Blood is an elixir of life, an essence that is integral to the very existence of beings with a circulatory system. From a historical perspective, blood has its roots in primordial cells that were responsible for consuming and sustaining life. Through eons of time and evolution, blood and the circulatory system have proven to be a catalyst, fueling the emergence of life in its most complex form.

What is Blood?

The Blood is a life-sustaining fluid that functions as a connective tissue, pulsing through the body like a network of veins and arteries, transporting oxygen, nutrients, and essential elements to every cell and tissue. Comprising 8% of an average adult’s body weight, this vital substance contains plasma, blood cells, and platelets and acts as the very essence of life. Possessing an impressive 5-6 liters, this blood circulatory system plays a crucial role in maintaining the health and well-being of the body, ensuring it is nourished and protected at all times.

Types of Blood Cells

We’ve seen that blood is made up of cells known as formed elements of blood. These cells serve distinct functions and play distinct roles in the body. The following are the blood cells that circulate throughout the body:

White Blood Cells (Leucocytes)

Leucocytes are blood cells that are colorless. They are colorless because they lack hemoglobin. Granulocytes and agranulocytes are two types of lymphocytes. WBCs primarily aid in immunity and defense mechanisms.

Red Blood Cells (Erythrocytes)

In humans, RBCs are biconcave cells with no nucleus, also known as erythrocytes. RBCs contain hemoglobin, an iron-rich protein that gives blood its red color. RBCs are the most abundant type of blood cell produced in the bone marrow. Their primary function is to move oxygen from and to different tissues and organs.

Red Blood Cells (Erythrocytes)
Red Blood Cells (Erythrocytes)

Types of White Blood Cells

White blood cells are classified into five types based primarily on the presence or absence of granules.

  • Granulocytes
  • Agranulocytes
five types of white blood cells


They are leukocytes, and their cytoplasm contains granules. Eosinophils, basophils, and neutrophils are examples of granulated cells.


  • They are leukocyte cells, which are found in the immune system.
  • These cells are in charge of fighting infections in vertebrate parasites as well as controlling mechanisms associated with allergy and asthma.
  • Eosinophil cells are small granulocytes that are produced in the bone marrow and account for 2 to 3% of total WBCs. The digestive tract contains a high concentration of these cells.


  • They are the least common granulocyte, accounting for 0.5 to 1% of WBCs.
  • They have large cytoplasmic granules that play an important role in mounting a non-specific immune response to pathogens as well as allergic reactions by releasing histamine and dilating blood vessels.
  • Because these white blood cells can be stained when exposed to basic dyes, they are known as basophils.
  • These cells are best known for their role in asthma, which results in airway inflammation and constriction.
  • Serotonin, histamine, and heparin are all secreted by them.


  • Normally, they are found in the bloodstream.
  • They are the most common cells found in pus.
  • Neutrophils with a diameter of 10 to 12 micrometers account for 60 to 65 percent of WBCs.
  • The nucleus has two to five-lobed nuclei, and the cytoplasm has very fine granules.
  • Neutrophil aids in the destruction of bacteria via lysosomes and functions as a powerful oxidant.
  • Neutrophils are only stained with neutral dyes. As a result, they are known as such.
  • Neutrophils are also the first immune system cells to respond to an invader such as a bacterium or virus.
  • These WBCs have a lifespan of up to eight hours and are produced in the bone marrow on a daily basis.


They are leukocytes without granules in their cytoplasm. Monocytes and lymphocytes are subsets of agranulocytes.


  • These cells typically have a large bilobed nucleus that measures 12 to 20 micrometers in diameter.
  • The nucleus is typically half-moon or kidney-shaped and accounts for 6 to 8% of WBCs.
  • They are the immune system’s garbage trucks.
  • Monocytes’ most important functions are to migrate into tissues and clean up dead cells, to protect against bloodborne pathogens, and to move quickly to infection sites in the tissues.
  • Because these white blood cells have a single bean-shaped nucleus, they are known as Monocytes.


