Blood – What We Need to Know

Tejasvani Knowledge Desk

Blood is a vital fluid in the human body that performs many essential functions necessary for life. It transports oxygen and nutrients, removes waste products, regulates body temperature, and protects the body against infections.

An average adult has about 5 litres of blood, which accounts for approximately 7–8% of total body weight. The density of blood is around 1060 kg/m³, which is very close to the density of water (1000 kg/m³).

Blood circulating through the arteries, veins, and capillaries is known as whole blood. It is composed of about 55% plasma and 45% blood cells.

Components of Blood

Blood has four main components:

• Plasma

• Red Blood Cells (RBCs)

• White Blood Cells (WBCs)

• Platelets

Each component plays a unique role in maintaining health.

Plasma

Plasma is the liquid portion of blood and makes up about 55% of total blood volume.

It is composed mainly of water but also contains:

• Proteins

• Sugars

• Fats

• Hormones

• Electrolytes

• Waste products

Functions of Plasma

• Transports blood cells throughout the body

• Carries nutrients to tissues

• Removes waste products

• Transports hormones and chemical messengers

• Maintains fluid balance in the body

• Supports immune defense through antibodies

Red Blood Cells (Erythrocytes)

Red blood cells are the most abundant cells in the blood, making up about 40–45% of blood volume.

They contain a protein called hemoglobin, which gives blood its red colour and allows it to carry oxygen.

Key Characteristics

• Produced in the bone marrow

• Regulated by the hormone erythropoietin (EPO) produced by the kidneys

• Average lifespan: 120 days

• Do not contain a nucleus

• Flexible shape helps them pass through tiny capillaries

Functions

• Transport oxygen from lungs to body tissues

• Carry carbon dioxide back to the lungs for removal

The percentage of blood made up of red blood cells is known as the hematocrit, an important measure used in blood tests.

White Blood Cells (Leukocytes)

White blood cells are responsible for protecting the body against infections and foreign invaders. They represent only about 1% of total blood volume but play a crucial role in immunity.

Neutrophils

Neutrophils are the most common type of white blood cell, making up 55–70% of WBCs.

They act as the first line of defence against infections and respond rapidly to invading microorganisms.

However, neutrophils have a very short lifespan, so the bone marrow must constantly produce new ones.

Lymphocytes

Another important group of white blood cells is lymphocytes, which include:

• T cells – regulate immune responses and destroy infected or abnormal cells

• B cells – produce antibodies that target bacteria and viruses

Platelets (Thrombocytes)

Platelets are small fragments of cells rather than complete cells. Their main role is blood clotting.

When a blood vessel is damaged, platelets:

• Gather at the injury site

• Stick to the damaged vessel wall

• Form a temporary plug

• Help create a fibrin clot that stops bleeding

This clot also acts as a framework for new tissue growth, allowing the wound to heal.

Platelet Imbalances

• High platelet count may increase the risk of clots, strokes, or heart attacks.

• Low platelet count may lead to excessive bleeding.

Functions of Blood

Blood performs several critical functions in the body.

1. Transportation

Blood transports many substances throughout the body.

• Oxygen delivery: RBCs carry oxygen from the lungs to body cells.

• Nutrient distribution: Glucose, amino acids, vitamins, and minerals are transported to tissues.

• Waste removal: Metabolic waste such as urea and carbon dioxide is transported to the kidneys and lungs for elimination.

• Hormone transport: Blood carries hormones from endocrine glands to target organs.

2. Regulation of Body Balance (Homeostasis)

Blood helps maintain internal stability.

• Temperature regulation: It distributes heat throughout the body.

• pH balance: Helps maintain the body's acid–base balance.

• Fluid balance: Maintains proper fluid levels in tissues.

3. Protection

Blood protects the body from harm.

• Immune defense: White blood cells fight bacteria, viruses, and other pathogens.

• Blood clotting: Platelets and clotting proteins prevent excessive blood loss after injury.

Common Blood Disorders

Several medical conditions can affect blood.

Anaemia

A condition where the number of red blood cells or hemoglobin is low, reducing oxygen delivery.

Symptoms:

• Fatigue

• Weakness

• Pale skin

Leukaemia

A cancer of blood-forming tissues leading to abnormal production of white blood cells.

Symptoms:

• Frequent infections

• Unexplained weight loss

• Easy bruising

Lymphoma

Cancer that begins in the lymphatic system.

Symptoms:

• Swollen lymph nodes

• Fatigue

• Weight loss

Myeloma

Cancer of plasma cells in the bone marrow.

Symptoms:

• Bone pain

• Fatigue

• Frequent infections

Haemophilia

A genetic disorder where blood does not clot properly.

Symptoms:

• Excessive bleeding

• Joint pain

• Prolonged bleeding after injury

Sickle Cell Disease

A genetic condition in which red blood cells become abnormally shaped, blocking blood flow.

Symptoms:

• Pain episodes

• Fatigue

• Frequent infections

Thrombocytopenia

A condition where platelet levels are low.

Symptoms:

• Easy bruising

• Frequent nosebleeds

• Excessive bleeding

Blood Viscosity – An Overlooked Parameter

Blood viscosity refers to the thickness and stickiness of blood. It plays a crucial role in how easily blood flows through arteries, veins, and capillaries.

Changes in viscosity can significantly affect cardiovascular health.

Factors Affecting Blood Viscosity

• Hematocrit levels

• Red blood cell aggregation

• Platelet aggregation

• Dehydration

• Low-density lipoprotein (LDL) levels

• Fibrinogen levels

• Ability of red blood cells to deform

Blood viscosity also changes with blood flow speed and vessel diameter.

• Faster blood flow → lower viscosity

• Slower blood flow → higher viscosity

Why Blood Viscosity Matters

When blood becomes too thick:

• Blood flow resistance increases

• The heart must work harder

• Friction against vessel walls increases

• Inflammation and plaque formation may occur

Plaque formation can narrow blood vessels and increase the risk of:

• Heart disease

• Stroke

• Circulatory disorders

Blood Viscosity and Chronic Disease

High blood viscosity has been linked to several chronic conditions, including:

• Cardiovascular disease

• Diabetes

• Obesity and metabolic syndrome

• Hypertension

• Vascular dementia

• Alzheimer’s disease

Research has shown that individuals with higher blood viscosity may have a significantly increased risk of cardiovascular events compared with those who have lower viscosity levels.