Blood: Structure, Function, and Disorders

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Blood: Structure, Function, and Disorders

Introduction to Blood

Blood is a specialized connective tissue that plays a vital role in the transport of substances, regulation of physiological processes, and protection against disease. It is the only fluid tissue in the body and is essential for maintaining homeostasis.

Major Components: Plasma (liquid matrix), formed elements (cells and cell fragments)
Volume: 4-6 liters in adults
pH: 7.35–7.45 (slightly alkaline)

Physical Characteristics of Blood

Color: Scarlet (oxygen-rich) to dark red (oxygen-poor)
Viscosity: 4–5 times thicker than water
Temperature: ~38°C (100.4°F)
Volume: 8% of body weight

Components of Blood

Plasma: 55% of whole blood; mostly water, proteins, nutrients, hormones, and waste products
Formed Elements: 45% of whole blood; includes erythrocytes (RBCs), leukocytes (WBCs), and platelets

Functions of Blood

Transport: Oxygen, carbon dioxide, nutrients, hormones, and waste products
Regulation: Body temperature, pH, and fluid volume
Protection: Against blood loss (clotting) and infection (immune response)

Formed Elements

Erythrocytes (Red Blood Cells, RBCs)

Structure: Biconcave discs, anucleate, filled with hemoglobin
Function: Transport oxygen and carbon dioxide
Lifespan: ~120 days

Hemoglobin

Structure: Four polypeptide chains (2 alpha, 2 beta in adults; 2 alpha, 2 gamma in fetuses)
Function: Binds and transports oxygen; each hemoglobin molecule can carry four O2 molecules
Equation:

RBC Production (Erythropoiesis)

Location: Red bone marrow
Stages: Hematopoietic stem cell → Proerythroblast → Erythroblast → Reticulocyte → Erythrocyte
Regulation: Erythropoietin (EPO) hormone from kidneys stimulates production in response to hypoxia

Dietary Requirements for Erythropoiesis

Iron: Required for hemoglobin synthesis
Vitamin B12 and Folic Acid: Necessary for DNA synthesis and cell division

Fate and Destruction of Erythrocytes

Old RBCs are phagocytized in the spleen and liver
Hemoglobin is broken down: iron is recycled, heme is converted to bilirubin

Erythrocyte Disorders

Anemia: Reduced oxygen-carrying capacity due to low RBCs or hemoglobin
Polycythemia: Excess RBCs increase blood viscosity

Causes of Anemia

Blood loss: Hemorrhagic anemia
Decreased RBC production: Iron-deficiency, pernicious anemia (B12 deficiency), aplastic anemia
Increased RBC destruction: Hemolytic anemia, sickle cell anemia, thalassemia

Leukocytes (White Blood Cells, WBCs)

Leukocytes are immune cells that defend the body against infection and foreign invaders. They are classified as granulocytes or agranulocytes based on the presence of cytoplasmic granules.

Types of Leukocytes

Granulocytes: Neutrophils, eosinophils, basophils
Agranulocytes: Lymphocytes, monocytes

Neutrophils

Most abundant WBC; phagocytize bacteria

Eosinophils

Combat parasitic infections; involved in allergic responses

Basophils

Release histamine; involved in inflammatory responses

Lymphocytes

B cells (produce antibodies), T cells (cell-mediated immunity), and NK cells (destroy abnormal cells)

Monocytes

Differentiate into macrophages; phagocytize pathogens and debris

Leukopoiesis (Production of WBCs)

Originates from hematopoietic stem cells in bone marrow
Stimulated by interleukins and colony-stimulating factors (CSFs)

Leukocyte Disorders

Leukemia: Cancer of WBCs; abnormal proliferation of immature leukocytes
Leukopenia: Abnormally low WBC count

Platelets and Hemostasis

Platelets (Thrombocytes)

Cell fragments derived from megakaryocytes
Essential for blood clotting (hemostasis)

Hemostasis

Hemostasis is the process that stops bleeding at the site of injury. It involves three major steps:

Vascular Spasm: Vasoconstriction reduces blood flow
Platelet Plug Formation: Platelets adhere to exposed collagen and aggregate
Coagulation: Fibrin mesh forms, stabilizing the clot

Coagulation Pathways

Intrinsic Pathway: Initiated by damage to blood vessel endothelium
Extrinsic Pathway: Triggered by external trauma and tissue factor release
Both pathways converge to activate prothrombin to thrombin, which converts fibrinogen to fibrin

Equation:

Clot Retraction and Repair

Clot contracts to bring wound edges together
Platelet-derived growth factor (PDGF) stimulates tissue repair

Fibrinolysis

Process of clot removal after healing
Plasmin digests fibrin mesh

Disorders of Hemostasis

Thromboembolic Conditions: Unwanted clot formation (thrombus, embolus)
Bleeding Disorders: Impaired clot formation (hemophilia, thrombocytopenia)
Disseminated Intravascular Coagulation (DIC): Both clotting and bleeding occur simultaneously

Blood Groups and Transfusions

ABO Blood Groups

Based on presence or absence of A and B antigens on RBCs
Type A: A antigen, anti-B antibody
Type B: B antigen, anti-A antibody
Type AB: Both antigens, no antibodies (universal recipient)
Type O: No antigens, both antibodies (universal donor)

Type	Antigen Present	Antibody Present	Can Give To	Can Receive From
A	A	Anti-B	A, AB	A, O
B	B	Anti-A	B, AB	B, O
AB	A, B	None	AB	All
O	None	Anti-A, Anti-B	All	O

Rh System

Rh+ has D antigen; Rh- lacks D antigen
Rh incompatibility can cause hemolytic disease of the newborn

Transfusion Reactions

Occur if mismatched blood is transfused; recipient antibodies attack donor RBCs

Blood Volume Replacement

Plasma expanders or isotonic saline can temporarily restore blood volume

Summary Table: Major Blood Disorders

Disorder	Description	Main Cause
Anemia	Low oxygen-carrying capacity	Blood loss, low production, high destruction
Polycythemia	Excess RBCs	Bone marrow cancer, high altitude
Leukemia	Cancer of WBCs	Uncontrolled WBC proliferation
Hemophilia	Impaired clotting	Genetic deficiency of clotting factors
Thrombocytopenia	Low platelet count	Bone marrow suppression, autoimmune

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