blood

Just about everyone knows that we can't live without blood. Without blood, our organs couldn't get the oxygen and nutrients they need to survive, we couldn't keep warm or cool off, we couldn't fight infections, and we couldn't get rid of our own waste products. Without enough blood, we'd weaken and die.
So how exactly does blood do these things? How is it made, and what's in it? How does blood clot? These questions and more are explained in this article about the mysterious, life-sustaining fluid called blood.
Two types of blood vessels carry blood throughout our bodies: The arteries carry oxygenated blood (blood that has received oxygen from the lungs) from the heart to the rest of the body. The blood then travels through the veins back to the heart and lungs, where it receives more oxygen. As the heart beats, you can feel blood traveling through the body at your 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 called whole blood. Whole blood contains three types of blood cells, including:
red blood cells
white blood cells
platelets
These three types of blood cells are mostly manufactured in the bone marrow (the soft tissue inside our bones), especially in the bone marrow of the vertebrae (the bones that make up the spine), ribs, pelvis, skull, and sternum (breastbone). These cells travel through the circulatory system suspended in a yellowish fluid called plasma (pronounced: plaz-muh). Plasma is 90% water and contains nutrients, proteins, hormones, and waste products. Whole blood is a mixture of blood cells and plasma.
Red blood cells (also called erythrocytes, pronounced: ih-rith-ruh-sytes) are shaped like slightly indented, flattened disks. Red blood cells contain an iron-rich protein called hemoglobin (pronounced: hee-muh-glow-bun). Blood gets its bright red color when hemoglobin in red blood cells picks up oxygen in the lungs. As the blood travels through the body, the hemoglobin releases oxygen to the tissues. The body contains more red blood cells than any other type of cell, and each red blood cell has a life span of about 4 months. Each day, the body produces new red blood cells to replace those that die or are lost from the body.
White blood cells (also called leukocytes, pronounced: loo-kuh-sytes) are a key part of the body's system for defending itself against infection. They can move in and out of the bloodstream to reach affected tissues. The blood contains far fewer white blood cells than red cells, although the body can increase production of white blood cells to fight infection. There are several types of white blood cells, 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 (pronounced: gran-yuh-low-sytes) and lymphocytes (pronounced: lim-fuh-sytes) travel along the walls of blood vessels. They fight germs such as bacteria and viruses and may also attempt to destroy cells that have become infected or have changed into cancer cells.
Certain types of white blood cells produce antibodies, special proteins that recognize foreign materials and help the body destroy or neutralize them. When a person has an infection, his or her white cell count (the number of cells in a given amount of blood) often is higher than when he or she is well because more white blood cells 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 enters the body again.
Platelets (also called thrombocytes, pronounced: throm-buh-sytes) 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.
Blood also contains important proteins called clotting factors, which 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 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 only one piece is missing, the final pieces can't come together.
When large blood vessels are severed (or cut), the body may not be able to repair itself through clotting alone. In these cases, dressings or stitches are used to help control bleeding.
In addition to the cells and clotting factors, blood contains other important substances, such as nutrients from the 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 also carries carbon dioxide and other waste materials to the lungs, kidneys, and digestive system, from where they are removed from the body.
An interesting thing about blood is that blood cells and some of the special proteins blood contains can be replaced or supplemented by giving a person blood from someone else. This process is called a transfusion. In addition to receiving whole blood transfusions, people can also receive transfusions of a particular component of blood that they need. For example, a person can receive only platelets, red blood cells, or a clotting factor. When a person donates blood, the whole blood can be separated into its different parts and used in this way.
Things That Can Go Wrong With Blood
Most of the time, blood functions without problems, but sometimes, blood disorders or diseases can cause illness in children and teens. Diseases of the blood that commonly affect children can involve any or all of the three types of blood cells (red blood cells, white blood cells, or platelets). Other types of blood diseases affect the proteins and chemicals in the plasma that are responsible for clotting.