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Blood Facts

Blood ['bləd]

The bright red liquid pumped by the heart.

hand with drop of blood

You've seen your own blood when you have a cut or a scrape. But how much do you know about your blood? Some people are “squeamish” about the sight of blood, but you couldn’t live without it! That's because just about every part of your body needs blood, from your muscles to your bones to your brain. Without blood, you couldn’t play, grow, speak, or learn. Blood performs many jobs for your body and is continuously working for you around the clock. Blood is fantastic!

Blood is the red liquid that is pumped by your heart through your veins, arteries, and capillaries. This is called the circulatory system. Blood takes food and oxygen to all of the cells in your body and removes waste away from the cells. How does it do all that? Let's learn more about this amazing tissue called blood.

What is blood made of?

blood cells
Red blood cell, platelet, white blood cell -- Image from scanning electron microscope

The average human adult has about 1½ gallons (5.5 liters) of blood traveling through their body’s circulatory system. An 80-lb child’s body might contain about 5 pints (2.5 liters) of blood. Whole blood makes up about 7% of a person's body weight and consists of red blood cells, white blood cells, platelets, and plasma. One drop of blood contains about 5 million red blood cells, about 6,000 white blood cells, and about 300,000 platelets.

diagram type of blood cells

Red Blood Cells

red blood cells

Red blood cells, also known as erythrocytes, are created in the bone marrow and are constantly being replaced. The human body produces about 2 million red blood cells per second. Each one lives about 120 days. Red blood cells make up about 44% of your blood. These cells have a concave shape ‐ like a doughnut with the hole slightly filled in. Red blood cells contain an iron-rich protein known as hemoglobin which gives blood its red color. Hemoglobin attaches to oxygen and carries it from the lungs to all other body parts. Oxygen is needed for all cells in the body to live. If body tissues do not get enough oxygen-rich blood, their cells cannot function. Red blood cells also remove the carbon dioxide which the body produces as a waste product, and transport it to the lungs where it is exhaled. Red blood cells are also responsible for taking nutrients from the food you eat to all of your cells -- from the tiny cell at the end of your big toe to the cells of your brain. These nutrients supply fuel for the cells’ work, which includes seeing, hearing, breathing, thinking, building muscles, and much more.

White Blood Cells

White Blood Cells in Blood Stream

White blood cells, or leukocytes, make up about 1% of your blood. They help your body fight infection and are a key part of the immune system. Like red blood cells, white blood cells are also created in the bone marrow. Some live only a few hours in the bloodstream, while others may live for months. Your body increases white blood cell production when more are needed to fight infection. There are several kinds of white blood cells and each kind has a special job. Some can attack and digest most types of bacteria. Some help to remove dead cells in the body and kill parasites and invading organisms. Some release histamines when pollen, food, or other matter that the body sees as an invader, enters and triggers an allergic response. Some release antibodies ‐- a kind of army that fights infection. Some even have the ability to “remember” certain invasive bacteria and keep the body from getting sick if that intruder appears again. Learn more about the immune system at Science Trek’s Virus page.


Platelets at work clotting blood
Platelets at work clotting blood

Platelets are also known as thrombocytes and are made in the bone marrow. They live about 6-9 days before they die. Their job is to coagulate blood and help it clot. When a cut or other injury takes place, the platelets cause the blood to become thicker and to bunch up to plug up the hole. This stops the bleeding, and a scab forms over the broken skin. When the injury is healed the clot dissolves and is swept away through the bloodstream as part of the waste products.


plasma separating from red blood

Plasma makes up about 55% of our blood volume. Plasma is a clear, yellow liquid that keeps the various other cells moving through the veins and arteries. Plasma is 90% water and also contains minerals, vitamins, nutrients, proteins and other dissolved materials necessary for the healthy functioning of the body.


diagram of the blood circulation system

Blood is part of the circulatory system, a network within the body which also includes the heart and the blood vessels. The one-way movement of blood through the heart and around the body is called circulation. Your blood flows nonstop every second of the day, whether you are awake or asleep. It takes less than 60 seconds for a drop of blood to travel from your heart, through your body, and back to your heart again.

