Structure and Function of the Hematologic System
Structure and Function of the Hematologic System
Composition of Blood
Blood is composed of water and solutes.
Adults typically have quarts ( L) of blood.
Chief functions of blood include:
Delivery of substances required for cellular metabolism.
Removal of metabolic wastes.
Defense against microorganisms and injury.
Maintenance of acid-base balance.
Plasma
Plasma constitutes to of the total blood volume.
It is the liquid portion, containing both organic and inorganic elements.
Key plasma proteins include:
Albumin: Functions as a carrier protein and is crucial for controlling plasma oncotic pressure.
Globulins: Serve as carrier proteins and include immunoglobulins (antibodies).
Clotting factors/proteins: Primarily fibrinogen, these proteins promote blood coagulation.
Cellular Components: Erythrocytes
Erythrocytes (red blood cells) are the most abundant cells found in the blood.
They are primarily responsible for tissue oxygenation.
Key characteristics include their biconcavity and reversible deformability, allowing them to navigate through small capillaries.
Erythrocytes have a life cycle of approximately to days.
Cellular Components: Leukocytes
Leukocytes (white blood cells) are essential for defending the body against infection and removing cellular debris.
While they act primarily in the tissues, they are transported via the circulatory system.
Leukocytes are classified by structure:
Granulocytes: Possess membrane-bound granules in their cytoplasm.
Agranulocytes: Lack visible granules in their cytoplasm.
They are also classified by function:
Phagocytes: Cells capable of ingesting microorganisms and other foreign particles.
Immunocytes: Cells involved in specific immune responses.
Granulocytes
Granulocytes have membrane-bound granules in their cytoplasm containing enzymes capable of destroying microorganisms.
They play significant roles in inflammatory and immune functions.
These cells are capable of ameboid movement, a process known as diapedesis, allowing them to migrate from blood vessels into tissues.
Specific types of granulocytes include:
Neutrophils: Also known as polymorphonuclear neutrophils (PMNs), they act as phagocytes in early inflammation.
Eosinophils: Primarily involved in ingesting antigen-antibody complexes. Their numbers often increase in response to IgE hypersensitivity reactions and parasitic infections.
Basophils: Structurally and functionally similar to mast cells, participating in allergic reactions and inflammation.
Agranulocytes
Specific types of agranulocytes include:
Monocytes: These are immature macrophages. Once they mature, they become part of the mononuclear phagocyte system (MPS).
Lymphocytes: These cells mature into T cells, B cells, and plasma cells, which are central to adaptive immunity.
Natural killer (NK) cells: These are a type of lymphocyte involved in innate immunity, capable of directly killing virus-infected cells and tumor cells.
Cellular Components: Platelets
Platelets (thrombocytes) are irregularly-shaped cytoplasmic fragments.
They are essential for blood coagulation (clotting) and the control of bleeding.
Lymphoid Organs
Primary Lymphoid Organs
These are the sites where lymphocytes are produced and mature.
Key primary lymphoid organs include the bone marrow and thymus.
Secondary Lymphoid Organs
These organs are where mature lymphocytes encounter antigens and immune responses are initiated.
Examples include the spleen, lymph nodes, tonsils, and Peyer patches.
The liver also performs some lymphoid functions.
Spleen
The spleen is the largest lymphoid organ.
Its diverse functions include:
Fetal hematopoiesis (blood cell formation during development).
Cleansing of blood by filtering out old and damaged cells, as well as microorganisms.
Initiating immune responses against blood-borne antigens.
Destroying aged and defective blood cells.
Serving as a blood reservoir.
The spleen consists of:
Splenic pulp: Masses of lymphoid tissue containing macrophages, lymphocytes, and lymphoid follicles.
Venous sinuses: Specific areas responsible for the phagocytosis of old, damaged, and dead blood cells. The spleen can store approximately ml of blood in its venous sinuses.
Lymph Nodes
Structurally, lymph nodes are part of the lymphatic system.
Functionally, they are integral to both the hematologic and immune systems.
Their roles include:
Transporting lymphatic fluid back to the systemic circulation.
Cleansing the lymphatic fluid of microorganisms and foreign particles.
