CIRCULATORY DISTURBANCES

Transcription

1 CIRCULATORY DISTURBANCES Shannon Martinson, January All lecture notes and slide shows are available online: Office: 418N REFERENCE TEXTS: Pathologic Basis of Veterinary Disease, Zachary, McGavin (ed): 5 th edition (2012), Chapter 2. Robbins & Cotran, Pathologic Basis of Disease, Kumar, et al (ed); 9 th edition (2015), Chapter 4. "The health of cells and organs critically depends on an unbroken circulation to deliver oxygen and nutrients and to remove wastes. The well-being of tissues also requires normal fluid balance; abnormalities in vascular permeability or hemostasis (stoppage of blood flow) can result in injury." (Pathologic Basis of Disease, 2005) NORMAL CIRCULATORY SYSTEM [for background information only] Important Concepts Distribution of fluid is a carefully controlled homeostatic mechanism. Deviations from normal may have profound pathological effects. Normal functions require intact blood and lymph vessels. Endothelial cells play an important role in fluid distribution, hemostasis, inflammation and healing. Components of the Circulatory System Heart pump. Arteries distribution system. Microcirculation system nutrient / waste exchange between blood and extravascular tissue. Veins & lymphatics collection system. Endothelial cells All components of the circulatory system are lined by a single layer of endothelial cells (endothelium). These cells synthesize & secrete substances which effect fluid balance, hemostasis, inflammation / immunity, angiogenesis / healing. Microcirculation Called microcirculation because it is microscopic. There are 3 main components 1) Arterioles Walls contain innervated smooth muscle cells (myocytes); contract to control blood flow. 2) Postcapillary venules Similar structure to capillary, but acquire thin layer of muscle as they move away from capillary bed. 3) Capillaries Enormous volume (1300 X cross-sectional area of aorta), but normally contain only ~5% of the blood; approximately 95% of capillary beds are not open during normal conditions. Site where nutrients and wastes are exchanged and a critical area in fluid balance. Mechanisms for substance transport across the capillary wall: The capillary wall is a semipermeable membrane; influences movement of fluid (water and solutes), nutrients and waste between blood and interstitial space. 1) Direct diffusion Most small molecules move by passive diffusion through the endothelial cell membrane (eg gas, lipid soluble molecules) or via interendothelial pores (eg water, ions, glucose, amino acids, waste). Normal interendothelial pores too small to allow escape of large proteins (eg albumin / globulins). o During inflammation, endothelial cells contract allowing larger molecules to escape.

2 2) Transcytosis With some endothelial cells, fluids / macromolecules can be transported across the cell by vesicles. Regional Differences in Capillary Lining: 1) Continuous capillaries Lined by a complete simple endothelium and a basal lamina. Found in muscle, brain, thymus, skin, bone, lung and other tissues. 2) Fenestrated capillaries Have many small openings; found in tissues with abundant fluid transport. Often have a diaphragm (egs intestinal villi, kidney interstitium, choroid plexus, ciliary process of eye). Without a diaphragm, can act as a filter (eg renal glomerulus). 3) Discontinuous capillaries Larger gaps than fenestrated with discontinuous basal lamina; allows large molecules or even cells to exit (eg red blood cells in the spleen). Hepatic and splenic sinusoids are lined by this type of endothelium. Fluid Distribution & Homeostasis Total Body Water (~65% of lean body weight) Extracellular fluid (25%) o Plasma (5%) o Interstitial tissue fluid (15%) o Transcellular fluid (5%) Intracellular fluid (40%) Interstitium Interstitium = space between microcirculation and the cells. Functions: o It binds most cellular and structural elements into discrete organs and tissues. o Acts as a medium through which all metabolic products must pass between microcirculation and cells. Structure: o Interstitium is composed of Extracellular Matrix (ECM) + supporting cells (eg fibroblasts) ECM provides structural support and has adhesive & absorptive properties: ➀ Structural molecules: collagen, reticulin & elastin fibers. ➁ Adhesive glycoproteins: fibronectin, laminin. ➂ Absorptive (hydroscopic) molecules: glycosaminoglycans, proteoglycans. Movement of Fluids Distribution of fluids, nutrients & wastes between blood interstitium cells is controlled by physical structures, pressures (hydrostatic), and ion concentration (osmotic) gradients. In most areas the capillary allows the free passage of water and ions and opposes the passage of plasma proteins. Water distribution between plasma and interstitium is primarily determined by the differences in hydrostatic and osmotic pressure between the two compartments.

3 Starling's equation Hydrostatic pressure in the vascular system (aided slightly by interstitial colloidal osmotic/oncotic pressure) is the force that moves fluid out of the vessels. Plasma oncotic pressure (= osmotic pressure exerted by plasma proteins - especially albumin), and to a lesser extent, tissue hydrostatic pressure around blood vessels are the forces that contain the fluid within the vascular system. ARTERIOLAR VENULAR Plasma Hydrostatic Psi 30 mm Hg 17 mm Hg Tissue Hydrostatic Psi 8 mm Hg 8 mm Hg Plasma Colloidal Osmotic Psi 25 mm Hg 25 mm Hg Tissue Colloidal Osmotic Psi 10 mm Hg 10 mm Hg (30-8)-(25-10) = 7 mm Hg (17-8)-(25-10) = 6 mm Hg Net filtration Pressure Net absorption Pressure EDEMA Edema = The abnormal (excess) accumulation of fluid in interstitial tissue spaces or in body cavities. Edema fluid is outside both the vascular fluid and cellular fluid compartments (ie within the interstitium). Gross: o Wet (gelatinous) and heavy, organs are swollen, fluid weeps from cut surface. o In several species (eg horses and some breeds of cattle) fluids are slightly yellow. Histology: o Can be lightly staining eosinophilic fluid (if protein is present) or clear/colorless (if low protein). o Tissue spaces are distended by the edema (collagen bundles separated by increased clear space) and lymphatics are dilated. Four Mechanisms of Edema Production: 1. Increased intravascular hydrostatic pressure Results from an impediment to venous blood flow o Can be generalized (eg heart failure) o Can be localized (eg tightly bandaged limb resulting in venous occlusion). 2. Decreased plasma colloidal osmotic (oncotic) pressure Results from hypoproteinemia o Proteins are not absorbed from the diet (egs starvation, GI malabsorption). o Proteins are not produced (eg liver disease). o Proteins are lost from the body (egs glomerular disease, intestinal damage). 3. Decreased lymphatic drainage / Lymphatic obstruction Results from damage or obstruction of lymphatics (egs surgery / trauma, neoplasm, or inflammation [lymphangitis]). Mechanisms 1, 2 and 3 result in "non-inflammatory edema" - protein poor edema fluid referred to as transudate. Transudate: o Protein content < 30 g/l o Specific gravity < o Total nucleated cell count < 1.5X10 9 /L

