Chapter 17: Blood – Structure, Function, and Clinical Aspects

Blood: The Internal Transport System

Overview and Functions

Blood is a life-sustaining fluid connective tissue that plays a central role in the cardiovascular system. It is responsible for transporting substances, regulating internal conditions, and protecting the body from threats.

• Transport: Delivers oxygen and nutrients to cells, removes metabolic wastes, and transports hormones.

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

• Protection: Prevents blood loss through clotting and fights infection via immune cells and proteins.

Composition of Blood and Hematopoiesis

Components of Blood

Blood consists of a liquid matrix called plasma and formed elements (cells and cell fragments). It is the only fluid tissue in the body.

• Plasma: Straw-colored, sticky fluid; about 90% water and 10% solutes (proteins, nutrients, electrolytes, gases, hormones, wastes).

• Formed Elements: Erythrocytes (RBCs), leukocytes (WBCs), and platelets.

Physical Characteristics and Volume

• Sticky, opaque fluid with a metallic taste.

• Color varies with oxygen content: scarlet (high O2), dark red (low O2).

• pH: 7.35–7.45 (slightly alkaline).

• Volume: 5–6 L in males, 4–5 L in females (~8% of body weight).

Plasma Composition

• 91% water, 7% proteins (albumin, globulins, fibrinogen), 2% other solutes (electrolytes, nutrients, gases, wastes, vitamins, regulatory substances).

• Albumin: Most abundant plasma protein; maintains osmotic pressure and acts as a carrier.

Formed Elements

• Erythrocytes (RBCs): Anucleate, biconcave discs specialized for gas transport.

• Leukocytes (WBCs): Complete cells with nuclei and organelles; function in immunity.

• Platelets: Cell fragments involved in clotting.

Hematopoiesis

Hematopoiesis is the process of blood cell formation, occurring primarily in red bone marrow. All formed elements arise from hematopoietic stem cells, which differentiate into myeloid or lymphoid lineages.

Erythrocytes (Red Blood Cells)

Structure and Function

RBCs are small, biconcave discs (7.5 μm diameter) lacking nuclei and organelles. Their shape and composition maximize surface area for gas exchange and allow flexibility in capillaries.

• Filled with hemoglobin (Hb), which binds and transports oxygen and carbon dioxide.

• No mitochondria; ATP is produced anaerobically, so RBCs do not consume the oxygen they carry.

Hemoglobin Structure and Gas Transport

• Each hemoglobin molecule consists of four polypeptide chains (2 alpha, 2 beta) and four heme groups, each with a central iron atom.

• Each Hb can carry four O2 molecules; each RBC contains about 250 million Hb molecules.

• Oxygen loading in lungs forms oxyhemoglobin; unloading in tissues forms deoxyhemoglobin.

• About 20% of CO2 binds to Hb, forming carbaminohemoglobin.

Erythropoiesis: Formation of RBCs

Erythropoiesis is the process of RBC production, taking about 15 days and involving several stages from stem cell to mature erythrocyte.

• Stages: Hematopoietic stem cell → myeloid stem cell → proerythroblast → erythroblast (basophilic, polychromatic, orthochromatic) → reticulocyte → erythrocyte.

• Reticulocyte count (1–2% of RBCs) indicates the rate of RBC formation.

Regulation of Erythropoiesis

RBC production is regulated by hormonal (mainly erythropoietin, EPO) and dietary factors. EPO is released by the kidneys in response to hypoxia (low oxygen levels).

• Stimuli for EPO release: decreased RBC count, decreased hemoglobin, reduced O2 availability.

• Testosterone increases EPO production, leading to higher RBC counts in males.

• Dietary requirements: iron, vitamin B12, folic acid, amino acids, lipids, carbohydrates.

Fate and Destruction of Erythrocytes

RBCs have a lifespan of about 120 days. Old RBCs are removed by macrophages in the spleen, and their components are recycled or excreted.

• Iron is stored and reused; heme is degraded to bilirubin (excreted in bile); globin is broken down to amino acids.

Erythrocyte Disorders

• Polycythemia: Excess RBCs increase blood viscosity; causes include bone marrow cancer, high altitude, or EPO abuse.

• Anemia: Low O2-carrying capacity due to blood loss, decreased RBC production, or increased RBC destruction (e.g., sickle-cell anemia).

Leukocytes (White Blood Cells)

Structure and Function

Leukocytes are complete cells with nuclei and organelles, making up less than 1% of blood volume. They defend the body against infection and can leave the bloodstream to enter tissues (diapedesis).

Classification of Leukocytes

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

• Agranulocytes: Lymphocytes, monocytes (lack visible granules).

Granulocytes

• Neutrophils: Most abundant; phagocytize bacteria; granules contain hydrolytic enzymes and defensins.

• Eosinophils: Attack parasitic worms; involved in allergies and asthma.

• Basophils: Rarest; granules contain histamine (inflammatory mediator).

Agranulocytes

• Lymphocytes: Second most abundant; crucial for immunity (T cells and B cells).

• Monocytes: Largest WBCs; differentiate into macrophages in tissues; phagocytic and activate lymphocytes.

Leukopoiesis

Leukopoiesis is the formation of WBCs, stimulated by interleukins and colony-stimulating factors. All WBCs originate from hematopoietic stem cells, differentiating into myeloid or lymphoid lines.

Leukocyte Disorders

• Leukemia: Cancerous overproduction of abnormal WBCs; classified by cell type and rate of progression.

• Leukopenia: Abnormally low WBC count, often drug-induced.

• Infectious Mononucleosis: Viral disease (Epstein-Barr virus) causing high numbers of atypical lymphocytes.

Platelets and Hemostasis

Platelets

Platelets are fragments of megakaryocytes, essential for blood clotting. They contain granules with clotting chemicals and form temporary plugs in damaged vessels.

Hemostasis

Hemostasis is the process of stopping bleeding, involving three steps:

1. Vascular Spasm: Vasoconstriction reduces blood flow after vessel injury.

2. Platelet Plug Formation: Platelets adhere to exposed collagen, become activated, and release chemicals to recruit more platelets (positive feedback).

3. Coagulation (Blood Clotting): Clotting factors lead to the conversion of fibrinogen to fibrin, forming a stable clot.

Clot Retraction and Removal

• Clot retraction draws wound edges together; plasmin digests fibrin to remove the clot after healing.

• Anticoagulants and removal of clotting factors prevent excessive clotting.

Hemostasis Disorders

• Thrombus: Stationary blood clot.

• Embolus: Dislodged clot that can cause blockages (e.g., pulmonary embolism, stroke).

Blood Loss, Transfusions, and Blood Typing

Blood Loss and Replacement

• Blood loss is minimized by vasoconstriction and increased RBC production.

• Severe loss (>30%) can cause shock or death; volume is restored with saline or plasma expanders, but only transfusions restore O2-carrying capacity.

Blood Groups and Typing

• Blood is classified by antigens (agglutinogens) on RBC membranes; the most important are ABO and Rh groups.

• Mismatched transfusions cause agglutination and hemolysis, which can be fatal.

Blood Type	Agglutinogens on RBC	Antibodies in Plasma
A	A	Anti-B
B	B	Anti-A
AB	A and B	None
O	None	Anti-A and Anti-B

• Rh+ means D antigen is present; Rh– means it is absent. Anti-Rh antibodies form after exposure to Rh+ blood.

• Hemolytic disease of the newborn can occur if an Rh– mother carries an Rh+ fetus.

Blood Typing and Cross-Matching

• Blood is typed by mixing with antibodies and observing agglutination.

• Cross-matching ensures compatibility between donor and recipient.