Why do shinigami bleed? This is a question really about how the spiritual body is "comprised"
How and why do shinigami bleed, have internal organs etc, it is shown they do not need food, so why a digestive tract at all.

All things in soul society is made of reishi, are we to believe reishi is the equivalent of "atoms" or "particles". 
"Spirit particles" is commonly used to describe energy that a quincy uses...
so the main question is...
In cannon, has it been explained why shinigami have a body that is still basically human...
I believe you've answered your own question - it's not really blood that's draining from a shinigami, it's the reishi of that person draining away, thus weakening their soul. Ichigo and co (and the viewers) only perceive it as blood because that's something our minds can actually grasp and understand. In more extreme cases, such as Ulquiorra, when critically injured, the person may simply disintegrate directly into particles before one's eyes, due to a sudden uncontrolled collapse of spiritual energy. Nell's wound results in rapid and ongoing "bloodloss" (reishi-loss) in her adult form, forcing her to 'reduce' her 'mass' back to a smaller size, that of a child.