4 4. Increased vascular permeability / Endothelial damage Mostly due to the initial reaction of the microvasculature to inflammatory / immunologic stimuli release of inflammatory mediators vasodilation and increased vascular permeability inflammatory edema o This mechanism will be further discussed in inflammation lectures. Note: The endothelium can also be damaged by non-inflammatory agents (eg viruses, toxins) resulting in increased vascular permeability. Mechanism 4 results in inflammatory edema" - protein rich edema fluid referred to as exudate. Exudate o Protein content > 30 g/l o Specific gravity > o Total nucleated cell count > 7.0X10 9 /L Localized versus Generalized Edema: 1) Local Edema a) Mechanisms ➀ Local impaired venous drainage. ➁ Local lymphatic obstruction. ➂ Local inflammation. 2) Generalized Edema a) Mechanisms ➀ Increased generalized hydrostatic pressure of blood (heart failure). ➁ Decreased colloid osmotic pressure of plasma proteins (protein-losing enteropathy/nephropathy). b) Common Locations of Generalized Edema With edema is generalized, we often see a combination of ascites, hydrothorax & subcutaneous ( dependent ) edema. o Dependent edema: Subcutaneous tissues of the ventrum ("brisket edema") Subcutaneous tissues of the ventral mandibular / cervical region ("bottle jaw") Terminology of Edema (non-inflammatory edema): Pitting Edema When pressure is applied to an area of subcutaneous edema a depression or dent results as excessive interstitial fluid is forced to adjacent areas. Anasarca Severe and generalized edema with profound subcutaneous tissue swelling (this term is often reserved for edema in fetuses). Hydrothorax Non-inflammatory fluid (transudate) in the thoracic cavity. Hydropericardium Non-inflammatory fluid (transudate) in the sac around the heart. Ascites (= Hydroperitoneum) Non-inflammatory fluid (transudate) in the peritoneal cavity.

5 Clinical Significance of Edema The clinical significance of edema is dependent upon: ➀ Extent - mild vs moderate vs marked/severe ➁ Location - site of accumulation: eg skin (relatively insignificant) vs lung or brain (often lethal). ➂ Duration - tissues may become more firm and distorted due to increased fibrous connective tissue after prolonged edema. Pulmonary Edema Definition = accumulation of edema fluid in the interstitium and alveoli of the lungs. This is a common cause of death in many disease processes. a) Mechanisms of pulmonary edema ➀ Circulatory failure (especially due to left-sided heart failure) Due to increased hydrostatic pressure of blood in the pulmonary veins (congestion) transudation of (noninflammatory) edema fluid into alveolar spaces. It is the most common cause of pulmonary edema. ➁ Damage to pulmonary capillary endothelium (microvascular injury) Usually occurs with peracute inflammation (ie inflammatory edema) or less commonly toxins. o Due to a sudden increase in vascular permeability. Often results in sudden death (when peracute) or can be followed by pneumonia if the animal survives. b) Dynamics of pulmonary edema Fluid accumulates in the interstitium Fluid moves through basement membranes into alveoli Fluid drains via lymphatics results in dilated interlobular and pleural lymphatics and may eventually lead to pleural fibrosis (when chronic). c) Gross appearance of pulmonary edema The lungs are heavy and wet. Froth (edema fluid + air bubbles) may be present within the trachea and bronchi and is obvious on cut section. The interlobular septa are prominent / thickened due to the increased fluid within this space. Often accompanied by congestion of the pulmonary vessels (especially when there is left-sided heart failure). d) Histopathology of pulmonary edema See edema fluid in the interstitium and alveolar spaces; eventually see dilated interlobular / pleural lymphatics. The edema fluid can be clear or pink in color (depending on the protein content: protein = pink fluid). e) Chronic Pulmonary Edema Most commonly seen with chronic cardiac failure and accompanying pulmonary congestion (discussed later). Over a long period, pleura and alveolar walls may become thickened with fibrous connective tissue. Edema of the Brain (Cerebral Edema) Can be caused by: trauma (head injury), obstruction of venous outflow, intracranial infections (eg meningitis, encephalitis). a) Gross appearance of cerebral edema Brain is heavier than normal. Gyri are swollen and become flattened and the sulci are narrow (due to swollen brain constricted by the bony cranium). When severe can see: ➀ Cerebellar coning - herniation of the cerebellum through the foramen magnum. ➁ Cerebral herniation - herniation of the caudal cerebral cortex beneath the tentorium cerebella.

