Composition of blood and their functions notes
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Composition of Blood and Their Functions
Blood is a specialized connective tissue that circulates through the cardiovascular system. In a healthy adult, total blood volume is approximately 5 liters. Blood consists of two major components: plasma (the liquid portion, ~55%) and formed elements (cells and cell fragments, ~45%).
Overview: Components of Blood
| Component | % of Blood Volume | Key Elements |
|---|---|---|
| Plasma | ~55% | Water, proteins, electrolytes, nutrients, hormones |
| Formed Elements | ~45% | RBCs, WBCs, Platelets |
The percentage of blood occupied by red blood cells is called the hematocrit (normal: ~42-47% in females, ~45-52% in males).
1. PLASMA
Plasma is the straw-colored liquid matrix of blood. It is approximately 90% water, with the remaining 10% consisting of dissolved substances.
Composition of Plasma
| Substance | Approximate Content | Functions |
|---|---|---|
| Water | ~90% | Solvent; transports dissolved substances; regulates temperature |
| Plasma Proteins | 6-8 g/dL | See below |
| Electrolytes | Na+, K+, Ca2+, Cl-, HCO3- | Osmotic balance, acid-base regulation, nerve/muscle function |
| Nutrients | Glucose, amino acids, lipids, vitamins | Energy source, tissue repair |
| Waste Products | Urea, creatinine, bilirubin | Transported to kidneys/liver for excretion |
| Hormones | Insulin, cortisol, thyroid hormones | Endocrine signaling |
| Dissolved Gases | O2, CO2, N2 | Respiratory exchange |
Plasma Proteins
Plasma proteins are produced mainly by the liver and constitute the major solutes in plasma.
| Protein | Normal Value | Functions |
|---|---|---|
| Albumin | ~4.5 g/dL (largest fraction) | Maintains colloid osmotic (oncotic) pressure; transports fatty acids, bilirubin, drugs, hormones |
| Globulins (alpha, beta) | ~2.5 g/dL | Transport lipids (lipoproteins), metals (transferrin), fat-soluble vitamins; complement proteins |
| Gamma-globulins (immunoglobulins) | Part of globulin fraction | Antibodies - immune defense |
| Fibrinogen | ~0.3 g/dL | Precursor to fibrin; essential for blood clotting |
| Clotting Factors | Trace | Factor V, VIII, IX, X, etc. - coagulation cascade |
Serum = plasma with fibrinogen and clotting factors removed (i.e., what remains after blood clots).
2. FORMED ELEMENTS
A. Erythrocytes (Red Blood Cells - RBCs)
Morphology:
- Biconcave discs, ~7.5 μm in diameter, 2.6 μm thick at the rim, 0.75 μm thick in the center
- Anucleate (lack a nucleus) and lack organelles
- Cytoplasm completely filled with hemoglobin
- Membrane: ~40% lipid, 10% carbohydrate, 50% protein (includes Band 3 protein, glycophorin A - basis of ABO blood group)
- Submembranous cytoskeleton: spectrin + actin + ankyrin network maintains shape and flexibility
Normal Count:
- Women: 3.9-5.5 million/μL
- Men: 4.1-6.0 million/μL
Life Span: ~120 days; removed by macrophages in the spleen, liver, and bone marrow
Functions:
- Oxygen transport - Hemoglobin binds O2 (as oxyhemoglobin) and delivers it to tissues
- Carbon dioxide transport - Hemoglobin binds CO2 (as carbaminohemoglobin) and returns it to the lungs
- Acid-base buffering - Hemoglobin acts as a buffer by binding/releasing H+ ions
- Internal size standard - Uniform dimensions allow estimation of other cell sizes in histology
Energy: Rely on anaerobic glycolysis (no mitochondria)
Clinical Note: Anemia = RBC count below normal range. Polycythemia = increased RBC count (e.g., high-altitude adaptation).
B. Leukocytes (White Blood Cells - WBCs)
Normal count: ~7,000 WBCs/μL (vs. 5 million RBCs/μL)
Leukocytes are the mobile units of the body's immune and defense system. Unlike RBCs, they leave the vasculature to function in tissues - entering via diapedesis (extravasation) through capillary walls.
