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You know what blood is - it's that red stuff that oozes out if you get a paper cut. But what is blood, really, and what does it do?

What Is Blood and What Does It Do?

Blood is needed to keep us alive. It brings oxygen and nutrients to all the parts of the body so they can keep working. Blood carries carbon dioxide and other waste materials to the lungs, kidneys, and digestive system to be removed from the body. Blood also fights infections, and carries hormones around the body.

Blood is made up of blood cells and plasma. Plasma (say: PLAZ-muh) is a yellowish fluid that has nutrients, proteins, hormones, and waste products. The different types of blood cells have different jobs.

How Blood Works

Blood helps the parts of our body work as they should. In this video, learn how blood brings oxygen and nutrients around the body and helps fight off infections.

What Are the Types of Blood Cells?

Red blood cells: Red blood cells (RBCs, also called erythrocytes; say: ih-RITH-ruh-sytes) are shaped like slightly indented, flattened disks. RBCs contain hemoglobin (say: HEE-muh-glow-bin), a protein that carries oxygen. Blood gets its bright red color when hemoglobin picks up oxygen in the lungs. As the blood travels through the body, the hemoglobin releases oxygen to the different body parts.

Each RBC lives for about 4 months. Each day, the body makes new RBCs to replace those that die or are lost from the body. RBCs are made in the inside part of bones called the bone marrow.

White blood cells: White blood cells (WBCs, also called leukocytes, say: LOO-kuh-sytes) are a key part of the immune system. The immune system helps the body defend itself against infection. Different types of WBCs fight germs, such as bacteria and viruses. Some types of WBCs make antibodies, which are special proteins that recognize foreign materials and help the body get rid of them.

There are several types of WBCs, and their life spans vary from hours to years. New cells are constantly being formed - some in the bone marrow and some in other parts of the body such as the spleen, thymus, and lymph nodes.

Blood contains far fewer WBCs than red blood cells, although the body can increase WBC production to fight infection. The white blood cell count (the number of cells in a given amount of blood) in someone with an infection often is higher than usual because more WBCs are being made or are entering the bloodstream to battle the infection.

Platelets: Platelets (also called thrombocytes, say: THROM-buh-sytes) are tiny oval-shaped cells that help in the clotting process. When a blood vessel breaks, platelets gather in the area and help seal off the leak. Platelets work with proteins called clotting factors to control bleeding inside our bodies and on our skin.

Platelets survive only about 9 days in the bloodstream and are constantly being replaced by new platelets made by the bone marrow.

How Does Blood Travel in the Body?

With each heartbeat, the heart pumps blood throughout our bodies, carrying oxygen to every cell. After delivering the oxygen, the blood returns to the heart. The heart then sends the blood to the lungs to pick up more oxygen. This cycle repeats over and over again.

The circulatory system is made up of blood vessels that carry blood away from and toward the heart.

Two types of blood vessels carry blood throughout our bodies:

1. Arteries carry oxygenated blood (blood that has gotten oxygen from the lungs) from the heart to the rest of the body.

2. Blood then travels through veins back to the heart and lungs, so it can get more oxygen to send back to the body via the arteries.

As the heart beats, you can feel blood traveling through the body at pulse points - like the neck and the wrist - where large, blood-filled arteries run close to the surface of the skin.

What if Someone Has Low Numbers of Blood Cells?

Sometimes medicine can be given to help a person make more blood cells. And sometimes blood cells and some of the special proteins blood contains can be replaced by giving a person blood from someone else. This is called a transfusion (say: trans-FEW-zyun).

People can get transfusions the part of blood they need, such as platelets, RBCs, or a clotting factor. When someone donates blood, the whole blood can be separated into its different parts to be used in these ways.

Hey, What's Your Type?

Everybody's blood is red, but it's not all the same. There are eight blood types, described using the letters A, B, and O. Those letters stand for certain proteins found on the red blood cells. Not everyone has the same proteins.

Besides getting a letter or two, a person's blood is either "positive" or "negative." This is a way of keeping track of whether someone's blood has a protein called Rh protein. If your blood is positive, you have this protein. If it's negative, you don't. Either way is totally fine.

