Introduction to Blood - Part 1 PDF

INTRODUCTION TO BLOOD – PART 1 Block: Foundations Block Director: James Proffitt, PhD Session Date: Monday, August 05, 2024 Time: 4:00 – 5:00 pm Instructor: Deborah Fuchs, MD Department: Pathology Email: [email protected]...

INTRODUCTION TO BLOOD – PART 1 Block: Foundations Block Director: James Proffitt, PhD Session Date: Monday, August 05, 2024 Time: 4:00 – 5:00 pm Instructor: Deborah Fuchs, MD Department: Pathology Email: [email protected] INSTRUCTIONAL METHODS Primary Method: IM10: Independent Learning ☐ Flipped Session ☐ Clinical Correlation Resource Types: RE18: Written or Visual Media (or Digital Equivalent) INSTRUCTIONS Please read lecture objectives, notes and watch pre-recorded video. READINGS REQUIRED Readings: Robbins & Cotran Pathologic Basis of Disease 10E (2021): 1) Ch 13: ‘Normal Hematopoiesis’ stop at ‘Disorders of White Cells’ 2) Ch 14: ‘Anemias’ (stop at ‘Anemias of Blood Loss’) and ‘Bleeding Disorders’ 3) Ch 4: ‘Hemostasis and Thrombosis’ (stop at ‘Thrombosis’). LEARNING OBJECTIVES 1. Describe the components of blood. 2. Describe the morphologic features that characterize differentiation of marrow precursors cells to the mature cells seen in the blood and the approximate time this process takes for platelets, RBCs, and neutrophils. 3. Define anemia and outline the classification of anemia. 4. Describe the components of the hemostasis system and their functions. 5. Compare and contrast disorders of primary hemostasis (platelet, vessel wall) and secondary hemostasis (coagulation factor) in terms of type of bleeding (location, severity) and onset of bleeding. 6. Describe the two main causes of thrombocytopenia. 7. Compare and contrast von Willebrand disease and Hemophilia A in terms of type of bleeding, defect, lab features and treatment. Block: Foundations | FUCHS [1 of 10] INTRODUCTION TO BLOOD – PART 1 CURRICULAR CONNECTIONS Below are the competencies, educational program objectives (EPOs), block goals, disciplines and threads that most accurately describe the connection of this session to the curriculum. Related Related Competency\EPO Disciplines Threads COs LOs CO-01 LO #1 MK--03: The molecular, cellular Hematoloy N/A and biochemical mechanisms of homeostasis CO-01 LO #2 MK--03: The molecular, cellular Hematoloy N/A and biochemical mechanisms of homeostasis CO-01 LO #3 MK-02: The normal structure and Hematoloy N/A function of the body as a whole and of each of the major organ systems CO-01 LO #4 MK-02: The normal structure and Hematoloy N/A function of the body as a whole and of each of the major organ systems CO-02 LO #5 MK-05: The altered structure and Pathology N/A function (pathology & pathophysiology) of the body/organs in disease CO-01 LO #6 MK-05: The altered structure and Pathology N/A function (pathology & pathophysiology) of the body/organs in disease CO-01 LO #7 MK-05: The altered structure and Hematoloy N/A function (pathology & pathophysiology) of the body/organs in disease CONTEXT: These lectures on blood will introduce you to the composition of blood, function of the blood cells and to the role of the bone marrow in producing blood cells. In addition, we will discuss the proteins of the coagulation system. Some common diseases will be used to illustrate the clinical consequences of derangements in these systems. These are pivotal lectures in a number of ways as they provide essential background for discussion of inflammation (week 3 and every block hereafter), wound repair, and diseases of the coagulation system, white blood cells and red blood cells. Some of these topics will be revisited in greater depth in future blocks, especially I&I. Block: Foundations | FUCHS [2 of 10] INTRODUCTION TO BLOOD – PART 1 OVERVIEW The average adult blood volume is 5-6 liters. Blood contains cellular (white blood cells [WBCs], red blood cells [RBCs] and platelets) and non-cellular elements. The cellular elements of the blood are largely derived from the bone marrow, lymph nodes, and thymus. The mature cells of the blood develop from a common progenitor cell (stem cell) in the bone marrow. This process, known as hematopoiesis (making blood) involves the differentiation of stem cells to mature erythrocytes (erythrocytopoiesis), platelets (megakaryocytopoiesis) and WBCs (granulocytopoiesis, lymphocytopoiesis, monocytopoiesis). The non- cellular element is known as plasma and contains water, sugars, lipid, vitamins, minerals, electrolytes and numerous proteins (enzymes, hormones, antibodies, blood clotting factors). Approximately 500 proteins