Genesis:
Genesis of white blood cells showing the myelocytic lineage (neutrophils, eosinophils, basophils, monocytes) and lymphocytic lineage (T cells, B cells, plasma cells). - Guyton & Hall Textbook of Medical Physiology
WBCs are divided into Granulocytes and Agranulocytes:
GRANULOCYTES (have prominent cytoplasmic granules; multilobed nuclei)
1. Neutrophils
- Most abundant WBC: 62% of total WBCs
- Nucleus: multilobed (3-5 lobes, "polymorphonuclear")
- Granules: azurophilic (primary - contain MPO, defensins, lysozyme) + specific/secondary (collagenase, lactoferrin)
- Life span: 4-8 hours in blood, 1-4 days in tissues
- Functions:
- Primary defense against bacterial infections
- Phagocytosis: engulf and destroy bacteria in phagolysosomes
- Release superoxide radicals (O2-) and H2O2 (oxidative burst) for microbial killing
- Lactoferrin binds iron, starving bacteria
- Short-lived; dying neutrophils + bacteria + tissue fluid = pus
- Release proresolving mediators and chemokines to recruit other immune cells
2. Eosinophils
- 2.3% of total WBCs (1-4% range)
- Nucleus: bilobed
- Granules: large, acidophilic (pink/red); contain Major Basic Protein (MBP), eosinophilic peroxidase
- Life span: similar to neutrophils; concentrated in gut mucosa and lung tissue
- Functions:
- Defense against parasitic infections (helminths)
- Modulate allergic responses (asthma, hay fever) by releasing chemokines, cytokines, and proresolving lipid mediators
- Phagocytose antigen-antibody complexes
- Increased in helminthic infections and allergic diseases (eosinophilia)
3. Basophils
- Least common: 0.4% of total WBCs
- Nucleus: bilobed or irregular, obscured by granules
- Granules: large, intensely basophilic (purple-blue); contain histamine and heparin
- Functions:
- Release histamine - triggers vasodilation and increased vascular permeability in allergic reactions
- Release heparin - local anticoagulant, promotes blood flow at inflammatory sites
- Role in immediate hypersensitivity (Type I) reactions
- Analogous to tissue mast cells
AGRANULOCYTES (few or no visible granules; non-lobed nuclei)
4. Monocytes
- 5.3% of total WBCs; largest WBC (12-20 μm diameter)
- Nucleus: kidney-shaped, indented, or C-shaped; basophilic cytoplasm with fine azurophilic granules
- Transit in blood: only 10-20 hours, then migrate to tissues
- In tissues: differentiate into macrophages (swelling up to 60-80 μm, much more potent)
- Functions:
- Phagocytosis of bacteria, cellular debris, and foreign particles (as macrophages)
- Antigen presentation to lymphocytes
- Tissue macrophages: long-lived tissue defense (months)
- Monocyte recruitment is key in both acute and chronic inflammation
5. Lymphocytes
- 30% of total WBCs; second most abundant
- Small cells (6-12 μm); large, round, dark nucleus occupying most of the cell
- Produced in bone marrow; mature in thymus (T cells) or bone marrow/lymphoid tissue (B cells)
- Circulate continuously: blood → lymph → tissues → lymph → blood
- Life span: weeks to months depending on activation state
- Functions:
- T lymphocytes (T cells): Cell-mediated immunity - destroy virus-infected/tumor cells (cytotoxic T cells); regulate immune responses (helper T cells)
- B lymphocytes (B cells): Humoral immunity - differentiate into plasma cells that produce antibodies (immunoglobulins)
- Natural Killer (NK) cells: Innate immunity against tumor and virus-infected cells
6. Plasma Cells (derived from B cells)
- Found mainly in tissues, rarely in blood
- Produce large quantities of specific antibodies
- Identified by "clock-face" chromatin pattern in nucleus
C. Platelets (Thrombocytes)
Morphology:
- Tiny, non-nucleated cell fragments, only 2-4 μm in diameter
- Derived by budding from megakaryocytes in bone marrow
- Contain alpha-granules (fibrinogen, vWF, growth factors) and dense granules (ADP, serotonin, Ca2+)
- Open canalicular system (OCS) for rapid granule secretion
Normal Count: 150,000-400,000/μL (average ~300,000/μL)
Life Span: ~10 days; ~150 billion platelets produced daily
Functions:
- Primary hemostasis - rapidly adhere to damaged vessel wall (collagen exposure) via vWF, aggregate to form a "platelet plug"
- Activate coagulation cascade - release ADP, thromboxane A2, and other factors that trigger secondary hemostasis (fibrin clot formation)
- Repair microvascular tears - plug minor leaks in capillaries and venules
- Wound healing - release growth factors (PDGF, VEGF) promoting tissue repair
Clinical notes:
- Thrombocytopenia (< 150,000/μL) = increased bleeding risk
- Thrombocytosis (> 400,000/μL) = increased clotting risk
Summary Table: Formed Elements
| Cell | Normal Count | Life Span | Key Function |
|---|---|---|---|
| Erythrocytes (RBCs) | 4-6 million/μL | 120 days | O2/CO2 transport, acid-base buffering |
| Neutrophils | 62% of WBCs | 4-8 h (blood); 1-4 days (tissue) | Phagocytosis, bacterial defense |
| Eosinophils | 2.3% of WBCs | Similar to neutrophils | Anti-parasitic, allergic modulation |
| Basophils | 0.4% of WBCs | Similar to neutrophils | Allergic reactions (histamine, heparin) |
| Monocytes | 5.3% of WBCs | 10-20 h (blood); months (tissue as macrophages) | Phagocytosis, antigen presentation |
| Lymphocytes | 30% of WBCs | Weeks to months | Adaptive immunity (T & B cell responses) |
| Platelets | 150,000-400,000/μL | 10 days | Hemostasis, clot formation, vessel repair |
Key Terminology
- Hematocrit: % of blood volume occupied by RBCs (~45%)
- Hemoglobin: Iron-containing protein in RBCs; each molecule carries 4 O2 molecules
- Diapedesis: WBC migration through capillary walls into tissues
- Chemotaxis: Directed WBC movement toward chemical signals at sites of infection/inflammation
- Phagocytosis: Engulfment and destruction of pathogens/debris by neutrophils and macrophages
- Serum: Plasma minus clotting factors (what remains after a clot forms)
- Rouleau: Stacking of RBCs in low-flow conditions, like coins in a roll
Sources: Guyton & Hall Textbook of Medical Physiology; Junqueira's Basic Histology, 17e; Henry's Clinical Diagnosis and Management by Laboratory Methods
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