Blood and Hemostasis: Structure, Function, and Clinical Relevance

Blood and Hemostasis

Overview of Blood

Blood is a specialized fluid connective tissue essential for transporting substances, regulating physiological parameters, and defending the body. It consists of plasma (the liquid matrix) and formed elements (cells and cell fragments).

• Blood volume: 5–6 liters in adult males, 4–5 liters in adult females.

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

• Viscosity: Blood is five times as viscous as water due to dissolved proteins and formed elements.

• pH: Slightly alkaline, ranging from 7.35 to 7.45 (average 7.4).

Functions of Blood

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

• Regulation: Maintains pH and ion composition of interstitial fluids by absorbing/neutralizing acids and balancing ion concentrations.

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

• 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 according to temperature needs.

Composition of Blood

Plasma

Plasma is the liquid matrix of blood, making up about 55% of its volume. It is similar to interstitial fluid but contains higher concentrations of dissolved proteins and respiratory gases.

• Water: Over 90% of plasma is water.

• Plasma Proteins:

  • Albumins (60%): Maintain osmotic pressure.

  • Globulins (35%): Include antibodies (immunoglobulins) and transport globulins for ions, hormones, and lipids.

  • Fibrinogen (4%): Essential for blood clotting, forming insoluble fibrin strands.

• Other Solutes:

  • Electrolytes: Na+, K+, Ca2+, Mg2+, Cl-, HCO3-, HPO42-, SO42-

  • Organic Nutrients: Lipids, carbohydrates, amino acids (for ATP production, growth, maintenance)

  • Organic Wastes: Urea, uric acid, creatinine, bilirubin, NH4+

Formed Elements

• Red Blood Cells (RBCs, Erythrocytes): 99.9% of formed elements; transport oxygen.

• White Blood Cells (WBCs, Leukocytes):

  • Platelets:

Table: Summary of Blood Composition

Component	Percentage	Main Function
Plasma	~55%	Transport, osmotic balance, clotting, immunity
RBCs	~45% of whole blood	Oxygen and CO2 transport
WBCs	<0.1%	Defense against pathogens
Platelets	<0.1%	Clotting

Hemopoiesis: Formation of Blood Cells

Stem Cells in Red Bone Marrow

• Hemocytoblasts (Hematopoietic Stem Cells): Give rise to all formed elements.

• Lymphoid Stem Cells: Produce lymphocytes (immune response).

• Myeloid Stem Cells: Produce RBCs, other WBCs, and platelets.

Colony-stimulating factors are hormones that stimulate blood cell formation during immune responses.

Pathways of Differentiation

• Lymphoid Pathway: Lymphoid stem cell → lymphoblast → prolymphocyte → lymphocyte

• Myeloid Pathway:

  • Monoblasts → promonocytes → monocytes

  • Myeloblasts → myelocytes → band cells → neutrophils, eosinophils, basophils

  • Megakaryocytes → platelets

  • Proerythroblasts → erythroblasts → reticulocytes → erythrocytes (RBCs)

Erythropoietin (EPO)

• Released in response to hypoxia (low tissue oxygen).

• Stimulates RBC production in red bone marrow.

Example: Athletes may train at high altitude to stimulate EPO production and increase RBC count for improved oxygen delivery.

Key Terms and Definitions

• Hematocrit: Percentage of whole blood volume occupied by formed elements (mainly RBCs).

• Hemopoiesis (Hematopoiesis): The process of blood cell formation.

• Plasma Proteins: Albumins, globulins, fibrinogen—each with specific roles in osmotic pressure, immunity, and clotting.

• Colony-Stimulating Factors: Hormones that regulate blood cell production.

Summary

• Blood is a vital fluid connective tissue with diverse functions in transport, regulation, defense, and homeostasis.

• It is composed of plasma and formed elements, each with distinct roles.

• Blood cell formation occurs in red bone marrow through the differentiation of stem cells.

Additional info: This summary covers the structure, function, and formation of blood as outlined in the provided lecture slides, suitable for college-level Anatomy & Physiology students.