have been identified in plasma and include the proteins of the hemostatic system, many of which are made in the liver. THE BONE MARROW – ROLE IN HEMATOPOEISIS Bone Marrow Structure: Encased by cortical bone, traversed by trabecular bone Highly organized meshwork of thin-walled capillary-venous sinuses with surrounding extracellular matrix, adipose and hematopoietic compartment o Newly formed cells exit the marrow via the sinuses Bone Marrow Function Site of production of all RBCs, granulocytes, monocytes, platelets, and early lymphocytes. Further differentiation of lymphocytes occurs in lymphatic organs. In children, all bones are used to maintain hematopoiesis In adults, hematopoietic activity is restricted to central bones (ribs, vertebrae, pelvis, sternum, skull), and proximal portions of humerus and femur All cells in the bone marrow develop from a common progenitor cell, or stem cell. This pluripotent stem cell has the capacity for self- renewal and can give rise to all mature cells of the blood and immune organs. This pluripotent stem cell gives rise to two types of multipotent progenitors, the common lymphoid and the common myeloid stem cell. The term myeloid refers to the bone marrow and cells derived from it (RBCs, platelets, granulocytes, and monocytes). The common myeloid stem cell gives rise to committed stem cells which differentiate along either the granulocytic/monocytic, megakaryocytic or erythroid pathways. The common lymphoid stem cell gives rise to precursors of T-cells, B-cells and natural killer cells. (see Figure 13-1 in Robbins and Cotran, Pathologic Basis of Disease, 10e) Block: Foundations | FUCHS [3 of 10] INTRODUCTION TO BLOOD – PART 1 Clinical correlate: Hematopoietic stem cell transplants are now routinely used in the treatment of hematolymphoid malignancies (leukemia, lymphoma) and other disorders of the bone marrow and immune systems. In addition, studies have shown that hematopoietic stem cells are able to form other kinds of cells, such as muscle, blood vessels, and bone. LEUKOCYTES (WHITE BLOOD CELLS) To be covered in Introduction to Blood 2 ERYTHROCYTES (RED BLOOD CELLS) Erythrocytopoiesis Characterized by increasing hemoglobin in cytoplasm and reduction in nuclear size; the nucleus is ultimately pitted toward the end of this process Takes ~7 days; RBC lasts 120 days in the circulation Driven by erythropoietin produced by kidneys in response to hypoxia (decreased oxygen in the blood) Reticulocytes are immature RBCs that have been released into the blood after extrusion of the nucleus. They can be demonstrated with special stains that highlight a reticular network of polyribosomes. Without special stains, they appear larger and more basophilic, than a mature RBC. They are also called polychromatophilic (“many colors”) erythrocytes. Marrow physiologically replaces 1% of RBCs/day = 2-4 x 109 RBCs/kg/day under steady state Clinical correlate: A reticulocyte count can be very helpful in determining the type of anemia. If the patient is anemic and there is a low number of reticulocytes, this suggests impaired marrow production of RBCs. On the other hand, if the reticulocyte count is increased, this is more consistent with loss of RBCs (bleeding) or peripheral destruction of RBCs. The bone marrow may increase RBC production up to 8-fold. In the blood, may see an increase in early RBCs (reticulocytes and even nucleated RBCs; similar to the left-shift to immaturity seen in neutrophils) Red cell morphology Mature RBCs are biconcave discs filled with hemoglobin. The nucleus is extruded during their final stage of development in the bone marrow. Block: Foundations | FUCHS [4 of 10] INTRODUCTION TO BLOOD – PART 1 Normal RBCs are about 8 microns in diameter. RBCs are larger than the diameter of some capillaries (4-10 microns). The biconcave shape gives the RBC the necessary flexibility to traverse the capillary bed. Red blood cells have two main functions: 1) Oxygen and carbon dioxide transport 2) Acid balance Laboratory evaluation of RBCs One of the most commonly ordered laboratory tests is the complete blood count (CBC) that includes the counts of the different cell types in the blood: WBC, platelets and RBC. In addition, information on hemoglobin content and RBC size is reported. A reticulocyte count is not part of the CBC and needs to be ordered separately. More on this in your Anemia and Hemoglobin biochemistry lectures. Diseases of RBCs: Anemia Definition: a reduction of the number of red blood cells and/or hemoglobin content. Leads to a reduction in oxygen transport