The functions of blood are numerous but can essentially be broken down into two subsets. Blood transports things to the various tissues of the body, and it removes things from the tissues of the body. It may be easiest to think of blood as a never-ending series of light rail cars or trains that travel through an immense transportation network. In this sense, the "cars" are constantly either delivering needed elements or carrying away things produced by the body.
As a delivery system, there are numerous functions of blood. Key among these is to deliver oxygen to all tissues, since no area of the body survives without a regular supply of oxygen. This is not the only thing on the "light rail system" that gets delivered, however. Other things that travel in blood include hormones, nutrients, temperature regulating elements, and various forms of nourishment, such as minerals and vitamins.
Certain types of blood cells go into action only if the body requires it, and this is one of the major functions of blood. If bacteria enter the body, white blood cells arrive to fight it. An injury requires cells and materials to help clot blood. These cells are not apart from the rest of blood and are constantly traveling with it. Looking at a cut bleeding, people can’t see the cells as different, but under a microscope, it's easy to see there are many elements that make up blood. By all of these elements traveling together, they are ready to work in the ways needed when required, and these additional "as needed" functions could be called special deliveries, based on individual bodily circumstances.
It's clear then, that one of the functions of blood is to work as a delivery system, but the transportation network has another important job. It also needs to be able to clear away things the body produces or doesn't need. Various types of waste are carried away by blood and may ultimately be excreted from the body or transformed through chemical processes. Blood must also get rid of excess carbon dioxide so that blood levels retain a normal pH balance.
When the functions of blood are suspected as being insufficient or abnormal, one thing doctors may do is perform a complete blood count (CBC) test or other medical tests to determine the efficacy of the cars and the network. Doctors could for instance look at veins and arteries to see if narrowing is creating problems with the "cars" traveling. Alternately, they could look at the blood itself to check it for insufficiencies. Since so much of what bodies are able to do is fully dependent on this vital transportation system, problems with it can be become hugely important and require immediate repair or medical treatment.
