BLOOD AND ITS COMPONENTS

What is Blood

Introduction

Blood is an essential component of the human body and plays an important role in maintaining normal body functions. It circulates continuously through the heart and blood vessels, transporting oxygen, nutrients, hormones and other substances to different parts of the body. At the same time, it carries carbon dioxide and metabolic wastes to organs such as the lungs and kidneys for removal.

From a biological perspective, blood is a specialised connective tissue consisting of a liquid component called plasma and formed elements, which include red blood cells (RBCs), white blood cells (WBCs), and platelets. Each component has a specific structure and function and together they help maintain the body's internal balance, known as homeostasis.

What Is Blood

Blood is a fluid connective tissue that circulates through the heart and blood vessels. In a healthy adult, the total blood volume is generally around 4–6 litres, although it varies according to body size and other factors.

Blood has two major parts:

• Plasma

• Formed elements

Plasma forms the liquid medium, while the formed elements consist mainly of red blood cells, white blood cells and platelets.

The major functions of blood can be grouped into three categories:

Transport: Blood carries oxygen, carbon dioxide, nutrients, hormones and metabolic wastes.

Protection: White blood cells help fight infections, while platelets and clotting factors help prevent excessive blood loss.

Regulation: Blood contributes to maintaining body temperature, pH, fluid balance and other conditions required for normal cellular activity.

Plasma: 

Plasma is the pale yellow liquid portion of blood. It makes up roughly 55% of blood volume and consists mainly of water.

However, plasma is much more than water. It contains proteins, salts, nutrients, hormones, gases and waste products.

Some important plasma proteins are:

Albumin : Albumin helps maintain the osmotic balance of blood and contributes to maintaining proper blood volume.

Globulins : Globulins include proteins involved in transport and immune defence. Some antibodies belong to the globulin group.

Fibrinogen : Fibrinogen is important in blood clotting. During clot formation, it is converted into fibrin, which forms a network that helps strengthen the clot.

Because so many substances travel through plasma, it can be considered the body's internal transport medium.

Red Blood Cells: 

Red blood cells (RBCs) also called erythrocytes, are the most numerous cells in normal blood.

Their most important function is transporting oxygen from the lungs to body tissues. They also contribute to transporting carbon dioxide from tissues back towards the lungs.

Human RBCs have a distinctive biconcave shape. This shape provides a large surface area and helps the cells move through narrow blood vessels.

Mature human RBCs lack a nucleus and most organelles. This gives them more space for haemoglobin, the protein responsible for carrying oxygen.

Red blood cells are produced primarily in the red bone marrow through a process called erythropoiesis. Their production is regulated largely by the hormone erythropoietin, which is produced mainly by the kidneys when oxygen availability is low.

Haemoglobin: 

Haemoglobin is an iron-containing protein found inside red blood cells. It gives blood much of its characteristic red colour and is essential for oxygen transport.

In the lungs, haemoglobin combines with oxygen to form oxyhaemoglobin. When blood reaches tissues where oxygen is needed, haemoglobin releases oxygen.

A simplified reaction is:

Haemoglobin + Oxygen ⇌ Oxyhaemoglobin

The ability of haemoglobin to pick up oxygen in the lungs and release it in tissues is influenced by factors such as oxygen concentration, carbon dioxide concentration, temperature and pH.

This makes haemoglobin an important link between the circulatory and respiratory systems.

If the blood has insufficient haemoglobin or abnormal red blood cells, its oxygen-carrying capacity may be reduced. One possible result is anaemia, although anaemia can have many different causes.

White Blood Cells: 

White blood cells (WBCs) or leukocytes, are essential to the body's immune defence.

Unlike mature human RBCs, WBCs contain nuclei. They can move through blood vessels and, in some cases, enter tissues to respond to infection or injury.

The major types of WBCs are:

• Neutrophils

• Eosinophils

• Basophils

• Lymphocytes

• Monocytes

Neutrophils : Neutrophils are important cells of the innate immune system. They can move towards infected areas and engulf microorganisms and cellular debris through phagocytosis.

Eosinophils : Eosinophils are involved in defence against certain parasites and participate in allergic and inflammatory responses.

Basophils : Basophils release substances such as histamine that are involved in inflammatory and allergic reactions.

Lymphocytes : Lymphocytes are central to adaptive immunity.

B lymphocytes can give rise to plasma cells that produce antibodies, while T lymphocytes perform several roles in cell-mediated immunity.

Monocytes : Monocytes circulate in the blood and can move into tissues, where they can develop into macrophages and other specialised immune cells.

Together, these cells form an important part of the body's defence network.

Platelets: 

Platelets, or thrombocytes, are small cell fragments found in blood. They are formed from large bone-marrow cells called megakaryocytes.

Their major function is to help control bleeding.

When a blood vessel is damaged, platelets become activated and gather at the injured area. They contribute to the formation of a platelet plug and participate in the series of reactions that ultimately produce a stable blood clot.

This process is called haemostasis.

A simplified sequence is:

Blood vessel injury → platelet activation → clotting reactions → fibrin formation → stable clot

Clotting factors, calcium ions and vitamin K are among the important components involved in normal coagulation.

Blood Clotting: 

Blood must remain fluid while circulating, but it must also be capable of clotting when a blood vessel is damaged. The body therefore maintains a careful balance.

During clotting, a series of reactions activates different clotting factors. Eventually, soluble fibrinogen is converted into insoluble fibrin. The fibrin forms a mesh that helps trap blood cells and strengthen the clot.

This mechanism is extremely useful after injuries. However, inappropriate clot formation inside blood vessels can also cause serious health problems.

The study of clotting therefore demonstrates an important biological principle: the body depends on precise regulation rather than simply increasing or decreasing a process.

Blood Groups: 

Not everyone's blood is chemically identical. Different blood groups are determined by specific molecules, known as antigens, present on the surface of red blood cells.

The most familiar system is the ABO blood group system.

The four major groups are:

• A

• B

• AB

• O

A person with blood group A has A antigens on their RBCs and anti-B antibodies in their plasma. Group B has B antigens and anti-A antibodies.

Group AB has both A and B antigens, while group O lacks both A and B antigens on its red blood cells.

Blood-group compatibility is extremely important during blood transfusion because an incompatible transfusion can trigger a serious immune reaction.

The Rh Factor

The Rh blood group system is another important blood-group system.

People who have the D antigen are generally called Rh-positive, while those who do not have it are described as Rh-negative.

The Rh factor has particular importance during pregnancy. If an Rh-negative mother carries an Rh-positive fetus, immune sensitisation can sometimes occur. Medical treatment and monitoring can greatly reduce the risk of complications associated with Rh incompatibility.

Common Blood-Related Disorders

Blood disorders can affect any of its major components.

Anaemia: A condition involving reduced oxygen-carrying capacity of blood, often associated with low haemoglobin or abnormal RBCs.

Leukaemia: A group of cancers affecting blood-forming tissues and the development of blood cells.

Haemophilia: An inherited bleeding disorder involving deficiency or dysfunction of certain clotting factors.

Thrombocytopenia: A condition in which the platelet count is lower than normal, potentially affecting normal clot formation.