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

Blood: Internal Transport System

Overview

Blood is a life-sustaining fluid tissue that serves as the transport vehicle of the cardiovascular system. It is essential for delivering nutrients, removing wastes, and supporting homeostasis and immunity.

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

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

• Protection: Prevents blood loss and infection via clotting and immune agents.

Composition of Blood

Main Components

Blood consists of plasma (the matrix) and formed elements (cells and cell fragments).

• Plasma: Straw-colored fluid, about 90% water, contains dissolved solutes.

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

Physical Characteristics and Volume

• Sticky, opaque fluid with metallic taste.

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

• pH: 7.35–7.45; makes up ~8% of body weight.

• Average volume: Males 5–6 L, Females 4–5 L.

Blood Plasma

Plasma is the liquid portion of blood, containing water, proteins, nutrients, gases, hormones, and wastes.

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

• Globulins: Transport proteins and antibodies.

• Fibrinogen: Essential for blood clotting.

Constituent	Description and Importance
Water	90% of plasma volume; dissolving and suspending medium for solutes of blood; absorbs heat
Electrolytes	Most abundant solutes by number; help maintain osmotic pressure, pH, and membrane potential
Plasma proteins	Albumin (60%), globulins (36%), fibrinogen (4%)

Constituent	Description and Importance
Nonprotein nitrogenous substances	By-products of cellular metabolism
Nutrients (organic)	Materials absorbed from digestive tract
Respiratory gases	Oxygen and carbon dioxide
Hormones	Steroid and thyroid hormones carried by plasma proteins

Formed Elements

Types and Characteristics

• Erythrocytes (RBCs): No nuclei or organelles; dedicated to gas transport.

• Leukocytes (WBCs): Complete cells; defense against disease.

• Platelets: Cell fragments; essential for clotting.

Erythrocytes (Red Blood Cells)

Structural Characteristics

Erythrocytes are small, biconcave, anucleate cells optimized for gas transport.

• Biconcave shape increases surface area for gas exchange.

• Filled with hemoglobin (Hb), the oxygen-carrying protein.

• No mitochondria; ATP production is anaerobic.

Function of Erythrocytes

• Dedicated to respiratory gas transport.

• Hemoglobin binds reversibly with oxygen.

• Each Hb molecule can transport four O2 molecules.

Structure of Hemoglobin

Hemoglobin consists of four polypeptide chains (two alpha, two beta) and four heme groups, each with an iron atom that binds O2.

Production of Erythrocytes (Erythropoiesis)

Erythropoiesis is the process of RBC formation, occurring in red bone marrow.

• Stem cell (hemocytoblast) differentiates into committed cells.

• Stages: proerythroblast → basophilic erythroblast → polychromatic erythroblast → orthochromatic erythroblast → reticulocyte → mature erythrocyte.

Regulation of Erythropoiesis

• Controlled by erythropoietin (EPO), a hormone released by kidneys in response to hypoxia.

• Balance between RBC production and destruction is crucial.

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

Fate and Destruction of Erythrocytes

RBCs have a lifespan of 100–120 days. Old RBCs are removed by macrophages in the spleen, liver, and bone marrow.

• Hemoglobin is broken down: iron is stored/reused, heme is converted to bilirubin, globin is metabolized to amino acids.

Erythrocyte Disorders

Anemia

Anemia is a condition of abnormally low O2-carrying capacity.

• Blood loss: Hemorrhagic anemia (acute or chronic).

• Not enough RBCs produced: Iron-deficiency, pernicious, renal, aplastic anemia.

• Too many RBCs destroyed: Hemolytic anemias, thalassemias, sickle-cell anemia.

Polycythemia

Polycythemia is an abnormal excess of RBCs, increasing blood viscosity.

• Polycythemia vera: bone marrow cancer.

• Secondary polycythemia: response to low O2 or increased EPO.

• Blood doping: artificial increase in RBCs for athletic performance.

Leukocytes (White Blood Cells)

General Structure and Function

Leukocytes are complete cells that defend against disease.

• Can leave capillaries (diapedesis) and move through tissues.

• Leukocytosis: elevated WBC count, normal response to infection.

Types of Leukocytes

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

• Agranulocytes: Lymphocytes, monocytes (lack granules).

Leukocyte Formation (Leukopoiesis)

Leukopoiesis is stimulated by interleukins and colony-stimulating factors.

• All leukocytes originate from hemocytoblast stem cells.

• Lymphoid stem cells produce lymphocytes; myeloid stem cells produce other WBCs.

Platelets

Structure and Function

Platelets are fragments of megakaryocytes, essential for clotting.

• Contain chemicals for clotting: serotonin, calcium, enzymes, ADP.

• Form temporary platelet plugs to seal vessel breaks.

Cell Type	Description	Function
Erythrocyte	Biconcave, anucleate	Transport O2 and CO2
Leukocyte	Complete cell	Defense against disease
Platelet	Cell fragment	Clotting

Hemostasis

Steps in Hemostasis

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

1. Vascular spasm: Vasoconstriction of injured vessel.

2. Platelet plug formation: Platelets adhere to exposed collagen and release chemicals.

3. Coagulation: Reinforcement of plug with fibrin threads.

Coagulation Pathways

• Intrinsic pathway: triggered by factors within blood.

• Extrinsic pathway: triggered by tissue factor outside blood.

• Both pathways lead to formation of prothrombin activator, conversion of prothrombin to thrombin, and formation of fibrin mesh.

Clot Retraction and Fibrinolysis

• Clot retraction: platelets contract, pulling vessel edges together.

• Fibrinolysis: plasmin digests fibrin, dissolving the clot.

Factor Number	Factor Name	Source	Function
I	Fibrinogen	Liver	Converted to fibrin
II	Prothrombin	Liver	Converted to thrombin
IV	Calcium ions	Diet	Essential for clotting

Disorders of Hemostasis

Thromboembolic Disorders

• Thrombus: Clot in unbroken vessel.

• Embolus: Clot that travels in bloodstream.

• Risk factors: atherosclerosis, inflammation, immobility.

Bleeding Disorders

• Impaired liver function: Inability to synthesize clotting factors.

• Hemophilia: Hereditary deficiency of clotting factors.

• DIC: Widespread clotting and bleeding.

Blood Transfusions

ABO Blood Groups

Blood groups are determined by antigens (agglutinogens) on RBC surfaces.

• Type A: A antigen

• Type B: B antigen

• Type AB: A and B antigens

• Type O: No antigens

Blood Group	RBC Antigens	Plasma Antibodies	Blood That Can Be Received
A	A	Anti-B	A, O
B	B	Anti-A	B, O
AB	A, B	None	A, B, AB, O
O	None	Anti-A, Anti-B	O

Blood Typing

Blood typing involves mixing blood with antibodies to detect agglutination.

Diagnostic Blood Tests

Clinical Applications

• Hematocrit, blood glucose, and WBC counts are used to diagnose anemia, diabetes, and infection.

• Complete blood count (CBC) assesses overall blood health.

Developmental Aspects of Blood

Fetal and Aging Blood

• Fetal blood cells form in yolk sac, liver, and spleen; red bone marrow becomes primary site by seventh month.

• Hemoglobin F has higher O2 affinity than adult hemoglobin.

• Blood diseases of aging include chronic leukemias, anemias, and clotting disorders.