Lesson Explainer: Components of the Blood Biology

In this explainer, we will learn how to describe the structure and function of the major components of the blood.

You are familiar with blood as the red liquid that flows from your body when you accidentally cut a finger or scrape a knee, but you may be less familiar with the fact that your blood is actually a type of connective tissue. It is made up of different types of cells suspended in a liquid medium we call plasma. Like other tissues, the function of blood is determined by the properties of its components. Blood has many functions, from transporting gasses to helping to maintain body temperature, which is an important part of homeostasis. We will learn more about the red blood cells, white blood cells, platelets, and plasma that make up our blood in this explainer.

Key Term: Blood

Blood is a fluid connective tissue in humans and other animals that transports cells and materials throughout the body.

Human blood is a slightly alkaline, or basic, fluid connective tissue. Your blood is contained within your circulatory system. The human circulatory system consists of the heart as well as a closed network of various-sized tubes called blood vessels. Blood vessels are highly branched and present in every part of your body. The heart is a muscular organ that pumps the blood through the vast network of vessels. Your body contains about 5 to 6 litres of blood, all of which is flowing constantly through your circulatory system. The human circulatory system is illustrated in Figure 1.

More than half of the blood is plasma by volume. Blood plasma is a yellow liquid that serves the purpose of carrying the blood cells; nutrients like glucose, which is used for cellular respiration; waste products, such as urea; electrolytes; hormones; and other essential materials like enzymes and antibodies throughout the body within the blood vessels.

Blood plasma is 90% water. It also contains inorganic salts like sodium and calcium ions as well as proteins such as albumin, fibrinogen, and globulin.

If we separated it into its components, about 45% of the blood would consist of red blood cells, about 1% would be white blood cells and platelets, and about 55% would be plasma, as shown in Figure 2.

Key Term: Blood Plasma

Blood plasma is a yellow liquid that blood cells are suspended in. It also carries materials like nutrients, wastes, and minerals throughout the body.

Example 1: Recalling the Percentages of the Main Components of Blood

The given diagram demonstrates the average percentages of the main components of blood in a sample. What component is label A representing?


This diagram shows blood in a test tube. The blood has been separated into its components in a lab. Blood is a liquid tissue made up of four main components. There are red blood cells, also called erythrocytes, that carry oxygen. White blood cells are responsible for identifying and fighting pathogens that can cause illness. Platelets are cell fragments that control blood clotting. Blood clots are important for repairing damaged blood vessels, preventing bleeding, and blocking pathogens from entering the blood. These three cellular components are suspended in a yellow liquid called blood plasma. Plasma is mostly water, but it contains proteins, ions, and nutrients like glucose. It also carries waste products and hormones from place to place in the body. Plasma is the largest component of the blood by volume.

Using this information and the information in the diagram, we can conclude that label A represents plasma.

Red blood cells are the second largest component of blood by volume. The scientific name for red blood cells is erythrocytes. “Erythro-” is a prefix that means red and “-cyte” is a suffix that means cell. Red blood cells are the most numerous cell type in the blood. There are about 5 billion red blood cells in just one millilitre of human blood, or 5 million in one cubic millimetre!

Red blood cells are formed in the bone marrow. Bone marrow is tissue found inside of bones. It takes about 7 days for a red blood cell to fully mature. Once it is mature, the red blood cell leaves the bone marrow and circulates within the blood for about 100–120 days, or 3–4 months. Aging red blood cells are removed from the bloodstream and destroyed by the liver and the spleen. Red blood cells are continually produced at a rate of about 2 million cells per second. Old red blood cells die and are removed at the same pace.

Red blood cells are highly specialized cells that are specifically adapted to carry oxygen from the lungs to the cells of the body that use it for cellular respiration.

Red blood cells, or RBCs for short, possess several adaptations that make them specialized for their function:

  • The cytoplasm of RBCs is rich in a molecule called “hemoglobin.” Hemoglobin is a protein that contains iron. The iron binds to oxygen molecules that diffuse into the blood from the lungs. Hemoglobin also gives RBCs their red color.
  • Mature RBCS do not possess a nucleus or most other organelles. This allows more space for hemoglobin and gives the cell a distinct shape.
  • RBCs are shaped like a flat disk with indentations on each side. This shape is called “biconcave.” “Bi-” is a prefix that means “two” and “concave” means “indented” or “curved inward.” The biconcave shape allows oxygen to diffuse into the cytoplasm of the cell efficiently since the surface area of the cell is large compared to the volume inside. It also helps the cells to flow smoothly through blood vessels.

Oxygen diffuses from the lungs into the cytoplasm of the RBCs where it binds with hemoglobin, forming oxyhemoglobin. The oxygen is carried and delivered to the tissues of the body where it is used for cellular respiration. Carbon dioxide, a waste by-product of cellular respiration, is exchanged for the oxygen. The carbon dioxide diffuses into the cytoplasm of the red blood cells where it binds with hemoglobin, forming carbaminohemoglobin. The red blood cells carry the carbon dioxide to the lungs where it is excreted. The structure of a red blood cell is shown in Figure 3.

Key Term: Erythrocyte (Red Blood Cell)

Erythrocytes are cells specialized for carrying oxygen and carbon dioxide from place to place. They have a biconcave shape and are rich in a protein called hemoglobin.

Key Term: Hemoglobin

Hemoglobin is an iron-rich protein that helps erythrocytes carry oxygen and gives them their red color.

Example 2: Describing the Shape of Red Blood Cells and Explaining Its Purpose

The diagram given shows multiple, normal red blood cells in a blood vessel.