  • They are essential in the production of antibodies.
  • Their size ranges between 8 and 10 micrometers.
  • They are commonly referred to as natural killer cells.
  • They are essential for body defense.
  • These white blood cells are colorless cells that form in lymphoid tissue and are thus known as lymphocytes.
  • Lymphocytes are classified into two types: B lymphocytes and T lymphocytes.
  • These cells play an important role in immune systems, as they are responsible for both humoral and cell-mediated immunity.

Platelets (Thrombocytes)

  • Thrombocytes are bone marrow-derived specialized blood cells.
  • When there is bleeding or hemorrhage, platelets come into play.
  • They aid in blood clotting and coagulation. Platelets aid in the coagulation of a cut or wound.

Components Of Blood

Blood contains a diverse range of cellular structures. When a blood sample is spun in a centrifuge machine, it separates into three components: plasma, buffy coat, and erythrocytes. RBC, WBC, platelets, and plasma are all found in the blood.

Components Of Blood


Plasma can contribute to the liquid state of blood because it makes up 55% of blood. When separated, it is pale yellow in colour. Salts, nutrients, water, and enzymes make up blood plasma. Blood plasma also contains important proteins and other components that are required for good health. As a result, patients with liver failure and life-threatening injuries are given blood plasma transfusions.

Components of Blood Plasma 

There are several protein components in blood plasma. Proteins found in blood plasma include:

  • Serum globulin
  • Serum albumin
  • Fibrinogen

The serum only contains globulin and albumin. Because fibrinogen is converted into fibrin during blood clotting, it is absent in serum.

Read Also: A Food Chain Overview

Red Blood Cells (RBC)

Haemoglobin, a protein, is found in red blood cells. They are produced by the bone marrow to transport oxygen into the body and carbon dioxide out.

White Blood Cells (WBC)

White blood cells are in charge of combating foreign pathogens that enter our bodies (such as bacteria, viruses, and fungi). They are produced in the bone marrow and circulate throughout the body.


Tiny disc-shaped cells help regulate blood flow when any part of the body is damaged, allowing for faster recovery through blood clotting.

The intricate composition of human blood is a marvel of nature, composed of plasma, blood cells, and platelets, each with a vital role in sustaining life. Yet, amidst the vast diversity of life on Earth, there exists one exceptional vertebrate that stands out from the rest, the crocodile icefish. With a unique adaptation, this fascinating species defies the norm, forgoing the presence of hemoglobin and instead, drawing its oxygen directly from the cold and oxygen-rich waters of its habitat, making it a truly remarkable exception in the animal kingdom.

Read Also: Diagram Of Water Cycle

Blood Vessels

Our bodies contain various types of blood vessels, each of which performs a specific function.

Blood Vessels
Photo from Wikimedia

Types of Blood Vessels

Three types of blood vessels are:

  • Arteries
  • Veins
  • Capillaries


Arteries are muscular tubes with strong walls. These blood vessels transport oxygen-rich blood from the heart to all of the body’s tissues. The aorta is one of the major arteries that branch out from the heart.


Veins are elastic blood vessels that transport deoxygenated blood to the heart from all parts of the body. The umbilical and pulmonary veins are exceptions. The pulmonary vein transports oxygenated blood from the lungs to the heart, while the umbilical vein transports oxygenated blood from the placenta to the fetus.


When arteries reach tissues, they divide into extremely thin tubes known as capillaries. Capillaries allow substances to be exchanged between blood and tissues.


Sinusoids are a type of larger capillary found in the bone marrow, liver, lymph nodes, spleen, and some endocrine glands. They can be continuous, interrupted, or fenestrated.

Layers of Blood Vessels

Both arteries and veins have three layers.

Tunica Intima: One of the arteries’ and veins’ innermost and thinnest layers. It is made up of endothelial cells. They have direct contact with the blood flow.

Tunica Media: An artery or vein’s middle layer. Smooth muscle cells make up the Tunica media.