capacity and organ hypoxia. Classified according to underlying mechanism or according to alterations in RBC morphology, including the size of the RBC (mean corpuscular volume, MCV). Many causes are known: 1) Blood loss, 2) Increased RBC destruction (hemolysis) and 3) Decreased production (refer to Table 14-1, Robbins and Cotran, Pathologic Basis of Disease, 10th Ed) PLATELETS Megakaryocytopoiesis Committed stem cell undergoes endomitotic division (nucleus divides, cytoplasm does not), resulting in a large cell with multiple nuclear lobes. Megakaryocytes straddle marrow sinusoids and shed platelets (cytoplasmic fragments) into the blood. Production time: ~5-10 days. Platelets live ~10 days in circulation. More on function below, under hemostasis. Block: Foundations | FUCHS [5 of 10] INTRODUCTION TO BLOOD – PART 1 THE HEMOSTASIS SYSTEM Hemo = blood; -stasis = a condition of balance, static Hemostasis System - 2 main functions: 1. Stop bleeding Rapid formation of a clot to stop bleeding from a damaged vessel; involves vessel wall, platelets, coagulation cascade 2. Prevent out of control clot formation (counter-regulatory mechanisms) This lecture will serve as an introduction to this system with an overview of normal hemostasis and a more in-depth discussion of two relatively common diseases of excess bleeding. Thrombosis will be covered in a subsequent lecture. Hemostasis involves four general components: 1. Vessel wall Primary Hemostasis 2. Platelets Hemostasis 3. Coagulation cascade (Secondary hemostasis) 4. Counter-regulatory mechanisms Normal hemostasis: Basic Overview / Sequence of Events (Robbins and Cotran, Pathologic Basis of Disease, 10 th Ed, Figure 4.4) After initial damage to a vessel, there is a brief period of arteriolar vasoconstriction that reduces the amount of blood flowing to the area. Damage to the endothelium exposes collagen, which is normally hidden from the blood. Circulating von Willebrand factor (vWF) binds to this collagen. Platelets then adhere to vWF to form an initial hemostatic plug. Binding of platelets to vWF activates platelets and they release granule contents which recruit additional platelets to the site and play a role in the coagulation cascade. During damage to the vessel wall, tissue factor (TF) is also exposed to the circulation. TF, along with the activated platelets, initiate the coagulation cascade (refer to Figure 1 below), which results in the formation of insoluble fibrin, a “cement” that holds the platelets together into a stable hemostatic plug. In order to limit the hemostatic response to the site of vascular injury and the final size of the clot, counter-regulatory mechanisms are set into motion. The counter-regulatory proteins slow formation of fibrin and limit the clot to the site of vascular damage (protein C, protein S and antithrombin). In addition, fibrinolysis acts to limit the size of the clot and contributes to clot dissolution. The liver is the site of synthesis of most of the proteins involved in coagulation (coagulation factors, natural anticoagulants and fibrinolytic proteins). Vitamin K is required to produce active forms of several of the coagulation factor proteins and natural anticoagulants (protein C and protein S). Block: Foundations | FUCHS [6 of 10] INTRODUCTION TO BLOOD – PART 1 A closer look at some of these components: Coagulation Cascade: The cascade is a series of conversions of inactive enzymes (aka factors) to active enzymes (from factor V to factor Va, for example) that culminates in the formation of the insoluble protein fibrin (“cement”). The cascade takes place on the phospholipid surface of activated platelets, thereby localizing the cascade to the site of vessel damage. Although the cascade is often described as consisting of three separate pathways (intrinsic, extrinsic and common), the intrinsic and extrinsic pathway are integrated in vivo and the extrinsic pathway is the primary route of initiation of secondary hemostasis (see Figure 1). Routine laboratory tests evaluate the cascade by measuring the time to fibrin formation from a patient blood sample. The prothrombin time (PT) measures the extrinsic and common pathways and the activated partial thromboplastin time (APTT) measures the intrinsic and common pathways (see Figure 2). Figure 1, Clotting factor cascade. Cornell University (http://eclinpath.com) Figure 2, Tests for the clotting pathway. (Hamad ALAssaf - http://slideplayer.com) Counter-regulatory mechanisms Once activated, the coagulation cascade must be regulated to prevent pathologic thrombosis and limit clotting to the site of vascular damage. Note the important antithrombotic properties of the endothelium. 