What is the difference between Plasma and Serum? Describe functions of blood in brief 5 mark question
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I now have comprehensive information from multiple authoritative textbooks. Here is a complete 5-mark answer:
Plasma vs. Serum & Functions of Blood
Plasma vs. Serum
Both plasma and serum are the liquid, cell-free portions of blood, but they differ in one key respect: the presence or absence of clotting factors.
| Feature | Plasma | Serum |
|---|---|---|
| Definition | Liquid portion of blood with cells removed by centrifugation with anticoagulant | Fluid remaining after blood has clotted and the clot has been removed |
| Fibrinogen | ✅ Present | ❌ Absent (consumed in clotting) |
| Clotting factors | ✅ Present (II, V, VIII, fibrinogen) | ❌ Absent (used up during coagulation) |
| Total protein | Higher (includes fibrinogen) | ~4% lower (fibrinogen removed) |
| Viscosity | Higher | Lower (due to absence of fibrinogen) |
| Platelets | Present (in uncentrifuged state) | Degranulated during clot formation |
| Analytes (K⁺, Ca²⁺, LD) | Lower | Higher (released from platelet granules during clotting) |
| How obtained | Blood collected in anticoagulant tube → centrifuged | Blood allowed to clot → clot removed → centrifuged |
| Clinical use | Required for coagulation studies (fibrinogen, PT/APTT) | Preferred for most chemistry assays, immunoassays, protein electrophoresis |
Serum retains albumin, immunoglobulins (antibodies), and non-coagulation proteins, and is essentially plasma minus fibrinogen and coagulation factors. — Histology: A Text and Atlas; Medical Physiology (Boron & Boulpaep); Quick Compendium of Clinical Pathology
Functions of Blood (Brief)
Blood is a fluid connective tissue (~6 L in adults; ~7–8% of body weight) that circulates through the cardiovascular system. Its major functions are:
1. Transport of Respiratory Gases
- Carries oxygen from the lungs to tissues (via haemoglobin in RBCs).
- Carries carbon dioxide from tissues back to the lungs for elimination.
2. Nutrient Delivery
- Transports glucose, amino acids, fatty acids, vitamins, and minerals absorbed from the GI tract to all body cells.
3. Waste Removal
- Carries metabolic waste products (urea, creatinine, bilirubin, lactic acid) to the kidneys, liver, and lungs for excretion.
4. Hormonal Communication
- Transports hormones and regulatory substances from endocrine glands to their target organs, enabling chemical signalling throughout the body.
5. Immune Defence
- Carries leukocytes (WBCs) and humoral agents (immunoglobulins, complement proteins) that protect against pathogens, foreign proteins, and cancer cells.
6. Haemostasis (Blood Clotting)
- Plasma clotting factors and platelets (thrombocytes) prevent excessive blood loss through the coagulation cascade, forming a fibrin plug at sites of vascular injury.
7. Maintenance of Homeostasis
- Thermoregulation: Redistributes heat from metabolically active organs to the skin.
- pH buffering: Plasma proteins and bicarbonate buffer system maintain blood pH at ~7.4.
- Osmotic regulation: Albumin (the major plasma protein) generates colloid osmotic (oncotic) pressure (~25 mmHg), preventing fluid loss from capillaries.
Sources: Histology: A Text and Atlas (Pawlina), p. 668–669; Medical Physiology (Boron & Boulpaep), p. 626; Quick Compendium of Clinical Pathology, 5th ed.
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