The heart is the main muscle that pumps blood to the rest of the body. First, it sends blood to the lungs to pick up oxygen. Then the blood delivers oxygen and nutrients to all the cells of the body. The blood returns to the heart, carrying waste products with it. The heart sends blood to the lungs to pick up more oxygen, and the cycle repeats over and over again. See a detailed diagram of this continuous action.

Blood travels through tubes called blood vessels. Arteries carry blood away from the

oxygenated and deoxygenated blood
Bright red oxygenated blood and darker returning blood

heart and veins carry blood back to the heart. These arteries and veins branch out all over your body, becoming smaller and smaller as they are located further away from the heart. Capillaries are very small blood vessels in each body part that deliver the oxygen and nutrients to the cells. Blood that is filled with oxygen appears a brighter red than blood that is returning in the veins, which is a darker red. If the entire system of blood vessels in your body were laid end-to-end they would measure about 60,000 miles (96,560 kilometers) – that’s enough to wrap around the earth twice! Learn more about blood vessels.

Blood circulation also helps to make sure your body stays at the right temperature. As body cells do their work, the heat given off is transferred to the blood and distributed throughout the body. In addition, your blood vessels can expand or contract, depending on your body’s needs: When the temperature outside is cold, the blood vessels narrow so that less heat is lost from the body. When it is hot outside, the blood vessels expand, causing more blood to flow near the surface and to be lost to the air, thus cooling the body down. By changing the blood flow to the skin, your body can control the heat exchange with your surroundings.

Learn more about your amazing circulatory system at Science Trek’s Heart page.

Blood Pressure

The body's ability to get the blood from the heart, through the lungs, and back to the

taking blood pressure

heart again is crucial to human health. The heart squeezes the blood and pushes it through the blood vessels. Your blood pressure is the force that the blood exerts against the walls of veins and arteries as it moves through them. Blood pressure is measured from a person’s upper arm using a sphygmomanometer. This tool measures two readings and shows them as a fraction. The top number is known as the systolic number and the bottom number is known as the diastolic number. Normal blood pressure is considered about 120/80. If the numbers are repeatedly higher than this guideline, then the person is said to have high blood pressure or hypertension. Repeated low readings are called hypotension. Blood pressure can change during the day and may read higher or lower due to temperature, activity level, age, stress, health, food intake, or medicines.


checking pulse

As blood travels through the body, it moves through the blood vessels in a rhythm with the beat of the heart. This rhythm, or heart rate, is your pulse. The pulse can be felt in specific locations of the body such as the wrist and the throat. The pulse is counted in beats per minute. Your normal resting heart rate could be anywhere between 60 to 100 beats per minute.

Your pulse will change depending on your activity level. The heart pumps more or less blood depending on the body’s needs. When you are sleeping, it pumps just enough blood to provide the lower amounts of oxygen needed. However, when you are exercising, the heart pumps faster so that your muscles get more oxygen and can work harder.

Blood Types

Diagram of the 4 types of blood

Although everyone’s blood performs the same function, not all human blood is the same. Red blood cells have certain proteins called antigens that determine a person’s blood type. There are four basic types of human blood: A, B, AB, and O. Blood is further identified as positive or negative based on something known as the Rh factor, which is found on the surface of some people’s red blood cells. People with Rh-positive blood have this factor, while people with Rh-negative blood do not. When human blood is identified, it is labeled with one of the four main blood types as well as the Rh positive or negative indicator. Blood types are identified as A+, A-, B+, B-, AB+, AB-, O+, or O-.

blood donation

Knowing blood type is important because sometimes, blood products need to be transferred from one person to another in what is known as a transfusion. If someone loses a lot of blood in an accident or during surgery, doctors can give them replacement blood. Blood cannot be manufactured, so blood donated from another human must be used. The blood is taken from the donor using a needle inserted into their arm. This blood is then kept cool and stored until required by someone in need. Portions of the blood can be used as needed. Red blood cells, white blood cells, platelets or plasma can be separated out and transfused for certain medical conditions. Donated blood is tested carefully to make sure it is healthy before it is used in a transfusion. Donating a pint of blood does not harm the donor. The donor’s red blood cells will be completely replaced within four to six weeks.

bag of donated blood

Even though blood can be shared between a donor and a patient, the two must have compatible blood types. For example, a person with Type B blood can receive blood only from people with Type B or Type O. Receiving blood from people with Type A or Type AB blood could cause a serious reaction. That is why doctors must check the patient's blood type and the donor’s blood type before any blood transfusion. Giving the patient the wrong blood type can cause life-threatening illness.

