Blood and Its Components: Complete Notes

Blood Corpuscles • Haemopoiesis • Formed Elements • Plasma Functions • Plasma Enzymes • Erythropoiesis

CSIR-NET • GATE • DBT • ICMR • MSc Biotechnology

Study Tip: Blood can be studied through a simple sequence: Blood → Plasma + Formed Elements → RBCs + WBCs + Platelets → Haemopoiesis → Erythropoiesis → Plasma Functions

For competitive examinations, understand the structure, origin and functions of each blood component rather than memorizing isolated facts.

1. Introduction to Blood

Blood is a specialized fluid connective tissue that circulates through the cardiovascular system. It continuously moves through the heart, arteries, arterioles, capillaries, venules and veins, allowing efficient communication between different tissues and organs.

Blood is essential for maintaining the internal environment of the body. It transports oxygen from the respiratory organs to tissues and carries carbon dioxide and other metabolic waste products away from tissues. It also transports nutrients absorbed from the digestive tract and distributes hormones and other signaling molecules.

In addition to transportation, blood performs important regulatory and protective functions. It participates in maintenance of body temperature, acid-base balance, fluid balance, immunity and prevention of excessive blood loss.

Major Functions of Blood

Transport: Transport of oxygen, carbon dioxide, nutrients, hormones and waste products.
Regulation: Regulation of pH, temperature, osmotic balance and fluid distribution.
Protection: Defense against pathogens and prevention of blood loss through hemostasis.

Key Point

Blood is a fluid connective tissue. Its extracellular matrix is represented by plasma, while its formed elements include erythrocytes, leukocytes and platelets.

2. Composition of Blood

Blood consists of two major components: plasma and formed elements. Plasma represents approximately 55% of blood volume, whereas formed elements account for approximately 45%, although the exact proportions vary between individuals and physiological conditions.

🩸 Basic Composition of Blood

Component	Approximate Proportion	Major Components	Major Functions
Plasma	~55%	Water, proteins, electrolytes, nutrients, hormones and wastes	Transport, regulation and homeostasis
Formed elements	~45%	RBCs, WBCs and platelets	Gas transport, immunity and hemostasis

3. Blood Corpuscles

Blood corpuscles or formed elements are the cellular and cell-derived components suspended in plasma. The three major formed elements are red blood cells, white blood cells and platelets.

Red Blood Cells

Also called erythrocytes. Their primary function is the transport of oxygen and contribution to carbon dioxide transport.

White Blood Cells

Also called leukocytes. They participate in immune defense, inflammation and protection against pathogens.

Platelets

Small cytoplasmic fragments derived from megakaryocytes and essential for hemostasis.

Formed Elements

Collective term for RBCs, WBCs and platelets.

4. Red Blood Cells / Erythrocytes

Red blood cells, or erythrocytes, are the most abundant formed elements of blood. Their major physiological function is the transport of oxygen from the lungs to tissues.

Mature human erythrocytes have a characteristic biconcave disc shape. They lack a nucleus and mitochondria. The absence of a nucleus provides additional space for hemoglobin, while the absence of mitochondria prevents the mature RBC from using the oxygen that it transports.

Structural Features of RBCs

Mature human RBCs are anucleate.
They have a biconcave disc shape.
They contain a high concentration of hemoglobin.
They lack mitochondria.
They generate ATP mainly through anaerobic glycolysis.
Their flexible membrane allows them to pass through narrow capillaries.
Their biconcave shape provides a relatively large surface area for gas exchange.

🔬 Structure of a Typical RBC

Major Functions of RBCs

Transport oxygen from lungs to tissues.
Participate in carbon dioxide transport from tissues to lungs.
Contribute to maintenance of blood pH.
Contribute to blood viscosity.

5. Hemoglobin and RBC Functions

Hemoglobin is the major respiratory pigment present inside erythrocytes. It is an iron-containing protein responsible for the majority of oxygen transport in blood.

Basic Structure of Hemoglobin

Hemoglobin consists of globin protein chains associated with heme groups. Each heme group contains an iron atom in the ferrous state (Fe²⁺), which can reversibly bind oxygen.

