Capillaries :-
Arteries branch into small passages called arterioles and then into the capillaries.The capillaries merge to bring blood into the venous system.
Veins :-
After their passage through body tissues, capillaries merge once again into venules, which continue to merge into veins.
The venous system finally coalesces into two major veins:
Superior vena cava
Inferior vena cava
These two great vessels empty into the right atrium of the heart.
Coronary vessels :-
The heart itself is supplied with oxygen and nutrients through a small "loop" of the systemic circulation and derives very little from the blood contained within the four chambers.
Portal veins :-
The general rule is that arteries from the heart branch out into capillaries, which collect into veins leading back to the heart. Portal veins are a slight exception to this. In humans the only significant example is the hepatic portal vein which combines from capillaries around the Gastrointestinaltract where the blood absorbs the various products of digestion; rather than leading directly back to the heart, the hepatic portal vein branches into a second capillary system in the liver.
