Structure and Function of the Hematologic System Chapter 19

Components of the Hematologic System Composition of blood 90% water and 10% solutes 6 quarts (5.5 L) Plasma 55% to 60% of the blood volume Organic and inorganic elements

Components of the Hematologic System Chief function Delivery of substances needed for cellular metabolism Removal of wastes Defense against microorganisms and injury Maintenance of acid-base balance

Components of the Hematologic System Composition of blood Plasma proteins Albumins Function as carriers and control the plasma oncotic pressure Globulins Carrier proteins and immunoglobulins (antibodies) Clotting factors Mainly fibrinogen

Components of the Hematologic System Composition of blood Cellular components Erythrocytes Most abundant cell in the body Responsible for tissue oxygenation Biconcavity and reversible deformity 120-day life cycle

Composition of Blood Cellular components Leukocytes (white blood cells) Defend the body against infection and remove debris Granulocytes Membrane-bound granules in their cytoplasm The granules contain enzymes capable of destroying microorganisms Inflammatory and immune functions Capable of ameboid movement (diapedesis)

Composition of Blood Granulocytes Neutrophils Eosinophils Polymorphonuclear neutrophil (PMN) Phagocytes in early inflammation Eosinophils Eosinophils ingest antigen-antibody complexes Induced by IgE hypersensitivity Increase in parasitic infections

Composition of Blood Granulocytes Mast cells Basophils Central cell in inflammation Found in vascularized connective tissue Basophils Structurally and functionally similar to mast cells

Composition of Blood Agranulocytes Monocytes and macrophages make up the mononuclear phagocyte system (MPS) Monocytes Macrophages Lymphocytes Natural killer (NK) cells

Composition of Blood Platelets Disk-shaped cytoplasmic fragments Essential for blood coagulation and control of bleeding Thrombopoietin Main regulator of platelets

Lymphoid Organs Spleen Largest secondary lymphoid organ Splenic pulp Masses of lymphoid tissue containing macrophages and lymphoid tissue Venous sinuses Phagocytosis of old, damaged, and dead blood cells Blood storage

Lymphoid Organs Lymph nodes Part of the immune and hematologic systems Facilitates maturation of lymphocytes Transports lymphatic fluid back to the circulation Cleanses the lymphatic fluid of microorganisms and foreign particles

Lymphoid Organs

Mononuclear Phagocyte System (MPS) The MPS consists of a line of cells that originate in the bone marrow, are transported into the bloodstream, differentiate into monocytes, and settle in the tissues as mature macrophages Cells of the MPS ingest and destroy microorganisms and foreign material The MPS is mostly the liver and spleen

Hematopoiesis Hematopoiesis is the process of blood cell production Two stages Mitosis Mitosis stops before the cell enters the peripheral blood Maturation and differentiation

Hematopoiesis Stem cell system Bone marrow Pluripotent stem cells Colony-stimulating factors Bone marrow Also called myeloid tissue Red and yellow bone marrow Adult active bone marrow Pelvic bones, vertebrae, cranium and mandible, sternum and ribs, humerus, and femur

Erythropoiesis Erythrocytes are derived from erythroblasts (normoblasts) Maturation is stimulated by erythropoietin

Erythropoiesis Sequence Uncommitted pluripotent stem cell, committed proerythroblast, normoblast, basophilic normoblast, polychromatophilic normoblast, orthochromic normoblast, reticulocyte (nucleus is lost), erythrocyte In each step the quantity of hemoglobin increases and the nucleus decreases in size

Erythropoiesis

Regulation of Erythropoiesis Numbers of circulating red cells in healthy individuals remain constant The peritubular cells of the kidney produce erythropoietin Hypoxia stimulates the production and release of erythropoietin

Regulation of Erythropoiesis Erythropoietin causes an increase in red cell production and release from bone marrow

Hemoglobin Synthesis Oxygen-carrying protein of the erythrocyte A single erythrocyte contains as many as 300 hemoglobin molecules Two pairs of polypeptide chains Globulins

Hemoglobin Synthesis Four colorful iron-protoporphyrin complexes Adult hemoglobin Two alpha chains and two beta chains

Hemoglobin Synthesis

Hemoglobin Synthesis Nutritional requirements Building blocks Proteins Amino acids Vitamins Vitamins B12, B6, B2, E, and C; folic acid; pantothenic acid; and niacin Minerals Iron and copper Folate

Hemoglobin Synthesis Iron cycle Total body iron is bound to heme or stored bound to ferritin or hemosiderin mononuclear phagocytes and hepatic parenchymal cells Less than 1 mg per day is lost in the urine, sweat, epithelial cells, or from the gut Transferrin Apotransferrin

Iron Cycle

Normal Destruction of Senescent Erythrocytes Aged red cells are sequestered and destroyed by macrophages of the MPS, primarily in the spleen The liver takes over if the spleen is absent Globin chains are broken down into amino acids

Normal Destruction of Senescent Erythrocytes Porphyrin is reduced to bilirubin, transported to the liver, and secreted in the bile

Iron Cycle

Development of Leukocytes Leukocytes arise from stem cells in the bone marrow Granulocytes mature in the bone marrow Agranulocytes and monocytes are released into the bloodstream before they fully mature

Development of Leukocytes Growth factors and colony-simulating factors encourage production and maturation of leukocytes

Development of Platelets Endomitosis The megakaryocyte undergoes the nuclear phase of cell division but fails to undergo cytokinesis The megakaryocyte expands due to the doubling of the DNA and breaks up into fragments

Development of Platelets Platelet levels are maintained by thrombopoietin and IL-11 Platelets circulate for 10 days before losing their functional capacity

Hemostasis Hemostasis means arrest of bleeding Requirements Platelets Clotting factors Blood flow and shear forces Endothelial cells Fibrinolysis

Platelet’s Role 1. Vasospasm 2. Plug 3. Activate coagulation cascade 4. Initiate repair

Hemostasis

Hemostasis Platelet plug formation Activation Adhesion Aggregation von Willebrand factor (vWF) Aggregation Secretion

Hemostasis Function of clotting factors Intrinsic pathway Activated when factor XII contacts subendothelial substances exposed by vascular injury Extrinsic pathway Activated when tissue factor (TF) (tissue thromboplastin) is released by damaged endothelial cells

Coagulation Cascade

Control of Hemostatic Mechanisms Clot retraction Fibrin strands shorten; become denser and stronger to approximate the edges of the injured vessel and site of injury Facilitated by large numbers of platelets within the clot and actin-like contractile proteins in the platelets

Control of Hemostatic Mechanisms Lysis of blood clots Fibrinolytic system Plasminogen and plasmin Fibrin degradation products D-dimers

Fibrinolytic System

Evaluation of the Hematologic System Tests of bone marrow function Bone marrow aspiration Bone marrow biopsy Measurement of bone marrow iron stores Differential cell count Blood tests Large variety of tests

Pediatrics and the Hematologic System Blood cell counts increase above adult levels at birth Trauma of birth and cutting the umbilical cord The hypoxic intrauterine environment stimulates erythropoietin production Results in polycythemia

Pediatrics and the Hematologic System Children tend to have more atypical lymphocytes as a result of frequent viral infections

Aging and the Hematologic System Erythrocyte life span is normal but erythrocytes are replaced more slowly Possible causes Iron depletion Decreased total serum iron, iron-binding capacity, and intestinal iron absorption Lymphocyte function decreases with age The humoral immune system is less responsive