6 b) Histopathology of cerebral edema Expansion of perivascular (Virchow-Robin) spaces. DEHYDRATION Definition = Deficiency of water; resulting from an imbalance between uptake and loss of water from the body. Causes include: uncontrolled diarrhea, vomiting, renal failure, diabetes, heat-stroke, water deprivation. a) Mechanisms A decrease in the total body water results in water deficit, which is shared among plasma, intracellular and interstitial fluid compartments. Tissue perfusion is reduced. Severe dehydration may result in hypovolemic shock as plasma water is drawn into the interstitium. b) Gross appearance of dehydration Folds of skin pulled out from the body hesitate before returning to their normal position ("skin tenting"). Eyes are sunken, mucous membranes and subcutaneous tissues are dry and sticky ALTERATIONS IN BLOOD FLOW AND PERFUSION HYPEREMIA Definition= Active engorgement of vascular beds due to increased arteriolar inflow. Gross appearance: o The affected tissue is red and warm, as arterioles and capillaries are filled with oxygenated blood. Types of Hyperemia 1. Physiologic Hyperemia examples: blood flow to the stomach and intestines during digestion blood flow in the muscles during exercise blood flow in skin to dissipate heat neurovascular hyperemia (blushing) 2. Pathologic Hyperemia Results from an underlying pathologic process (usually inflammation). Arteriolar dilation is a response to inflammatory stimuli / mediators. Red coloration is a cardinal sign of inflammation = "Hyperemia of Inflammation" Often accompanied by edema. The effect of hyperemia: Hastens movement of metabolites into an area Flushes catabolites from the area CONGESTION Definition = Passive engorgement of a vascular bed generally caused by a decreased outflow of blood. Gross appearance: o Tissues are dark red to blue/black (cyanotic), depending on degree of stagnation (deoxygenated blood). o Cut surfaces ooze blood and tissues are often wet (due to accompanying edema from increased hydrostatic pressure).

7 Histology: o Acute: Capillaries engorged with blood and usually some edema. o Chronic: Engorgement by poorly oxygenated venous blood chronic local hypoxia atrophy, degeneration or even necrosis of parenchymal cells. The effect of congestion: o Leads to hypoxia and accumulation of catabolites o Often edema occurs (due to increased hydrostatic pressure) o Interference with normal tissue function o May get thrombosis of congested veins o +/- Proliferation of connective tissue (if chronic) Two factors are considered in defining the type of congestion: 1) Duration: Acute: Implies abrupt onset with rapid development. Chronic: Slowly developing or present for a long time. 2) Extent: Localized congestion: Confined to a discrete area (eg isolated venous obstruction). Generalized congestion: Indicates a systemic change (eg cardiac failure). Examples of Congestion: Localized Congestion Local obstruction to venous drainage (eg intestinal torsion / intussusception / strangulation). Blood backs up into the microvascular bed passive venous engorgement of the drainage area. Acute Generalized Congestion Most commonly seen with acute heart failure or following euthanasia with barbiturates. Congestion associated with pathology of the heart or lung tends to be generalized and can be acute or chronic: With left-sided heart failure Congestion of lungs (= pulmonary congestion). With right-sided heart failure Systemic congestion (especially the liver) and generalized edema (eg ascites, dependent edema). With certain types of primary pulmonary disease progressive loss of pulmonary vascular bed pulmonary hypertension right heart failure secondary to pulmonary disease = Cor Pulmonale. PULMONARY CONGESTION Most commonly caused by left heart failure. Left ventricular failure impedes forward flow of blood from the lungs congestion of the lungs alveolar capillaries become engorged with blood ( pressure in alveolar capillaries). Gross appearance of acute congestion: Diffuse red lungs (congestion) which are wet (edema) and heavy (from extra blood + edema). Histology of acute congestion: The capillaries are filled with RBCs, +/- proteinaceous fluid in the alveoli (edema) Gross appearance of chronic congestion: Lungs can be lightly tan in color (due to presence of "heart failure cells"- see below). Histology of chronic congestion: The capillaries are filled with RBCs and there is proteinaceous fluid (edema), RBCs and hemosiderin laden macrophages (heart failure cells) in the alveolar spaces

8 Consequences of Chronic Pulmonary Congestion: 1. Intra-alveolar hemorrhage o Small capillaries rupture focal hemorrhage into the alveolar spaces RBCs are phagocytized by alveolar macrophages the iron from the heme of RBCs is stored as hemosiderin pigment within macrophages = "heart failure cells". 2. Pulmonary Edema (see previous discussion on edema) o Causes interference with gaseous exchange. 3. Fibrosis of Interstitium o Fibroblasts secrete excess collagen in response to increased pressure in alveolar capillaries and chronic edema in alveolar interstitium. 4. Pulmonary Hypertension Increased pressure in the alveolar capillaries increased pressure in pulmonary arteries pulmonary hypertension. Can result in right heart failure (cor pulmonale) HEPATIC CONGESTION Most commonly due to right heart failure (eg pulmonic valve disease, RV myocardial damage, cor pulomale) a) Gross appearance of hepatic congestion Liver is usually enlarged and dark red-brown with rounded edges. o Acutely there is an overall increase in liver size due to increased volume of added blood. o Chronically, there is low-grade hypoxia & blood pressure atrophy & death of centrilobular hepatocytes & fibrosis (with a possible decrease in size of the liver). On cut surface has a reticular appearance (= regular dark red and tan-brown zonal pattern) = "nutmeg liver". o Dark red areas correspond to the congested zones around the central veins (zone 3) and the tan-brown areas correspond to the less affected parenchyma around the portal / midzonal areas (zone 1 & 2). b) Histology Acute congestion o Central veins and sinusoids in zone 3 are distended with erythrocytes. o Centrilobular hepatocyte (zone 3) atrophy, degeneration and/or necrosis due to hypoxia in stagnant blood. o Midzonal hepatocyte (zone 2) fatty change due to partial hypoxia. o Periportal hepatocytes (zone 1) mostly normal. Chronic congestion o As above with additional changes in the centrilobular region (zone 3) Hemosiderin-filled macrophages (Kupffer cells) due to erythrocyte phagocytosis. Dilation of sinusoids atrophy/ loss of centrilobular hepatocytes. Low-grade hypoxia and increased pressure in zone 3 fibrous connective tissue deposition ("Cardiac Cirrhosis"). HYPEREMIA CONGESTION vs HEMORRHAGE Hyperemia / congestion blood is inside a blood vessel (ie: intravascular) Hemorrhage blood is outside the vessel wall (ie: extravascular)

