up vote 6 down vote. Start by reading a standard for SMTP protocol. This is (originally) a very simple protocol with no strings attached and no confusing or bogus statements. HTTP and FTP are more complicated (for beginner) as they are more feature-rich, non-linear (in many aspects) etc. Upd: I've put the link to the original RFC 821, which is now obsolete.
The circulatory system functions in the delivery of oxygen, nutrient molecules, and hormones and the removal of carbon dioxide, ammonia and other metabolic wastes. Capillaries are the points of exchange between the blood and surrounding tissues.
The endocrine system is a collection of specialized cells that are built to produce and release hormones-chemical messengers who travel around the body through blood and other fluids. Hormones cooperate and interact with the nervous system to control many of the body's functions.