Tunica Externa: This is the tunica that surrounds the tunica media. It is composed of collagen and is supported by the elastic lamina found in arteries.

Functions of Blood

Blood is in charge of the following bodily functions:

Tissue Connective Fluid
Blood is a fluid connective tissue that is 55% plasma and 45% formed elements such as WBCs, RBCs, and platelets. Because these living cells are suspended in plasma, blood is referred to as a fluid connective tissue rather than just fluid.

It gives the cells oxygen
Blood transports oxygen from the lungs to various cells throughout the body. Waste carbon dioxide is exhaled after moving from the blood to the lungs.

Hormones and nutrients are transported
Glucose, vitamins, minerals, and proteins are absorbed into the bloodstream via the capillaries in the villi lining the small intestine.

Hormones secreted by the endocrine glands are also transported to various organs and tissues by the blood.


By absorbing and releasing heat, blood helps to keep the internal body temperature stable.

Clotting of the Blood at the Site of Injury
Platelets aid in blood clotting at the site of injury. Platelets and fibrin combine to form a clot at the wound site.

Waste transport to the Kidney and Liver
Blood is filtered in the kidney to remove nitrogenous waste from the blood plasma. The liver also removes toxins from the blood.

Body protection against pathogens
Infections are fought by White Blood Cells. They proliferate rapidly during infections.

Frequently Asked Questions FAQs on Blood

What is blood?

Blood is a fluid connective tissue that circulates through the body in the circulatory system. It is composed of plasma, red blood cells, white blood cells, and platelets. Blood transports oxygen and nutrients to various cells and tissues and helps remove waste products. Blood also plays a role in the immune system, blood clotting, and maintaining blood pressure.

There are three main types of blood cells found in human blood: red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).

State the types of blood cells found in human blood

Red blood cells contain hemoglobin, a protein that binds to oxygen and carries it to cells throughout the body. White blood cells are part of the body’s immune system and help fight infections and diseases. Platelets are involved in blood clotting and help stop bleeding.

State the different types of white blood cells found in the blood

There are five types of white blood cells (leukocytes) in human blood: neutrophils, lymphocytes, monocytes, eosinophils, and basophils.

What are granulocytes?

Granulocytes are a type of white blood cell that contain granules in their cytoplasm and are involved in the immune response to infections. There are three main types of granulocytes: neutrophils, eosinophils, and basophils. They play a crucial role in fighting against bacterial and fungal infections.

What are agranulocytes?

Agranulocytes are a type of white blood cell that do not have granules in their cytoplasm. They are also known as monocytes and lymphocytes, and play a crucial role in the body’s immune system. Agranulocytes help the body fight off infections by identifying and destroying foreign invaders, such as bacteria, viruses, and cancer cells.

What are the 4 main functions of blood?

The four main functions of blood are:

  1. Transportation: Blood carries oxygen, nutrients, and hormones to cells and removes waste products from them.
  2. Regulation: Blood helps regulate body temperature, pH, and fluid balance.
  3. Protection: Blood plays a crucial role in protecting the body against infections and diseases through its clotting mechanism and by carrying white blood cells that fight against foreign invaders.
  4. Storage: Blood stores important nutrients, hormones, and minerals, including iron and calcium, that are essential for the body’s functions.

Who discovered blood?

The discovery of blood is not credited to one specific person. The knowledge about blood has been accumulated over time through observation, experimentation, and medical advancements. The ancient Greeks and Romans, for example, recognized the importance of blood in maintaining life and health, and they developed early theories about its role in the body. The structure and functions of blood were further understood and documented during the scientific revolution in the 16th and 17th centuries.

Name the various components of blood

The various components of blood are:

  • Plasma: the yellowish fluid component of blood that carries dissolved substances and blood cells.
  • Red blood cells (erythrocytes): responsible for carrying oxygen from the lungs to the rest of the body.
  • White blood cells (leukocytes): involved in fighting infections and diseases.
  • Platelets (thrombocytes): play a role in blood clotting.

What are the various types of blood vessels present in our body?