9 HEMORRHAGE Definition: Escape of blood from the cardiovascular system (extravasation). Can be discharge of blood from the vascular compartment to the exterior of the body or enclosed within a tissue or body cavity. Causes of Hemorrhage 1. Trauma Causes subcutaneous, body cavity, intramuscular or tissue hemorrhage. 2. Septicemia, viremia or toxic conditions widespread petechiae and ecchymoses. 3. Abdominal neoplasia rupture of masses can cause hemoperitoneum. 4. Coagulation Disorders often causes large hemorrhages. 5. Thrombocytopenia (decreased numbers of platelets) often causes small mucosal hemorrhages. 6. Severe congestion can cause capillary bleeding. Outcome / Significance of Hemorrhage Depends on: 1) Location: There are two critical sites: a. Central nervous system (CNS): Subdural (or epidural) hematomas Blood accumulation beneath (or above) the dura; can compress brain b. Heart: Cardiac Tamponade due to hemopericardium Acute right heart failure due to massive blood accumulation within the pericardial sac causing restriction of diastolic cardiac filling compressive effect. 2) Rate and Volume of Blood Loss a. High rates and volumes are worse Can cause anemia and inadequate oxygenation of tissues With rapid severe blood loss (more than 1/3 of the volume lost over minutes-hours), can lead to hemorrhagic shock (hypovolemic shock). TERMINOLOGY OF HEMORRHAGE Cardiac Tamponade Acute right heart failure due to massive blood accumulation within the pericardial sac causing restriction of diastolic cardiac filling Hemorrhage by Rhexis Hemorrhage due to a substantial tear (rip / rent) in a blood vessel or heart. o Occurs with trauma, necrosis of the vessel wall, vascular invasion by neoplasia, etc o Moderate / marked flow of blood out of vascular system. Tends to result in massive or submassive hemorrhage involving all or most of the affected organ or body cavity or hematomas Hemorrhage by Diapedesis Hemorrhage due to small defect o RBCs passing through the vessel wall in inflammation or with congestion. Can also occur with hypoxia, toxic injury, and with coagulation abnormalities. o Hemorrhage is usually mild. o Tends to result in petechiae, purpura, ecchymoses, and paint brush hemorrhages (see definitions below) Hemorrhagic Diathesis Increased tendency to hemorrhage from usually insignificant injuries. Seen in a wide variety of clinical disorders (eg coagulation deficiency / platelet disorders).

10 Hematoma Accumulation of blood in tissue (3 dimensional extravascular clot); may be small or very large. Hemopericardium Blood in the pericardial sac. Hemothorax Blood in the pleural cavity. Hemoperitoneum Blood in the peritoneal cavity. Hemarthrosis Blood in a joint space. Hemoptysis Coughing up of blood from the lungs or airways. Epistaxis Bleeding from the nose. Hematemesis Vomiting up blood. Hematochezia Presence of (fresh) blood in the stool. Melena Presence of tarry (digested) blood in the stool. Petechia (pl. petechiae) Small, up to 1-2 mm, hemorrhages; most often occur on skin, mucosal / serosal surfaces. Often caused by platelet disorders Purpura Hemorrhages measuring 3 mm to 1 cm; most often occur on skin, mucosal / serosal surfaces. Often seen with diseases that cause petechiae (platelet disorders) but also with vasculitis / blood vessel damage. Ecchymosis (pl. ecchymoses) Hemorrhages larger than petechiae & purpura (>1 cm). Often blotchy or irregular, as seen in bruises (contusions). Occur with vasculitis / moderate blood vessel damage. Paint Brush Hemorrhages Hemorrhages which look as though red paint was hastily applied with a paint brush; most commonly found on serosal or mucosal surfaces. Suffusive Hemorrhage Affected areas of hemorrhage are larger than ecchymosis and are contiguous.

11 Agonal Hemorrhages Small hemorrhages (petechiae and ecchymoses) associated with the death struggle (ie terminal hypoxia). RESOLUTION OF HEMORRHAGE Arrest of hemorrhage occurs as a result of hemostasis (covered in the next lecture) Resolution depends on amount of hemorrhage: Resorption Small amount of hemorrhage can be resorbed. Organization Larger amounts of hemorrhage require phagocytosis and degradation by macrophages. Pigments from degraded hemoglobin form sequentially: Hemoglobin Bilirubin Hemosiderin (red-blue) (blue-green) (yellow-brown) Organizing hematoma: Central mass of fibrin and RBCs surrounded by vascularized connective tissue (supplies nutrients and support) Macrophages phagocytize and degrade the fibrin and RBCs (see pigment from degraded hemoglobin) HEMOSTASIS Refers to the arrest of bleeding. Normally is a well-regulated process which maintains blood in a fluid, clot-free state within a normal vessel. Rapid clot formation (hemostatic plug) will occur at the site of vessel injury. Thrombosis can be considered an inappropriate activation of the normal hemostatic processes. Three general components are required for hemostasis and thrombosis: o Endothelial cells (vascular wall) o Platelets o Coagulation Cascade Normal Hemostasis = Sequence of events following vascular injury: 1. Arteriolar vasoconstriction Transient effect mediated by the ENDOTHELIUM Due to reflex neurogenic mechanism and local secretion of endothelin. 2. Primary hemostasis Mediated by PLATELETS Damage to the endothelium exposes the subendothelial ECM (extracellular matrix). Causes platelets to react: I. Adhere to the ECM (adhesion) and change shape (from round discs to flat plates) II. Secretion of granules III. Recruit other platelets to the site and aggregate forming a primary hemostatic plug Primary hemostatic plug: o Covers and seals the small area of vascular damage o If injury is minimal, the platelet plug may be adequate o If injury is severe, secondary hemostasis occurs 3. Secondary hemostasis Mediated by the COAGULATION CASCADE Tissue factor (TF)

