HUMAN BLOOD- Exam-Ready Notes

 

HUMAN BLOOD

Exam-Ready Notes

INTRODUCTION

Blood is a specialized fluid connective tissue that circulates throughout the body in blood vessels. It transports respiratory gases, nutrients, hormones, metabolic wastes, and immune cells, thereby maintaining homeostasis.

Definition

Blood is a fluid connective tissue composed of:

1. Plasma → fluid matrix
2. Formed elements (blood cells) → RBCs, WBCs, and platelets

GENERAL CHARACTERISTICS OF BLOOD

Feature	Description
Nature	Fluid connective tissue
Color	Bright red in arteries, dark red in veins
pH	Slightly alkaline (7.35–7.45)
Average volume	5–6 L in adults
Specific gravity	1.050–1.060
Temperature	About 38°C
Viscosity	4–5 times that of water

COMPOSITION OF BLOOD

Blood consists of:

Component	Percentage
Plasma	~55%
Formed elements	~45%

COMPONENTS OF BLOOD

1. PLASMA

Plasma is the straw-colored fluid part of blood in which blood cells remain suspended.

Composition of Plasma

Component	Percentage
Water	90–92%
Plasma proteins	6–8%
Other substances	1–2%

A. WATER

Functions:

• Solvent for nutrients and wastes
• Maintains blood volume
• Helps regulate body temperature

B. PLASMA PROTEINS

Major plasma proteins are:

Protein	Percentage	Function
Albumin	~60%	Maintains osmotic pressure
Globulins	~35%	Immunity and transport
Fibrinogen	~4%	Blood clotting

1. Albumin

• Produced by liver
• Smallest and most abundant plasma protein
• Maintains colloidal osmotic pressure
• Helps transport substances

2. Globulins

Types:

• Alpha globulins
• Beta globulins
• Gamma globulins

Functions:

• Immunity
• Transport of lipids and metals

Gamma globulins act as antibodies.

3. Fibrinogen

• Synthesized in liver
• Essential for blood coagulation
• Converts into fibrin during clotting

C. OTHER SUBSTANCES IN PLASMA

Nutrients

• Glucose
• Amino acids
• Fatty acids
• Vitamins

Electrolytes

• Sodium
• Potassium
• Calcium
• Chloride
• Bicarbonate

Hormones

Transported from endocrine glands to target organs.

Waste Products

• Urea
• Uric acid
• Creatinine

Respiratory Gases

• Oxygen
• Carbon dioxide

SERUM

Definition

Serum is the fluid remaining after blood clotting.

Difference Between Plasma and Serum

Plasma	Serum
Contains fibrinogen	Does not contain fibrinogen
Obtained from anticoagulated blood	Obtained after clotting
Used in transfusion	Used in diagnostic tests

Important Point

Serum = Plasma – Clotting factors

FORMED ELEMENTS (BLOOD CELLS)

The formed elements include:

1. RBCs (Erythrocytes)
2. WBCs (Leukocytes)
3. Platelets (Thrombocytes)

ERYTHROCYTES (RBCs)

Definition

RBCs are biconcave, non-nucleated cells containing hemoglobin.

FEATURES OF RBCs

Feature	Description
Shape	Biconcave disc
Nucleus	Absent in mammals
Diameter	~7.2 µm
Lifespan	~120 days
Site of formation	Red bone marrow
Destruction site	Spleen and liver
Number	5–5.5 million/mm³ in males

FUNCTIONS OF RBCs

1. Transport of Oxygen

Hemoglobin binds oxygen forming oxyhemoglobin.

2. Transport of Carbon Dioxide

Some CO₂ is transported as carbaminohemoglobin.

3. Acid–Base Balance

Hemoglobin acts as a buffer.

HEMOGLOBIN

Definition

Hemoglobin is the iron-containing respiratory pigment of RBCs.

Structure

• Protein part → Globin
• Pigment part → Heme (contains iron)

Normal Values

Person	Hb level
Adult male	13–18 g/dL
Adult female	12–16 g/dL

FUNCTIONS OF HEMOGLOBIN

1. Oxygen transport
2. CO₂ transport
3. Buffer action

ERYTHROPOIESIS

Definition

Formation of RBCs is called erythropoiesis.

Site

• Fetus → liver and spleen
• Adult → red bone marrow

Hormone Involved

Erythropoietin from kidneys stimulates RBC formation.

LIFE CYCLE OF RBCs

1. Produced in bone marrow
2. Circulate for 120 days
3. Destroyed mainly in spleen

Fate of Hemoglobin

• Iron recycled
• Globin converted to amino acids
• Heme converted to bilirubin

ANEMIA

Definition

Decrease in hemoglobin concentration or RBC count.

Causes

• Iron deficiency
• Blood loss
• Vitamin B12 deficiency
• Bone marrow disorders

Symptoms

• Fatigue
• Pallor
• Weakness

LEUKOCYTES (WBCs)

Definition

WBCs are nucleated colorless cells involved in body defense.

Normal Count

4,000–11,000/mm³

TYPES OF WBCs

Two major groups:

Granulocytes	Agranulocytes
Neutrophils	Lymphocytes
Eosinophils	Monocytes
Basophils

GRANULOCYTES

These contain cytoplasmic granules.

1. NEUTROPHILS

Features

• Most abundant WBCs
• Multi-lobed nucleus
• 60–70% of WBCs

Functions

• Phagocytosis of bacteria
• First line of defense

2. EOSINOPHILS

Percentage

2–4%

Functions

• Defense against parasites
• Allergic reactions

3. BASOPHILS

Percentage

<1%

Functions

Release:

• Histamine
• Heparin
• Serotonin

Role in inflammation and allergy.

