Chapter 19: Blood – Structure, Function, and Clinical Relevance

Chapter 19: Blood

Learning Objectives

• Understand the characteristics and functions of blood.

• Describe the composition of plasma.

• Describe the characteristics and functions of the RBCs and WBCs found in blood.

• Understand the ABO blood groups and blood typing.

• Understand the process of hemostasis.

Characteristics and Functions of Blood

Overview

Blood is a specialized connective tissue that plays a vital role in the transport of substances, regulation of homeostasis, and protection against disease. It consists of plasma and formed elements (cells and cell fragments).

• Transportation: Delivers oxygen, nutrients, hormones, and removes wastes.

• Regulation: Maintains pH, ion composition, and body temperature.

• Prevention of Blood Loss: Through clotting mechanisms.

• Defense: Protects against toxins and pathogens via immune cells.

• Stabilization of Body Temperature: Distributes heat throughout the body.

Physical Characteristics of Blood

• pH: 7.35–7.45 (slightly alkaline)

• Temperature: 38°C (100.4°F)

• Viscosity: 5x that of water

• Equivalent Salt Concentration: 0.9% NaCl

• Percent of Body Weight: ~7%

• Volume: Varies by sex (typically 4–5 L in females, 5–6 L in males)

Hematocrit

Hematocrit is the percentage of whole blood volume occupied by red blood cells (RBCs). It is determined by centrifuging blood and measuring the proportion of packed RBCs.

• Formula:

$\text{Hematocrit} = \frac{\text{Volume of packed RBCs}}{\text{Volume of whole blood}} \times 100$

• Normal Values: Males: ~40–54%; Females: ~37–47%

Composition of Plasma

Major Components

Plasma is the liquid matrix of blood, making up about 55% of its volume. It is composed mainly of water, proteins, and other solutes.

Component	Percentage	Examples
Water	~92%	Solvent for carrying other substances
Plasma Proteins	~7%	Albumins (60%): Osmotic pressure, transport of lipids, hormones Globulins (35%): Immune function (antibodies), transport Fibrinogen (4%): Blood clotting Regulatory Proteins (<1%): Enzymes, hormones
Other Solutes	~1%	Electrolytes, organic nutrients, organic wastes

Formed Elements: Erythrocytes (Red Blood Cells)

Structure and Function

Erythrocytes are biconcave, anucleate cells specialized for oxygen transport. Their structure supports their function in several ways:

• Large surface area to volume ratio for efficient gas exchange.

• Ability to form stacks (rouleaux) for smooth flow through vessels.

• Flexibility to bend and pass through small capillaries.

RBCs have a lifespan of about 120 days. Their main function is to transport oxygen (via hemoglobin) and carbon dioxide.

• Anemia: Condition with reduced RBC count or hemoglobin, leading to decreased oxygen delivery.

• Polycythemia: Increased RBC count, which can increase blood viscosity.

Hemoglobin

Structure and Function

Hemoglobin is a protein in RBCs responsible for oxygen and carbon dioxide transport.

• Composed of four protein chains (globins), each with a heme group.

• Each heme contains an iron ion that binds oxygen.

• Oxyhemoglobin: Hemoglobin bound to oxygen.

• Deoxyhemoglobin: Hemoglobin not bound to oxygen.

• Carbaminohemoglobin: Hemoglobin bound to carbon dioxide.

Erythropoiesis: Formation of Red Blood Cells

Process and Regulation

Erythropoiesis is the process of RBC production, occurring in the red bone marrow. It requires amino acids, iron, and vitamins (especially B12).

• Stimulated by the hormone erythropoietin (EPO), primarily released by the kidneys in response to hypoxia (low oxygen), anemia, or decreased blood flow.

• EPO increases RBC production and accelerates hemoglobin synthesis.

RBC Degradation

Fate of Old RBCs

Old or damaged RBCs are removed by macrophages in the liver, spleen, or bone marrow. Hemoglobin is broken down as follows:

• Globin → amino acids (recycled)

• Heme → iron (recycled) + biliverdin → bilirubin (excreted in bile)

• Bilirubin is processed by the liver and excreted in feces (as stercobilins) and urine (as urobilins).

Blood Groups and Transfusion

ABO Blood Groups

Blood types are determined by the presence or absence of specific antigens (A, B) on RBC surfaces. Antibodies in plasma react with foreign antigens, causing agglutination (clumping) if incompatible blood is transfused.

Type	Distribution (US)	Antigen	Antibody
A	40%	A	Anti-B
B	10%	B	Anti-A
AB	4%	A and B	None
O	46%	None	Anti-A and Anti-B

Transfusion Reactions

Transfusion reactions occur when recipient antibodies attack donor antigens. Cross-matching is essential to prevent these reactions.

Recipient's Antibodies	Donor's Antigens
Type A: Anti-B	Cannot receive B or AB blood
Type B: Anti-A	Cannot receive A or AB blood
Type AB: None	Universal recipient
Type O: Anti-A and Anti-B	Can only receive O blood

Rh Factor

The Rh antigen (D antigen) is another important blood group. Individuals are Rh-positive (have the antigen) or Rh-negative (lack the antigen). Hemolytic disease of the newborn can occur if an Rh-negative mother carries an Rh-positive fetus, leading to maternal antibodies attacking fetal RBCs.

