The systemic circulation is the portion of the circulatory system is the network of veins, arteries and blood vessels that transports blood from heart, services the body's cells and then re-enters the heart, the Mayo Clinic noted. [Image Gallery: The BioDigital Human]
A southern pine (2) 2x12 beam can span 8 feet when supporting joists spanning a maximum of 14 feet, and it can cantilever 2 feet beyond the posts on each side. With the allowable cantilever of the joists, this beam would support a deck 12 feet by 17 1/2 feet.
As for spacing, 16 on center is the most common. If the span is long for the material used, 12 on center is possible, or even 8. If the span is short relative to the material used for joists, 19.2 (19 3/8) spacing or even 24 spacing may be acceptable. Proper spacing depends upon what kind and size of floor joists you have and the span of the floor joists.