HA 5- Hematology (blood)

Blood: Composition, Functions, and Cellular Elements

Chapter Objectives

• Understand the composition of blood.

• Describe the functions of red blood cells, white blood cells, and plasma.

• Explain the role of hemoglobin in oxygen transport.

Composition of Blood

Blood is a specialized body fluid that performs vital transport and regulatory functions. It is composed of plasma and formed elements (cells and cell fragments).

• Plasma: The liquid matrix, making up about 55% of blood volume.

• Formed Elements: Cells and cell fragments, including red blood cells (RBCs), white blood cells (WBCs), and platelets, comprising about 45% of blood volume.

Component	Percentage of Blood	Main Constituents
Plasma	~55%	Water (91%), Proteins (7%), Other solutes (2%)
Formed Elements	~45%	RBCs, WBCs, Platelets

Additional info: Plasma proteins include albumins, globulins, and fibrinogen. Other solutes include ions, nutrients, waste products, gases, and regulatory substances.

Blood Function

Blood is a type of liquid connective tissue with several essential functions:

• Transports oxygen, carbon dioxide, nutrients, hormones, and waste products.

• Regulates pH, body temperature, and water content of cells.

• Protects against blood loss (hemostasis) and infection (immune response).

The average adult human contains approximately 5 liters of blood.

Formed Elements of Blood

The cellular components of blood are classified as follows:

Type	Examples	Function
Red Blood Cells (Erythrocytes)	RBCs	Transport oxygen and some carbon dioxide
White Blood Cells (Leukocytes)	Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils	Immune defense
Platelets (Thrombocytes)	Platelets	Blood clotting

Two Forms of Blood Cells: Myeloid and Lymphoid Lineages

Blood cells originate from hematopoietic stem cells in the bone marrow and differentiate into two main lineages:

• Myeloid lineage: Produces erythrocytes, platelets, monocytes, neutrophils, eosinophils, and basophils.

• Lymphoid lineage: Produces lymphocytes (T cells, B cells, and natural killer cells).

Plasma

Plasma is the straw-colored, liquid portion of blood in which cells are suspended.

• Composed mainly of water, proteins, and dissolved substances.

• Transports soluble food molecules, waste products, hormones, and antibodies.

Red Blood Cells (Erythrocytes)

Red blood cells (RBCs) are the most abundant formed elements in blood.

• Main function: Transport oxygen from the lungs to tissues and carry some carbon dioxide back to the lungs.

• Contain the protein hemoglobin, which binds oxygen.

Hemoglobin is a globular protein with four subunits, each containing an iron atom that can bind one molecule of oxygen.

Equation for oxygen binding:

$\text{Hb} + 4\,\text{O}_2 \rightleftharpoons \text{Hb}(\text{O}_2)_4$

Additional info: The biconcave shape of RBCs increases surface area for gas exchange and flexibility for passage through capillaries.

White Blood Cells (Leukocytes)

White blood cells (WBCs) are involved in the body's defense mechanisms.

• Types include neutrophils, lymphocytes, monocytes, eosinophils, and basophils.

• Functions: Phagocytosis, antibody production, and immune regulation.

Platelets (Thrombocytes)

Platelets are small cell fragments essential for blood clotting (hemostasis).

• Aggregate at sites of blood vessel injury to form a temporary plug.

• Release chemicals that activate the coagulation cascade.

Summary Table: Main Components and Functions of Blood

Component	Main Function
Plasma	Transport of nutrients, wastes, hormones, and antibodies
Red Blood Cells	Oxygen and carbon dioxide transport
White Blood Cells	Immune defense
Platelets	Blood clotting