Chapter 19: Blood – Structure, Function, and Clinical Significance

Blood and the Cardiovascular System

Overview of the Cardiovascular System

The cardiovascular system is essential for transporting substances throughout the body and maintaining homeostasis. It consists of three main components: blood, the heart, and blood vessels.

• Blood: Specialized connective tissue containing cells suspended in a fluid matrix.

• Heart: A muscular pump that circulates blood.

• Blood Vessels: Conducting passageways that carry blood throughout the body.

Functions of Blood

• Transport: Dissolved gases (O2, CO2), nutrients, hormones, and metabolic wastes.

• Regulation: pH and ion composition of interstitial fluids.

• Restriction: Limits fluid losses at injury sites.

• Defense: Protects against toxins and pathogens.

• Stabilization: Redistributes heat to maintain body temperature.

Characteristics of Blood

• Temperature: 38°C (100.4°F)

• Viscosity: 5 times thicker than water

• pH: Slightly alkaline (7.35–7.45)

• Volume: Approximately 7% of body weight in kilograms

Fractionation of Blood

Fractionation is the process of separating whole blood into plasma and formed elements.

The Composition of Whole Blood

Plasma

Plasma is the fluid component of blood, making up 46–63% of blood volume. It consists primarily of water, plasma proteins, and other solutes.

• Water: 92% of plasma

• Plasma Proteins: Albumins, globulins, fibrinogen

• Other Solutes: Organic nutrients, wastes, electrolytes

Plasma is similar to interstitial fluid but differs in protein and gas concentrations.

Formed Elements

Formed elements are the cellular components of blood, including red blood cells, white blood cells, and platelets.

• Red Blood Cells (RBCs): Erythrocytes, responsible for oxygen transport

• White Blood Cells (WBCs): Leukocytes, involved in immune defense

• Platelets: Cell fragments important for clotting

Red Blood Cells (RBCs)

Structure and Function

Red blood cells are highly specialized for oxygen transport. They are biconcave discs, which increases their surface-area-to-volume ratio and allows for efficient gas exchange.

• Biconcave Shape: Facilitates absorption and release of oxygen

• Rouleaux Formation: RBCs stack for smooth flow through capillaries

• Anucleate: Lack nuclei, cannot divide or repair

• No Mitochondria/Ribosomes: Rely on anaerobic metabolism

Hemoglobin

Hemoglobin is the protein responsible for oxygen and carbon dioxide transport in RBCs. It has a quaternary structure with four polypeptide chains, each containing a heme group.

• Oxyhemoglobin (HbO2): Hemoglobin bound to oxygen

• Deoxyhemoglobin: Hemoglobin not carrying oxygen

• Carbaminohemoglobin: Hemoglobin bound to carbon dioxide

RBC Formation and Turnover

RBCs are produced through erythropoiesis in red bone marrow. They have a lifespan of about 120 days and are continuously replaced.

• Hemocytoblasts: Stem cells that give rise to RBCs

• Erythropoiesis: Process of RBC formation

• Stages: Proerythroblast → Erythroblast → Normoblast → Reticulocyte → Mature RBC

Hemoglobin Recycling

When RBCs are destroyed, their components are recycled. Iron is stored or transported, and heme is converted to biliverdin and then bilirubin, which is excreted in bile.

• Hemolysis: RBC rupture

• Macrophages: Phagocytize old RBCs

• Bilirubin: Excreted by the liver; buildup causes jaundice

• Iron: Transported by transferrin, stored as ferritin/hemosiderin

Blood Types and Cross-Reactions

Blood Type Determination

Blood type is determined by the presence or absence of surface antigens (agglutinogens) on RBCs. There are four main blood types: A, B, AB, and O.

• Type A: Surface antigen A; anti-B antibodies

• Type B: Surface antigen B; anti-A antibodies

• Type AB: Both antigens; no anti-A or anti-B antibodies

• Type O: No antigens; both anti-A and anti-B antibodies

Rh Blood Group

The Rh blood group is based on the presence or absence of the Rh (D) antigen. Rh+ individuals have the antigen, while Rh- individuals do not.

• Rh+: Has Rh antigen; no anti-Rh antibodies

• Rh-: Lacks Rh antigen; can develop anti-Rh antibodies if exposed

• Hemolytic Disease of the Newborn: Occurs with Rh incompatibility between mother and fetus

Transfusion Reactions and Compatibility Testing

Transfusion reactions occur when incompatible blood types are mixed, leading to agglutination and hemolysis. Compatibility testing is essential before transfusions.

• Cross-match Testing: Checks for reactions between donor and recipient

• Blood Typing: Identifies A, B, and Rh antigens

• Universal Donor: Type O-

White Blood Cells (WBCs)

Types and Functions

White blood cells, or leukocytes, are crucial for immune defense. They are classified as granular or agranular based on the presence of cytoplasmic granules.

• Granular Leukocytes: Neutrophils, eosinophils, basophils

• Agranular Leukocytes: Monocytes, lymphocytes

Classes of Lymphocytes

• T Cells: Cell-mediated immunity

• B Cells: Humoral immunity (antibody production)

• Natural Killer (NK) Cells: Immune surveillance

WBC Production and Regulation

WBCs are produced from hemocytoblasts in bone marrow. Colony-stimulating factors (CSFs) regulate their production and differentiation.

• Leukopoiesis: Formation of WBCs

• Lymphocytopoiesis: Formation of lymphocytes

• CSFs: Multi-CSF, GM-CSF, G-CSF, M-CSF

Platelets and Hemostasis

Platelets

Platelets are cell fragments essential for blood clotting. They are produced from megakaryocytes in bone marrow and circulate for 9–12 days.

• Functions: Release clotting chemicals, patch vessel walls, reduce vessel break size

• Thrombocytopoiesis: Platelet production

• Regulation: Thrombopoietin, Interleukin-6, Multi-CSF

Hemostasis

Hemostasis is the process of stopping bleeding, involving vascular, platelet, and coagulation phases.

• Vascular Phase: Vascular spasm reduces blood flow

• Platelet Phase: Platelet adhesion and aggregation form a plug

• Coagulation Phase: Clotting factors activate pathways to form a blood clot

• Clot Retraction: Stabilizes and repairs the vessel

• Fibrinolysis: Gradual dissolution of the clot

Coagulation Pathways

Coagulation involves three pathways: extrinsic, intrinsic, and common. Each pathway activates clotting factors, leading to the conversion of fibrinogen to fibrin.

• Extrinsic Pathway: Initiated by tissue factor from damaged cells

• Intrinsic Pathway: Initiated by exposure to collagen and platelet factors

• Common Pathway: Activation of Factor X, prothrombin to thrombin, fibrinogen to fibrin

Key Equation:

$\text{Prothrombin} \xrightarrow{\text{Prothrombin Activator}} \text{Thrombin}$

$\text{Fibrinogen} \xrightarrow{\text{Thrombin}} \text{Fibrin}$

Regulation and Disorders

• Anticoagulants: Inhibit clotting (e.g., antithrombin III, heparin, thrombomodulin)

• Calcium and Vitamin K: Essential for clotting factor synthesis

• Disorders: Thrombocytopenia, hemophilia, thrombophilia, deep vein thrombosis, pulmonary embolism

Summary Table: Blood Components

Component	Main Function	Key Features
Plasma	Transport, regulation	92% water, proteins, solutes
RBCs	Oxygen transport	Biconcave, hemoglobin, anucleate
WBCs	Immune defense	Granular/agranular, nucleated
Platelets	Clotting	Cell fragments, short lifespan