Blood Components
~8 min read
- Composition: 55% plasma + 45% formed elements (RBC, WBC, platelets).
- RBC: Carries O₂ via haemoglobin; lacks nucleus; lifespan 120 days.
- Clotting: Platelets release clotting factors. Vitamin K essential for clot formation.
Blood is the body's transport medium. CDS/OTA tests components (RBC, WBC, platelets, plasma), haemoglobin function, and clotting.
What Blood Contains
| Component | Role | Notes |
|---|---|---|
| Plasma (~55%) | Liquid medium — water + dissolved proteins, salts, hormones, waste | ~90% water |
| Red blood cells (RBC, erythrocytes) | O₂ and CO₂ transport via haemoglobin | No nucleus in mature RBC; ~120-day lifespan |
| White blood cells (WBC, leukocytes) | Immunity — fight infection | Types: neutrophils, lymphocytes, monocytes, eosinophils, basophils |
| Platelets (thrombocytes) | Blood clotting — release clotting factors | No nucleus; fragments of megakaryocytes |
Haemoglobin and Oxygen Transport
- Haemoglobin is the iron-containing protein in RBC that carries both oxygen and carbon dioxide.
- Iron is the central atom — deficiency causes anaemia.
- Carbon monoxide (CO) binds haemoglobin ~250× more strongly than O₂, blocking oxygen transport — causes suffocation and death.
- Normal Hb: men 13.5–17.5 g/dL; women 12–15.5 g/dL.
Blood Clotting
- Platelets release clotting factors at the site of injury.
- Cascade converts prothrombin → thrombin, which converts fibrinogen → fibrin mesh that traps cells and forms the clot.
- Vitamin K is essential for synthesising clotting factors in the liver — deficiency causes prolonged bleeding.
- Calcium ions (Ca²⁺) are also required at multiple steps.
- Dengue virus causes a drop in platelet count by interfering with platelet production in bone marrow and infecting endothelial cells.
Lymph — The Other Body Fluid
- Lymph is formed when fluid leaks out of blood capillaries into surrounding tissues.
- It is colourless, contains lymphocytes (a type of WBC), but lacks red blood cells and platelets.
- Lymph flows in lymphatic vessels (separate from blood vessels) and is filtered through lymph nodes before joining venous blood.
- Fat absorption: dietary fat enters lymph via lacteals in intestinal villi.
CDS/OTA PYQ Examples
Q: Which vitamin has a role in blood clotting?
(a) Vitamin A (b) Vitamin B (c) Vitamin D (d) Vitamin K
Answer: (d) Vitamin K. [CDS-I 2016]
Q: Which air pollutant combines with haemoglobin and reduces oxygen-carrying capacity?
(a) Chlorofluorocarbon (b) Fly ash (c) Carbon monoxide (d) Sulphur dioxide
Answer: (c) Carbon monoxide. [CDS-II 2016]
Q: Which statement on lymph is correct?
(a) Lymph forms due to leakage of blood through capillaries (b) Lymph contains RBCs (c) Lymph is circulated by heart (d) Lymph only transports hormones
Answer: (a) Lymph forms by leakage of fluid through capillaries. [CDS-I 2018]
Q: Which statement on haemoglobin is correct?
(a) Carries only oxygen (b) Carries both oxygen and carbon dioxide (c) Carries only carbon dioxide (d) Used only for blood clotting
Answer: (b) Carries both O₂ and CO₂. [CDS-I 2019]
Q: From which are blood clotting factors released?
(a) RBCs (b) Eosinophils (c) Platelets (d) Monocytes
Answer: (c) Platelets. [CDS-I 2024]
Q: Which cells do NOT contain nucleus?
(a) Root hair cells only (b) RBC and Platelets only (c) Root hair, RBC and Monocytes (d) RBC, Platelets and Monocytes
Answer: (b) Mature RBC and Platelets are anucleate. [CDS-II 2016]
Drill Blood Components for CDS/OTA
CDS/OTA-pattern items on Blood Components with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why do mature RBCs have no nucleus?
To maximise space for haemoglobin and increase flexibility so they can squeeze through capillaries. They live ~120 days and cannot divide.
How does dengue cause low platelet count?
Dengue virus suppresses platelet production in bone marrow and damages endothelial cells, both reducing platelet survival. Severe thrombocytopenia causes the haemorrhagic phase.
What is the difference between plasma and serum?
Plasma is the liquid part of blood including clotting factors. Serum is plasma minus the clotting factors (what's left after blood has clotted).