Naturally, as an internal organ it looks rather bloody, so I'm primarily going to suggest sanguine
Of the color of blood; red; ruddy: as, a sanguine complexion; the sanguine francolin, Ithaginis cruentatus; specifically, in heraldry, same as murrey.
The Century Dictionary and Cyclopedia 1890-1914
It is a bit problematic though since blood color can come in many shades depending on the state it's in: It can be very bright when it's thin and fresh or very dark when it's clotting and despite being very apt by definition for many contexts, I have never seen anything directly described with this word.
The alleged synonym Murrey is quite helpful though since the C.D.C. specifies it's the color of a mulberry, which should be relatively easy to observe in reality:
Of a mulberry (dark-red) color.
n. In heraldry, a tincture of a dark-reddish brown, also called sanguine, indicated in heraldic representations in black and white by lines crossing each other
The Century Dictionary and Cyclopedia, 1890-1914
Given that these are colors of heraldic tradition, you might also be able to examine a coat-of-arms with the color. The Heraldry Society's Education Pack P.D.F. pack may be helpful. Although the C.D.C. considers these words synonymous, which makes sense given that berries may have been used for such pigmentation, Sanguine seems to be a brighter color than Murrey according to the H.S. P.D.F., so despite the semantic appeal of Sanguine, Murrey would be more accurate if both words don't refer to the whole spectrum of reds blood and mulberries can be.
For your convenience and as a contingency for if the Hearldry Society's website goes offline, here's an image file with both colors, with a Sanguine backdrop and a Murrey heart. It looks fairly close to me. It also seems like the colors of this website's unbolded and bolded links:
Speaking of contingencies and convenience both of the above Wordnik links also contain entries in The American Heritage Dictionary, 4th edition which is a newer and presumably less rare book than the multi-volume C.D.C. if you need to verify with a print source.