• Prevention: Administration of Rho(D) immune globulin to Rh-negative mothers.

Formed Elements: Leukocytes (White Blood Cells)

General Properties

• Less numerous than RBCs

• Have a nucleus

• No hemoglobin

• Function in defense against pathogens

Classification of Leukocytes

• Granular leukocytes: Neutrophils, eosinophils, basophils

• Agranular leukocytes: Monocytes, lymphocytes

Structure and Function of Granular Leukocytes

Type	Structure	Function
Neutrophil (50–70%)	Multi-lobed nucleus, pale granules	Phagocytosis of bacteria, first responders to infection
Eosinophil (2–4%)	Bi-lobed nucleus, red-orange granules	Combat parasitic infections, modulate allergic responses
Basophil (<1%)	Bi-lobed or S-shaped nucleus, dark blue granules	Release histamine and heparin during inflammatory reactions

Structure and Function of Agranular Leukocytes

Type	Structure	Function
Monocyte (2–8%)	Large, kidney-shaped nucleus	Differentiate into macrophages, phagocytize pathogens and debris
Lymphocyte (20–40%)	Large, round nucleus, little cytoplasm	Specific immunity (B cells, T cells, NK cells)

Production and Regulation of WBCs

WBCs are produced in the bone marrow from hematopoietic stem cells. Their production is regulated by colony-stimulating factors (CSFs) and interleukins in response to infection or inflammation.

Formed Elements: Platelets

Production and Function

Platelets are cell fragments derived from megakaryocytes in the bone marrow. They play a crucial role in hemostasis by forming platelet plugs and releasing factors that promote clotting.

• Thrombocytopoiesis: Platelet production

• Thrombocytopenia: Abnormally low platelet count

• Thrombocytosis: Abnormally high platelet count

Hemostasis

Phases of Hemostasis

1. Vascular Phase: Immediate vasoconstriction after vessel injury (lasts ~30 min)

2. Platelet Phase: Platelet adhesion and aggregation (begins ~15 sec after injury)

3. Coagulation Phase: Formation of a fibrin clot (begins ~30 sec after injury)

Coagulation Pathways

Blood clotting involves a cascade of reactions leading to the conversion of fibrinogen to fibrin. There are two initial pathways (extrinsic and intrinsic) that converge on a common pathway.

Pathway	Trigger	Key Steps
Extrinsic	Tissue damage	Tissue factor (III) + Factor VII → Activates Factor X
Intrinsic	Exposed collagen at injury site	Activated proenzymes (Factor XII) + Platelet factors → Activates Factor X
Common	Activated Factor X	Prothrombin activator → Thrombin → Fibrinogen → Fibrin

Key Equations

$\text{Prothrombin} \xrightarrow{\text{Prothrombin Activator}} \text{Thrombin}$ $\text{Fibrinogen} \xrightarrow{\text{Thrombin}} \text{Fibrin}$

Control of Coagulation

Requirements and Regulation

• Calcium ions (Ca2+) and Vitamin K are essential for clotting factor activation.

• Clot Retraction (Syneresis): Platelets contract to tighten the clot.

• Fibrinolysis: Dissolution of the clot by plasmin.

$\text{Plasminogen} \xrightarrow{\text{Activation}} \text{Plasmin} \rightarrow \text{Digests Clot}$

• Anticoagulants: Substances that prevent unwanted clotting (e.g., antithrombin III, heparin, thrombomodulin, protein C).

Unwanted Blood Clots

• Embolus: A clot that travels in the bloodstream.

• Thrombus: A stationary clot within a vessel.

Key Terms

• Whole blood, Plasma, Formed elements, Hematocrit, Buffy coat, Albumin, Globulin, Fibrinogen, Hemoglobin, Oxyhemoglobin, Deoxyhemoglobin, Carbaminohemoglobin, Erythropoiesis, Erythropoietin, Biliverdin, Bilirubin, Urobilins, Stercobilins, Agglutination, Hemolysis, Rh Factor, Leukocyte, Neutrophil, Eosinophil, Basophil, Monocyte, Lymphocyte, Thrombocyte, Thrombocytopoiesis, Hemostasis, Factor X, Prothrombin Activator, Prothrombin, Thrombin, Fibrin, Fibrinolysis, Embolus, Thrombus, Antithrombin III, Thrombomodulin/Protein C, Plasminogen, Plasmin

Clinical Applications and Examples

• Blood Typing: Essential for safe transfusions; mismatches can cause agglutination and hemolysis.

• Hemolytic Disease of the Newborn: Prevented by Rho(D) immune globulin in Rh-negative mothers.

• Anemia and Polycythemia: Disorders of RBC count affecting oxygen delivery and blood viscosity.

• Leukocyte Disorders: Leukopenia (low WBC count), leukocytosis (high WBC count), leukemia (cancer of WBCs).

• Platelet Disorders: Thrombocytopenia (low platelets), thrombocytosis (high platelets).

• Coagulation Disorders: Hemophilia (deficiency of clotting factors), vitamin K deficiency, liver disease.

Additional info: This summary expands on the provided notes with definitions, normal values, and clinical context for clarity and completeness.