Important Forms of Hemoglobin

Oxyhemoglobin: Hemoglobin bound to oxygen.
Deoxyhemoglobin: Hemoglobin without oxygen bound at the oxygen-binding sites.
Carbaminohemoglobin: Hemoglobin carrying carbon dioxide bound to globin amino groups.

Exam Point:

Oxygen transport is mainly carried out by hemoglobin inside RBCs. Carbon dioxide is transported in several forms, with bicarbonate representing the major form in blood.

6. White Blood Cells / Leukocytes

White blood cells, also called leukocytes, are nucleated blood cells involved primarily in immune defense. Unlike erythrocytes, leukocytes contain nuclei and organelles.

Leukocytes can leave the bloodstream and migrate into tissues. The movement of leukocytes through the vessel wall is known as diapedesis.

Major Classification of WBCs

Granulocytes: Neutrophils, eosinophils and basophils.
Agranulocytes: Lymphocytes and monocytes.

7. Types of White Blood Cells

7.1 Neutrophils

Neutrophils are major components of the innate immune system and are important phagocytic cells. They are particularly important in acute inflammatory responses and defense against many bacterial infections.

Granulocyte.
Multilobed nucleus.
Strong phagocytic activity.
Important in acute inflammation.
Often among the first leukocytes recruited to sites of infection.

7.2 Eosinophils

Eosinophils participate in defense against parasites and are associated with allergic and hypersensitivity reactions.

Granulocyte.
Usually possess a bilobed nucleus.
Important in defense against helminthic parasites.
Associated with allergic inflammation.

7.3 Basophils

Basophils are granulocytes involved in inflammatory and allergic responses. Their cytoplasmic granules contain biologically active mediators such as histamine.

7.4 Lymphocytes

Lymphocytes are essential components of the immune system. Major populations include B lymphocytes, T lymphocytes and natural killer cells.

B cells: Important for humoral immunity and antibody production.
T cells: Important for cell-mediated immunity and immune regulation.
Natural killer cells: Important in innate cytotoxic responses.

7.5 Monocytes

Monocytes are large circulating leukocytes. After entering tissues, they can differentiate into macrophages and other specialized cells. They participate in phagocytosis, antigen presentation and inflammation.

WBC	Classification	Major Function
Neutrophils	Granulocyte	Phagocytosis and acute inflammation
Eosinophils	Granulocyte	Parasite defense and allergy
Basophils	Granulocyte	Allergic and inflammatory responses
Lymphocytes	Agranulocyte	Adaptive and innate immunity
Monocytes	Agranulocyte	Phagocytosis and antigen presentation

8. Platelets / Thrombocytes

Platelets, also called thrombocytes, are small cytoplasmic fragments derived from large bone marrow cells called megakaryocytes. They do not contain a nucleus but possess granules and cellular machinery required for hemostasis.

Functions of Platelets

Adhere to damaged blood vessel surfaces.
Become activated following vascular injury.
Release mediators from platelet granules.
Aggregate with other platelets.
Form the initial platelet plug.
Provide a surface supporting coagulation reactions.
Participate in tissue repair.

Important:

Platelets are not complete cells. They are cytoplasmic fragments produced from megakaryocytes.

9. Haemopoiesis

Haemopoiesis, also written as hematopoiesis, is the process of formation, development and maturation of blood cells. It includes the production of erythrocytes, leukocytes and platelets.

All major blood-cell lineages originate from hematopoietic stem cells. These stem cells have the ability to self-renew and differentiate into progenitor cells that eventually produce mature blood cells.

Hematopoietic Stem Cells

Hematopoietic stem cells are multipotent stem cells capable of generating multiple blood-cell lineages. Their descendants progressively become committed to specific developmental pathways.

Two Major Hematopoietic Lineages

Myeloid lineage: Gives rise to erythrocytes, megakaryocytes/platelets, granulocytes and monocytes.
Lymphoid lineage: Gives rise mainly to B cells, T cells and natural killer cells.

🧬 Simplified Haemopoiesis Pathway

Important Hematopoietic Growth Factors

Erythropoietin (EPO): Stimulates erythroid production.
Thrombopoietin (TPO): Important in megakaryocyte development and platelet production.
G-CSF: Promotes production and maturation of neutrophils.
GM-CSF: Supports development of several myeloid cell types.
Interleukins: Participate in proliferation, differentiation and survival of hematopoietic cells.