People have one of these eight different blood types:

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It swirls through your veins and arteries, but what is it really? Find out about blood in this article for kids.

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Human Blood Video | Blood Components | Blood Cells

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What are Variables?

What are Variables?

In science, a variable is any factor, trait, or condition that can exist in differing amounts or types.

Scientists try to figure out how the natural world works.To do this they use experiments to search for cause and effect relationships. Cause and effect relationships explain why things happen and allow you to reliably predict the outcomes of an action. Scientists use the scientific method to design an experiment so that they can observe or measure if changes to one thing cause something else to vary in a repeatable way.

These factors that change in a scientific experiment are variables.

A properly designed experiment usually has three kinds of variables: independent, dependent, and controlled.

What is an Independent Variable?

The independent variable is the one that is changed by the scientist. Why just one? Well, if you changed more than one variable it would be hard to figure out which change is causing what you observe. For example, what if our scientific question was: "How does the size of a dog affect how much food it eats?"; then, during your feeding experiments you changed both the size of the dog and the time of day the dogs were fed. The data might get a bit confusing- did the larger dog eat less food than the smaller dog because of his size or because it was the middle of the day and dogs prefer to eat more in the morning?

Sometimes it is impossible to just change one variable, and in those cases, scientists rely on more-complicated mathematical analysis and additional experiments to try to figure out what is going on. Older students are invited to read more about that in our Experimental Design for Advanced Science Projects page. To be clear though, for a science fair, it is usually wise to have only one independent variable at a time. If you are new to doing science projects and want to know the effect of changing multiple variables, do multiple tests where you focus on one independent variable at a time.

What is a Dependent Variable?

The dependent variables are the things that the scientist focuses his or her observations on to see how they respond to the change made to the independent variable. In our dog example, the dependent variable is how much the dogs eat. This is what we are observing and measuring. It is called the "dependent" variable because we are trying to figure out whether its value depends on the value of the independent variable. If there is a direct link between the two types of variables (independent and dependent) then you may be uncovering a cause and effect relationship. The number of dependent variables in an experiment varies, but there can be more than one.

What is a Control Variable?

Experiments also have controlled variables. Controlled variables are quantities that a scientist wants to remain constant, and she or he must observe them as carefully as the dependent variables. For example, in the dog experiment example, you would need to control how hungry the dogs are at the start of the experiment, the type of food you are feeding them, and whether the food was a type that they liked. Why? If you did not, then other explanations could be given for differences you observe in how much they eat. For instance, maybe the little dog eats more because it is hungrier that day, maybe the big dog does not like the dog food offered, or maybe all dogs will eat more wet dog food than dry dog food. So, you should keep all the other variables the same (you control them) so that you can see only the effect of the one variable (the independent variable) that you are trying to test. Similar to our example, most experiments have more than one controlled variable. Some people refer to controlled variables as "constant variables."

In the best experiments, the scientist must be able to measure the values for each variable. Weight or mass is an example of a variable that is very easy to measure. However, imagine trying to do an experiment where one of the variables is love. There is no such thing as a "love-meter." You might have a belief that someone is in love, but you cannot really be sure, and you would probably have friends that do not agree with you. So, love is not measurable in a scientific sense; therefore, it would be a poor variable to use in an experiment.

Examples

Examples of Variables

Time as an Example of an Independent Variable

In some experiments, time is what causes the dependent variable to change. The scientist simply starts the process, then observes and records data at regular intervals.

The Independent Variable for Surveys and Tests of Different Groups

When a scientist performs a test or survey on different groups of people or things, those groups define the independent variable. For example:

Either/Or (Binary) Variables

Sometimes a variable simply represents an either/or (binary) condition. For example, something might be either present or not present during an experiment.

Sample

Sample

Here is a sample containing the variables and hypothesis.

Checklist

Science Fair Project Variables Checklist

Assign these with Google Classroom:

Explore Our Science Videos

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How to use dependent, independent, and controlled variables in your science experiments.

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