12 o A membrane-bound procoagulant factor is exposed at the site of injury (damaged endothelium) which initiates the coagulation cascade (culminating in thrombin formation). Platelets o Exposed phospholipid complexes on platelets provide sites for coagulation reactions; helps localize the coagulation process. Thrombin activation o Thrombin converts fibrinogen (soluble) to fibrin monomers. Fibrin polymerization / stabilization o Fibrin monomers polymerize into an insoluble gel called fibrin which cements & anchors the primary platelet aggregate forming the 2 o hemostatic plug. 4. Antithrombotic Counter-Regulation Release of components to limit the size of hemostatic plug. ROLE OF ENDOTHELIAL CELLS IN HEMOSTASIS Injury to the endothelium is the major initiating event for thrombosis and coagulation. The endothelium modulates many aspects of normal hemostasis. o Provides a surface that promotes the smooth, laminar (non-turbulent) flow of blood ( antithrombotic property). o When required it can also enhance vasodilation and inhibit platelet adhesion, aggregation and coagulation ( antithrombotic property). o When necessary it produces and responds to substances to form a thrombus or blood clot ( prothrombotic property). Antithrombotic Properties of Endothelial Cells 1. Antiplatelet Acts as a barrier - prevent platelets and plasma factors from being exposed to subendothelial ECM. Prostacyclin & nitric oxide (NO) - inhibit platelet adhesion / aggregation and maintains vascular relaxation. Adenosine diphosphatase (ADPase) - degrades ADP (ADP promotes platelet aggregation). 2. Anticoagulant properties Heparin-like molecules o Are membrane binding sites for antithrombin III inactivates thrombin + clotting factors (Xa). Thrombomodulin o Thrombomodulin binds to thrombin converting it to an anticoagulant which can activate Protein C. Active Protein C (with Protein S) cleaves / inhibits clotting factors (Va and VIIIa). Tissue factor pathway inhibitor (TFPI) o Synthesized and expressed on endothelial cell membrane complexes and inactivates TF and clotting factors (VIIa and Xa). 3. Fibrinolytic properties Tissue plasminogen activator (tpa) o Synthesize tpa activates plasmin (fibrinolytic cascade) removes fibrin from endothelial surfaces. Prothrombotic (Procoagulant) Properties of Endothelial Cells Endothelial cells may be injured directly or activated by infectious agents (eg bacterial endotoxin), hemodynamic factors, plasma mediators and cytokines. When this happens prothrombotic factors are expressed. von Willebrand factor (vwf) o Endothelial cells synthesize, store & release vwf; essential cofactor for platelet binding to collagen

13 and other surfaces Tissue Factor (TF = Factor III = thromboplastin) o Injured endothelial cells are induced to secrete TF which activates the extrinsic coagulation cascade. Plasminogen activator inhibitor (PAI) o Endothelial cells secrete PAI which suppresses fibrinolysis (via counteracting tpa). The Role of Endothelial Cells in Vascular Repair Numerous growth factors are secreted by endothelial cells: o Platelet Derived Growth Factor (PDGF) stimulates smooth muscle and fibroblasts proliferation. o Fibroblast Growth Factor (FGF) stimulates angiogenesis and fibroblasts in wound healing. o Transforming Growth Factor-β (TFG-β) modulates vascular repair. ROLE OF PLATELETS IN HEMOSTASIS Derived from megakaryocytes; circulate as round, smooth discs with glycoprotein receptors. o Platelets are also referred to as thrombocytes Play a central role in normal hemostasis. o Major role is to form the initial (1 o hemostatic) plug that covers and seals a small damaged area. o Contain mostly procoagulant (& few anticoagulant) mediators in their granules or at other cell sites. Platelet Response Vascular injury exposes extracellular matrix (ECM); especially collagen, which is normally hidden by the intact endothelium. Platelets + ECM 3 reactions to injury 1. Adhesion and shape change Adhesion mediated via interactions with vwf acts as bridge for platelet surface receptors and ECM. 2. Secretion (release reaction) of granules Release of dense granules is important because Ca 2+ is required for coagulation cascade and adenosine diphosphate (ADP) is an important mediator of platelet aggregation. Leads to surface expression of phospholipid complexes (binding site for Ca 2+ & coagulation factors) 3. Aggregation Thromboxane A 2 (TxA 2 ) secreted by platelets; induces vasoconstriction & platelet aggregation. ADP + TxA 2 starts the reaction that leads to enlarging platelet aggregation = 1 o hemostatic plug. Platelet Defects 1. Thrombocytopenia Definition: Circulating platelet numbers are decreased when compared to normal reference ranges (<200 x 10 9 /L is thrombocytopenia in most species; horses <100 x 109/L). Diagnosis History of bleeding low platelet counts Mechanisms Deficient formation of platelets (eg estrogen toxicity suppresses marrow production) Excessive utilization of platelets (eg consumptive coagulopathies) Premature destruction of platelets (eg antibodies to platelets) 2. Thrombocytopathy Definition: Defective platelet function. Mechanisms

