Blood: Structure, Function, and Clinical Relevance

Blood: Structure and Function

Overview of Blood as a Connective Tissue

Blood is a specialized connective tissue composed of cells (formed elements) suspended in a liquid matrix called plasma. It plays a vital role in transport, immunity, and homeostasis.

• Cells: Red blood cells (RBCs), white blood cells (WBCs), and platelets

• Matrix: Plasma (liquid component)

• Definition of Connective Tissue: Characterized by scattered cells, matrix, ground substance, and fibers

Functions of Blood

• Transport: Delivers oxygen to tissues and removes carbon dioxide

• Immune Function: Mediated by WBCs and antibodies

• Buffering: Maintains pH via the bicarbonate buffer system and carbonic anhydrase

Characteristics of Blood

• Total volume: 5–6 L in adults

• RBC count: 4–6 million/mm3

• WBC count: 5,000–10,000/mm3

• pH: 7.35–7.45 (slightly alkaline)

• Viscosity: More viscous than water due to dissolved proteins

Hematocrit

Hematocrit is the percentage of RBCs in a blood sample. Normal range: 38–55% (higher in males).

Plasma

Composition and Function

Plasma is a colloid, primarily water (~90%), containing dissolved substances such as carbohydrates, proteins, ions, and lipoproteins.

• Proteins: Albumin (osmotic pressure, transport), globulins (immune function), fibrinogen (clotting)

• Ions: K+, Na+, Cl-, HCO3-, Ca2+

• Other solutes: Glucose, amino acids, lipids, hormones, waste products

Albumin

• Maintains osmotic pressure, preventing edema

• Produced in the liver

• Transports substances such as bilirubin and fatty acids

Formed Elements

Red Blood Cells (Erythrocytes)

RBCs are anucleate cells specialized for oxygen transport. They are produced in the bone marrow and have a lifespan of ~120 days.

• Hemoglobin: Protein with four globin chains, each containing a heme group with iron that binds oxygen

• Normal hemoglobin: 13.5–17.5 g/dL (men), 12–15.5 g/dL (women)

• Enzyme: Carbonic anhydrase (converts CO2 to bicarbonate)

Hematopoiesis and Erythropoiesis

Blood cells are formed in red bone marrow from pluripotent stem cells (hemocytoblasts). Erythropoiesis is the process of RBC formation, stimulated by erythropoietin (EPO) from the kidneys in response to hypoxia.

• Reticulocyte: Last immature stage before mature RBC; increased reticulocytes indicate increased erythropoiesis

Fate of Red Blood Cells

• Destroyed in the liver and spleen when aged

• Hemoglobin breakdown: iron is recycled, globin is degraded to amino acids, heme is converted to bilirubin (excreted in bile)

Red Blood Cell Disorders

• Polycythemia: Excess RBCs; increases blood viscosity

• Anemia: Decreased oxygen-carrying capacity (various types: hemorrhagic, hemolytic, sickle cell, pernicious, aplastic)

Blood Typing and Transfusion

ABO Blood Group System

Blood types are determined by the presence of specific glycoprotein antigens (agglutinogens) on RBC surfaces. The main types are A, B, AB, and O.

• Type A: A antigen, anti-B antibodies

• Type B: B antigen, anti-A antibodies

• Type AB: Both A and B antigens, no antibodies (universal recipient)

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

Rh Factor and Hemolytic Disease of the Newborn

The Rh (D) antigen determines positive or negative blood type. Rh incompatibility can cause erythroblastosis fetalis in newborns if an Rh-negative mother carries an Rh-positive fetus.

• Prevention: Rh-negative mothers receive Rh immunoglobulin (Rhogam) to prevent antibody formation

White Blood Cells (Leukocytes)

General Characteristics

• Contain nuclei and organelles

• 5,000–10,000/mm3 in blood; many reside in tissues

• Functions: immune defense, diapedesis (migration through vessel walls), chemotaxis, phagocytosis

Types of White Blood Cells

• Granulocytes: Neutrophils, eosinophils, basophils

• Agranulocytes: Lymphocytes, monocytes

Neutrophils

• Most abundant (50–70%)

• First responders to infection; phagocytize bacteria

Eosinophils

• 2–5% of WBCs

• Bilobed nucleus, reddish-orange granules

• Combat parasitic infections and mediate allergic responses

Basophils

• 0–1% of WBCs

• Release histamine and heparin; mediate inflammation and allergic reactions

Monocytes

• 3–8% of WBCs

• Phagocytic; become macrophages in tissues

Lymphocytes

• 20–40% of WBCs

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

WBC Disorders

• Leukopenia: Low WBC count; increased infection risk

• Leukocytosis: High WBC count; often due to infection or inflammation

• Leukemia: Cancer of blood-forming tissues; abnormal proliferation of WBCs

Platelets and Hemostasis

Platelets (Thrombocytes)

• Cell fragments derived from megakaryocytes

• 200,000–400,000/mm3

• Essential for blood clotting (hemostasis)

Hemostasis: Stopping Bleeding

• Vasoconstriction: Narrowing of blood vessels to reduce blood loss (mediated by serotonin)

• Platelet Plug Formation: Platelets adhere to exposed collagen, aggregate, and release chemicals (ADP, serotonin, thromboxane A2)

• Coagulation: Formation of a stable fibrin clot via the coagulation cascade

• Clot Retraction: Fibrin strands contract, reducing clot size and aiding tissue repair

Coagulation Cascade

The coagulation cascade involves intrinsic and extrinsic pathways leading to the formation of fibrin, which stabilizes the clot.

• Key steps:

  • Formation of prothrombinase

  • Conversion of prothrombin to thrombin

  • Conversion of fibrinogen to fibrin

Clot Dissolution (Fibrinolysis)

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

Clotting Disorders

• Hemophilia: Genetic deficiency of clotting factors

• Thrombosis: Abnormal clot formation in unbroken vessels

• Embolus: A clot fragment traveling in the bloodstream

Key Vocabulary and Reference Values

• RBC count: 4–6 million/mm3

• Hemoglobin: 12–18 g/dL

• Hematocrit: 38–55%

• Total blood volume: 4–6 L

• pH: 7.35–7.45

• Platelets: 200,000–400,000/mm3

• WBC count: 5,000–10,000/mm3

Additional info: This guide covers the essential structure, function, and clinical relevance of blood, including its cellular and plasma components, hematopoiesis, blood typing, immune function, and hemostasis. It is suitable for ANP college-level study and exam preparation.