10. Formed Elements of Blood

Formed elements are the cellular and cell-derived components of blood. They include erythrocytes, leukocytes and platelets.

Formed Element	Origin	Major Function
Erythrocytes	Erythroid lineage	Oxygen and carbon dioxide transport
Neutrophils	Myeloid lineage	Phagocytosis and acute inflammation
Eosinophils	Myeloid lineage	Parasite defense and allergic responses
Basophils	Myeloid lineage	Inflammatory and allergic responses
Lymphocytes	Lymphoid lineage	Immune defense
Monocytes	Myeloid lineage	Phagocytosis and antigen presentation
Platelets	Megakaryocytes	Hemostasis and clot formation

11. Blood Plasma

Plasma is the liquid component of blood in which the formed elements remain suspended. It consists primarily of water together with proteins, electrolytes, nutrients, hormones, dissolved gases and metabolic wastes.

Major Components of Plasma

Water: Acts as the major solvent and provides the liquid medium for transport.
Proteins: Include albumin, globulins, fibrinogen and many other proteins.
Electrolytes: Include sodium, potassium, calcium, chloride and bicarbonate.
Nutrients: Include glucose, amino acids, lipids, vitamins and other molecules.
Hormones: Chemical messengers transported to target tissues.
Metabolic wastes: Include urea, uric acid, creatinine and other waste products.

Plasma vs Serum:

Plasma is obtained from anticoagulated blood and contains fibrinogen and other clotting factors. Serum is the fluid remaining after blood has clotted and therefore lacks fibrinogen consumed during clot formation.

12. Major Plasma Proteins

12.1 Albumin

Albumin is the most abundant plasma protein. It is synthesized mainly by the liver and contributes significantly to plasma colloid osmotic pressure.

Functions of Albumin

Maintains colloid osmotic pressure.
Helps regulate fluid distribution between blood and tissues.
Transports several molecules.
Can bind fatty acids, hormones, bilirubin and various drugs.

12.2 Globulins

Globulins represent a diverse group of plasma proteins. They include antibodies, complement proteins and various transport proteins.

Immunoglobulins participate in immune defense.
Complement proteins contribute to innate immunity.
Other globulins transport different substances.

12.3 Fibrinogen

Fibrinogen is an important plasma coagulation protein. During blood clotting, fibrinogen is converted into fibrin, which forms a network that contributes to stabilization of the blood clot.

Protein	Major Function
Albumin	Osmotic pressure and transport
Globulins	Immunity and transport
Fibrinogen	Blood coagulation

13. Functions of Plasma

13.1 Transport Function

Plasma acts as the major transport medium for nutrients, hormones, electrolytes, metabolic waste products and numerous proteins.

13.2 Maintenance of Osmotic Balance

Plasma proteins, especially albumin, contribute to colloid osmotic pressure and help regulate water movement between blood vessels and surrounding tissues.

13.3 Regulation of pH

Plasma contains buffering systems that help maintain blood pH. The bicarbonate buffer system is particularly important, while proteins and phosphate also contribute to acid-base regulation.

13.4 Electrolyte Balance

Plasma contains sodium, potassium, calcium, chloride, bicarbonate and other ions. These electrolytes are essential for nerve activity, muscle contraction, osmotic balance and acid-base homeostasis.

13.5 Temperature Regulation

Because plasma is largely water and continuously circulates throughout the body, blood can distribute heat between different regions of the body.

13.6 Immune Function

Plasma contains antibodies, complement proteins and other immune mediators that participate in defense against infectious agents.

13.7 Blood Clotting

Plasma contains coagulation factors such as fibrinogen. These proteins participate in the coagulation cascade and help prevent excessive blood loss after vascular injury.

13.8 Hormonal Communication

Many hormones circulate through plasma from endocrine organs to their target tissues.

Summary of Plasma Functions

Transport
pH regulation
Osmotic regulation
Temperature distribution
Immune defense
Blood coagulation
Hormone transport
Waste transport

14. Enzymes in Plasma

Plasma contains numerous enzymes and enzyme activities. Some enzymes have physiological functions in the circulation, whereas many enzymes measured in plasma or serum originate primarily from tissues.