1. Antithrombin, after activation by heparin-like molecules on the endothelium, inhibits thrombin and other coagulation factors; this results in inhibition of the coagulation cascade 2. Protein C is activated by a complex of thrombin and thrombomodulin (expressed on the endothelium). Activated protein C (APC) with its cofactor Protein S, inhibits the coagulation cascade by inactivating factors Va and VIIIa. Block: Foundations | FUCHS [7 of 10]  INTRODUCTION TO BLOOD – PART 1 3. Tissue plasminogen activator (t-PA), produced by endothelial cells, promotes fibrinolysis to clear fibrin deposits from the endothelial surface. **Defects in any of these components can result in hemorrhage or thrombosis** Defects of the vessels and/or underlying collagen can cause vascular fragility and bleeding Deficiencies in the coagulation factors results in bleeding (Ex: Hemophilia) Low, or defective, platelets can result in bleeding Defects in the counter-regulatory mechanisms result in failure to limit the coagulation response and cause abnormal clotting (pathologic thrombosis). BLEEDING DISORDERS Caused by defects in primary or secondary hemostasis. Clinical features vary based on the defective component: Deep tissue hematomas and bleeding into joints (hemarthrosis); seen in coagulation factor deficiencies Mucosal bleeding (hematuria, menorrhagia, epistaxis) is more common in disorders of primary hemostasis. Skin Bleeding (purpura) o Petechiae – minute, pinpoint hemorrhage (1-2mm) – capillary bleeding; defect in primary hemostasis (platelet number or function or vascular integrity) o Purpura – slightly larger than petechiae. See in primary hemostasis disorders o Ecchymoses, subcutaneous hemorrhage (‘bruise”). Larger confluent areas (>1-2 cm); due to trauma, abnormalities in primary (small, superficial) or secondary hemostasis (large, deep) The onset of bleeding after trauma or surgery can also be a clue to the underlying disorder. Patients with platelet disorders tend to bleed immediately after trauma whereas with coagulation factor deficiencies, the bleeding is often delayed. Why? Primary Hemostasis Coagulation Factor Disorder Characteristic locations of bleeding Bleeding after minor cuts? Block: Foundations | FUCHS [8 of 10]  INTRODUCTION TO BLOOD – PART 1 Petechiae Ecchymoses (small or large? deep or superficial?) Hemarthrosis Timing of bleeding after surgery Bleeding due to defects of primary hemostasis Thrombocytopenia Thrombocytopenia is the most common acquired bleeding disorder with numerous and diverse causes, including decreased production and accelerated destruction. In some patients, a definitive etiology cannot be determined (idiopathic). 1) Decreased production: drugs, bone marrow neoplasms, vitamin deficiency, congenital disorders, infection (HIV) 2) Accelerated destruction. More common than decreased production. In these disorders, platelets are destroyed by an antibody-mediated process or mechanical (non-immune) process. Von Willebrand Disease (vWD) The Robbins text classifies vWD as a hemorrhagic disorder related to abnormalities in clotting factors. Keep in mind that the defect in vWD is in platelet adhesion to damaged endothelium; DO NOT confuse with the disorders of the coagulation factors (hemophilia). Common inherited bleeding disorders (frequency 1%) Multiple variants have been described: decrease in vWF number (quantitative) or abnormal function (qualitative) Hallmark clinical presentation is platelet-type bleeding: mucocutaneous bleeding (epistaxis, menorrhagia, purpura and petechiae). Laboratory findings: Normal platelet count Decreased vWF protein and/or function ▪ Treatment (dictated by variant), options include: DDAVP (desmopressin): causes release of vWF and FVIII from endothelial cells vWF concentrates Bleeding due to coagulation factor deficiencies: Deficiencies of all coagulation factors have been described. FVIII deficiency is the most common inherited coagulation factor deficiency. Block: Foundations | FUCHS [9 of 10] INTRODUCTION TO BLOOD – PART 1 Acquired Coagulation Factor Deficiencies Liver disease Vitamin K deficiency Medications: warfarin Hemophilia A (FVIII Deficiency) X-linked inheritance; frequency of 1/5,000 males. Clinical: easy bruising, hemorrhage after surgery or trauma, hemarthrosis Laboratory: prolonged aPTT, normal PT and low factor VIII activity (dictates severity of disease). Treatment: FVIII replacement Block: Foundations | FUCHS [10 of 10]