Blood: Introduction and Overview

Definition and Role in the Cardiovascular System

Blood is a specialized bodily fluid that circulates through the heart, arteries, veins, and capillaries, delivering essential substances to cells and removing waste products. It is a critical component of the cardiovascular system, which also includes the heart and blood vessels.

• Heart: Pumps blood and maintains blood pressure.

• Blood Vessels: Arteries carry blood away from the heart, capillaries permit exchange between blood and interstitial fluids, and veins return blood to the heart.

• Blood: The fluid portion of the cardiovascular system.

Functions of Blood

Major Physiological Functions

• Transport: Moves dissolved gases (O2, CO2), nutrients, hormones, and metabolic wastes throughout the body.

• Restriction of Fluid Loss: Initiates clotting to prevent excessive blood loss at injury sites.

• Regulation: Maintains pH and ion composition of interstitial fluids, absorbs/neutralizes acids, and balances the body’s internal environment.

• Defense: Transports white blood cells and antibodies to fight infection and defend against toxins and pathogens.

• Stabilization of Body Temperature: Distributes heat generated in one area to other tissues, directing blood flow to regulate temperature.

Composition of Blood

Major Components

• Whole Blood: All parts of blood mixed together.

• Plasma: The liquid matrix, making up about 55% of whole blood. Composed mostly of water (92%), with dissolved proteins, electrolytes, nutrients, hormones, and waste products.

• Formed Elements: Cells and cell fragments, making up about 45% of whole blood. Includes red blood cells (RBCs), white blood cells (WBCs), and platelets.

Average Blood Volume:

• Males: 5–6 liters

• Females: 4–5 liters

Properties of Whole Blood

• Temperature: About 38°C (100.4°F), slightly above normal body temperature.

• Viscosity: Five times as viscous as water due to interactions among dissolved proteins, formed elements, and water molecules.

• pH: Slightly alkaline, with a pH between 7.35 and 7.45 (average: 7.4).

Plasma: Composition and Function

Components of Plasma

• Water (92%): Maintains blood fluidity and volume.

• Electrolytes: Essential for cellular activities (e.g., Na+, K+, Ca2+).

• Organic Nutrients: Lipids, carbohydrates, amino acids for ATP production, growth, and maintenance.

• Organic Wastes: Urea, uric acid, creatinine, bilirubin, NH3—carried to sites of breakdown or excretion.

• Plasma Proteins (7%): Large, globular proteins (mainly produced by the liver) that cannot cross capillary walls.

Major Plasma Proteins and Their Functions

• Albumins: Transport substances and maintain fluid balance.

• Globulins: Include antibodies (immunoglobulins) for immune defense and transport globulins for ions, hormones, and lipids.

• Fibrinogen: Essential for blood clotting; forms fibers that create scabs.

• Enzymes and Hormones: Both active and inactive forms are present in plasma.

Formed Elements: Types and Functions

Overview

• Formed Elements: Blood cells and cell fragments, measured as hematocrit (percentage of whole blood from formed elements).

• Healthy Hematocrit Range: 37–54% (Males average 47%, Females average 42%).

Types of Formed Elements

• Platelets (<0.1%): Small membrane-bound fragments involved in clotting.

• White Blood Cells (WBCs, <0.1%): Involved in immune defense; five classes with different functions.

• Red Blood Cells (RBCs, 99.9%): Required for oxygen transport.

Hematopoiesis: Formation of Blood Cells

Location and Process

• Occurs in red bone marrow (medulla) of vertebrae, ribs, sternum, skull, scapulae, pelvis, and ends of long bones.

• All blood cells originate from hemocytoblasts (stem cells).

Pathways of Hematopoiesis

• Lymphoid Stem Cells: Produce lymphocytes (T cells, B cells, NK cells) for immune response.

• Myeloid Stem Cells: Produce all other formed elements (other WBCs, platelets, RBCs).

