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.
Next, learn about the different blood components and why artificial blood might be necessary.