AGRANULOCYTES

1. LYMPHOCYTES

Percentage

20–25%

Types

• B lymphocytes
• T lymphocytes

Functions

• Antibody production
• Cellular immunity

2. MONOCYTES

Features

• Largest WBCs
• Kidney-shaped nucleus

Functions

• Phagocytosis
• Become macrophages in tissues

ABNORMALITIES OF WBCs

Leukocytosis

Increase in WBC count.

Leukopenia

Decrease in WBC count.

Leukemia

Cancer of blood-forming tissues causing excessive abnormal WBC production.

PLATELETS (THROMBOCYTES)

Definition

Small, non-nucleated cell fragments involved in clotting.

FEATURES

Feature	Description
Shape	Irregular discs
Number	1.5–4 lakh/mm³
Lifespan	7–10 days
Formation	Bone marrow

FUNCTIONS OF PLATELETS

1. Blood clotting
2. Repair of damaged vessels
3. Release clotting factors

BLOOD CLOTTING (COAGULATION)

Definition

Conversion of blood from liquid to gel to prevent blood loss.

MECHANISM OF BLOOD CLOTTING

Step 1: Formation of Prothrombin Activator

Occurs through intrinsic and extrinsic pathways.

Step 2: Conversion of Prothrombin into Thrombin

Step 3: Conversion of Fibrinogen into Fibrin

Fibrin threads form clot meshwork.

ANTICOAGULANTS

Substances preventing clotting:

• Heparin
• Sodium citrate
• EDTA

BLOOD GROUPS

ABO BLOOD GROUP SYSTEM

Discovered by Karl Landsteiner.

Blood Group	Antigen on RBC	Antibody in Plasma
A	A	Anti-B
B	B	Anti-A
AB	A and B	None
O	None	Anti-A and Anti-B

Rh FACTOR

Rh Positive

Rh antigen present.

Rh Negative

Rh antigen absent.

ERYTHROBLASTOSIS FETALIS

Occurs when:

• Mother is Rh negative
• Fetus is Rh positive

Maternal antibodies destroy fetal RBCs.

FUNCTIONS OF BLOOD

1. TRANSPORT FUNCTION

Blood transports:

• Oxygen
• CO₂
• Nutrients
• Hormones
• Waste products

2. REGULATORY FUNCTION

Maintains:

• pH
• Temperature
• Water balance

3. PROTECTIVE FUNCTION

Provides:

• Immunity
• Blood clotting
• Defense against pathogens

IMPORTANT TERMS

Term	Meaning
Hematocrit	Percentage of RBC volume in blood
Hemolysis	Destruction of RBCs
Hemostasis	Prevention of blood loss
Thrombosis	Formation of clot in vessel
Embolism	Blockage by detached clot

IMPORTANT NEET POINTS

Very Important Facts

• Blood is a fluid connective tissue.
• Plasma forms ~55% of blood.
• Serum lacks fibrinogen.
• RBC lifespan = 120 days.
• Spleen is called “graveyard of RBCs.”
• Neutrophils are most abundant WBCs.
• Monocytes are largest WBCs.
• Platelets help in clotting.
• Universal donor = O negative.
• Universal recipient = AB positive.

HIGH-YIELD COMPARISON TABLES

RBC vs WBC

Feature	RBC	WBC
Color	Red	Colorless
Nucleus	Absent	Present
Hemoglobin	Present	Absent
Function	Gas transport	Defense
Number/mm³	Millions	Thousands

Plasma vs Serum

Plasma	Serum
Contains clotting factors	No clotting factors
Yellow fluid	Clear yellow fluid
Used for transfusion	Used in diagnostics

Granulocytes vs Agranulocytes

Granulocytes	Agranulocytes
Granules present	Granules absent
Lobed nucleus	Non-lobed nucleus
Neutrophils, eosinophils, basophils	Lymphocytes, monocytes

FLOW CHART: COMPONENTS OF BLOOD

Blood

│

├── Plasma (55%)

│   ├── Water

│   ├── Proteins

│   ├── Nutrients

│   ├── Hormones

│   └── Electrolytes

│

└── Formed Elements (45%)

    ├── RBCs

    ├── WBCs

    └── Platelets

FREQUENTLY ASKED NEET QUESTIONS

1. Why are mammalian RBCs biconcave?

• Increases surface area for gas exchange
• Provides flexibility

2. Why are platelets called thrombocytes?

Because they help in thrombus (clot) formation.

3. Why is AB positive called universal recipient?

Because it lacks anti-A and anti-B antibodies.

4. Why is O negative called universal donor?

Because it lacks A, B, and Rh antigens.

MEMORY TIPS (MNEMONICS)

WBC Abundance

Never Let Monkeys Eat Bananas

• Neutrophils
• Lymphocytes
• Monocytes
• Eosinophils
• Basophils

ABO Blood Groups

Group	Antigen	Antibody
A	A	B
B	B	A
AB	Both	None
O	None	Both

Mnemonic:
“Type A attacks B; Type B attacks A.”

SUMMARY

Blood is an essential fluid connective tissue responsible for transport, protection, and regulation in the body. It consists of plasma and formed elements. RBCs transport gases, WBCs provide immunity, and platelets aid clotting. Plasma contains proteins, nutrients, hormones, and electrolytes necessary for homeostasis. Understanding blood composition and functions is fundamental for NEET biology and human physiology.