Blood

Blood: Overview and Physical Characteristics

Definition and Composition

Blood is a specialized connective tissue composed of formed elements (cells and cell fragments) suspended in a liquid matrix called plasma. It is essential for the transport of substances, regulation of physiological parameters, and protection against disease.

• Whole Blood = Formed Elements + Plasma

• Viscosity: Blood is more viscous than water (about 5 times thicker).

• pH: 7.35 – 7.45 (average pH: 7.4)

• Temperature: ~38°C (100.4°F)

• Volume: 4–6 L (average adult)

• Color: Oxygen-rich blood is bright red; oxygen-poor blood is deep red (burgundy).

• Hematocrit: Percentage of blood volume occupied by red blood cells (RBCs). Males: 47% ± 5% Females: 42% ± 5%

Components of Whole Blood

Plasma

Plasma is the liquid matrix of blood, making up about 55% of its volume. It serves as the medium for transporting nutrients, hormones, and waste products.

• Water: 92% of plasma

• Plasma Proteins: 7% (albumins, globulins, fibrinogen)

• Other Solutes: 1% (electrolytes, enzymes, gases, nutrients, wastes)

Major Plasma Proteins

• Albumins (60%): Maintain osmotic pressure (Blood Colloidal Osmotic Pressure, BCOP), transport fatty acids, steroid hormones, and some drugs.

• Globulins (35%): Transport proteins (alpha, beta), immune proteins (gamma globulins/antibodies).

• Fibrinogen (4%): Involved in blood clotting; can be activated to fibrin to form a mesh during coagulation.

Formed Elements

Formed elements are the cellular components of blood, comprising about 45% of its volume.

• Red Blood Cells (Erythrocytes): ~45% of whole blood; transport oxygen.

• White Blood Cells (Leukocytes): <1% of whole blood; defend against pathogens.

• Platelets: <1% of whole blood; cell fragments involved in clotting.

Types of Leukocytes

• Agranulocytes: Lymphocytes (T, B, NK cells), Monocytes

• Granulocytes: Neutrophils, Eosinophils, Basophils

General Functions of Blood

Transportation

• Delivers oxygen and nutrients to tissues

• Transports metabolic waste to lungs and kidneys for removal

• Transports hormones from endocrine glands to target sites

Regulation

• Maintains body temperature by absorbing and distributing heat

• Maintains normal pH using buffers (alkaline reserve of bicarbonate ions)

• Maintains adequate plasma volume in the circulatory system

Protection

• Prevents blood loss (hemostasis)

• Prevents infection (immunoglobulins, leukocytes, complement proteins)

Formation of Blood Cells: Hematopoiesis (Hemopoiesis)

Location and Process

Hematopoiesis is the process of blood cell formation, occurring primarily in the red bone marrow (spongy bone) of the axial skeleton, pelvic girdle, and proximal epiphyses of humerus and femur.

• All formed elements originate from a hemocytoblast (hematopoietic stem cell).

• Hemocytoblasts differentiate into myeloid stem cells and lymphoid stem cells.

Hematopoiesis Pathways

Stem Cell	Lineage	Final Cell Types
Myeloid Stem Cell	Progenitor Cell → Megakaryoblast → Megakaryocyte → Platelets	Platelets
Myeloid Stem Cell	Progenitor Cell → Monoblast → Monocyte → Macrophage	Monocytes, Macrophages
Myeloid Stem Cell	Progenitor Cell → Myeloblast → Neutrophil/Eosinophil/Basophil	Neutrophils, Eosinophils, Basophils
Lymphoid Stem Cell	T Lymphoblast → T Lymphocyte	T Cells
Lymphoid Stem Cell	B Lymphoblast → B Lymphocyte → Plasma Cell	B Cells, Plasma Cells
Lymphoid Stem Cell	NK Lymphoblast → NK Lymphocyte	Natural Killer (NK) Cells

Key Terms and Definitions

• Hematocrit: Percentage of blood volume occupied by RBCs.

• Hemocytoblast: Multipotent stem cell that gives rise to all blood cell types.

• Plasma: Fluid matrix of blood containing water, proteins, and solutes.

• Formed Elements: Cellular components of blood (RBCs, WBCs, platelets).

• Hemopoiesis/Hematopoiesis: Formation of blood cells.

• Osmotic Pressure: Pressure exerted by plasma proteins to maintain fluid balance.

Important Equations

• Osmotic Pressure (BCOP): $\text{BCOP} = \text{Concentration of plasma proteins}$

• Hematocrit Calculation: $\text{Hematocrit} = \frac{\text{Volume of RBCs}}{\text{Total Blood Volume}} \times 100$

Summary Table: Components of Whole Blood

Component	Percentage of Whole Blood	Main Function
Plasma	~55%	Transport medium for nutrients, hormones, waste
Red Blood Cells (RBCs)	~45%	Transport oxygen and carbon dioxide
White Blood Cells (WBCs)	<1%	Defense against pathogens
Platelets	<1%	Blood clotting (hemostasis)

Additional info:

• Blood is classified as a connective tissue due to its origin from mesenchyme and its matrix (plasma).

• Hematology is the study of blood and its disorders.

• Plasma proteins are primarily produced by the liver.