Blood is made up of both cells and liquid, and the liquid part of the blood is called plasma.

Plasma is made up mostly of water and is yellow in color.

There are different substances that are inside the plasma such as glucose, gasses and proteins and these substances are dissolved into the plasma.

The plasma also contains blood cells; red blood cells, platelets and white blood cells.

Red Blood Cells

The red blood cells are shaped like small, flattened disks so that they can flow through the body easily.

The body has trillions and trillions of red blood cells and each red blood cell contains millions and millions of molecules of hemoglobin.

Hemoglobin helps the body to carry oxygen and because it contains an iron protein, the hemoglobin helps to give red blood cells their color, and this is why they are called red blood cells.

The iron protein combines with the oxygen molecules so that it can be carried by the red blood cells.


The platelets are small, pieces of cell that are sticky. The stickiness of the platelets helps the blood to clot.

A blood clot is just a solid mass of cells and other substances that stop leaks when a blood vessel is damaged.

The platelets stick to tears that happen in the blood vessels and they form a clot by making chemicals that are needed to clot the blood. This helps to stop leaks in the blood vessels.

White Blood Cells

White blood cells help to protect the body. They are larger than the red blood cells but there are way less of them.

For example, white blood cells that are called phagocytes, help to destroy microorganisms and debris that is found in the blood. These cells are our bodies defense.


The blood circulates throughout the body by blood vessels and because of how the heart pumps.

Your body has to have at least 4.5 liters of blood in order to survive. As we read about in the circulatory system, the main function of the blood is to transport.

The blood carries oxygen and nutrients through the arties to different parts of the body and the veins carry waste and carbon dioxide away from the cells so that it can be excreted or moved out of the body.

Arteries and Veins

When the blood is in the organs, there are smaller arteries that branch off.

The walls are made up of only one layer of cells and these blood vessels are then called capillaries, or tiny blood vessels that connect arteries to veins.

Functions of the Blood

Blood carries hormones throughout the body so that the body can regulate the functions of the body, the blood defends against infections, regulates the body temperature, controls the pH of the body, and helps to repair body tissues.

The blood has many jobs!

What is Blood Type?

Blood type changes depending on the proteins called antigens that the red blood cells carry on their surface.

The specific proteins that the blood carries changes depending on your genes that you get from your mom and dad.

The antigens that you inherit determine what blood type you are.

Why is Blood Type Important?

Blood type is important because if you need to get blood in the hospital, the hospital has to know what type of blood to give you, or your blood type.

If blood other than your blood type is “transfused” or put into your body, then your body will think it is something bad in the body and your body will fight the blood.

When your body rejects the blood, the red blood cells go together, and the blood vessels block them, and this can be bad for someone that needs blood.

ABO and Rhesus Blood Type

ABO and Rhesus are the most known antigens. In ABO blood type, there has to be two common antigens called antigen A and antigen B.

If your red blood cells only have antigen A then you have the blood type called A.

If your red blood cells only carry antigen B, then you have blood type B.

If your red blood cells carry both A and B antigens, then you have the blood type called AB and if your red blood cells carry neither A or B antigens then you have the blood type called O.

Rhesus blood type is when you only have one common antigen, and this is called the Rh antigen.

If your red blood cells carry the Rhesus antigen, then you have the blood type Rh+ and if your blood lacks the Rhesus antigen then you have the blood type Rh- (+ is positive and – is negative).

Facts About Blood:

  • Blood diseases sometimes affect certain people.
  • Anemia is a disease when the body does not have enough hemoglobin or iron and so the body cannot carry enough oxygen to the cells.
  • Leukemia is a blood disease which is a type of cancer in the bone marrow and it is caused by an unusual amount of white blood cells. In this disease, the white blood cells cannot fight off the infections.
  • Hemophilia is a blood disease that stops the blood from clotting. A small injury can cause a person with hemophilia to bleed too much. This disease is a genetic disease inherited from a family member.
  • Sickle-Cell disease is another blood disease. This disease is also a genetic disease and it mostly affects people that are African American. This disease causes hemoglobin to shape like a sickle and causes these sickle-cells to get stuck in the capillaries and block the blood from flowing.

What Did You Learn?

  • What does the blood consist of that is a yellow color, made mostly of water and other dissolved substances? The plasma is the liquid part of the blood that is made mostly of water and other dissolved substances.
  • What are the three types of cells that the blood has? The blood contains white blood cells, red blood cells and platelets.
  • What is the main function that the blood has? The main function of the blood is to transport.
  • What causes the different blood types? Antigens help to determine what blood type someone will be.
  • What are the three main blood types? The three main blood types are blood type A, blood type B, or blood type O.
  • What is blood? Blood is a substance made up of liquid and cells.