Development of Blood Cells (Hematopoiesis)
Bone Marrow
Red (active or hematopoietic) marrow: This is the primary site of blood cell production in adults, found mainly in flat bones and is highly vascularized.
Yellow (inactive) marrow: Found in other bones, it consists primarily of adipose tissue and can convert to red marrow if needed.
Bone Marrow Niches
Microenvironments within the bone marrow that support hematopoietic stem cells (HSCs).
Endosteal niches (osteoblastic niches): Involve osteoblasts and osteoclasts, which contribute to the HSC environment.
Perivascular niches: These are more active in supporting hematopoiesis.
Mesenchymal stem cells (MSCs): These multipotent cells develop into various cell types like osteoclasts, fibroblasts, chondrocytes, and adipocytes. They also play a crucial role in maintaining HSCs.
Hematopoietic stem cells (HSCs): These are the progenitor cells for all blood cells. Their development is influenced by colony-stimulating factors (CSFs).
Cellular Differentiation
Multipotent stem cells: These are intermediate groups of stem cells with a limited ability to differentiate into several different cell types.
Hematopoietic stem cells (HSCs): All blood cells originate from these self-renewing stem cells, which further differentiate into hematopoietic progenitor cells.
Hematopoiesis Process
Hematopoiesis is the process of blood cell production.
It occurs in the liver and spleen during fetal development, and after birth, it primarily shifts to the bone marrow (medullary hematopoiesis).
Proliferation into various cell types occurs simultaneously.
This process is regulated by Colony-stimulating factors (CSFs), also known as hematopoietic growth factors, and Erythropoietin.
Hematopoiesis involves two stages: proliferation and differentiation/maturation.
It occurs only in the bone marrow after birth (not in the liver and spleen after birth).
Hematopoiesis increases in response to conditions like hemolytic anemia.
Erythropoiesis (Red Blood Cell Development)
This is the specific process of red blood cell development.
The sequence of development is: Progenitor cell $\rightarrow$ Proerythroblast $\rightarrow$ Erythroblast/Normoblast $\rightarrow$ Reticulocyte $\rightarrow$ Erythrocyte.
During each step, the quantity of hemoglobin increases, and the nucleus size decreases.
Regulation: Erythropoiesis is controlled by a feedback loop involving erythropoietin, which causes an increase in red cell production under conditions of tissue hypoxia.
Hemoglobin (Hb) Synthesis
Hemoglobin is the oxygen-carrying protein within erythrocytes.
It consists of two pairs of polypeptide chains (globulins); the most common adult hemoglobin has two alpha and two beta chains.
Hemoglobin also contains four colorful iron-protoporphyrin complexes called heme. Each heme molecule carries one molecule of oxygen.
Oxygen binds to reduced ferrous iron () within the heme. Methemoglobin, which contains nonreduced ferrous iron (), cannot bind oxygen.
Nutritional Requirements for Erythropoiesis
Adequate nutrition is critical for red blood cell production:
Proteins: Essential amino acids are required.
Vitamins: Including B12, B6, B2, E, C; folic acid; pantothenic acid; and niacin.
Minerals: Primarily iron and copper.
Destruction of Senescent Erythrocytes
Old or senescent erythrocytes become increasingly fragile and lose their reversible deformability.
Normal destruction of these aged red cells occurs through sequestration and destruction by macrophages of the mononuclear phagocyte system (MPS).
This process primarily takes place in the spleen, but if the spleen is dysfunctional or absent, the liver takes over this role.
Iron Cycle
Iron is crucial for hemoglobin synthesis.
It is bound to heme within blood and muscle cells, and stored bound to ferritin or hemosiderin, or within mononuclear phagocytes.
The iron cycle involves the recycling of iron from destroyed erythrocytes.
This cycle is controlled by hepcidin.
Macrophages of the MPS (predominantly in the spleen) break down ingested erythrocytes and return the salvaged iron to the bloodstream directly or after storage.
Leukocytes Development
Myelopoiesis: This is the development of granulocytes (neutrophils, eosinophils, basophils) and monocytes.
These cells mature in the bone marrow.
Granulocytes form two pools: a functional and circulating pool, and a marginating storage pool stored in blood vessel walls.
Lymphopoiesis: This is the development of lymphocytes.
