Blood and the Cardiovascular System

Overview of the Cardiovascular System

The cardiovascular system is essential for transporting substances throughout the body. It consists of three main components:

• Blood – the fluid connective tissue that transports materials.

• Heart – the muscular pump that circulates blood.

• Blood vessels – the network of conduits that carry blood to and from tissues.

Components and Functions of Blood

Main Components of Blood

Blood is a specialized connective tissue composed of cells suspended in a fluid matrix. It consists of:

• Plasma – the liquid matrix, making up 46–63% of blood volume.

• Formed elements – cells and cell fragments, including red blood cells (RBCs), white blood cells (WBCs), and platelets.

Functions of Blood

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

• Regulation of pH and ion composition of interstitial fluids.

• Restriction of fluid losses at injury sites (hemostasis).

• Defense against toxins and pathogens (immune response).

• Stabilization of body temperature by redistributing heat.

Physical Characteristics of Blood

• Temperature: Slightly higher than body temperature.

• Viscosity: Higher than water due to cells and proteins.

• pH: Slightly alkaline (7.35–7.45).

• Volume: About 7% of body weight in liters.

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

Plasma Composition

• 92% water

• 7% plasma proteins (albumins, globulins, fibrinogen)

• 1% other solutes (nutrients, wastes, electrolytes)

Plasma is similar to interstitial fluid but has a higher concentration of dissolved proteins and different gas concentrations.

Plasma Proteins

• Albumins: Most abundant; maintain osmotic pressure and transport substances.

• Globulins: Include antibodies (immunoglobulins) and transport proteins.

• Fibrinogen: Soluble protein involved in clotting; converted to fibrin during coagulation.

Most plasma proteins are synthesized by the liver, except immunoglobulins, which are produced by WBCs.

Formed Elements

• Red blood cells (erythrocytes)

• White blood cells (leukocytes)

• Platelets (thrombocytes)

Hemopoiesis is the process of producing formed elements, primarily in red bone marrow.

Red Blood Cells (Erythrocytes)

Characteristics and Functions

• Make up 99.9% of formed elements.

• Contain hemoglobin, a red pigment that binds and transports oxygen and carbon dioxide.

• Normal RBC count: Adult males 4.5–6.3 million/μL; females 4.2–5.5 million/μL.

• Hematocrit: Percentage of RBCs in whole blood (normal: 37–54%).

Structure of RBCs

• Biconcave discs – increase surface area for gas exchange.

• Form stacks (rouleaux) for smooth flow through capillaries.

• Flexible to pass through small vessels.

• Anucleate (no nucleus), lack mitochondria and ribosomes – cannot divide or repair.

Hemoglobin Structure and Function

• Composed of four polypeptide chains (2 alpha, 2 beta).

• Each chain contains a heme group with an iron ion that binds oxygen.

• Oxygen-bound form: Oxyhemoglobin; without oxygen: Deoxyhemoglobin.

• Can also bind carbon dioxide (carbaminohemoglobin).

RBC Life Cycle and Turnover

• Average lifespan: ~120 days.

• Destroyed by macrophages in spleen, liver, and bone marrow.

• Hemoglobin is recycled: iron is stored or transported, heme is converted to biliverdin and then bilirubin (excreted in bile).

• Disorders: Anemia (low hematocrit or hemoglobin), hemoglobinuria (hemoglobin in urine), hematuria (RBCs in urine).

Erythropoiesis (RBC Formation)

• Occurs in red bone marrow (myeloid tissue) in adults.

• Stem cells (hemocytoblasts) differentiate into myeloid and lymphoid stem cells.

• Stages: Proerythroblast → Erythroblast → Normoblast (nucleus ejected) → Reticulocyte → Mature RBC.

• Regulated by erythropoietin (EPO), secreted by kidneys and liver in response to hypoxia.

• Requires amino acids, iron, and vitamins (especially folic acid).

Blood Types and Transfusion Compatibility

ABO Blood Group System

• Determined by presence or absence of surface antigens (agglutinogens) A and B on RBCs.

• Four main types: A, B, AB, O.

• Plasma contains antibodies (agglutinins) against foreign antigens.

