Blood: Structure, Function, and Cellular Components

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BLOOD

Overview

Blood is a specialized connective tissue that plays a vital role in the cardiovascular system. It is essential for transporting substances, regulating physiological parameters, and protecting the body against disease and injury.

Components of Whole Blood

Main Components

Plasma: The liquid matrix of blood, making up about 55% of its volume. Contains water, proteins, nutrients, hormones, and waste products.
Formed Elements: Cellular components including erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets).

Component	Percentage by Volume	Main Functions
Plasma	~55%	Transport, osmotic balance, clotting
Buffy coat (leukocytes & platelets)	<1%	Immunity, clotting
Erythrocytes	~45%	Oxygen and carbon dioxide transport

Functions of Blood

Key Roles

Transportation: Carries O2, CO2, nutrients, wastes, hormones, and heat throughout the body.
Regulation: Maintains temperature, pH (7.4), and osmotic fluid balance.
Protection: Provides immune defense and enables clotting to prevent blood loss.

Blood Features

Physical and Chemical Properties

Temperature: 100.4°F (38°C)
Viscosity: 5 times more viscous than water
pH: 7.4 (slightly alkaline)
Volume: Men: 5-6 liters (1.3-1.8 gallons); Women: 4-5 liters (1.1-1.3 gallons)

Plasma Proteins

Types and Functions

Albumens (55-60%): Responsible for viscosity, osmotic pressure, and transport of substances.
Globulins (35-38%): Includes immunoglobulins (antibodies for disease defense) and transport globulins.
Fibrinogen (4-7%): Essential for blood clotting.

Protein	Percentage	Main Function
Albumens	55-60%	Osmotic pressure, transport
Globulins	35-38%	Immunity, transport
Fibrinogen	4-7%	Clotting

Origins of Plasma Proteins

Liver: Produces about 90% of plasma proteins, including albumens, fibrinogen, and transport globulins.
Lymphocytes: Produce immunoglobulins (antibodies).

Formed Elements of Blood

Cellular Components

Thrombocytes: Platelets, involved in clotting.
Leukocytes: White blood cells, involved in immune defense.
Erythrocytes: Red blood cells, responsible for gas transport.

Erythrocytes (Red Blood Cells)

Functions

Transport O2 and CO2
Major determinant of blood viscosity

Hemoglobin Structure

Each erythrocyte contains about 280 million hemoglobin molecules.
Hemoglobin consists of four globin (amino acid chains) and four heme groups, each with an iron atom at the center.

Structure

Biconcave shape
No nucleus

Advantages of Erythrocyte Structure

Large surface area for gas exchange
Form stacks called rouleaux for smooth flow
Flexible to bend through capillaries
More space to carry O2
Do not use O2 for their own metabolism

Disadvantages

Lack of nucleus and mitochondria
Cannot divide
Average lifespan: 120 days

Hemopoiesis

Formation of Blood Cells

Occurs in red bone marrow
Hemocytoblasts: Stem cells that divide to form all blood cell types

Types of Stem Cells

Totipotent: Can become a complete organism (early embryonic stage)
Multipotent: Can form different tissues (ectoderm, mesoderm, endoderm)
Pluripotent: Can form all cell types of a specific tissue (e.g., hemocytoblast forms all blood cells)

Red Blood Cell Formation (Erythropoiesis)

Process

Rate: ~3 million cells/sec
Duration: 7-15 days
Regulated by erythropoietin (hormone produced by kidneys)
Negative feedback loop controls production

Stages

Hemocytoblasts become Erythrocyte colony forming unit (CPU) (nucleated, begin hemoglobin synthesis)
CPU becomes Erythroblast (hemoglobin increases, nucleus and cell shrink)
Becomes Reticulocyte (loses nucleus, enters circulation)
Matures into Erythrocyte

Red Blood Cell Breakdown

Process

Macrophages engulf dying RBCs
Hemoglobin is broken down:
- Globin: amino acids reused
- Heme: converted to bilirubin
Bilirubin: transported to liver, excreted in bile, removed via feces and urine
Iron: binds to plasma protein, returned to bone marrow for reuse

Leukocytes (White Blood Cells)

Functions

Defend against infections
Remove toxins and wastes
Move out of blood into tissues
Attracted to chemicals from pathogens/damaged cells
Some are phagocytotic (engulf cells/wastes)

Types of Leukocytes

Type	Subtype	Main Function
Granular	Neutrophils	Phagocytosis, inflammation
Granular	Basophils	Release histamine/heparin
Granular	Eosinophils	Kill parasites, reduce inflammation
Agranular	Lymphocytes	Immunity (T, B, NK cells)
Agranular	Monocytes	Become macrophages, clean debris

Neutrophils

60-70% of WBCs
First responders at injury site
Phagocytize antibody-marked bacteria
Release prostaglandins (inflammation) and leukotrienes (attract phagocytes)
Short lifespan (~10 hours)

Eosinophils

2-4% of WBCs
Phagocytize antibody-marked bacteria, protozoa, and debris
Release toxins to kill invaders
Kill parasitic worms
Increase during allergic reactions
Reduce inflammation

Basophils

1% of WBCs
Release histamine (inflammation) and heparin (prevents clotting)
Attract other cells to injury site
Increase during allergies, leukemias, diabetes, hypothyroidism

Monocytes

3-8% of WBCs
Leave blood to become macrophages
Engulf microbes, especially viruses
Attract other WBCs to attack site
Clean debris and dead cells

Lymphocytes

25-33% of WBCs
Move between tissues and blood
Three types: T cells (cellular immunity), B cells (humoral immunity), NK cells (destroy abnormal cells)

T Cells

Attack foreign cells
Attract other lymphocytes

B Cells

Produce antibodies
Mark or destroy foreign antigens

NK Cells

Detect and destroy abnormal tissue cells (e.g., cancer)

Platelets (Thrombocytes)

Structure and Function

Cell fragments
Circulate for 9-12 days
Release chemicals to initiate blood clotting
Dissolve old clots
Form plugs at injury sites
Contract wounds to aid healing

Hemostasis

Process of Stopping Bleeding

Vascular Spasm: Rapid constriction of injured vessel, reduces blood loss
Platelet Plug Formation: Platelets adhere to exposed collagen, aggregate, and form a plug
Blood Clotting (Coagulation): Formation of a fibrin mesh that traps blood cells

Platelet Plug Formation

Occurs within 15 seconds of injury
Platelets contact sticky collagen fibers, enlarge, and become sticky
Release clotting factors (ADP, thromboxane A2, serotonin)

Blood Clotting (Coagulation)

Begins in 15-30 seconds, completes in 3-5 minutes
Requires fibrinogen, vitamin K, calcium, and clotting factors
Fibrin threads entrap red cells, platelets, and plasma in the clot

Pathways of Coagulation

Pathway	Trigger	Time	Main Steps
Extrinsic	Tissue damage	Seconds	Tissue factor + Ca2+ → Factor X activation
Intrinsic	Blood vessel damage	Minutes	Collagen exposure, platelet factors → Factor X activation
Common	Factor X activation	-	Prothrombinase → Thrombin → Fibrinogen → Fibrin

Factors Preventing Clotting

Heparin and Antithrombin: Inhibit thrombin formation
Smooth lining of blood vessels: Repels platelets, secretes prostacyclin
Rapid blood flow: Prevents platelet aggregation

Example: Clinical Application

Kidney failure can reduce erythropoietin production, leading to anemia due to decreased RBC formation.
Vitamin K deficiency impairs clotting, increasing bleeding risk.

Additional info: Expanded explanations and tables were added for clarity and completeness, including the coagulation pathways and clinical relevance.