Measurement of plasma or serum enzyme activity is particularly important in clinical biochemistry because increased enzyme activity can provide information about tissue damage, disease or altered metabolism.

14.1 Alanine Aminotransferase — ALT

ALT catalyzes the transfer of an amino group between alanine and an alpha-keto acid. ALT activity is commonly measured as an indicator of hepatocellular injury and is relatively associated with the liver.

14.2 Aspartate Aminotransferase — AST

AST is present in several tissues, including liver, heart and skeletal muscle. Increased AST activity can occur following tissue injury and therefore is not completely specific to the liver.

14.3 Alkaline Phosphatase — ALP

ALP catalyzes hydrolysis of phosphate-containing substrates under alkaline conditions. Major sources include liver and bone.

14.4 Gamma-Glutamyl Transferase — GGT

GGT participates in transfer of gamma-glutamyl groups and is associated particularly with the hepatobiliary system.

14.5 Lactate Dehydrogenase — LDH

LDH catalyzes the reversible conversion of lactate and pyruvate coupled with NADH/NAD⁺. It is widely distributed in tissues, so increased activity can occur in many conditions involving tissue injury.

14.6 Creatine Kinase — CK

Creatine kinase is involved in cellular energy metabolism through the creatine-phosphocreatine system. Different CK isoenzymes are associated with different tissues.

14.7 Amylase

Amylase participates in carbohydrate digestion by hydrolyzing starch and related polysaccharides. Plasma or serum amylase measurement can be useful in evaluating pancreatic and salivary gland conditions.

Enzyme	Important Association
ALT	Hepatocellular injury
AST	Liver, heart and skeletal muscle injury
ALP	Liver/biliary tract and bone
GGT	Hepatobiliary system
LDH	Widely distributed; tissue injury
CK	Muscle and energy metabolism
Amylase	Pancreatic and salivary gland function

Important Concept:

Many enzymes measured in plasma are not primarily intended to perform their main physiological function in plasma. Their increased plasma activity may occur because of release from damaged or stressed cells.

15. Sites of Erythropoiesis

Erythropoiesis is the process of production of red blood cells. The major site of erythropoiesis changes during embryonic, fetal and postnatal development.

15.1 Early Embryonic Stage — Yolk Sac

During early embryonic development, blood formation begins primarily in the yolk sac. Primitive erythrocytes are produced during this early developmental phase.

15.2 Fetal Stage — Liver

As embryonic development progresses, hematopoietic activity shifts largely toward the fetal liver. The spleen can also contribute to fetal hematopoiesis.

15.3 Late Fetal Stage — Bone Marrow

During later fetal development, bone marrow gradually becomes an increasingly important site of blood-cell production.

15.4 Adult — Red Bone Marrow

In healthy adults, erythropoiesis occurs predominantly in red bone marrow, especially in bones of the axial skeleton and selected proximal regions of long bones.

🩸 Developmental Sites of Erythropoiesis

High-Yield Sequence:

Yolk Sac → Fetal Liver ± Spleen → Bone Marrow

16. Regulation of Erythropoiesis

Erythropoiesis is regulated primarily according to the oxygen needs of tissues. When oxygen delivery to tissues decreases, the kidneys increase production of erythropoietin (EPO).

EPO acts mainly on erythroid progenitor cells in the bone marrow. It promotes survival, proliferation and differentiation of erythroid precursor cells.

Negative Feedback Mechanism

Factors Required for Erythropoiesis

Iron: Required for heme and hemoglobin synthesis.
Vitamin B12: Important for DNA synthesis and normal RBC maturation.
Folate: Required for nucleotide synthesis and cell division.
Amino acids: Required for globin protein synthesis.
Erythropoietin: Major hormonal regulator of erythroid production.
Healthy bone marrow: Required for normal erythrocyte production.

