Blood

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.

A Life-Sustaining Fluid

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 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, and it contains three types of blood cells:

1. red blood cells (RBCs)
2. white blood cells (WBCs)
3. platelets

In babies and young children, blood cells are made within the bone marrow (the soft tissue inside our bones) of lots of bones throughout the body. But, as kids get older, blood cells are made mostly in the bone marrow of the vertebrae (the bones of the spine), ribs, pelvis, skull, sternum (the breastbone), and parts of the humerus (the upper arm bone) and femur (the thigh bone).

The cells travel through the circulatory system suspended in a yellowish fluid called plasma. Plasma is 90% water and contains nutrients, proteins, hormones, and waste products. Whole blood is a mixture of blood cells and plasma.

What are Red Blood Cells?

Red blood cells (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 red blood cells to replace those that die or are lost from the body.

What are White Blood Cells?

White blood cells (also called leukocytes) 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 WBCs than red cells, although the body can increase production of WBCs 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 may also attempt to destroy cells that have become infected or have changed into cancer cells.

Certain types of WBCs produce antibodies, special proteins that recognize foreign materials and help the body destroy or neutralize them. The white 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 enters the body again.