Blood Composition: Components, Functions, and Clinical Significance

Blood is a specialized connective tissue (~7–8% body weight) comprising cellular elements suspended in plasma.

Components and Functions

Component	% Volume	Key Functions
Plasma (92% water)	55%	Transport medium: proteins (albumin, globulins, fibrinogen), electrolytes, nutrients, hormones, waste, gases, clotting factors.
Red Blood Cells (Erythrocytes)	45% (hematocrit)	O₂ transport (hemoglobin); CO₂ transport; acid-base buffering; lifespan ~120 days.
White Blood Cells (Leukocytes)	<1% (buffy coat)	Immune defense: neutrophils (acute bacteria), lymphocytes (adaptive), monocytes/macrophages, eosinophils (parasites/allergy), basophils (allergy).
Platelets (Thrombocytes)	<1%	Hemostasis: adhesion, activation, aggregation; release growth factors for repair; lifespan 7–10 days.

Key Plasma Proteins

• Albumin (60%): oncotic pressure, transport.
• Globulins: α/β (transport), γ (immunoglobulins/antibodies).
• Fibrinogen: clot formation.

Physiologic Variation

• Sex: Males have higher Hgb/Hct (testosterone stimulates erythropoietin).
• Age: Neonates high Hct (fetal Hb); declines to nadir at 2–3 months; adult levels by puberty.
• Altitude: Chronic hypoxia → ↑ EPO → ↑ Hct.
• Pregnancy: Plasma volume ↑ > RBC mass → physiologic anemia (Hct ~33–38%).

Clinical Assessment

• CBC with differential: screens anemia, infection, leukemia, thrombocytopenia.
• Coagulation studies (PT/INR, aPTT): plasma factors.
• Blood typing, crossmatch for transfusion.

Blood’s integrated functions—transport, defense, hemostasis, homeostasis—make it a vital diagnostic window.