Blood is created in bone marrow and is made up of cells suspended in liquid plasma. Red blood cells carry oxygen, white blood cells fight infection, and platelets are cell fragments that allow blood to clot.

Arteries are thick-walled because they carry oxygenated blood at high pressure, veins are comparatively thin-walled as they carry low-pressure deoxygenated blood.

Some plant cells produce their own energy through photosynthesis which is the process by which sunlight, carbon dioxide, and water react to make sugar and oxygen. Animal cells cannot produce their own energy and, instead, generate energy when mitochondria consume outside sugar and oxygen through aerobic respiration.

In the metric system, milli is the prefix for 10^-3.

Blood is categorized into four different types (A, B, AB, and O) based on the type of antigens found on the outside of the red blood cells. Additionally, each type can be negative or positive based on whether or not the cells have an antigen called the Rh factor.