Blood and the Cardiovascular System

Overview of the Cardiovascular System

The cardiovascular system is essential for transporting substances throughout the body. It consists of the heart (a pump), blood vessels (conducting hoses), and blood (a fluid connective tissue).

• Heart: Pumps blood throughout the body.

• Blood Vessels: Network of arteries, veins, and capillaries.

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

Components and Functions of Blood

Major Functions of Blood

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

• Regulation: pH, ion composition of interstitial fluids, and body temperature.

• Restriction: Restricts fluid losses at injury sites (clotting).

• Defense: Against toxins and pathogens (immune response).

• Stabilization: Maintains body temperature by absorbing and redistributing heat.

Physical Characteristics of Blood

• Temperature: 38ºC (100.4ºF)

• Viscosity: High (thicker than water)

• pH: Slightly alkaline (7.35–7.45)

• Volume: Approximately 7% of body weight (in kg); e.g., a 75-kg person has about 5.25 L of blood.

Composition of Blood

• Whole Blood: Consists of plasma (fluid) and formed elements (cells and cell fragments).

• Fractionation: The process of separating whole blood into plasma and formed elements.

Plasma

• Makes up about 55% of blood volume.

• Over 90% is water; also contains dissolved proteins and solutes.

• Similar in composition to interstitial fluid due to exchange across capillary walls.

Plasma Proteins

• Origins: Most plasma proteins are produced by the liver; antibodies by plasma cells; peptide hormones by endocrine organs.

Formed Elements

• Red Blood Cells (RBCs): Erythrocytes

• White Blood Cells (WBCs): Leukocytes

• Platelets: Cell fragments involved in clotting

• Hemopoiesis: 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 O2 and CO2.

• RBC Count: Adult male: 4.5–6.3 million/μL; Adult female: 4.2–5.5 million/μL.

• Hematocrit: Percentage of formed elements in blood (Male: 46%, Female: 42%).

Structure of RBCs

• Small, biconcave discs (thin center, thick edge).

• Large surface-area-to-volume ratio for efficient gas exchange.

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

• Flexible to pass through small capillaries (as small as 4 μm).

• Mature RBCs are anucleate (no nucleus), lack mitochondria and ribosomes, and live about 120 days.

Hemoglobin (Hb)

• Protein responsible for transporting respiratory gases.

• Normal levels: Male: 14–18 g/dL; Female: 12–16 g/dL.

• Structure: Four globular subunits (2 alpha, 2 beta), each with a heme group containing iron.

• Each iron binds one O2 molecule (forms oxyhemoglobin, HbO2).

• Fetal hemoglobin binds O2 more readily than adult hemoglobin.

Hemoglobin Function

• Each RBC contains ~280 million Hb molecules (can carry over a billion O2 molecules).

• In tissues (low O2): Hb releases O2, binds CO2 (forms carbaminohemoglobin).

• In lungs (high O2): Hb binds O2, releases CO2.

RBC Formation and Turnover

• ~1% of RBCs replaced daily (~3 million new RBCs/sec).

• Erythropoiesis: RBC formation; in adults, occurs in red bone marrow (myeloid tissue).

• Hemocytoblasts (HSCs): Stem cells that produce myeloid and lymphoid stem cells.

• Stages: Myeloid stem cell → Proerythroblast → Erythroblast → Reticulocyte → Mature RBC.

• Erythropoietin (EPO): Hormone from kidneys/liver that stimulates erythropoiesis in response to hypoxia.

• Requires amino acids, iron, folic acid, vitamins B12 and B6.

• Lack of vitamin B12 causes pernicious anemia.

Hemoglobin Recycling

• Macrophages in spleen, liver, and bone marrow engulf old RBCs and break down Hb.

• Iron is recycled; heme is converted to biliverdin (green), then bilirubin (yellow), excreted in bile.

• Bilirubin buildup causes jaundice; further breakdown produces urobilins and stercobilins.

• Iron is transported by transferrin, stored as ferritin and hemosiderin.

• Hemoglobinuria: Hb in urine (from excessive hemolysis).

• Hematuria: Whole RBCs in urine (from kidney/vessel damage).