General Erythropoietic Sequence

Hematopoietic Stem Cell → Myeloid Progenitor → Erythroid Progenitor → Erythroblast → Reticulocyte → Mature RBC

17. Important Clinical and Examination Points

RBCs are the most abundant formed elements of blood.
Mature human RBCs lack a nucleus.
Mature RBCs lack mitochondria.
Hemoglobin is the major oxygen-carrying protein of RBCs.
WBCs are primarily involved in immune defense.
Neutrophils are important phagocytic cells.
Lymphocytes are important in adaptive immunity.
Monocytes can differentiate into macrophages in tissues.
Platelets originate from megakaryocytes.
Platelets are important for hemostasis.
Albumin is the major plasma protein.
Albumin contributes significantly to plasma colloid osmotic pressure.
Fibrinogen participates in blood coagulation.
Plasma contains clotting factors; serum lacks fibrinogen consumed during clotting.
EPO is a major regulator of erythropoiesis.
Adult erythropoiesis occurs mainly in red bone marrow.
Early embryonic hematopoiesis occurs in the yolk sac.
Fetal liver is an important site of fetal hematopoiesis.
ALT is commonly associated with hepatocellular injury.
ALP is associated particularly with liver/biliary tract and bone.

18. Important Comparisons for Competitive Exams

Feature	RBC	WBC	Platelets
Full name	Erythrocyte	Leukocyte	Thrombocyte
Nucleus in mature human component	Absent	Present	Absent
Origin	Erythroid lineage	Myeloid/lymphoid lineages	Megakaryocytes
Major function	Gas transport	Immune defense	Hemostasis

Concept	Key Point
Plasma	Liquid component of blood containing water, proteins and solutes.
Serum	Fluid remaining after clot formation; lacks fibrinogen consumed during clotting.
Albumin	Major plasma protein; important for colloid osmotic pressure.
Fibrinogen	Plasma coagulation protein converted to fibrin.
EPO	Major hormonal regulator of erythropoiesis.
Adult erythropoiesis	Mainly in red bone marrow.
Early embryonic hematopoiesis	Yolk sac.

19. Quick Revision Notes

⭐ Must-Remember Points

Blood is a specialized fluid connective tissue.
Blood = Plasma + Formed Elements.
Plasma is approximately 55% of blood volume.
Formed elements account for approximately 45%.
Formed elements include RBCs, WBCs and platelets.
RBCs are called erythrocytes.
WBCs are called leukocytes.
Platelets are called thrombocytes.
Mature human RBCs are anucleate.
Mature RBCs lack mitochondria.
Hemoglobin transports most oxygen in blood.
WBCs are primarily involved in immune defense.
Neutrophils are important phagocytes.
Lymphocytes are essential for immune responses.
Platelets originate from megakaryocytes.
Platelets participate in primary hemostasis and coagulation.
Haemopoiesis is the formation of blood cells.
Hematopoietic stem cells are multipotent.
EPO stimulates erythropoiesis.
Albumin is the major plasma protein.
Albumin contributes to colloid osmotic pressure.
Fibrinogen participates in coagulation.
Plasma contains clotting factors.
Serum is obtained after blood clotting and lacks fibrinogen consumed during clot formation.
ALT is commonly associated with hepatocellular injury.
Adult erythropoiesis occurs mainly in red bone marrow.
Early embryonic hematopoiesis occurs in the yolk sac.
Fetal liver is an important hematopoietic organ.
High-yield sequence: Yolk sac → Liver/Spleen → Bone marrow.

21. Final Exam-Oriented Summary

Blood is a specialized fluid connective tissue consisting of plasma and formed elements. Plasma provides the liquid environment for transport and regulation, whereas formed elements perform specialized functions such as gas transport, immunity and hemostasis.

Blood: Plasma + formed elements.
RBC: Oxygen transport.
WBC: Immune defense.
Platelets: Hemostasis.
Plasma: Liquid component containing water, proteins and solutes.
Albumin: Major plasma protein; important for colloid osmotic pressure.
Fibrinogen: Important coagulation protein.
Haemopoiesis: Formation of blood cells.
Erythropoiesis: Formation of RBCs.
EPO: Major regulator of erythropoiesis.
Adult erythropoiesis: Mainly red bone marrow.
Early embryonic hematopoiesis: Yolk sac.
Fetal hematopoiesis: Liver is an important site, with spleen also contributing.
High-yield sequence: Yolk sac → Liver/Spleen → Bone marrow.

For competitive examinations, remember the relationship between structure and function. The biconcave, anucleate RBC is specialized for gas transport; leukocytes are specialized for immune defense; platelets are specialized for hemostasis; and plasma provides the transport and regulatory environment for all these components.

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Sunday, 23 August 2026