Myeloid Pathway Details

• WBCs (non-lymphocytes): Myelocytes and band cells mature into neutrophils, eosinophils, basophils, and monocytes.

• Platelets: Megakaryocytes shed cytoplasm as platelets (cell fragments, not true cells).

• RBCs: Proerythroblasts mature into erythrocytes, stimulated by erythropoietin (EPO) from the kidneys.

Red Blood Cells (Erythrocytes)

Structure and Function

• Biconcave disc shape: High surface area-to-volume ratio for efficient gas exchange; flexible to pass through capillaries.

• No nucleus or organelles: Maximizes space for hemoglobin; cannot divide or repair themselves.

• Hemoglobin (Hb): Main protein for oxygen transport; each RBC contains ~280 million Hb molecules.

Hemoglobin Structure

• Composed of four protein chains (2 alpha, 2 beta), each with a heme group containing iron (Fe2+).

• Each Hb molecule binds up to 4 O2 molecules.

• Oxygenated Hb (oxyhemoglobin) is bright red; deoxygenated Hb (deoxyhemoglobin) is dark red.

RBC Lifespan and Turnover

• Average lifespan: ~120 days.

• Old RBCs are removed by macrophages in the spleen, liver, or bone marrow.

• Hemolysis: Rupture of RBCs releases contents for recycling or excretion.

Blood Typing and Transfusions

Blood Group Antigens

• RBCs have surface antigens (A, B, Rh) that determine blood type.

• ABO System: Type A (A antigen), Type B (B antigen), Type AB (both), Type O (neither).

• Rh System: Presence (+) or absence (–) of Rh antigen.

Antibodies and Compatibility

• Plasma contains antibodies against foreign antigens (e.g., Type A blood has anti-B antibodies).

• Mixing incompatible blood types causes agglutination (clumping) and hemolysis.

• Rh-negative individuals develop anti-Rh antibodies only after exposure to Rh-positive blood (important in pregnancy and transfusions).

White Blood Cells (Leukocytes)

Types and Functions

• Granulocytes: Neutrophils (bacterial defense), eosinophils (parasites/allergies), basophils (inflammation/allergies).

• Agranulocytes: Monocytes (become macrophages, phagocytosis), lymphocytes (T cells, B cells, NK cells; specific immunity).

WBC Actions

• Can leave the bloodstream (diapedesis) and migrate to tissues.

• Phagocytosis: Engulf and digest pathogens, debris, or dead cells.

• Follow chemical signals to sites of infection or injury.

Hemostasis: Blood Clotting

Phases of Hemostasis

1. Vascular Phase: Blood vessel constricts to reduce blood loss; endothelial cells release chemicals to promote spasm and make the area sticky for clotting.

2. Platelet Phase: Platelets adhere to damaged vessel, release chemicals to recruit more platelets (positive feedback), and form a temporary plug.

3. Coagulation Phase: Cascade of reactions converts fibrinogen to insoluble fibrin, forming a stable clot. Requires calcium and clotting factors (proteins I–XIII).

Clot Retraction and Fibrinolysis

• After tissue repair, the clot is dissolved by fibrinolysis. Plasminogen is activated to plasmin, which digests fibrin and breaks down the clot.

Blood Testing and Hematology

Common Blood Tests

• Complete Blood Count (CBC): Measures numbers and types of RBCs, WBCs, and platelets; assesses hemoglobin concentration and hematocrit.

• WBC Differential: Breaks down the percentage of each type of WBC; helps diagnose infections, inflammation, and blood disorders.

• Blood Typing: Determines ABO and Rh status for transfusions.

Clinical Relevance

• Abnormal values can indicate anemia, infection, clotting disorders, leukemia, or other medical conditions.

Summary Table: Major Components of Blood

Key Equations and Definitions

• Hematocrit:

• Oxygen Carrying Capacity:

Additional info: These notes provide a comprehensive overview of blood structure, function, and clinical relevance, suitable for college-level Anatomy & Physiology students.