Blood is a fluid that travels through the circulatory system of the body to deliver oxygen and nutrients to every cell of the body. It is pumped by the heart through arteries to the body's capillaries, and is returned to the heart through the veins. It is re-oxygenated in the lungs.
Blood is mostly salt water, and the salinity of blood is approximately the same as that of sea water. The main solid constituent of blood is red blood cells, which contain hemoglobin - a molecule that attaches itself to oxygen where it is plentiful and to carbon dioxide when it is plentiful. The other primary solid constituent is white blood cells, which are part of the body's immune system. The blood also contains platelets, which assist in the clotting of blood. The rest of blood is primarily made up of dissolved sugar, proteins and other trace elements. The liquid part of blood is referred to as plasma.
In order to keep the body healthy, both the volume of fluid in the blood and the number of red blood cells must be sufficient. The body can lose a fair amount of both without ill effects - one pint more or less of fluid can easily be replaced. However, loss of fluid either through dehydration or bleeding can be a life threatening condition. The loss of red blood cells from any cause where the amount of fluid does not change is referred to as anemia and can also be life threatening.
Blood can be transfused from one person to another as long as their blood types match. There are three antigens that can be present on human blood (A, B and Rhesus plus or minus), and it is typed as to how many of those antibodies are present. A person with all three antigens is referred to as AB+, and a person with none is O-. The first is the rarest blood type, the latter the most common. People can accept any blood as a donor that does not contain an unacceptable antigen. As such, persons with AB+ can accept any blood type, while persons with O- can donate to anyone.