inflammatory disease process & to monitor the activity of a known inflammatory disease.

• The main inflammatory markers that suggest inflammation are increase erythrocyte sedimentation rate, elevated C reactive protein and leukocytosis.

• Leukocytosis is especially common in infections & some clue about the cause can be obtained by identifying which type of leukocyte is responsible for the increase.

• Most bacterial infections induce a relatively selective increase in neutrophilic granulocytes, while parasitic infections as well as allergic reactions induce increased eosinophils count.

• Many viruses cause selective increase in lymphocyte count, often at the same time reducing the count of circulating neutrophils leading to leukopenia.

Acute inflammation is marked by five classic signs: heat, redness, swelling, pain, and loss of function. Systemic effects include fever, loss of appetite, headache, and muscle and joint aches. TNF-𝜶 and IL-1 increase temperature set point through prostaglandins, while antipyretic drugs lower fever by inhibiting prostaglandin synthesis. Acute phase proteins like C-reactive protein (aids opsonisation and phagocytosis), fibrinogen (enhances erythrocyte stickiness), hepcidin (limits iron for pathogens), and ferritin (restricts free iron) are released from the liver. Inflammatory markers, including elevated erythrocyte sedimentation rate, C-reactive protein, and leukocytosis, help assess inflammation. Leukocytosis, common in infections, can indicate the infection type based on leukocyte changes: neutrophils in bacterial infections, eosinophils in allergic or parasitic reactions, and lymphocytosis in viral infections, sometimes reducing neutrophil counts, leading to leukopenia.

Tissue Repair

• This is the process by which tissues replace their dead cells after damage.

• It is tightly linked with inflammation & relies on successful completion of the inflammatory process.

• It can occur in two different forms but a combination of them is found in most instances.

  • Regeneration of injured tissue by parenchymal cells of the same type

  • Replacement by connective tissue, resulting in a permanent scar.
    Tissue repair is the process through which dead cells are replaced after injury, closely associated with inflammation.

Tissue repair replaces dead cells following damage and is closely linked to inflammation, depending on its successful completion. It generally occurs in two forms: regeneration of the same type of parenchymal cells and replacement with connective tissue, leading to permanent scarring.

Regeneration

• The cells of the body are divided into three groups on the basis of mitotic capacity:

  • Continuously dividing (labile) cells proceed from one mitosis to the next & proliferate throughout life, replacing cells that are continuously dying in the process of cell renewal. These include the stratified squamous surfaces of the skin, oral cavity, oesophagus, vagina and cervix. The columnar epithelium of the GI tract, uterus and fallopian tubes & the transitional epithelium of the bladder. The haematopoietic cells of the bone marrow continuously replenish the circulating peripheral blood cells.

  • Quiescent (stable) cells most of the time are in resting state but after cell loss they can be driven into mitosis. This includes the hepatocytes, the cells of the kidney and pancreas. Also the endothelial cells, fibroblasts and smooth muscle cells.

  • Non-dividing cells cannot undergo mitotic division at all. This includes nerve cells and cardiac muscle cells which can only regenerate a portion of a lost cell. Skeletal muscle is generally placed in this category but has limited regenerative capacity due to the presence of satellite cells (stem cells).

• Regeneration means the replacement of lost cells with identical new ones through the mitotic division of remaining cells.

• This way the tissue structure and function are restored to pre-injury state.

• This can only occur in tissues composed of labile and stable cells, & even in these tissues, they stromal connective tissue framework must be largely preserved for regeneration to take place.

• Regulation of regeneration is extremely complex and involves a variety of polypeptide growth factors and other mechanisms that operate inside cells or between neighbouring cells.

• Growth factors influence cell growth and division through regulation of gene transcription eventually leading to protein and DNA synthesis.

  • Examples of growth factors with effects of various cells are: Epidermal growth factor (EGF), Platelet derived growth factor (PDGF), basic fibroblast growth factor (b-FGF), transforming growth factor-beta (TGF-𝜷), vascular endothelial growth factor (VEGF) & tumour necrosis factor-alpha (TNF-𝜶).

The cells in the body are categorised by mitotic capacity into three groups: continuously dividing (labile) cells, which proliferate throughout life such as skin and blood cells; quiescent (stable) cells that mostly remain inactive but can divide when necessary like liver and kidney cells; and non-dividing cells that cannot mitotically divide, which include nerve and cardiac muscle cells. Regeneration involves replacing lost cells with identical ones, restoring tissue function, but requires preservation of the connective tissue framework. This process is regulated by complex growth factors, such as Epidermal growth factor (EGF) and Platelet-derived growth factor (PDGF), that influence cell growth and division through gene transcription, leading to protein and DNA synthesis.