A person with one blood type usually has antibodies against other blood types, so if the patient’s blood includes antibodies against the donor’s blood, the body's immune system may attack the new blood cells as invaders. However, if there are no anti-donor antibodies in the patient’s blood, then the two blood types are compatible and the transfusion can proceed. Learn more about compatible blood types and see a chart showing how donation works.

Keeping Your Blood Healthy

bandage covering blood wound

You can help keep your blood stay by eating healthy foods. Foods rich in iron such as meats, fish, eggs, raisins, nuts, dried beans, and leafy greens (like spinach) help your body make hemoglobin for red blood cells. Drinking lots of water helps your heart pump blood more easily and helps maintain a healthy blood pressure. Exercise and physical activity are also important for a healthy circulatory system.

If you get a scrape or a cut that bleeds, press a clean tissue or cloth over the cut until the bleeding stops. Wash the area with soap and water to prevent infection and cover it with a bandage. If another person has an injury, don’t touch their blood. Wear gloves or wrap your hands before you try to help them.


hand with bruises

Bruises can occur when someone falls or is hit with something such as a baseball. Blood vessels under the skin are broken and blood empties into the tissue without leaving the body. It isn't like a cut where the blood comes out onto the skin; instead, the blood pools under the skin causing abnormal coloring. As those blood cells begin to die, they change color from the red we associate with blood, to shades of purple, green, and brown. This is known as a bruise. Over time, the body absorbs those dead cells into the blood stream where they are carried away as waste.

Blood Disorders

Blood is an incredible material that is responsible for so many processes in your body. It

blue gloved hand holding blood in test tube

works so well that we usually don’t even think about it. But sometimes things can go wrong with blood. A medical professional may take a sample of a person’s blood, and then perform tests on the sample to see how well the body is working. Testing can show abnormal substances in blood that indicate something might be wrong in the body. Testing can also reveal problems with blood cells. Take a look at just a few conditions that can impact blood health.

  • AnemiaAnemia is the most common disorder and is a condition in which the red blood cells are not able to carry enough oxygen. This may be due to a lack of nutrients or severe loss of blood. Anemia can cause tiredness, shortness of breath, and dizziness among other symptoms.
  • Sickle Cell AnemiaSickle cell anemia is an inherited condition in which the hemoglobin is defective and causes the red blood cells to become sticky and misshapen. They do not carry oxygen well and can even get stuck in veins, arteries, and capillaries, which can slow or block the blood flow. Sickle cell patients require blood transfusions frequently.
  • Clotting Disorders — In some people, the blood does not clot properly. Hemophilia is one such disorder. This inherited disease causes excessive bleeding from even the most minor cut. It can also result in excess bruising and internal bleeding.
  • Neutropenia — White blood cells can be the victim of disease too. Neutropenia happens when there are not enough white blood cells to protect against infection. HIV is a condition in which viruses attack the white blood cells and destroy them. Leukemia is a cancer of the blood that causes abnormal white blood cells.
doctor in lab holding vial of blood

Scientists who study blood and look for ways to treat blood disorders are known as hematologists. Learn more about the field of hematology and the work that hematologists do.

Fun Facts About Blood

Photo of an octopus
An octopus with blue veins

  • A fact which makes mammals’ red blood cells different to all other cells is that, when they are mature, red blood cells do not have a nucleus. All other vertebrates have red cells with nuclei.
  • Red blood cells can change shape without breaking. They squeeze single-file through the capillaries.
  • Not all blood is red. Crabs, octopi, and squid have blue blood, some earthworms, lizards, and leeches have green blood, peanut worms have purple blood, and sea cucumbers have yellow blood. The blackfin icefish has white blood, making it the only known vertebrate that does not have red blood. 
  • Some invertebrate animals such as coral, jellyfish, and flatworms do not have blood because they are able to absorb nutrients and move wastes directly to the outside of their bodies.
  • Scary stories about vampires who want to suck your blood are just pretend tales. But a few creatures do indeed prey upon your blood, such as mosquitoes!

