Blood Composition and Plasma Proteins

Overview of Blood and Plasma Proteins

Blood is a specialized connective tissue composed of plasma and formed elements. Plasma proteins play critical roles in maintaining homeostasis, transporting substances, and defending the body against pathogens.

• Albumin: Synthesized in the liver, albumin maintains blood colloid osmotic pressure, helps retain fluid in the bloodstream, and transports substances such as fatty acids, hormones, drugs, and bilirubin.

• Globulins (α, β, γ): α and β globulins are produced in the liver, while γ globulins are produced by plasma cells/lymphoid tissues. They transport lipids, metal ions, and vitamins, contribute to immunity, and γ globulins function as antibodies.

• Fibrinogen: Produced in the liver, fibrinogen is essential for blood clotting and is converted to fibrin during coagulation to form blood clots.

Erythropoiesis and Erythropoietin (EPO)

Regulation of Red Blood Cell Production

Erythropoiesis is the process of red blood cell (RBC) production, primarily regulated by the hormone erythropoietin (EPO). EPO ensures adequate oxygen delivery by stimulating RBC formation in response to hypoxia.

• Site of EPO Production: Primarily the kidneys (renal cells); a small amount is produced by the liver.

• Stimulus for EPO Release: Low blood oxygen levels (hypoxia), such as during anemia, blood loss, high altitude exposure, or reduced oxygen delivery to tissues.

• Target Tissue(s): Red bone marrow, specifically erythroid progenitor and precursor cells that develop into red blood cells.

• Action of EPO: Stimulates erythropoiesis by promoting the survival, proliferation, and differentiation of erythroid precursor cells, increasing the number of circulating RBCs and improving oxygen-carrying capacity.

Platelets and Thrombopoiesis

Formation and Function of Platelets

Platelets, or thrombocytes, are small cell fragments essential for blood clotting. They are produced through the process of thrombopoiesis, regulated by the hormone thrombopoietin (TPO).

• Origin: Platelets are derived from megakaryocytes, which develop from hematopoietic stem cells in the bone marrow.

• Developmental Pathway: Hematopoietic stem cell → Megakaryoblast → Megakaryocyte (stages I-IV) → Platelets (by cytoplasmic fragmentation).

• Function: Platelets help stop bleeding by forming plugs in blood vessel injuries and releasing factors that promote clotting.

Leukocytes (White Blood Cells): Types and Functions

Classification and Roles of Leukocytes

Leukocytes are the body's primary defense against infection and disease. They are classified into granulocytes and agranulocytes based on the presence or absence of cytoplasmic granules.

• Granulocytes: Neutrophils, eosinophils, basophils

• Agranulocytes: Lymphocytes, monocytes

Relative Abundance and Blood Smear Appearance

The five types of leukocytes vary in abundance and can be identified in blood smears by their distinct morphology and staining characteristics.

• Neutrophils: 50–70% of WBCs; multi-lobed nucleus, pale granules; phagocytize bacteria.

• Lymphocytes: 25–45%; large, round nucleus; involved in immune responses (T and B cells).

• Monocytes: 3–8%; kidney-shaped nucleus; differentiate into macrophages.

• Eosinophils: 2–4%; bi-lobed nucleus, red granules; combat parasites, involved in allergies.

• Basophils: 0.5–1%; U/S-shaped nucleus, dark granules; release histamine in inflammation.

Normal Cell Counts of WBCs in Adults

Normal ranges for each type of white blood cell are important for diagnosing various conditions.

Summary Table: Leukocyte Types and Functions

Blood Smear Histology

Microscopic Identification of Blood Cells

Blood smears stained with Wright's stain allow for the identification of different blood cells based on their morphology and staining properties. Neutrophils, eosinophils, basophils, lymphocytes, and monocytes can be distinguished by their nuclear shape and cytoplasmic granules.

Additional info:

• Granulocytes are characterized by the presence of cytoplasmic granules and include neutrophils, eosinophils, and basophils. Agranulocytes lack visible granules and include lymphocytes and monocytes.

• Platelets are not true cells but fragments of megakaryocytes, essential for hemostasis.

• EPO is a key hormone in the regulation of erythropoiesis, especially in response to hypoxic conditions.