Blood: Structure and Functions

Functions of Blood

Blood is a vital connective tissue that performs several essential functions in the human body.

• Transport: Carries oxygen, nutrients, hormones, and waste products.

• Regulation: Maintains pH, temperature, and fluid balance.

• Protection: Defends against pathogens and prevents blood loss through clotting.

Example: Blood transports oxygen from the lungs to tissues and removes carbon dioxide for exhalation.

Physical and Chemical Properties of Blood

Blood has specific measurable properties that are important for homeostasis.

• pH: Normally ranges from 7.35 to 7.45.

• Specific Gravity: Approximately 1.060.

• Volume: Average adult has 4-6 liters.

• Hematocrit: Percentage of blood volume occupied by red blood cells (RBCs); normal values are 38-48% for females, 40-54% for males.

Classification and Composition of Blood

Blood is classified as a connective tissue due to its cellular components suspended in plasma.

• Living Matrix: The formed elements (RBCs, WBCs, platelets).

• Non-living Matrix: Plasma, the liquid component.

Whole Blood and Plasma

Whole blood consists of plasma and formed elements.

• Plasma: Straw-colored fluid, 55% of blood volume, contains water, proteins (albumin, globulins, fibrinogen), electrolytes, nutrients, hormones, and waste products.

• Formed Elements: RBCs, WBCs, and platelets.

Example: Plasma transports hormones and nutrients throughout the body.

Formed Elements of Blood

Red Blood Cells (RBCs)

RBCs, or erythrocytes, are specialized for oxygen transport.

• Structure: Biconcave discs, lack nuclei in mature form.

• Function: Carry oxygen via hemoglobin; remove carbon dioxide.

• Lifespan: Approximately 120 days.

Example: RBCs deliver oxygen to muscle cells during exercise.

White Blood Cells (WBCs)

WBCs, or leukocytes, are involved in immune defense.

• Types: Neutrophils, lymphocytes, monocytes, eosinophils, basophils.

• Functions: Phagocytosis, antibody production, inflammation, allergic responses.

Example: Neutrophils engulf bacteria during infection.

Platelets

Platelets, or thrombocytes, are cell fragments essential for blood clotting.

• Function: Initiate clot formation by adhering to damaged blood vessels.

• Regulation: Platelet production is controlled by the hormone thrombopoietin.

Hematopoiesis: Blood Cell Formation

Sites of Hematopoiesis

Blood cells are produced in the red bone marrow.

• Adults: Vertebrae, sternum, ribs, pelvis, proximal femur and humerus.

• Children: All bones contain active marrow.

Development of RBCs

RBCs develop from pluripotent stem cells in the bone marrow.

• Stages: Proerythroblast → Erythroblast → Reticulocyte → Mature erythrocyte.

• Reticulocyte: Immature RBC released into circulation; matures within 1-2 days.

• Nucleus Ejection: RBCs eject their nucleus during maturation.

Example: Reticulocyte count is used clinically to assess bone marrow activity.

Hormonal Regulation

Erythropoiesis is regulated by the hormone erythropoietin (EPO), produced by the kidneys in response to hypoxia.

• Negative Feedback: Increased RBCs reduce EPO production.

Hemoglobin and Oxygen Transport

Structure and Function of Hemoglobin

Hemoglobin (Hb) is a protein in RBCs that binds oxygen.

• Structure: Four polypeptide chains, each with a heme group containing iron.

• Function: Transports oxygen and carbon dioxide.

Equation:

Fetal vs. Adult Hemoglobin

Fetal hemoglobin (HbF) has a higher affinity for oxygen than adult hemoglobin (HbA), facilitating oxygen transfer from mother to fetus.

• HbF: Two alpha and two gamma chains.

• HbA: Two alpha and two beta chains.

Blood Cell Lifespan and Removal

RBC Lifespan and Breakdown

RBCs circulate for about 120 days before being removed by macrophages in the spleen and liver.

• Breakdown Products: Hemoglobin is split into heme and globin; iron is recycled, and bilirubin is excreted in bile.

White Blood Cells: Types and Functions

Classification of WBCs

WBCs are classified as granulocytes or agranulocytes.

• Granulocytes: Neutrophils, eosinophils, basophils (contain granules).