Top 10 Questions

January 2015

Thanks to Dr. Roger Turcotte, internist, St. Luke's Internal Medicine; and Dr. Alicia Lachiondo, pediatrician, Treasure Valley Pediatrics for the answers.

  1. Why is blood so important in your body?

    Blood has many functions. Blood is actually an organ just like other organs in your body such as your heart and liver. Its function is to circulate nutrients. It circulates oxygen, sugar and proteins, which you need in certain areas of your body, and gets rid of waste products, like carbon dioxide. (From Leah at Owyhee Harbor Elementary School in Boise)

  2. How is blood made?

    Blood is actually made in your bones. Some blood is made in your spine. Some is made in the big leg bone, called the femur and some in your long arm bones. Blood can also be made in your liver. And, a part of your blood is made in an organ called the thymus. (From Taylor at Owyhee Harbor Elementary School in Boise)

  3. What substances make up blood?

    Blood is made up of many different substances. There is plasma, which is a little over half of your blood's content. It is filled with sugar, proteins, and many other essential parts of your immune system. The other major component of your blood is composed of red blood cells. These cells carry oxygen to your tissues and also carry carbon dioxide. A smaller fraction of your blood content is made up of white blood cells. These cells fight infections and perform many other functions. The fourth component is made up of platelets, which help your body stop bleeding when you get cut. (From Colton at Kamiah Elementary School in Kamiah)

  4. What happens when blood gets infected? What color does it turn?

    When you have an infection, you can get really sick. Your blood will have a lot more white blood cells during the time of infection to help fight off the infection, but your blood will still be red because you will have a lot more red blood cells than white blood cells. (From Laurel at Cynthia Mann Elementary School in Boise)

  5. How do you figure out the blood type of animals?

    In any animal, including people, we use a blood sample to figure out blood type. Then, in a lab, we determine what blood type that person or animal has. Animals have different types of blood than people have. (From Haden at Owyhee Harbor Elementary School in Boise)

  6. What is the difference between the types of blood?

    There are four main types of blood that we consider to be important. They are: A, B, AB, and O. It is a bit complicated how we know this and make this distinction. Basically, on the surface of your red blood cells are proteins. We call these antigens. There is an A and a B antigen. People who have just the A antigen are type A, and people who have just the B antigen are type B. For people whose blood protein has both present, their blood is type AB. People who have neither protein present are type O. (From Steph at Owyhee Harbor Elementary School in Boise)

  7. How does your blood pressure get too low or too high?

    There are a lot of factors and many organs that affect your blood pressure. When we measure your blood pressure, we are measuring how much blood is pumping through your arteries and veins (little tubes that blood flows through), and how much pressure is inside those arteries and veins. We don't want the pressure to get too high or too low because it can affect the pressure in your organs. The most common way you can have low blood pressure is by being dehydrated because you are not drinking enough water. You can also have low blood pressure if you lose blood, like from a major injury. High blood pressure is more complicated. It is affected by salts that you eat, foods that you have eaten over the course of your life, how much you exercise, and how healthy your kidneys and other organs are. (From Cody at Kamiah Elementary School in Kamiah)

  8. Do you always get your parents' blood type?

    Your parents determine what your blood type will be, but you won't always have exactly the same type of blood as your parents. (From Abby at Owyhee Harbor Elementary School in Boise)

  9. What happens to blood when we die?

    When you die, your blood decomposes, just like your other tissues. When you are alive, blood has to be continuously replenished. That is, your body is continuously making blood. Red blood cells are destroyed in various places, usually in your spleen. A spleen is a special blood organ. As the red blood cells are destroyed, they have to be remade in your marrow, which is inside your bones. When you die, the production of red blood cells stops and your body simply decomposes. (From Izaih at Cynthia Mann Elementary School in Boise)

  10. How does the heart pump blood?

    The heart pumps blood by squeezing. Every time your heart squeezes, it squeezes blood out from the heart muscle to a great big blood vessel called the aorta, which then branches into many smaller vessels of your circulatory system. Each time your heart beats, it not only pumps blood out of your heart, but new blood comes from your lungs into your heart to be ready to pump out in that next cycle. (From Isabella at Owyhee Harbor Elementary School in Boise)