What is the difference between plasma and serum in blood composition and function
Plasma and serum are two closely related terms in biology and medicine, often leading to confusion among students and teachers. Both are liquid components derived from blood after the removal of blood cells, but understanding their precise differences is fundamental to lab diagnostics, physiology, and even biotechnology research.
What Is Plasma?
Plasma is the liquid portion of blood that remains when clotting is prevented. This means, before a blood sample has had a chance to clot, the fluid you obtain by removing blood cells is known as plasma. Plasma contains a range of dissolved substances, including water, proteins (like albumin and globulins), hormones, nutrients, waste products, and—most importantly—clotting factors like fibrinogen.
What Is Serum?
Serum is the liquid remaining once the blood has had time to clot. After clot formation and removal of the clot, the fluid is termed serum. Serum is similar in composition to plasma but crucially lacks fibrinogen and other clotting factors, as these are consumed during clotting. It contains water, proteins, antibodies, electrolytes, hormones, and various metabolites.
How Are Plasma and Serum Obtained?
To obtain plasma, a blood sample is collected in a tube containing an anticoagulant (such as EDTA, citrate, or heparin) to prevent clotting. The tube is then centrifuged, separating blood cells at the bottom and leaving plasma at the top.
Serum, in contrast, is collected by drawing blood into a plain tube (without anticoagulant), allowing it to clot naturally. After clot formation, centrifugation separates the serum (top) from the coagulated material.
Biological Significance and Applications
Plasma and serum are both used extensively in clinical labs and research. Plasma, containing clotting factors, is essential for studies involving blood coagulation. Serum, without clotting proteins, is preferred for certain biochemical tests, blood typing, antibody assays, and supplementation of cell culture media.
Using both plasma and serum in experiments can help verify results, as the presence or absence of certain factors may influence observed outcomes.
Comparative Table: Plasma vs. Serum
| Feature | Plasma | Serum |
|---|---|---|
| Definition | Liquid part of blood when clotting is prevented | Liquid part remaining after blood has clotted |
| How it's obtained | Blood + Anticoagulant → Centrifuge | Clotted blood → Centrifuge |
| Clotting Factors (e.g., Fibrinogen) | Present | Absent |
| Appearance | Light yellow, clear | Light yellow, clear |
| Density | 1.025 g/ml | 1.024 g/ml |
| Main Uses | Coagulation studies, diagnostics, research | Blood typing, antibody detection, cell cultures |
| Notable Components | Water, proteins (albumin, globulin, fibrinogen), hormones | Similar to plasma but without fibrinogen and other clotting proteins |
Why Does This Difference Matter?
The main practical difference is the presence or absence of clotting proteins. Certain lab tests require the consistency provided by these proteins, while others demand their absence to avoid false readings. For example, plasma is used for tests requiring clotting factors, whereas serum is vital for antibody and metabolite testing.
For academic and exam purposes, remembering that plasma contains fibrinogen (due to no clotting) and serum does not (because the fibrinogen has been used up to form the blood clot), is fundamental.
Explanation of the Clotting Process
Clotting is activated when blood is allowed to stand. This triggers a cascade leading to the conversion of prothrombin to thrombin, which turns fibrinogen into fibrin. Fibrin forms the mesh of the blood clot, and platelets, activated during this process, release compounds that further change the composition of serum.
Collecting plasma avoids this by using anticoagulants, such as EDTA or citrate, to block the process and maintain all proteins in solution.
Case Example
A doctor orders a blood test to investigate clotting disorders. Plasma will be used to ensure that all necessary clotting factors are available for study.
For detection of antibodies (such as in infection diagnosis), serum will be preferred because clotting proteins are unnecessary and could interfere.
Quick Recap: Key Points
- Plasma is obtained from blood with an anticoagulant (contains fibrinogen).
- Serum comes from clotted blood (no fibrinogen present).
- Both are clear, light yellow fluids used in medical and research labs.
Practice Question
A blood sample is collected in a test tube without any anticoagulant and left at room temperature. What will the fluid at the top be after centrifugation: plasma or serum?
Answer: Serum
Continue Learning
- Practice more questions on cell biology, physiology, and genetics with Vedantu’s free resources.
Understanding the distinction between plasma and serum not only enhances conceptual clarity but also builds a foundation for exams and experiments related to human physiology, diagnostic medicine, and biotechnology.
FAQs on Understanding the Differences Between Plasma and Serum
1. What is the main difference between plasma and serum?
The main difference between plasma and serum is that plasma contains clotting factors while serum does not. Plasma is the liquid part of blood obtained before clotting, whereas serum is the fluid left after blood has clotted.
- Plasma contains clotting proteins like fibrinogen.
- Serum lacks fibrinogen and other clotting factors.
- Both contain water, electrolytes, hormones, antibodies, and nutrients.
2. What is plasma in blood?
Plasma is the pale yellow liquid component of blood that suspends blood cells and contains clotting factors. It makes up about 55% of total blood volume and plays a key role in transport and homeostasis.
- Contains water (90–92%).
- Includes proteins such as albumin, globulins, and fibrinogen.
- Transports nutrients, hormones, gases, and waste products.
3. What is serum in blood?
Serum is the clear fluid that remains after blood has clotted and the clot has been removed. It is essentially plasma without clotting factors like fibrinogen.
- Obtained after coagulation.
- Contains antibodies, hormones, and electrolytes.
- Commonly used in diagnostic tests and serology.
4. Does plasma contain fibrinogen?
Yes, plasma contains fibrinogen, a key clotting protein required for blood coagulation. During clot formation, fibrinogen is converted into fibrin to form a blood clot.
- Fibrinogen is produced by the liver.
- It is absent in serum because it is used up during clotting.
5. Why does serum not contain clotting factors?
Serum does not contain clotting factors because they are consumed during the process of blood clotting. When blood coagulates, fibrinogen and other clotting proteins form a clot, leaving behind serum.
- Blood is allowed to clot naturally.
- Fibrin forms a mesh that traps blood cells.
- The remaining liquid is serum.
6. How is plasma obtained from blood?
Plasma is obtained by centrifuging blood that has been treated with an anticoagulant to prevent clotting. The anticoagulant preserves clotting factors in the liquid portion.
- Add anticoagulant such as EDTA or heparin.
- Centrifuge the blood sample.
- The upper yellow layer is plasma.
7. How is serum obtained from blood?
Serum is obtained by allowing blood to clot naturally and then centrifuging it to remove the clot. The remaining supernatant fluid is serum.
- No anticoagulant is added.
- Blood is left undisturbed to clot.
- Centrifugation separates the serum from the clot.
8. What are the functions of plasma in the body?
Plasma functions mainly in transport, immunity, and maintenance of blood volume and pH. It serves as the medium in which blood cells circulate.
- Transports nutrients, hormones, and wastes.
- Contains antibodies for immune defense.
- Maintains osmotic balance via albumin.
- Provides clotting factors for hemostasis.
9. What is serum used for in medical tests?
Serum is commonly used in diagnostic tests to detect antibodies, hormones, enzymes, and disease markers. Its lack of clotting factors makes it suitable for many biochemical and serological analyses.
- Used in ELISA and antibody tests.
- Helps diagnose infections and autoimmune diseases.
- Used for measuring glucose, cholesterol, and enzymes.
10. Is plasma or serum used for blood transfusion?
Plasma is used for blood transfusion because it contains essential clotting factors needed to treat bleeding disorders. Serum is not typically used for transfusions.
- Fresh frozen plasma (FFP) treats clotting deficiencies.
- Provides fibrinogen and other coagulation proteins.
- Serum is mainly used for laboratory testing, not transfusion.
