Doctors and scientists have come up with lots of gadgets that can take over for parts of the body that break or wear out. A heart, for example, is basically a pump; an artificial heart is a mechanical pump that moves blood. Similarly, total knee replacements substitute metal and plastic for bones and cartilage. Prosthetic limbs have become increasingly complex, but they're still essentially mechanical devices that can do the work of arms or legs. All of these are fairly easy to comprehend -- swapping out an organ for a manmade replacement usually makes sense.

Artificial blood, on the other hand, can be mind boggling. One reason is that most people think of blood as more than just connective tissue that carries oxygen and nutrients. Instead, blood represents life. Many cultures and religions place special significance on it, and its importance has even affected the English language. You might refer to your cultural or ancestral traits as being in your blood. Your family members are your blood relatives. If you're outraged, your blood boils. If you're terrified, it runs cold.

Blood carries all these connotations for good reason -- it's absolutely essential to the survival of vertebrate life forms, including people. It carries oxygen from your lungs to all the cells in your body. It also picks up the carbon dioxide you don't need and returns it to your lungs so you can exhale it. Blood delivers nutrients from your digestive system and hormones from your endocrine system to the parts of your body that need them. It passes through the kidneys and liver, which remove or break down wastes and toxins. Immune cells in your blood help prevent and fight off illnesses and infections. Blood can also form clots, preventing fatal blood loss from minor cuts and scrapes.

Photographers Bruce Wetzel/Harry Schaefer, courtesy National Cancer Institute A scanning electron microscope image from normal circulating human blood.

It can seem improbable, or even impossible, that an artificial substance could replace something that does all this work and is so central to human life. To understand the process, it helps to know a little about how real blood works. Blood has two main components -- plasma and formed elements. Nearly everything that blood carries, including nutrients, hormones and waste, is dissolved in the plasma, which is mostly water. Formed elements, which are cells and parts of cells, also float in the plasma. Formed elements include white blood cells (WBCs), which are part of the immune system, and platelets, which help form clots. Red blood cells (RBCs) are responsible for one of blood's most important tasks -- carrying oxygen and carbon dioxide.

RBCs are numerous; they make up more than 90 percent of the formed elements in the blood. Virtually everything about them helps them carry oxygen more efficiently. An RBC is shaped like a disc that's concave on both sides, so it has lots of surface area for oxygen absorption and release. Its membrane is very flexible and has no nucleus, so it can fit through tiny capillaries without rupturing.

Photo courtesy Garrigan.Net Red blood cells, also called erythrocytes, are shaped like biconcave discs.

A red blood cell's lack of nucleus also gives it more room for hemoglobin (Hb), a complex molecule that carries oxygen. It's made of a protein component called a globin and four pigments called hemes. The hemes use iron to bond to oxygen. Inside each RBC are about 280 million hemoglobin molecules.

If you lose a lot of blood, you lose a lot of your oxygen delivery system. The immune cells, nutrients and proteins that blood carries are important, too, but doctors are generally most concerned with whether your cells are getting enough oxygen.

In an emergency situation, doctors will often give patients volume expanders, like saline, to make up for lost blood volume. This helps restore normal blood pressure and lets the remaining red blood cells continue to carry oxygen. Sometimes, this is enough to keep the body going until it can produce new blood cells and other blood elements. If not, doctors can give patents blood transfusions to replace some of the lost blood. Blood transfusions are also fairly common during some surgical procedures.

This process works pretty well, but there are several challenges that can make it difficult or impossible to get patients the blood they need:

• Human blood has to be kept cool, and it has a shelf life of 42 days. This makes it impractical for emergency crews to carry it in ambulances or for medical staff to carry it onto the battlefield. Volume expanders alone may not be enough to keep a badly bleeding patient alive until he reaches the hospital.
• Doctors must make sure the blood is the right type -- A, B, AB or O -- before giving it to a patient. If a person receives the wrong type of blood, a deadly reaction can result.
• The number of people who need blood is growing faster than the number of people who donate blood.
• Viruses like HIV and hepatitis can contaminate the blood supply, although improved testing methods have made contamination less likely in most developed countries.

Thank You Thanks to Scott Bernstein for his assistance with this article.

This is where artificial blood comes in. Artificial blood doesn't do all the work of real blood -- sometimes, it can't even replace lost blood volume. Instead, it carries oxygen in situations where a person's red blood cells can't do it on their own. For this reason, artificial blood is often called an oxygen therapeutic. Unlike real blood, artificial blood can be sterilized to kill bacteria and viruses. Doctors can also give it to patients regardless of blood type. Many current types have a shelf life of more than a year and don't need to be refrigerated, making them ideal for use in emergency and battlefield situations. So even though it doesn't actually replace human blood, artificial blood is still pretty amazing.

We'll look at where artificial blood comes from and how it works in a person's bloodstream next.