Blood Physiology: Structure, Function, and Cellular Components

The Blood: Structure and Function

Parts of Blood and Their Functions

Blood is a specialized bodily fluid essential for transporting substances, defending against pathogens, and maintaining homeostasis.

• Plasma: The liquid component, comprising water, proteins (albumin, globulins, fibrinogen), electrolytes, nutrients, hormones, and waste products. Function: Transports dissolved substances, maintains osmotic balance, and serves as a medium for cellular elements.

• Red Blood Cells (Erythrocytes): Biconcave, anucleate cells rich in hemoglobin. Function: Transport oxygen from lungs to tissues and carbon dioxide from tissues to lungs.

• White Blood Cells (Leukocytes): Diverse group of nucleated cells. Function: Defend against infection and foreign substances.

• Platelets (Thrombocytes): Small, anucleate cell fragments. Function: Initiate blood clotting and repair damaged blood vessels.

Blood Volume and Component Proportions

• Plasma makes up about 55% of blood volume.

• Formed elements (RBCs, WBCs, platelets) comprise about 45%.

• Clinical Relevance: Changes in these proportions can indicate dehydration, anemia, or polycythemia.

Hematocrit

Definition: The percentage of blood volume occupied by red blood cells.

• Normal Range: Approximately 40-50% in adults.

• Clinical Application: Used to diagnose anemia and polycythemia.

Plasma Proteins and Their Functions

• Albumin: Maintains osmotic pressure and transports substances.

• Globulins: Include antibodies (immunoglobulins) and transport proteins.

• Fibrinogen: Essential for blood clotting.

Hematopoiesis: Blood Cell Formation

Hematopoiesis is the process of blood cell production, occurring primarily in the bone marrow.

• Hematopoietic Stem Cells (HSCs): Multipotent cells that give rise to all blood cell types.

• Sites: In adults, mainly in the axial skeleton (vertebrae, ribs, pelvis, sternum).

• Growth Factors: Colony-stimulating factors (CSFs) and interleukins regulate differentiation.

Red Blood Cell (Erythrocyte) Life Cycle

• Production: Stimulated by erythropoietin (EPO) from the kidneys.

• Lifespan: Approximately 120 days.

• Destruction: Old RBCs are removed by macrophages in the spleen and liver.

White Blood Cells (Leukocytes): Types and Functions

• Granulocytes: Contain cytoplasmic granules; include neutrophils, eosinophils, basophils.

• Agranulocytes: Lack visible granules; include lymphocytes and monocytes.

• Function: Immune defense, phagocytosis, antibody production.

Platelets and Hemostasis

• Platelet Function: Adhere to damaged vessel walls, aggregate, and release factors to initiate clot formation.

• Hemostasis: The process of stopping bleeding, involving vascular spasm, platelet plug formation, and coagulation.

Blood Clot Formation

• Steps:

  1. Vascular spasm

  2. Platelet plug formation

  3. Coagulation cascade (conversion of fibrinogen to fibrin)

• Key Equation: $\text{Prothrombin} \xrightarrow{\text{Thrombin}} \text{Fibrinogen} \xrightarrow{\text{Fibrin}}$

Summary Table: Blood Cell Types and Functions

Cell Type	Main Function	Key Features
Red Blood Cell (Erythrocyte)	Oxygen transport	Biconcave, no nucleus, contains hemoglobin
White Blood Cell (Leukocyte)	Immune defense	Granulocytes (neutrophils, eosinophils, basophils); Agranulocytes (lymphocytes, monocytes)
Platelet (Thrombocyte)	Blood clotting	Cell fragments, no nucleus

Additional info:

• Blood cell formation (hematopoiesis) is regulated by hormones and growth factors such as erythropoietin, thrombopoietin, and colony-stimulating factors.

• Disorders of blood include anemia (low RBCs), leukopenia (low WBCs), and thrombocytopenia (low platelets).