Reasons for deep-red versus light red-blood in stool?

This came up from when my dog had to go the vet, but I think I read that it applies to humans as well.

This is my understanding: if it’s light-color blood, it is from surface blood vessels, ie in itself a not so immediately serious symptom, and indicates some sort of capillary, hemorrhoid, and/or near to the anus blood-vessel break. Those three are either independent or combined.

If the blood is dark, I was told–by the vet for dogs–the bleeding is starting higher up from the colon directly.

  1. is this more or less correct for people too?

  2. what is the reason for two different type colors of blood? I know that venous blood looks different–but does it macroscopically?–due to relative lack of oxygenation (but would love to be corrected and learn more).

I am not seeking medical advice.

  1. It was explained to me this way by a doctor for humans.

  2. I was told it’s darker because it’s coagulated. (and I’m guessing somewhat stained also).

If the blood comes from far enough up the digestive tract that it gets partially digested, you get black or tarry stools. The black color comes from the iron in the blood. Iron supplements can also cause black stools. This is true for people too.

And it means SEE YOUR DOCTOR.

For humans, it also means a colonoscopy in your future. And THAT, Dear Friends, is why many people set it aside and think, “Maybe it’s nothing, maybe it will go away.”

Colonoscopy = no fun

Getting dead from colon cancer = even less fun

Bright red = hemorrhoids

Dark red = ulcers


This was the 25 words-or-less answer I got when I finally remembered to ask the nurse why she wanted to know.

Colonsocopy actually = some serious drug-induced short term diahrhea, followed by a non-event at the Dr’s office followed by a lazy day sleeping off the meds.

It’s not anything anybody ought to dread. It’s a bit more hassle than a teeth cleaning, which itself is only slightly more hassle than a haircut.

We define “hassle” very differently. :slight_smile:

I’m still not clear on the why there is a color difference. Dark–>higher in the gut–>more time to trickle down and therefore oxidize?

I don’t mean dark stool. And I get what each may be a sign of, now (thanks).

I mean threads of blood in the toilet bowl. Could be dark red, could be light red. Why?

Same thing - just that the blood is trickling down so slowly it becomes a vein within the stool instead of covering it.
If there is no stool, just blood coming from your anus, see a doc.