Blood: Components and Functions

Overview of Blood

• Blood is a specialized connective tissue composed of cells suspended in a fluid matrix called plasma.

• It serves multiple essential functions in the body, including transport, regulation, protection, and homeostasis.

Functions of Blood

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

• Regulation: Maintains pH and ion composition of interstitial fluids.

• Restriction of Fluid Loss: Initiates clotting to prevent blood loss at injury sites.

• Defense: Protects against toxins and pathogens via immune cells and antibodies.

• Stabilization of Body Temperature: Distributes heat throughout the body.

Physical Characteristics of Blood

• Temperature: 38ºC (100.4ºF)

• Viscosity: High, due to cells and plasma proteins

• 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 is 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 plasma proteins and other solutes.

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

Plasma Proteins

• Most plasma proteins are synthesized in the liver; antibodies are made by plasma cells, and peptide hormones by endocrine organs.

Formed Elements

• Red blood cells (RBCs) or erythrocytes

• White blood cells (WBCs) or leukocytes

• Platelets (cell fragments)

Hemopoiesis

• The process of producing formed elements, primarily in red bone marrow (myeloid tissue).

Red Blood Cells (Erythrocytes)

Structure and Function

• RBCs make up 99.9% of formed elements and contain hemoglobin, a red pigment that binds and transports O2 and CO2.

• Normal RBC count (per microliter): Males 4.5–6.3 million; Females 4.2–5.5 million.

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

• RBCs are small, biconcave discs, which increases surface area for gas exchange and allows flexibility in capillaries.

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

Hemoglobin (Hb)

• Normal Hb: Males 14–18 g/dL; Females 12–16 g/dL.

• Composed of four globular protein subunits (2 alpha, 2 beta), each with a heme group containing iron.

• Iron binds O2 to form oxyhemoglobin (HbO2); releases O2 to form deoxyhemoglobin.

• Each RBC contains about 280 million Hb molecules, allowing transport of over a billion O2 molecules per cell.

Gas Exchange

• In peripheral capillaries (low O2): Hb releases O2 and binds CO2 (forms carbaminohemoglobin).

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

Anemia

• Occurs when hematocrit or Hb content is reduced, impairing O2 delivery to tissues.

RBC Formation and Turnover

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

• Erythropoiesis: RBC formation; in adults, occurs in red bone marrow from hemocytoblasts (hematopoietic stem cells).

• Stages: Myeloid stem cell → Proerythroblast → Erythroblast stages → 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; deficiency in B12 causes pernicious anemia.

Hemoglobin Recycling

• Macrophages in spleen, liver, and bone marrow engulf aged RBCs and recycle Hb components.

• Iron is stored or transported by transferrin; excess stored as ferritin or hemosiderin.

• Heme is converted to biliverdin (green), then bilirubin (yellow), excreted in bile; buildup causes jaundice.

• Hemoglobinuria: Hb in urine (high hemolysis); Hematuria: RBCs in urine (kidney/vessel damage).

Blood Types

Blood Group Antigens and Antibodies

• Surface antigens on RBCs determine blood type and are recognized by the immune system.

• Main antigens: A, B, and Rh (D).

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

• Only sensitized Rh– individuals have anti-Rh antibodies.

Transfusion Reactions

• Agglutinins (antibodies) in plasma attack foreign antigens, causing agglutination (clumping) and hemolysis.

• Compatibility testing (cross-match) is essential before transfusions.

• Type O– is the universal donor, but other antigens may still cause reactions.

White Blood Cells (Leukocytes)

Types and Functions

• WBCs have nuclei and organelles but lack hemoglobin.

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

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

Characteristics of Circulating WBCs

• Can migrate out of bloodstream (diapedesis).

• Capable of amoeboid movement.

• Attracted to chemical stimuli (positive chemotaxis).

• Some are phagocytic.

Types of WBCs

• T cells: Cell-mediated immunity; attack foreign cells.

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

• NK cells: Destroy abnormal cells.

WBC Disorders

• Leukopenia: Low WBC count.

• Leukocytosis: High WBC count.

• Leukemia: Cancer of WBCs; extreme leukocytosis.

WBC Production (Leukopoiesis)

• Hemocytoblasts produce myeloid and lymphoid stem cells.

• Myeloid stem cells give rise to all formed elements except lymphocytes.

• Lymphoid stem cells produce lymphocytes (in bone marrow and lymphatic tissues).

• Colony-stimulating factors (CSFs) regulate WBC populations.

Platelets (Thrombocytes)

Structure and Function

• Platelets are 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

Phases of Hemostasis

• Hemostasis is the process of stopping bleeding, involving three phases:

  1. Vascular phase: Vascular spasm (smooth muscle contraction) reduces blood flow; endothelial changes promote clotting.

  2. Platelet phase: Platelet adhesion and aggregation form a temporary plug; activated platelets release clotting compounds.

  3. Coagulation phase: Cascade of reactions leads to formation of a stable blood clot.

Coagulation Pathways

• Involves three pathways:

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

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

  • Common pathway: Factor X activates prothrombin activator, converting prothrombin to thrombin, which converts fibrinogen to fibrin, forming the clot.

Key Equations

• Prothrombin (inactive) Thrombin (active)

• Fibrinogen (soluble) Fibrin (insoluble)

Feedback and Regulation

• Positive feedback: Thrombin stimulates tissue factor and PF-3 release, accelerating clotting.

• Anticoagulants (e.g., antithrombin-III, heparin, thrombomodulin, prostacyclin) inhibit clotting.

• Calcium ions (Ca2+) and vitamin K are essential for clotting factor synthesis and function.

Clot Retraction and Fibrinolysis

• Clot retraction pulls vessel edges together, reducing bleeding and aiding repair.

• Fibrinolysis: Gradual dissolution of clot; thrombin and tissue plasminogen activator (t-PA) activate plasminogen to plasmin, which digests fibrin.

Bleeding and Clotting Disorders

• Thrombocytopenia: Low platelet count.

• Hemophilia: Genetic deficiency in clotting factors.

• Thrombophilia: Tendency to form clots.

• Deep vein thrombosis (DVT): Clot formation in deep veins.