Blood around our body to our tissues and

Blood is a connective tissue vital to our body’s survival. In this lesson, we will explore the three functions of blood by delving into red blood cells, white blood cells and platelets – the three individual components that make up blood as a whole.

Blood, A Connective Tissue

Types of living cells in the blood

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In the human body, one of the most important connective tissues is blood. Blood is considered a connective tissue because it consists of a non-living fluid in which living cells are suspended. The blood matrix surrounding the cells is known as plasma, which accounts for about 55% of our blood volume. There are three types of living cells in blood: red blood cells (or erythrocytes), white blood cells (or leukocytes) and platelets (or thrombocytes). These make up the remaining 45% of our blood volume.

Red Blood Cells

Blood has three main functions: transportation, regulation and protection.

Our erythrocytes, or red blood cells, are the most abundant cell type in the human body. Additionally, erythrocytes are anucleated, which means they don’t have a nucleus. This extra room allows for more hemoglobin to be stored in our red blood cells. Hemoglobin is a respiratory pigment, which binds to either oxygen or carbon dioxide. This allows oxygen to be transported around our body to our tissues and organs (and carbon dioxide to be taken away).

Hemoglobin is largely comprised of iron, which when combined with oxygen, gives blood its red color. Additionally, blood helps cycle nutrients and hormones throughout our bodies.

Process of transporting oxygen throughout the body
Oxygen Transportation

Blood also helps us maintain homeostasis by regulating our internal body pH and temperature as well as how much water is in our bodies at a given time. Plasma, our connective tissue matrix, is about 90% water. Blood is also vital in protecting our bodies. Blood loss is controlled with clotting mechanisms, and white blood cells provide immune response.

Let’s examine this function in more detail.

White Blood Cells

The types of leukocytes in the blood
White Blood Cell Types

There are five different types of white blood cells, or leukocytes, in our bodies, and we can break them up into two main categories. These are granulocytes and agranulocytes. These terms simply refer to how these cells look when stained.

Granulocytes have visible granules in their cell bodies, and agranulocytes do not. There are three types of granulocytes. They are neutrophils, eosinophils and basophils.



Neutrophils are the most abundant type of white blood cell in our body. These are primarily responsible for killing off foreign bacteria. Neutrophils are the first responders when we are injured or sick, and they help keep us healthy by phagocytizing (or consuming) bacteria in our bodies.


Eosinophils are responsible for fighting infections of parasitic worms.

These cells release toxins that kill the worms and are also involved in the inflammatory response when we have an allergic reaction.


Basophils release two chemicals, histamine and heparin. Histamine is also involved in allergic reactions, and heparin is an anticoagulant. Anticoagulants keep blood from clotting, which helps bring more blood to a damaged area in our body, which, in turn, helps promote healing.


The two remaining types of leukocytes are agranulocytes. These are monocytes and lymphocytes.


Monocytes are the largest of the white blood cells. These are the cells that ‘clean up’ our blood the most. Monocytes also employ phagocytosis to ‘eat’ foreign particles, bacteria and dead neutrophils.


Lymphocytes are broken down into two main types, B-lymphocytes, which produce antibodies, and T-lymphocytes, which are responsible for cell-mediated resistance to infections. Lymphocytes are the ‘last responders’ in our immune system and allow for long-term resistance.Have you ever heard that you can only catch the chicken pox once, or you never catch the same cold twice? That’s because our bodies ‘remember’ diseases that we have previously fought, which makes it nearly impossible to catch the exact same disease again. Lymphocytes are one of the tools our bodies use to ‘remember’ these diseases, so we don’t become infected with them again.

Remembering White Blood Cells

The phrase ‘Never Let Monkeys Eat Bananas’ is a useful tool that not only helps you remember the name of the white blood cells, but also the order they are found in our bodies from greatest to least abundant.


The last blood component, which is involved in protection, is platelets, or thrombocytes.

These are small, irregularly shaped cells, which do not have a nucleus. These cells contain fibrinogen, which helps our blood to clot and helps our skin form scabs when we’re cut.

Lesson Summary

In summary, our bodies rely on blood connective tissue for transportation of oxygen, carbon dioxide and nutrients, regulation of our internal pH and temperature, and protection against disease and foreign particles. Red blood cells (or erythrocytes) are the tool our bodies use for transportation, and white blood cells (or leukocytes) are responsible for keeping us free of disease and healthy.

Finally, platelets (or thrombocytes) are responsible for clotting our blood when we have a cut or open wound.

Learning Outcome

After watching this lesson, you will be able to:

  • Explain how red blood cells contribute to the functions of blood
  • Describe the function of each of the five types of white blood cells
  • List a mnemonic device to remember the white blood cells
  • Summarize the function of platelets

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