Lymphocytes are released into the bloodstream and mature in various lymphoid organs.
Platelet Development
Platelets originate from megakaryocytes.
A megakaryocyte undergoes DNA replication but does not divide. Instead, its cell surface elongates and fragments into thousands of platelets.
One large megakaryocyte can produce thousands of platelets.
Platelet levels are regulated by the hormone thrombopoietin.
Platelets typically circulate for about days before losing their functional capacity.
Senescent platelets are primarily destroyed in the spleen.
Mechanisms of Hemostasis
Hemostasis is the process of arresting bleeding through the formation of blood clots.
The sequence of events in hemostasis includes:
Vascular injury occurs, leading to vasoconstriction.
Damage to endothelial cells causes platelet adherence and the formation of a hemostatic plug.
The clotting system is activated to form stable fibrin clots.
The fibrin/platelet clot then contracts to form a more permanent plug.
Role of Blood Vessels
Endothelial cells normally adhere to the subendothelial matrix of connective tissue.
They produce nitric oxide and prostacyclin, which regulate blood flow and prevent the clotting system from activating inappropriately.
Damage to the endothelium exposes the underlying subendothelial matrix and prompts endothelial cells to release platelet activators, such as von Willebrand factor (vWF or clotting factor VIII).
Role of Platelets
Platelets perform several critical functions in hemostasis:
Regulate blood flow by inducing vasoconstriction at the injury site.
Form a platelet plug to physically stop bleeding.
Activate the coagulation cascade to stabilize the platelet plug with fibrin.
Initiate the repair process, including clot retraction and clot dissolution.
Vessel damage triggers platelet activation, which involves:
Adhesion to the damaged wall.
Activation leading to degranulation and release of clotting factors.
Aggregation to form the platelet plug.
Clotting Factors
A blood clot is a meshwork of protein (fibrin) strands that stabilizes the initial platelet plug.
These fibrin strands are produced by the clotting (coagulation) system, which involves a cascade of protein activations:
Intrinsic pathway: Activated when Factor XII (Hageman factor) comes into contact with subendothelial substances exposed by vascular injury.
Extrinsic pathway: Activated when tissue thromboplastin is released by damaged endothelial cells.
Common pathway: Both intrinsic and extrinsic pathways converge here, leading to the activation of Factor X and subsequent clot formation.
Control of Hemostatic Mechanisms
Several factors on the endothelial cell surface prevent spontaneous hemostasis and limit clot formation to the site of injury:
Thrombin inhibitors (e.g., antithrombin) neutralize thrombin.
Tissue factor inhibitors (e.g., tissue factor pathway inhibitor) block the extrinsic pathway.
Mechanisms for degrading activated clotting factors (e.g., protein C) break down active clotting factors.
Retraction and Lysis of Blood Clots
Clot Retraction
After clot formation, fibrin strands shorten, becoming denser and stronger.
This process helps to approximate the edges of the injured vessel and the site of injury, promoting wound healing.
Clot retraction is facilitated by the large number of platelets within the clot.
Lysis of Blood Clots (Fibrinolytic System)
The fibrinolytic system is responsible for the breakdown and removal of blood clots.
It involves plasminogen and its active form, plasmin, an enzyme that degrades fibrin.
This process results in the formation of fibrin degradation products, which are then cleared from the circulation.
Hematologic System in Infants and Children
Blood cell counts are typically increased above adult levels at birth due to the trauma of birth and the cutting of the umbilical cord.
The hypoxic intrauterine environment stimulates erythropoietin production in the fetus, leading to polycythemia (an abnormally high concentration of hemoglobin in the blood) at birth.
Newborns are at increased risk for:
Impaired phagocytosis.
Bacterial infections.
Delayed wound healing.
However, newborns receive protection from many diseases through passive IgG antibody from their mother.
Hematologic System in Older Adults
The life span of erythrocytes remains normal in older adults, but their replacement rate slows down.
This slower replacement is often caused by iron deficiency, which includes iron depletion, decreased total serum iron, reduced iron-binding capacity, and diminished intestinal iron absorption.
Lymphocyte function generally decreases with age, impacting cellular immunity.
The humoral immune system also becomes less responsive in older adults, affecting antibody production.