Rh Blood Group

• Based on presence (Rh+) or absence (Rh−) of the D antigen.

• Rh− individuals can develop anti-Rh antibodies if exposed to Rh+ blood (sensitization).

• Hemolytic disease of the newborn (HDN) can occur if an Rh− mother carries an Rh+ fetus.

Transfusion Reactions and Compatibility Testing

• Cross-reaction: Occurs if donor and recipient blood types are incompatible, leading to agglutination and hemolysis.

• Compatibility testing: Includes cross-match and blood typing for A, B, and Rh antigens.

• Type O− is the universal donor; AB+ is the universal recipient.

White Blood Cells (Leukocytes)

Types and Functions

• Defend against pathogens, remove toxins and wastes, attack abnormal cells.

• Have nuclei and organelles, but lack hemoglobin.

• Most are found in connective tissues and lymphatic organs; only a small fraction circulates in blood (5,000–10,000/μL).

Classification of WBCs

• Granular leukocytes: Neutrophils, eosinophils, basophils.

• Agranular leukocytes: Monocytes, lymphocytes.

Neutrophils

• 50–70% of WBCs; multilobed nucleus; pale granules.

• First responders to injury; phagocytize bacteria; form pus.

Eosinophils

• 2–4% of WBCs; attack parasites; involved in allergic responses.

• Release enzymes to reduce inflammation.

Basophils

• <1% of WBCs; release histamine (vasodilation) and heparin (anticoagulant).

• Enhance inflammation at injury sites.

Monocytes

• 2–8% of WBCs; become macrophages in tissues; aggressive phagocytes.

• Attract other immune cells and fibroblasts for tissue repair.

Lymphocytes

• 20–40% of WBCs; specific immunity; migrate between blood and tissues.

• Types: T cells (cell-mediated immunity), B cells (antibody production), NK cells (immune surveillance).

WBC Production and Regulation

• Leukopoiesis: Formation of WBCs from hemocytoblasts in bone marrow.

• Regulated by colony-stimulating factors (CSFs): Multi-CSF, GM-CSF, G-CSF, M-CSF.

• Lymphocytes mature in lymphatic tissues (T cells in thymus, B and NK cells in bone marrow).

Platelets (Thrombocytes)

Structure, Function, and Production

• Cell fragments involved in clotting; circulate 9–12 days; removed by spleen.

• Normal count: 150,000–500,000/μL.

• Functions: Release clotting chemicals, form temporary plugs, reduce vessel break size.

• Produced by thrombocytopoiesis in bone marrow from megakaryocytes.

• Regulated by thrombopoietin (TPO), interleukin-6 (IL-6), and Multi-CSF.

Hemostasis: Control of Blood Loss

Phases of Hemostasis

• Vascular phase: Vascular spasm reduces blood flow; endothelial cells release factors for repair.

• Platelet phase: Platelets adhere to exposed collagen, aggregate, and release chemicals to promote clotting.

• Coagulation phase: Cascade of clotting factors leads to fibrin formation and blood clot stabilization.

Coagulation Pathways

• Extrinsic pathway: Triggered by tissue factor from damaged cells.

• Intrinsic pathway: Initiated by exposure of blood to collagen.

• Common pathway: Both pathways activate Factor X, leading to conversion of prothrombin to thrombin, and fibrinogen to fibrin.

Regulation and Disorders of Clotting

• Anticoagulants (e.g., antithrombin III, heparin, thrombomodulin, prostacyclin) prevent excessive clotting.

• Calcium ions and vitamin K are essential for clotting factor synthesis and function.

• Disorders: Thrombocytopenia (low platelets), hemophilia (inherited bleeding), thrombophilia (excess clotting), deep vein thrombosis (DVT), pulmonary embolism.

Clot Retraction and Fibrinolysis

• Clot retraction: Platelets contract to reduce the size of the clot and facilitate tissue repair.

• Fibrinolysis: Plasminogen is converted to plasmin, which digests fibrin and dissolves the clot.

Additional info: These notes provide a comprehensive overview of blood structure, function, and clinical relevance, suitable for exam preparation in Anatomy & Physiology courses.