14 Defect in adhesion (eg von Willebrand s disease) Defect in aggregation Defect in release of granules ROLE OF THE COAGULATION CASCADE IN HEMOSTASIS The coagulation cascade is the third arm in the hemostatic process and is an amplifying series of enzymatic conversions. An enzyme (activated coagulation factor) + a substrate (next unactivated coagulation factor) newly activated coagulation factor. Reactions are assembled on a phospholipid complex which is held together by calcium ions. The cascade culminates in the production of thrombin (bound to platelet surface) thrombin converts soluble fibrinogen to fibrin which stabilizes the hemostatic plug. Generation of thrombin is the most important factor in the progression and stabilization of the clot (thrombus). Thrombin can be generated at the site of injury by either the intrinsic or extrinsic coagulation pathway which converge where factor X is activated common coagulation pathway. 1. Intrinsic Coagulation Pathway All factors of the intrinsic system (factors XII, XI, IX, and VIII) are present in normal plasma; the cascade is activated by contact of factor XII (Hageman factor) with the subendothelial collagen (ECM) following vascular damage. 2. Extrinsic Coagulation Pathway Tissue factor (= Factor III = thromboplastin) is a cell surface protein on injured endothelial cells that interacts with circulating factor VII to initiate the extrinsic pathway. 3. Common Pathway Activated factor X (Xa) is produced by proteolysis of Factor X, which occurs at the end of both the intrinsic & extrinsic coagulation pathways. Calcium and platelet surface phospholipids are necessary for factor Xa to be activated. Xa converts prothrombin to thrombin (factor II IIa). o Thrombin cleaves peptides from plasma fibrinogen (factor 1, which is soluble) to form fibrin monomers (factor 1a). Monomers self-polymerize into larger fibrin polymers (insoluble); then are cross-linked/stabilized by factor XIIIa. Contraction of fibrin-platelet thrombus reduced size of thrombus (restore blood flow) and draws damaged vessels edges closer (for healing). Coagulation Disorders Coagulation disorders can be inherited or acquired In general, large hematomas suggest a coagulation disorder whereas petechial or ecchymotic hemorrhage on a mucosal surface may indicate a platelet deficiency or abnormality. Inherited Deficiencies of Coagulation Numerous (see powerpoint slide - for your information only) Acquired Deficiencies of Coagulation Can be due to decreased production of coagulation factors: o Accompany many severe diseases: As a transitory depression of factor synthesis o Can be general

15 Liver failure causes a general decrease in production (most coagulation factors are produced in the liver) o Can be more specific: Vitamin K deficiency causes a more specific decrease in production Factors II, VII, IX, X (and protein C and S) are vitamin K dependent Can be due to increased use (ie consumptive coagulopathy): o Accompany many severe diseases: Disseminated intravascular coagulation (DIC)* Severe trauma or deep burns Sepsis FIBRINOLYTIC SYSTEM / ANTICOAGULATION Coagulation must be restricted to the site of vascular injury to prevent extensive clotting away from the site. See previous discussion of "Antithrombotic Properties of Endothelial Cells". Fibrinolytic cascade limits the size and/or dissolves the thrombus (which is a temporary patch). o Primarily by the activation of circulating inactive precursor plasminogen Plasminogen is activated by tissue plasminogen activator (tpa) and the coagulation pathway (XIIakallikrein). o Plasminogen once activated plasmin Plasmin breaks down fibrin and fibrinogen (and some clotting factors and plasma proteins) dissolving the hemostatic plug. Fibrin degradation products (FDPs) have anticoagulant activity and can be used as a measure of thrombotic states THROMBOSIS Terminology Thrombosis o Inappropriate activation (occurring in uninjured or mildly injured vessels) of the hemostatic process resulting in the formation or presence of a solid mass (thrombus) within the blood vessels or heart. Thrombus (pl. thrombi) o Aggregate of blood factors, primarily platelets & fibrin, with entrapment of cellular elements (RBCs/WBCs) causing partial or complete vascular obstruction at the point of formation. Often adherent to the vessel wall (differentiating feature from a post-mortem blood clot). Pathogenesis of Thrombosis: Three primary influences = Virchow's triad. 1. Endothelial injury Dominant influence = can lead to thrombosis by itself. Eg Inflammation of heart valves (endocarditis) exposure of the subendothelial ECM platelet adherence / release of tissue factor primary and secondary hemostatic plug formation thrombosis. 2. Alterations in normal blood flow Normal blood flow is laminar with the cellular elements in the middle of the vessel lumen and surrounded by plasma. With turbulence or stasis disruption of normal laminar flow allows platelets to contact endothelium: o Turbulence also promotes endothelial cell injury / activation

16 o Stasis also prevents dilution of activated clotting factors by fresh-flowing blood and allows the build-up of thrombi (causes hypercoagulability). 3. Hypercoagulability Definition = any alteration of the coagulation pathways that predisposes to thombosis. Due to: o Increased prothrombotic factors (eg with sepsis) o Decreased inhibitory factors (eg loss of antithrombin III with glomerular disease). Locations for thrombi Thrombi may develop anywhere in cardiovascular system: on the valves, in the cardiac chambers or within the lumina of arteries, veins and capillaries. Thrombi grow in the direction of blood flow (ie, arterial thrombi grow away from the heart, while venous thrombi grow toward the heart) and pieces can break off forming emboli. Arterial thrombi o Arterial thrombi usually form at sites of turbulence or endothelial injury. o Grow away from the heart. o Often paler & "meatier" than venous thrombi. Composed mainly of platelets & fibrin, because rapid blood flow tends to exclude RBCs. o Can have alternating dark and pale laminations called lines of Zahn: Reflects continued waves of thrombosis: pale layers (mostly fibrin / platelets) alternating with dark red layers (more entrapped RBCs). Venous thrombi o Grow towards the heart. o Usually form in static (slow flow) environment. o Contain more entrapped RBCs and therefore are more uniformly dark red. o Attachment to wall is often focal and loose which can make it difficult to differentiate from a postmortem blood clot. Blood Clot = Clotted blood within a blood vessel (blood clot can refer to thrombus or post-mortem blood clot, so be specific). Post-mortem blood clots are usually not associated with a pathological condition and usually are not attached to the vessel wall. A thrombus and a post-mortem blood clot can look similar, since the two are clearly related. Chicken-Fat Clot = Common gelatinous, yellow, post-mortem blood clot seen at necropsy (especially horses). Plasma clot that develops because of rapid erythrocyte sedimentation in animals with high fibrinogen. Yellow areas represent fibrin and plasma and dark red areas at the margins represent sedimented RBCs. Morphological Differentiation of Thrombi and Post-Mortem Clots Arterial Thrombus Venous Thrombus Postmortem Clot Colour Pale to dark red Red Yellow or red Lamination Yes Not frequent No Attachment Yes Focal / loose (can be difficult to detect) No Size and location Often small Often fill lumen Fill lumen

