16.2 Blood Consists of Plasma and Formed Elements

Overview of Blood

Blood is a specialized connective tissue with unique physical and chemical properties. It plays a vital role in transporting substances, regulating body functions, and protecting against disease.

• Definition: Blood is composed of plasma (the liquid matrix) and formed elements (cells and cell fragments).

• Physical Characteristics: Blood is slightly basic (pH 7.35–7.45), denser than water, and has a bright red color when oxygenated.

• Volume: Normal blood volume is 5–6 liters in males and 4–5 liters in females.

Components of Blood

• Plasma: The liquid portion, making up about 55% of blood volume. Contains water, proteins (albumin, globulins, fibrinogen), nutrients, gases, hormones, wastes, and electrolytes.

• Formed Elements: Includes erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (cell fragments).

Separation of Blood Components

• When centrifuged, blood separates into three layers:

  1. Plasma (top, ~55%)

  2. Buffy coat (middle, <1%; contains leukocytes and platelets)

  3. Erythrocytes (bottom, ~45%)

• Hematocrit: The percentage of blood volume occupied by erythrocytes; a key indicator of oxygen-carrying capacity.

16.3 Erythrocytes Transport Oxygen

Structure and Function of Erythrocytes

Erythrocytes, or red blood cells (RBCs), are specialized for oxygen transport.

• Shape: Biconcave discs, increasing surface area for gas exchange.

• Contents: Lack nuclei and most organelles; filled with hemoglobin.

• Hemoglobin: A protein that binds and transports oxygen and carbon dioxide.

• Normal Hemoglobin Levels: 13–18 g/dL (males), 12–16 g/dL (females).

Hemoglobin Structure and Function

• Composed of four polypeptide chains, each with a heme group containing iron.

• Each iron atom binds one O2 molecule; thus, each hemoglobin can carry four O2 molecules.

• Oxygen binds reversibly to hemoglobin, allowing efficient transport and release.

• About 20% of CO2 is also transported by hemoglobin.

Production and Life Cycle of Erythrocytes

• Hematopoiesis: Formation of blood cells in red bone marrow.

• All blood cells arise from hematopoietic stem cells.

• Erythropoiesis is the specific process of RBC formation, regulated by erythropoietin (EPO) from the kidneys.

• RBCs have a lifespan of 100–120 days; old cells are removed by macrophages in the spleen.

Erythrocyte Disorders

• Anemia: Deficiency in RBCs or hemoglobin, leading to reduced oxygen-carrying capacity. Causes include blood loss, iron deficiency, vitamin B12 deficiency, or genetic disorders (e.g., sickle cell anemia).

• Polycythemia: Excess RBCs, increasing blood viscosity and risk of clotting.

16.4 Leukocytes Defend the Body

Types and Functions of Leukocytes

Leukocytes, or white blood cells (WBCs), are crucial for immune defense and make up less than 1% of total blood volume.

• Granulocytes: Contain visible cytoplasmic granules.

  • Neutrophils: Most abundant; phagocytize bacteria.

  • Eosinophils: Combat parasites; involved in allergies.

  • Basophils: Release histamine; involved in inflammation and allergic responses.

• Agranulocytes: Lack visible granules.

  • Lymphocytes: B cells (produce antibodies), T cells (attack infected/tumor cells).

  • Monocytes: Differentiate into macrophages; phagocytize pathogens and debris.

Leukocyte Disorders

• Leukopenia: Abnormally low WBC count; increases infection risk.

• Leukocytosis: Elevated WBC count; often indicates infection.

• Leukemia: Cancer of WBCs; uncontrolled proliferation of abnormal cells.

• Infectious Mononucleosis: Viral infection causing excess atypical lymphocytes.

16.5 Platelets Are Cell Fragments That Help Stop Bleeding

Structure and Function of Platelets

Platelets are small, anucleate cell fragments derived from megakaryocytes. They play a key role in hemostasis (prevention of blood loss).

• Platelets form the temporary platelet plug in damaged vessels.

• Thrombopoietin regulates platelet production.

• Platelets circulate for about 10 days before being removed by the spleen.

16.6 Hemostasis Prevents Blood Loss

Phases of Hemostasis

Hemostasis is the process that stops bleeding and involves three main steps:

1. Vascular Spasm: Vasoconstriction reduces blood flow to the injured area.

2. Platelet Plug Formation: Platelets adhere to exposed collagen fibers, become activated, and aggregate to form a plug.

3. Coagulation (Blood Clotting): A cascade of reactions leads to the conversion of fibrinogen to fibrin, forming a stable clot.

Coagulation Pathways

• Two main pathways: Intrinsic (triggered by damage inside the vessel) and Extrinsic (triggered by external trauma).

• Both pathways lead to the activation of factor X, which converts prothrombin to thrombin, and then fibrinogen to fibrin.

• Equation:

Clot Retraction and Fibrinolysis

• Clot retraction stabilizes the clot; platelets contract to pull wound edges together.

• Fibrinolysis dissolves the clot after healing, primarily via the enzyme plasmin.

Hemostatic Disorders

• Thromboembolic Disorders: Unwanted clot formation (e.g., deep vein thrombosis).

• Bleeding Disorders: Impaired clot formation (e.g., hemophilia, thrombocytopenia).

16.7 Transfusion Can Replace Lost Blood

Blood Groups and Transfusion Reactions

Blood transfusions require compatibility between donor and recipient blood types to prevent immune reactions.

• ABO Blood Groups: Determined by presence of A and/or B antigens on RBCs.

  • Type A: A antigen, anti-B antibodies

  • Type B: B antigen, anti-A antibodies

  • Type AB: A and B antigens, no anti-A or anti-B antibodies (universal recipient)

  • Type O: No antigens, both anti-A and anti-B antibodies (universal donor)

• Rh Factor: Presence (+) or absence (–) of D antigen. Rh– individuals can develop anti-Rh antibodies after exposure.

Transfusion Reactions

• Occur if recipient's antibodies attack donor RBCs, causing agglutination and hemolysis.

• Cross-matching and blood typing are essential before transfusion.

16.8 Blood Tests Give Insights into a Patient's Health

Diagnostic Importance of Blood Tests

Blood tests are essential for diagnosing and monitoring various health conditions.

• Visual Properties: Changes in color, clarity, or viscosity can indicate disease.

• Differential WBC Count: Assesses proportions of different leukocyte types to detect infection or immune disorders.

• Prothrombin Time and Platelet Count: Evaluate clotting ability and hemostatic status.

• Comprehensive Metabolic Panel (CMP) and Complete Blood Count (CBC): Provide overall assessment of blood health and organ function.

Additional info: These notes are based on standard Anatomy & Physiology curriculum and expand on the provided outline for clarity and completeness.