= Cardiac vein
A \b[cardiac vein] carries <blood> away from the <heart>'s own tissue after passage through its supplying vessels. This is the return side of the <coronary circulation>, distinct from the large volume of <blood> passing through the ventricular cavities. Much of this return reaches the <right atrium> through the <coronary sinus>; <anterior cardiac veins> can return directly to the <right atrium>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[§19.1, Coronary Veins]{ref}
The <great cardiac vein>, <middle cardiac vein> and <small cardiac vein> provide named routes through this network. An illuminating anatomical exercise pairs each route with its connections rather than simply matching it to the nearest <coronary artery>. The <artery> supplies tissue and the <vein> drains tissue, while their branching and crossings need not be identical.
Primary evidence for that qualification comes from a study dissecting 37 fixed human hearts. It documented varying relationships among the <coronary sinus>, its tributaries and neighboring arteries.<The anatomy of the coronary sinus and its tributaries by J. R. Ortale and others>[Abstract, methods and vessel relationships]{ref} Such a study observes actual connected vessels in specimens; a schematic drawing deliberately compresses the variation into a readable route map.
For a transport measurement, identifying the correct <cardiac vein> matters because a regional sample has a regional <venous oxygen content>. Substituting it for <mixed venous oxygen content> in a whole-body <Fick principle> calculation would change the system being sampled. The anatomical return route and the corresponding <blood flow> must match the oxygen-balance question.
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