17 Outcome Of Thrombi 1. Lysis (dissolution) Especially when thrombi are small and in the early phases due to potent thrombolytic / fibrinolytic activity of blood. 2. Propagation An increase in size of the thrombus may eventually obstruct the vessel. 3. Embolization Can occur if pieces (thromboemboli) break off the thrombus. 4. Organization and recanalization The presence of a thrombus induces inflammation and fibrosis (organization); the latter reduces the size of the thrombus. New small blood vessels can penetrate / grow within the organizing thrombus (recanalization). Both of the above aid in the restoration of blood flow. EMBOLISM Terminology of Embolism Embolism = Passage through the venous or arterial circulations of any material capable of lodging in a blood vessel and thereby obstructing the lumen. o Most common form of embolism is thromboembolism: ie from piece(s) broken off of a thrombus. Embolus (pl. emboli) = Detached intravascular material (solid, liquid, or gaseous) carried via the blood to a site distant from its origin. Thromboembolism = Occlusion of a blood vessel by an embolus that has broken away from a thrombus. o Localizes at point where it can no longer "fit" through. Thromboembolus (pl. thromboemboli) o The piece of thrombotic material transported in the bloodstream to another site. Composition of Emboli Remember that most emboli are thromboemboli, however many other types exist: o Parasites Nematodes - Dirofilaria immitis (heartworm) Nematode larvae - Ascarid or Strongyle larvae o Fibrocartilaginous emboli Originate from intervertebral disk material (traumatic implantation into spinal vessels) Causes necrotizing myelopathy (spinal cord infarction) o Fat (or bone marrow) Can originate from several sources: Eg bone fractures, surgery, osteomyelitis, hyperlipidemia. o Other Foreign material (air bubbles, hair, etc), tumour cell clusters ("tumor emboli"), amniotic fluid Infectious causes of thrombosis or thromboembolism Infectious agents can damage endothelium, thereby causing thrombosis and/or thromboembolism:

18 o Many bacteria can cause valvular endocarditis with resultant thrombosis and thromboembolism. o Several viral agents can damage endothelium leading to thrombosis. DISSEMINATED INTRAVASCULAR COAGULATION (DIC) DIC = The sudden or insidious onset of widespread fibrin thrombi in the microcirculation. o These thrombi are not visible on gross inspection but are readily apparent microscopically. o Can cause diffuse circulatory insufficiency, particularly in the brain, lungs, heart, and kidneys. o With the development of the multiple thrombi, there is a rapid consumption of platelets and coagulation proteins; in addition fibrinolytic mechanisms are activated, thus an initial thrombotic disorder can evolve into a serious bleeding disorder (hence the term consumptive coagulopathy). o Note: DIC is not a primary disease but rather a potential complication of any condition associated with widespread activation of thrombin. Some causes of DIC include: severe burns, heatstroke, systemic viral disease, shock, toxemia, sepsis, widespread metastatic tumours, heartworm disease (dogs), etc. INFARCTION Infarct = an area of ischemic necrosis caused by occlusion of either the arterial supply or the venous drainage. Most infarcts result from thrombotic or embolic events or vascular occlusion due to compression of a vessel (eg intestinal volvulus, testicular torsion, etc). 40% of all human deaths in N. Am. are due to cardiovascular disease, mostly from myocardial or cerebral infarction; whereas pulmonary, intestinal and renal infarction are more common in domestic animals. Factors that Influence the Development / Characteristics of an Infarct 1. Nature of the vascular supply. Tissues with a single blood supply (egs kidney, brain, heart, spleen) are more prone to infarction than those with dual or collateral blood supply (egs lung, liver, intestine, skeletal muscle). 2. Rate of development of occlusion of the vessel. If slow, allows time for the collateral supply to fully open. 3. Vulnerability to hypoxia. Certain cell/tissue types are more vulnerable to ischemic damage (especially the brain and heart). 4. Oxygen content of blood at time of infarct. Underlying anemia would increase the likelihood of ischemia resulting in infarction. Gross appearance of an Infarct Often wedge-shaped, with the base at the periphery and the occluded vessel at the apex. Early they are ill defined (+/- irregular margins) and often hyperemic. Later (by 48 hours) most become pale. 1. Red Infarct Due to the presence of blood in the infarcted region (also called hemorrhagic infarct). o Occurs in some acute infarcts due to RBCs leaking in from adjacent arteries and veins (eventually the RBCs lyse pale). o In venous occlusions, where blood is prevented from draining from the organ (eg volvulus, strangulations); called a venous infarct. o Also occurs in organs with dual blood supply (eg lung, liver) or where blood collects in loose tissue. 2. White Infarct Lack of blood in the infarct (also called pale or anemic infarct). o Mostly occurs with arterial occlusions in solid organs with end arterial circulation (eg heart, kidney).

