Why aren't our brains red? Perfusion, "suffusion"(?), and stuff gushing out

Cut my finger trimming some steak last night. First-aid thoughts:

  1. Where did the blood go in my steak (raw) since the cow was using that muscle?

  2. Because my finger’s bleeding, why not it? Answer: ok, maybe I have a blood vessel(s) in finger flesh that isn’t quadricep flesh…OK, if that’s true, I’ve seen surgery films, they’re always mopping stuff bubbling up around retractors, and I know about major and minor blood vessels, down to capillaries…

Everything (everything? in our body needs oxygen always? Does capillary action carry out the blood cell’s magic even without it looking, you know, bloody?
Brains are greyish, bones are white, I think our liver, if you sliced it to go with Chianti, would be dark liver colored. When does blood stop looking like blood, or some of its functions get taken over by other fluids?

  1. Is there a maximum distance for O2 perfusion (distance from capillaries) such that the cells or organelles can say “It’s all right, I can take it from here…”? I’m hoping the answer is yes, which was the conclusion I reached when I went back to prep work.

  2. Is blood suffusion a word?

(I am aware of the treatment of meat following slaughter to change its color. OP is different.)

OK, I’ll give it a shot.

  1. The blood was drained from the cow when it was slaughtered. The red color you see in the raw steak is not from blood, but is the color of the muscle.

  2. The steak is not bleeding because it does not have any blood in it. It is a dead piece of meat. Your finger is alive and has blood circulating through it. If you were to chop your finger off and drain the blood out of it, it wouldn’t bleed when you cut it.

Circulating blood does a lot more than just provide oxygen to the cells. It also removes wastes. It also fights infections. It also transfers heat. It also supplies nourishment.

  1. Different cells have different demands for these services, so the “distance” the blood is from the different cells really depends on what those cells need. In particular with oxygen, diffusion is the process by which oxygen goes from the hemoglobin to the individual cells. As with most diffusion mechanisms, the speed at which it occurs depends on the concentration gradient, or, in other words, oxygen need.

  2. No, “blood suffusion” is not a word. It is two words. I am not sure what you mean.

Some random factoids that excavating didn’t already cover:

Oxygen is needed by any cell carrying out metabolic activity. The more active it is, the more blood it needs. Some tissues are dead and therefore need no blood - for example hair, the outermost layer of teeth and skin. Other tissues like bone may be mostly composed of inert material, with just enough cells hanging around in maintenance mode. Corneas are interesting in that they get oxygen from the air directly.

A cell with a very high demand for oxygen will die quickly without it - brain cells being one of the first to go. Other tissues can survive without oxygen for much longer periods of time. Less oxygen is used at low temperature, so the cells can last longer before they die, which is why you’re better off drowning in cold water.

Meat is red largely because of myoglobin. This is similar to hemoglobin and it’s the “blood” you see when you cut into a steak.

Brain tissue, has no myoglobin and thus no red color. Fat tends to appear white (just like it would in your steak) and so that’s the predominant coloring of the brain, which uses fat to insulate neurons.

Thanks. The thing is, I knew/know that the lovely juices even in the rarest steak is not blood, just like I know bones aren’t bloody (despite raw head’s ever present companions).

I’m off to Wiki hemoglobin and myoglobin…but before I do, I think my question about what is the minimum channel/medium for O2 transport is still percolating.

I also realized that I knew the answer to “everything living” was hair, which I guess is called excreta in old books because it’s dead dead, as opposed to necrotic dead.

I asked because using regular (layman) English, in context of OP I was getting at a spectrum of “organ-wetness” on an increasing scale of “perfusion” to “basically just a dripping sponge of red stuff.”

Note that very small animals, like insects, don’t need a circulatory system, because oxygen can just diffuse directly into the cells. An insect has spiracles (holes) in their exoskeleton that lead to trachea and tracheoles (tubes) that carry air into the body. From there the air diffuses directly into the cells that need it, without any intermediary fluid like blood to carry it around. Some larger insects actively pump air into the trachea but smaller ones don’t even need to do this.


I’m not really sure what the OP is asking, but in the dermis there are around 100 capillaries per mm2 and around 500 per mm2 of muscle.

Fruit flies, at the very least, do have a simple open circulatory system. Their body cavity is filled with hemolymph, which is pumped around both by a heart-like organ and normal movement of the body wall muscles. The hemolymph even has hemocyanin as an oxygen carrier.

I don’t know enough about other tiny invertebrates to confidently state whether this is the exception or the rule.

Well, according to Wikipedia, “Unlike vertebrates, insects do not generally carry oxygen in their haemolymph.” (Cite) That’s what I’d always understood. Maybe fruit flies are unusual in this respect?