There are three types of blood vessels in the human body: arteries, veins, and capillaries. Arteries carry oxygenated blood away from the heart to the body’s tissues. Veins carry deoxygenated blood back to the heart. Capillaries are small, thin-walled vessels that allow the exchange of nutrients, oxygen, and waste products between the bloodstream and the body’s tissues.

What are sinusoids?

Sinusoids are specialized types of blood vessels that are found in the liver, bone marrow, and spleen. They are wide, thin-walled, and have irregular shapes that allow for easy exchange of blood between the vessels and surrounding tissues. They help in the filtration of blood, removal of waste and foreign material, and production of blood cells. The unique structure of sinusoids enables them to regulate the flow of blood, which allows for effective regulation of the exchange of nutrients, gases, and waste products between the blood and tissues.

Name the various layers of blood vessels

The various layers of blood vessels are:

  • Tunica intima: This is the innermost layer and is made up of a single layer of smooth endothelial cells that line the lumen of the vessel.
  • Tunica media: This is the middle layer and is made up of smooth muscle cells and elastic fibers.
  • Tunica adventitia: This is the outermost layer and is made up of connective tissue and supportive fibers.

Name the major functions of blood

The major functions of blood include:

  1. Transport of Oxygen and Nutrients: Blood delivers oxygen and nutrients to the body’s cells and tissues.
  2. Removal of Waste and Carbon Dioxide: Blood carries waste and carbon dioxide away from cells and tissues, helping to keep the body healthy.
  3. Regulation of Body Temperature: Blood helps regulate the body temperature by carrying heat from warm areas to cooler areas.
  4. Maintenance of Fluid Balance: Blood helps regulate fluid balance in the body by controlling the volume and concentration of fluid.
  5. Immune Defense: Blood contains white blood cells and antibodies that help protect the body from infections and diseases.
  6. Blood Clotting: Blood has clotting factors that help to stop bleeding when there is an injury.
  7. Hormone Transport: Blood helps transport hormones throughout the body to regulate various functions.

What are sinusoids?

Sinusoids are a type of blood vessel that is found in certain organs, such as the liver and spleen. They are characterized by their large and irregular shape, as well as their large diameter and thin walls. Sinusoids are unique among blood vessels in that they allow for the exchange of fluids, cells, and small molecules between blood and surrounding tissues. This allows for the efficient exchange of materials, such as nutrients and waste products, between the blood and organs.

Name the various layers of blood vessels

The various layers of blood vessels are:

  1. Endothelium: This is the innermost layer of blood vessels. It is made up of a single layer of cells that line the lumen of the vessel.
  2. Basement membrane: It is a thin layer of the extracellular matrix that provides structural support to the endothelium.
  3. Smooth Muscle Layer: This layer contains smooth muscle cells that help regulate blood flow by contracting and relaxing.
  4. Adventitia: The outermost layer of blood vessels, the adventitia provides additional structural support and contains connective tissue, nerve fibers, and small blood vessels.

Name the major functions of blood

The major functions of blood are:

  1. Transport of oxygen and nutrients to tissues
  2. Removal of waste products from tissues
  3. Regulation of body temperature
  4. Maintenance of fluid and electrolyte balance
  5. Transport of hormones and signaling molecules
  6. Protection against infection and injury through the immune system.

What gives blood its red color?

The red color of blood is due to the presence of hemoglobin, a protein in red blood cells (RBCs) that carries oxygen from the lungs to the body’s tissues and transports carbon dioxide from the tissues back to the lungs to be exhaled. The iron within hemoglobin molecules is what gives blood its red color.

Does plasma contain hemoglobin?

No, plasma does not contain hemoglobin. Hemoglobin is the iron-containing protein in red blood cells that carries oxygen from the lungs to the tissues and returns carbon dioxide from the tissues to the lungs to be exhaled. Plasma, on the other hand, is the yellowish fluid component of blood that makes up about 55% of total blood volume and transports blood cells and other substances throughout the body.

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