19 Usually has a red zone at periphery because the capillaries at the border of infarct undergo dissolution and blood seeps into this marginal area. Histopathology of an Infarct Ischemic necrosis of affected parenchyma: o Discrete areas of coagulative necrosis (+/- hemorrhage) in all tissues except brain where liquefactive necrosis will occur o Peripheral rim of inflammation and hemorrhage. o Infarcts arising from septic (bacterially infected) emboli may be converted to an abscess with time. Repair of Infarcts Fibrous connective tissue (scar tissue) replaces necrotic parenchyma. As fibrous tissue matures and condenses (contracts), it forms a depression / indentation on the organ surface. Septic Infarct Develop from a bacterially infected thromboembolus or when the necrotic tissue of an infarct is seeded by bacteria (necrotic tissue is a good growth medium for these pathogenic organisms). Venous Infarct Severe venous obstruction can cause venous infarction. Mostly due to twisting of vessels (eg intestinal volvulus / torsion / strangulation) shock / death. Also seen with obstruction (eg thrombosis or tumor invasion) of the cranial or caudal vena cava o Often obstruction is incomplete causing slowly developing stasis with engorgement of the tributary veins. Important examples of venous obstruction/infarction Acute Blockage of the Portal Venous System o Mostly with twists in portions of GI tract / portal venous system venous infarction of stomach or intestine; twisted vessels are compressed, but because arterial pressure > venous pressure blood enters the tissue but cannot leave. Sequelae: shock and death unless corrected with surgery. Eg: gastric torsion (dogs) obstruction of gastric portion of portal vein severe venous congestion vascular stasis ischemic necrosis (infarction) loss of endothelial integrity hemorrhage shock. Blockage of the posterior vena cava o Possible etiologies: Severe dirofilariasis (heartworm) or adrenal tumors in dogs Hepatic abscesses in ruminants. o Possible result: Acute and complete occlusion death. Chronic occlusion possibility of collateral circulation developing from azygous vein. Important example of arterial blockage / infarction Pulmonary Artery Thrombosis / Thromboembolism o Thrombosis / thromboembolism of the pulmonary artery can be due to a variety of causes, eg pneumonia, parasite infestations (eg heartworm), hypercoagulability (eg hyperadrenocorticism, nephrotic syndrome), liver abscess rupture into the vena cava with subsequent thromboembolism to the lungs, etc

20 o Possible result: If acute and involves large branch of artery death. If incomplete and smaller branches variably altered circulation or possibly pulmonary infarcts. SHOCK Shock is characterized by systemic hypotension due either to reduced cardiac output or to reduced effective circulating blood volume which results in impaired tissue perfusion and cellular hypoxia. Brain and heart are the organs most susceptible to ischemic damage from shock. Shock is the final common pathway for many potentially lethal clinical events which include microbial sepsis, severe hemorrhage, extensive trauma or burns, myocardial infarction, severe pulmonary embolism, etc. Three General Categories of Shock 1. Cardiogenic Shock Results from failure of the heart to adequately pump blood; ie cardiac output is decreased. Can occur with a variety of heart diseases. o Eg: myocardial infarction, ventricular tachycardia, arrhythmias, cardiomyopathy, or an obstruction of the flow of blood from the heart 2. Hypovolemic Shock Results from decreased circulating blood volume. Can be due to blood loss from hemorrhage (internal or external) or fluid loss (dehydration) secondary to vomiting, diarrhea or burns. 3. Blood Maldistribution (Vasogenic shock) See a decrease in peripheral vascular resistance and resultant pooling of blood in peripheral tissues. Many causes, including neural or cytokine induced vasodilation, trauma, systemic hypersensitivity to allergens (anaphylaxis) or endotoxemia. Three main causes: o Anaphylactic Shock - vasodilation due to release of vasoactive amines (eg histamine), etc. o Neurogenic Shock - vasodilation due to loss of the autonomic nervous system signals to the smooth muscle in vessel walls. o Septic Shock*- vasodilation due to release of inflammatory mediators associated with overwhelming infections (especially gram-negative sepsis). Pathogenesis of Septic Shock Inflammatory cells have a number of receptors (Toll-like receptors) that respond to a variety of substances derived from microbes (such as endotoxin from gram-negative bacteria or peptidoglycan from gram-positive bacteria). Endotoxin is bacterial wall lipopolysaccharide (LPS) which is released when bacterial cell walls are degraded (septic shock can be reproduced with LPS injection alone). When microbial components (eg LPS) bind to a WBC surface receptors WBCs release cytokines activation/injury of endothelial cells vasodilation, coagulation cascade ( DIC), complement activation, etc. Three Stages of Shock 1. Nonprogressive (compensated) shock Reflex compensatory mechanisms are activated and perfusion of vital organs is maintained (eg increased heart rate, peripheral vasoconstriction, etc).

21 2. Progressive shock Characterized by tissue hypoperfusion and onset of worsening circulatory and metabolic imbalances, including acidosis. 3. Irreversible shock Sets in after the body has incurred cellular and tissue injury so severe that even if the hemodynamic defects are corrected, survival is not possible. Lesions of Shock Shock is characterized by failure of multiple organ systems: Pulmonary congestion and edema: the lung is a prominent shock organ in cattle & horses. Liver congestion: liver is a prominent shock organ of dogs. Kidneys : Acute tubular necrosis. Heart: Subendocardial hemorrhage and myocardial necrosis. Blood vessels: Endothelial damage with possible thrombosis / DIC. Brain: Neuronal cell death. Adrenal glands: Hemorrhage. Gastrointestinal tract: Mucosal congestion and necrosis. Skeletal muscle: Pallor (probably due to peripheral vasoconstriction).