Repair by connective tissue

• Severe or persistent tissue injury and inflammation with damage to both parenchymal cells & the stromal framework means repair can’t be accomplished by parenchymal regeneration alone.

• Large amounts of exudate and a lack of renewable cell population favour fibrosis.

• Under these conditions, destroyed parenchymal cells begin being replaced by so-called granulation tissue.

• Structurally granulation tissue contains proliferating fibroblasts & new thin-walled, delicate capillaries, in a loose extracellular matrix.

• This repair has three main components:

  • Angiogenesis is the formation of new blood vessels that develop by budding from pre-existing vessels.

    • These vessels are leaky from incomplete inter-endothelial junctions that allows the emigration of inflammatory cells & filtration of fluid to deliver nutrients & oxygen needed for inflammation and repair.

  • Fibrosis is the formation of connective tissue on the granulation tissue framework that develops early at the repair site.

    • The recruitment & stimulation of fibroblasts is driven by the various growth factors.

    • In time newly formed blood vessels disappear with the departure of inflammatory cells & reduced metabolic activity at the injury site.

    • The granulation tissue evolves into a scar composed of largely inactive, fibroblasts, dense collagen & elastic fibres and other ECM components.

  • Scar remodelling is a slow process that leads to some rearrangement & reduction in size of the scar.

    • Initially due to the absence of traction in the area of inflammation and repair, collagen fibres are laid chaotically in all directions.

    • As more traction & pressure is applied, the scar will undergo some remodelling to become firmer, tougher & more resilient.

    • This is achieved by the action of enzymes proteinases like collagenases or gelatinases & others that ca degrade collagen & ECM components, so that new collagen fibres may be produced & arranged in a more efficient fashion.

• Mature scars are composed of inactive fibroblasts, dense collagen, ECM & very few blood vessels.

• A scar provides sufficient stability that the injured tissue is usually able to function, even with reduced capacity.

Repair of severe or persistent tissue injury, which damages both parenchymal and stromal components, cannot be achieved solely through regeneration. In such scenarios, granulation tissue—comprising proliferating fibroblasts and delicate capillaries—replaces the damaged cells. This repair involves three main processes: angiogenesis, the formation of new, leaky blood vessels that facilitate nutrient delivery; fibrosis, where connective tissue forms on the granulation framework; and scar remodeling, which gradually organizes collagen fibers for improved strength and stability. Mature scars consist mainly of inactive fibroblasts and dense collagen, providing sufficient stability for the injured tissue to function despite its reduced capacity.

Abnormalities of Repair

• In wound healing many factors may reduce the quality & adequacy of the reparative process.

• Infection is the most important cause of delay in healing, by prolonging the inflammation phase & potentially increasing the tissue injury.

• Nutrition has profound effects on wound healing. Adequate intake of proteins & certain vitamins is very important.

  • For example protein deficiency & particularly vitamin C deficiency inhibit collagen synthesis & retard healing.

  • Vitamin A is essential for the replacement of epithelial cells, while vitamin E appears to have a role in healing & prevention of excessive scar formation.

• Glucocorticoids have well-known anti-inflammatory effects & they also suppress synthesis of collagen & scar formation. Their administration may result in delayed healing & poor wound strength owing to diminished fibrosis.

• Mechanical factors such as increased local pressure or stretching, may cause wounds to pull apart, forcing the healing process to follow the healing pattern dominated by scarring.

• Insufficient blood flow due to either peripheral blood vessel disease or insufficiency also impairs healing by decreasing the delivery of oxygen & essential nutrients.

In wound healing, various factors can negatively impact the quality and effectiveness of the repair process. Infection is the primary cause of healing delays, as it prolongs inflammation and exacerbates tissue damage. Nutrition plays a critical role; adequate protein and certain vitamins are essential for healing. For instance, deficiencies in protein and vitamin C can hinder collagen synthesis, while vitamin A is crucial for epithelial cell replacement, and vitamin E helps prevent excessive scarring. The use of glucocorticoids, which have anti-inflammatory properties, can suppress collagen synthesis and impair healing, resulting in weaker scars. Additionally, mechanical factors such as local pressure and stretching can disrupt healing, and poor blood circulation due to vascular issues can further delay recovery by limiting oxygen and nutrient supply.