Humans can't live without blood. Without blood, the body's organs couldn't get the oxygen and nutrients they need to survive, we couldn't keep warm or cool off, fight infections, or get rid of our own waste products. Without enough blood, we'd weaken and die.
Here are the basics about the mysterious, life-sustaining fluid called blood.
Two types of blood vessels carry blood throughout our bodies:
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.
The blood that flows through this network of veins and arteries is whole blood, which contains three types of blood cells:
In babies and young kids, blood cells are made within the bone marrow (the soft tissue inside of bones), particularly in the long bones like the humerus (the upper arm bone) and femur (the thigh bone). But, as kids get older and approach adulthood, blood cells are made mostly in the bone marrow of the vertebrae (the bones of the spine), ribs, pelvis, skull, sternum (the breastbone).
The cells travel through the circulatory system suspended in a yellowish fluid called plasma, which is 90% water and contains nutrients, proteins, hormones, and waste products. Whole blood is a mixture of blood cells and plasma.
Red blood cells (RBCs, also called erythrocytes) are shaped like slightly indented, flattened disks. RBCs contain the iron-rich protein hemoglobin. 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 tissues.
The body contains more RBCs than any other type of cell, and each has a life span of about 4 months. Each day, the body produces new RBCs to replace those that die or are lost from the body.
White blood cells (WBCs, also called leukocytes) are a key part of the body's system for defending itself against infection (this system is called the immune system). They can move in and out of the bloodstream to reach affected tissues.
Blood contains far fewer WBCs than red blood cells, although the body can increase WBC production to fight infection. There are several types of WBCs, and their life spans vary from a few days to months. New cells are constantly being formed in the bone marrow.
Several different parts of blood are involved in fighting infection. White blood cells called granulocytes and lymphocytes travel along the walls of blood vessels. They fight germs such as bacteria and viruses and also may attempt to destroy cells that have become infected or have changed into cancer cells.
Certain types of WBCs make antibodies, which are special proteins that recognize foreign materials and help the body destroy or neutralize them. 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 produced or are entering the bloodstream to battle the infection.
After the body has been challenged by some infections, lymphocytes "remember" how to make the specific antibodies that will quickly attack the same germ if it ever enters the body again.
Platelets (also called thrombocytes) are tiny oval-shaped cells made in the bone marrow. They help in the clotting process. When a blood vessel breaks, platelets gather in the area and help seal off the leak. Platelets survive only about 9 days in the bloodstream and are constantly being replaced by new cells.
Important proteins called clotting factors are critical to the clotting process. Although platelets alone can plug small blood vessel leaks and temporarily stop or slow bleeding, the action of clotting factors is needed to produce a strong, stable clot.
Platelets and clotting factors work together to form solid lumps (called blood clots) to seal leaks, wounds, cuts, and scratches and to prevent bleeding inside and on the surfaces of our bodies. The process of clotting is like a puzzle with interlocking parts. When the last part is in place, the clot happens — but if even one piece is missing, the final pieces can't come together.
When large blood vessels are cut, the body may not be able to repair itself through clotting alone. In these cases, dressings and stitches are used to help control bleeding.
Blood contains other important substances, such as nutrients from food that has been processed by the digestive system. Blood also carries hormones released by the endocrine glands and carries them to the body parts that need them.
Blood is essential for good health because the body depends on a steady supply of fuel and oxygen to reach its billions of cells. Even the heart couldn't survive without blood flowing through the vessels that bring nourishment to its muscular walls.
Blood cells and some of the special proteins blood contains can be replaced or supplemented by giving a person blood from someone else via a transfusion. Besides receiving whole-blood transfusions, people also can receive transfusions of a particular component of blood, 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.
Most of the time, blood functions without problems. But sometimes, blood disorders or diseases can cause illness. Diseases of the blood that commonly affect kids can involve any or all of the three types of blood cells. Other types of blood diseases affect the proteins and chemicals in the plasma that are responsible for clotting.
The most common condition affecting RBCs is anemia, a lower-than-normal number of red cells in the blood. The symptoms of anemia — such as pale skin, weakness, a fast heart rate, and poor growth in infants and children — happen because of the blood's reduced ability to carry oxygen.
Anemia usually is caused by either inadequate RBC production or unusually rapid RBC destruction. In severe cases of chronic anemia, or when a large amount of blood is lost, someone may need a transfusion of RBCs or whole blood.
Anemia from inadequate RBC production. Conditions that can cause a reduced production of red blood cells include:
Anemia resulting from increased RBC destruction. Conditions that can cause increased RBC destruction in kids include::
The body's clotting system depends on platelets as well as many clotting factors and other blood components. If a hereditary defect affects any of these components, it can cause a bleeding disorder.
Common bleeding disorders include:
Other causes of clotting problems include chronic liver disease (clotting factors are produced in the liver) and vitamin K deficiency (the vitamin is necessary for the production of certain clotting factors).
Reviewed by: Yamini Durani, MD
Date reviewed: May 2015
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|National Heart, Lung, and Blood Institute (NHLBI) The NHLBI provides the public with educational resources relating to the treatment of heart, blood vessel, lung, and blood diseases as well as sleep disorders.|
|Sickle Cell Disease Association of America This group provides education, advocacy, and other initiatives to promote awareness of and support for sickle cell disease programs.|
|American Association of Blood Banks This site of the American Association of Blood Banks describes blood banking and transfusions.|
|National Hemophilia Foundation (NHF) The NHF's Web site contains information on bleeding disorders such as hemophilia.|
|American Society of Hematology This group provides information relating to blood, blood-forming tissues, and blood diseases.|
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