Blood Types

Surface Antigens and Blood Groups

• Surface antigens on RBCs identify cells to the immune system.

• Major antigens: A, B, and Rh (D).

• Rh Group: Rh+ (antigen present), Rh– (antigen absent).

Transfusion Reactions

• Agglutinogens: Surface antigens on RBCs.

• Agglutinins: Antibodies in plasma that attack foreign RBC antigens, causing agglutination (clumping).

• Cross-reactions can occur if incompatible blood is transfused, leading to hemolysis.

• Type O– is the universal donor, but cross-reactions are still possible due to other antigens.

White Blood Cells (Leukocytes)

Characteristics and Functions

• Have nuclei and organelles, but lack hemoglobin.

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

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

Types of WBCs

Lymphocyte Classes

• T cells: Cell-mediated immunity; attack foreign cells, regulate other lymphocytes.

• B cells: Humoral immunity; differentiate into plasma cells that produce antibodies.

• Natural Killer (NK) cells: Detect and destroy abnormal cells.

WBC Production and Regulation

• Leukopoiesis: Production of WBCs from hemocytoblasts (myeloid and lymphoid stem cells).

• Colony-Stimulating Factors (CSFs): Hormones that regulate WBC populations (e.g., Multi-CSF, GM-CSF, G-CSF, M-CSF).

• Lymphocytopoiesis: Production of lymphocytes in lymphatic tissues (thymus, spleen, lymph nodes).

WBC Disorders

• Leukopenia: Low WBC count.

• Leukocytosis: High WBC count.

• Leukemia: Cancer of WBCs, marked by extreme leukocytosis.

Platelets (Thrombocytes)

Structure, Function, and Production

• Cell fragments involved in clotting; circulate for 9–12 days.

• Normal count: 150,000–500,000/μL; one-third stored in spleen and other vascular organs.

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

Platelet Production (Thrombocytopoiesis)

• Occurs in red bone marrow from megakaryocytes (giant cells that shed cytoplasmic fragments).

• Regulated by hormones: Thrombopoietin (TPO), Interleukin-6 (IL-6), Multi-CSF.

Hemostasis (Control of Blood Loss)

Phases of Hemostasis

• Vascular Phase: Vascular spasm (smooth muscle contraction) reduces blood flow; endothelial cells release factors and become sticky to seal the break.

• Platelet Phase: Platelets adhere to exposed surfaces, aggregate to form a plug, and release clotting compounds (ADP, thromboxane A2, serotonin, PDGF, Ca2+).

• Coagulation Phase: Cascade of reactions involving clotting factors leads to conversion of fibrinogen to fibrin, forming a stable clot.

Coagulation Pathways

• Extrinsic Pathway: Triggered by tissue factor (Factor III) from damaged tissues; activates Factor X.

• Intrinsic Pathway: Triggered by exposure of blood to collagen; platelets release PF-3; activates Factor X.

• Common Pathway: Factor X activates prothrombin activator, which converts prothrombin to thrombin; thrombin converts fibrinogen to fibrin.

Regulation and Resolution of Clotting

• Positive Feedback: Thrombin stimulates formation of tissue factor and PF-3, accelerating clotting.

• Anticoagulants: Antithrombin-III, heparin, thrombomodulin, and prostacyclin inhibit clotting.

• Calcium and Vitamin K: Essential for synthesis and function of clotting factors.

• Clot Retraction: Platelets contract, pulling torn vessel edges together for repair.

• Fibrinolysis: Gradual breakdown of clot by plasmin (activated from plasminogen by thrombin and t-PA).

Bleeding and Clotting Disorders

• Thrombocytopenia: Low platelet count.

• Hemophilia: Genetic deficiency of clotting factors.

• Thrombophilia: Tendency to form clots.

• Deep Vein Thrombosis (DVT): Formation of clots in deep veins, often in the legs.

Summary Table: Blood Components and Functions

Additional info: This guide expands on the original notes with definitions, normal values, and explanations of processes such as erythropoiesis, hemoglobin recycling, and the coagulation cascade for clarity and completeness.