Blood: Composition, Plasma, Blood Cells & Functions | MBBS
Blood is a specialized connective tissue in fluid form. It is often described as the “fluid of life” because it transports oxygen and nutrients to tissues and carries carbon dioxide and metabolic waste away. It also contributes to immunity, temperature regulation and maintenance of the body's internal environment.
1. Properties of Blood
Blood is normally red because of hemoglobin present in red blood cells. Arterial blood is bright red because it contains more oxygen, while venous blood is darker red because of its lower oxygen content.
| Property | Normal Feature |
|---|---|
| Blood volume | Approximately 7–8% of body weight in a typical adult |
| pH | Approximately 7.35–7.45 |
| Specific gravity | Approximately 1.050–1.060 |
| Viscosity | About 4–5 times that of water |
2. Composition of Blood
Blood consists of plasma and formed elements. Formed elements include red blood cells, white blood cells and platelets.
| Component | Approximate Proportion |
|---|---|
| Plasma | ~55% |
| Formed elements | ~45% |
Blood Cells
- Red blood cells (erythrocytes) – transport oxygen and contribute to carbon dioxide transport.
- White blood cells (leukocytes) – provide immune and defensive functions.
- Platelets (thrombocytes) – play an essential role in hemostasis and blood clotting.
3. Hematocrit or Packed Cell Volume
The hematocrit (Hct), also called packed cell volume (PCV), is the percentage of blood volume occupied by red blood cells after centrifugation.
In a typical adult, hematocrit is roughly 40–45%, although normal values vary with sex, age, altitude and laboratory reference range. A thin layer called the buffy coat, containing mainly white blood cells and platelets, lies between the plasma and packed RBCs.
4. Plasma
Plasma is the clear, straw-colored liquid portion of blood. It consists mainly of water, together with proteins, electrolytes, nutrients, hormones, gases and metabolic waste products.
Major plasma proteins include albumin, globulins and fibrinogen. Albumin contributes importantly to plasma colloid osmotic pressure, while globulins include antibodies and other transport proteins. Fibrinogen is essential for blood coagulation.
5. Serum
Serum is the fluid that remains after blood has clotted. During clotting, fibrinogen is converted into fibrin and becomes part of the clot. Consequently, serum lacks fibrinogen and several other clotting factors consumed during coagulation.
6. Functions of Blood
Nutritive Function
Blood transports glucose, amino acids, lipids, vitamins and other nutrients absorbed from the gastrointestinal tract to tissues.
Respiratory Function
Hemoglobin carries oxygen from the lungs to tissues. Blood also transports carbon dioxide from tissues back to the lungs for elimination.
Excretory Function
Metabolic waste products are transported by blood to organs responsible for their elimination, particularly the kidneys, lungs, liver and skin.
Hormone and Enzyme Transport
Blood carries hormones released by endocrine glands to their target tissues and transports various biologically active substances.
Regulation of Water and Acid–Base Balance
Blood exchanges water with the interstitial fluid and helps maintain body fluid balance. Plasma proteins and hemoglobin also act as important buffers that help maintain acid–base balance.
Temperature Regulation
Blood distributes heat throughout the body and participates in heat loss and heat conservation, helping maintain core body temperature.
Storage Function
Blood serves as a readily available circulating reservoir for water, electrolytes, glucose and other substances required by tissues.
Defensive Function
White blood cells provide cellular defense. Neutrophils and monocytes can engulf microorganisms, while lymphocytes are important in adaptive immunity. Platelets and plasma clotting factors also help prevent blood loss after vascular injury.
7. High-Yield MBBS Revision
- Blood = Plasma + Formed elements.
- Plasma ≈ 55% and formed elements ≈ 45% of blood volume.
- RBCs are mainly responsible for oxygen transport.
- WBCs are important for body defense and immunity.
- Platelets are essential for hemostasis.
- Hematocrit/PCV represents the percentage of blood volume occupied by RBCs.
- Plasma contains fibrinogen; serum does not.
- Hemoglobin and plasma proteins contribute to acid–base buffering.
Quick Takeaway
Blood is a multifunctional connective tissue that links virtually all organs of the body. Its cellular components perform oxygen transport, immunity and hemostasis, while plasma provides the fluid medium for transporting nutrients, hormones, electrolytes and waste products. Together, these components help maintain homeostasis, metabolism, defense and internal stability.
Medical Disclaimer: The information provided in this article is strictly for educational, study, and exam-preparation purposes. It does not constitute professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for clinical decisions.