• Agranulocytes: Lymphocytes, monocytes (lack granules).

Functions and Characteristics

• Neutrophils: Most numerous; phagocytize bacteria.

• Lymphocytes: Second most numerous; involved in immune responses.

• Monocytes: Become macrophages in tissues.

• Eosinophils: Combat parasites and mediate allergic responses.

• Basophils: Release histamine and heparin; involved in inflammation.

WBC Movement and Response

• Diapedesis: Movement of WBCs out of blood vessels into tissues.

• Amoeboid Motion: WBCs move by changing shape.

• Chemotaxis: WBCs are attracted to sites of infection by chemical signals.

Platelets and Hemostasis

Platelet Function

Platelets are essential for initiating hemostasis, the process that stops bleeding.

• Platelet Adhesion: Platelets stick to exposed collagen at injury sites.

• Platelet Aggregation: Platelets clump together to form a plug.

• Platelet Release: Release chemicals that promote clotting.

Hemostasis: Stages and Mechanisms

Hemostasis involves three main stages:

• Vascular Spasm: Vasoconstriction reduces blood flow.

• Platelet Plug Formation: Platelets adhere and aggregate.

• Coagulation: Fibrin forms a stable clot.

Coagulation Pathways

• Intrinsic Pathway: Initiated by damage inside the vessel.

• Extrinsic Pathway: Initiated by external trauma.

• Common Pathway: Both pathways converge to activate prothrombin to thrombin, which converts fibrinogen to fibrin.

Equation:

Fibrinolysis

Fibrinolysis is the process of breaking down clots after healing.

• Plasmin: Enzyme that digests fibrin.

Blood Groups and Transfusion

ABO Blood Group System

The ABO system classifies blood based on the presence of antigens (agglutinogens) on RBCs.

Transfusion Compatibility

Blood transfusions require matching donor and recipient blood types to prevent reactions.

• Universal Donor: Type O (no antigens).

• Universal Recipient: Type AB (no antibodies).

Rh Factor

The Rh system is based on the presence (+) or absence (−) of the D antigen.

• Rh incompatibility: Can cause hemolytic disease of the newborn (erythroblastosis fetalis).

Clinical Terms and Disorders

Common Blood Disorders

• Anemia: Reduced oxygen-carrying capacity due to low RBCs or hemoglobin.

• Polycythemia: Excess RBCs increase blood viscosity.

• Leukemia: Cancer of WBCs.

• Leukopenia: Low WBC count.

• Thrombocytopenia: Low platelet count.

Hemolytic Disease of the Newborn

Occurs when an Rh-negative mother carries an Rh-positive fetus, leading to antibody-mediated destruction of fetal RBCs.

Additional Terms

• Buffy coat: Layer of WBCs and platelets in centrifuged blood.

• Immunoglobulin: Antibody protein.

• Transferrin: Iron transport protein.

• Ferritin: Iron storage protein.

• Bilirubin: Breakdown product of heme.

• Urobilin: Urine pigment from bilirubin.

• Stercobilin: Fecal pigment from bilirubin.

• Jaundice: Yellowing due to excess bilirubin.

• Serum: Plasma without clotting factors.

• Fibrinogen: Clotting protein.

• Prothrombin: Clotting factor precursor.

• Reticulocyte count: Measure of immature RBCs.

• Oxyhemoglobin: Hemoglobin bound to oxygen.

• Deoxyhemoglobin: Hemoglobin not bound to oxygen.

• Hypoxia: Low oxygen levels.

• Megakaryocyte: Platelet precursor cell.

• Tissue plasminogen activator: Enzyme that activates plasmin.

• Anticoagulant: Substance that prevents clotting.

• Procoagulant: Substance that promotes clotting.

• Platelet adhesion: Platelet binding to vessel wall.

• Agglutinin: Antibody causing agglutination.

• Agglutinogen: Antigen causing agglutination.

• Embolus: Traveling blood clot.

• Thrombus: Stationary blood clot.

• Transfusion reaction: Immune response to incompatible blood.

• Erythroblastosis fetalis: Hemolytic disease of the newborn.

Additional info: Academic context and definitions were expanded for clarity and completeness.