In insects, the respiratory system consists of tubes that carry air in from the outside. The circulatory system just moves food and waste around, not oxygen.

  1. Some drains as the meat is cut and transported. The rest of the plasma evaporates away leaving behind the solid components.

  2. Your finger is still being supplied with blood since you’re presumably alive. Surgery has more blood because you’re cutting a square amount more capillaries open (depth x length of incision)

Not every structure in your body needs blood, but all the cells do. A lot of your body is what’s called inter-cellular matrix. It can be made up of stuff like blood plasma, protein structures, collagen fibers, cartilage, and bone. All of these still contain much fewer blood vessels than your skin, but they’re still capillarized.

Brains are grey because of the constituent cells are predominantly neurons and their associated cells. The schwann cells in the brain and spinal cord produce fat and wrap around the neurons to aid in protection and transmission of signals. This fat concentration gives it the darker color and higher and lower levels make light and dark matter.

Bones are white because they’re mostly intercellular matrix. They’re still alive though! They have 2.5 types of cells, osteoblasts, osteoclasts, and osteocytes, that all receive blood and help build up and break down bone.

I’m not sure about the liver but iirc the hepatic portal system (capillary structure in the liver) causes a large concentration of blood cells, making it seem darker than it really would be.

As an aside, blood isn’t really red itself either. The red blood cells are, but the liquid component (60-70%) is actually yellow. The only really “red” cells in your body are ones that have shitloads of hemoglobin, which holds iron. Muscle cells and RBCs have lots cause the former needs a lot of energy and the latter carries it.

  1. Yep. Oxygen can be transported, passively I think, between cells. But physics becomes a problem. Concentration decreases with the square of distance so you don’t want to rely on this if you’re a large organism. That said, a lot of cells lie above the capillaries and don’t receive direct blood flow. Most of them are very squished (squamous epithelium is the term if you wanna read more) allowing them to easily diffuse oxygen through them. Alveolar cells in the lung are this type, as well as most of the cells in the upper parts of your skin

As far as organelles, there’s no real transport mechanism inside the cell to carry oxygen. The concentration gradients are lower around the mitochondria as respiration turns it to CO2, so the O2 naturally flows there. Respiration is a lot more complex tho so I might be misremembering that part.

  1. no clue

Lots of invertebrates (some exceptions: octopi, earth worms, and starfish) have open circ systems if any at all. But they don’t really rely on it for oxygen transport. Its used to supply food to the cells, the hemocyanin is just a nice side benefit.

Insofar that cut or injured tissue is red due to oxygenated blood, “fresh” brain and bones can also appear red. I’m not going to link to any images here, but a google image search of humans involved in traffic accidents will demonstrate that brain and bone can indeed be red.
Sent from my iPhone using Tapatalk

Btw, in my experience, the surface of a live brain is between white and pale yellow. Surface blood vessels contrast sharply on it.

That red colour in your steak is actually mixture of water and a protein called myoglobin. If it came prepacked from a supermarket, it was kept in CO to stop it from going brown.

Bone outside is white (well, more like off yellow, the joints are more truly white) due to the mineralization and calcium deposits, but the bone marrow can be quite vascular and bloody, depending on bone type and age.

Brain of course has vessels, especially in surface areas. When those vessels are cut, the blood spills out.

I won’t link anything either but I’ve been to enough ‘gore’ sites and seen plenty of massive head wound photos to know that, ahem, freshly spilled brain matter is indeed red with blood. Hell the Zapruder film showed that…

One of the more interesting things I learned but then never needed to know (so I’m probably wrong on the details) is that the concentration of oxygen in the tissues dictates how much of the hemoglobin inside each blood cell opens up to release the oxygen trapped inside. That prevents the tissues nearer to the lungs from taking all the oxygen, leaving none for the tips of your toes.

Can anyone clarify that hazy memory from Biology class many years ago?

Eh? “Rawhead and Bloody Bones” is one creature. It’s an overall descriptor, not a team.

I know that. It’s “raw head['s]” companion in words.

And a propos, since the sentence mentions bloody bones, which both of us recognize as a striking juxtaposition. Goddamn but we’re cultured and witty.


So let’s take bone, the bony part (:)), with the mineralization and calcium deposits. How much of the bone, on an ongoing basis, receives/needs O2? Does some part of it (the surface, maybe) amount to, physiologically, excreta, having got past the part where the rest of the organism has facilitated such mineralization/calcification?

To add to my confusion/lack of knowledge, I’m aware of scoliosis and the ever present structural responses due to gravity or applied stress ( astronauts, resistance training for the elderly), which either complicates my question above or answers it in the negative.