  1. What scientific term best describes the shape of a mature red blood cell?
    1. Biconcave
    2. Circular
    3. Biconvex
    4. Convex
  2. Why does a red blood cell have this shape?
    1. To decrease the surface area, therefore restricting the volume of oxygen lost from the cell
    2. To provide a large surface area for the active transport of oxygen in and out of the cell
    3. To increase the available surface area, so more oxygen can diffuse in and out of the cell
    4. To increase the volume of the cell, so more genetic material can be carried around the body


Part 1

In the photo provided with the question, we see that the red blood cells are flat, disk-like shapes with indentations on each side. A surface that is indented is sometimes called “concave,” and “bi-” is a prefix that means “two.”

So, this shape is commonly referred to by scientists as “biconcave.”

Part 2

To illustrate the answer to the second part of this question, a comparison of the cross sections of a round cell shape and the typical, biconcave shape of human red blood cells is shown below.

Red blood cells function to carry oxygen from the lungs to the cells of the body. The cytoplasm of red blood cells contains a protein called hemoglobin, which is particularly efficient at carrying oxygen. However, the oxygen must diffuse into the cytoplasm of the cell to make contact with the hemoglobin. You can see in the round cell’s cross section that the oxygen has trouble diffusing all the way to the center of the cell. This is because the surface area of the cell compared to the volume inside is quite small, so the cell is less efficient at carrying oxygen. In contrast, the biconcave cell’s cross section shows that the oxygen is able to diffuse throughout all of the cytoplasm of the cell, making contact with all of the hemoglobin. This cell has a larger surface area in comparison to the volume of cytoplasm inside and is much more efficient at carrying oxygen.

Therefore, the correct answer is “to increase the available surface area, so more oxygen can diffuse in and out of the cell.”

The next blood component we will discuss is white blood cells. White blood cells are also called leukocytes. “Leuko-” is a prefix that means “white.” White blood cells are responsible for protecting our bodies from infection by identifying and destroying pathogens, which are microorganisms that can cause illness.

There are between 4 million and 11 million white blood cells in one millilitre of blood. Because of their function fighting disease, this number will increase during an infection. Unlike red blood cells, white blood cells are colorless and possess a nucleus. White blood cells are formed in the bone marrow, but they may mature elsewhere in the body. The average lifespan of a white blood cell is around 13 days, although some white blood cells will live for much shorter periods of time and some may persist for several months.

There are several different types of white blood cells and they come in different shapes. One common example is called a phagocyte. The function of phagocytes is to surround, engulf, and break down pathogens in the blood. Another type of white blood cell is a lymphocyte. Lymphocytes carry out several different immune functions, such as identifying and “remembering” specific pathogens, making antibodies, and targeting and destroying infected cells. White blood cells are an important part of our immune system, the organ system that fights illness and infection. Some white blood cells are shown in the illustration in Figure 4.

Key Term: White Blood Cell (Leukocyte)

Leukocytes, or white blood cells, are a part of the immune system that help the body fight against disease.

The last of the four major blood components is platelets. Platelets are also known as thrombocytes. “Thrombo-” is a prefix that means “blood clot,” and platelets serve the function of causing blood clotting. We know that “-cyte” means “cell,” but platelets are actually small fragments of much larger cells called megakaryocytes. Platelets are formed from megakaryocytes in the bone marrow. A typical platelet will remain in circulation for about 10 days.

Clotting is a very important function of our blood. Platelets clump together around an injury to a blood vessel. You are probably familiar with the scab that forms after you accidentally cut or scrape your skin. The scab is made of dried blood clots! Blood clotting helps prevent the loss of too much blood. It also functions to block pathogens from entering the blood. The formation of a blood clot is shown in Figure 5.

Key Term: Platelet (Thrombocyte)

Platelets are small cell fragments that circulate in the blood and function to cause blood clotting.

Example 3: Identifying Blood Components Based on Their Functions

The given table shows some of the functions of different components of the blood. State the correct component for each function.

FunctionClotting bloodTargeting and destroying pathogensCarrying oxygenTransporting waste substances and nutrients


There are four main components in human blood. These four components are red blood cells, white blood cells, platelets, and plasma. Each of these components has a particular function that contributes to the function of blood as a tissue.

Blood carries oxygen, nutrients like glucose, waste, and other materials throughout the body. It transports white blood cells that fight infection. Blood also contains components that cause clotting. Blood clots prevent blood loss and block pathogens from entering the blood.

Since we know the four components of blood, we can match them to the various functions. Red blood cells have a special biconcave shape and are filled with a protein called hemoglobin. Both of these adaptations allow red blood cells to effectively carry oxygen from our lungs to the cells of our bodies. White blood cells are immune cells; they are responsible for identifying and destroying pathogens to prevent illness. Platelets are cellular fragments that form a plug called a blood clot. Finally, the plasma is the liquid medium that the three cellular components are suspended in. Blood plasma transports soluble substances like waste products and nutrients from place to place in the body.

Then, in order to complete the table, the correct responses are as follows:
1: platelets, 2: white blood cells, 3: red blood cells, 4: plasma.

Blood is essential to the proper function of the human body. Its main function is to carry blood cells and other materials from place to place in the body. Human blood consists of components that are adapted to specific functions.

Let’s review what we have learned about blood from this explainer.

Key Points

  • The major components of blood are red blood cells (erythrocytes), plasma, platelets, and white blood cells (phagocytes and lymphocytes).
  • Plasma is the liquid medium in which the blood cells and platelets are carried.
  • Red blood cells are specifically adapted to carrying oxygen. They possess cytoplasm rich in hemoglobin and a flat, biconcave shape.
  • White blood cells, such as lymphocytes and phagocytes, fight infection.
  • Platelets cause clotting of the blood at the site of an injury.
  • Red blood cells and platelets are formed and mature in the bone marrow, and old red blood cells are removed and destroyed by the spleen and liver.
  • White blood cells are formed in the bone marrow, but they may mature elsewhere in the body.

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