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I have read many articles about how the brain is the most power-hungry organ in any living complex organism, requiring about 70% of it's oxygen supplies in the resting state.

Since the usual medium of oxygen delivery is through respiration, this should mean that a lot of blood is supplied to (and hence present in) the brain.

Why then is the color of the brain almost always white? There is usually a little red tainting on the exterior and that's it, you can see the interior is still white. Should it not be red? Presuming that for a higher surface area to volume ratio, most of the brain cells are in direct contact with the bloodstream.

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  • $\begingroup$ If I recall correctly, most of the brain cells are not in direct contact with the bloodstream, there is a famous blood-brain barrier that aims to shield most of the brain from the blood. The barrier is that thin, that fat cannot pass through it. Therefore brains can only work on sugar supply. $\endgroup$ – Willem Van Onsem Aug 18 '15 at 8:55
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    $\begingroup$ @CommuSoft - The blood-brain barrier shields of constituents in the blood from the brain. It is formed by tight junctions between the epithelial cells lining the blood vessels in the brain. The only difference with the brain is that these tight junctions are absent elsewhere. Hence, less molecules pass into the brain from the bloodstream than elsewhere. Sugar is imported in every cell with dedicated transporter proteins - i.e., sugar cannot pass the bloodstream passively anywhere, not in the brain, not in the rest of the body. $\endgroup$ – AliceD Aug 18 '15 at 13:02
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The brain is indeed stacked with blood vessels, as shown in a 3D model in Fig. 1.

Biobot brain
Fig. 1. 3D-printed model of blood vaculature. Source: Biobots.

The blood supply on the surface of a live brain is readily seen during a craniotomy (Fig. 2.)

enter image description here
Fig. 2. Surface of the brain. Source: The Sterile Eye.

When freshly prepared, the interior of the brain appears pink, indicative of the presence of blood (Fig. 3):

Sections
Fig. 3. Coronal sections of a non-fixated brain. Source: Documenting Reality.

Mostly, images of the brain are taken after fixation of the brain. The fixation process includes removal of the blood in the brain and the fixative pales the color of the tissue markedly. Fixation results in the more familiar pale-appearing pictures of the brain as shown in Fig. 4.

Fixated brain
Fig. 4. Coronal section through a fixated brain. Source: Wikimedia.

Also note that the blood supply mainly targets the areas where the cell bodies are located, i.e., the gray matter. The white matter is pretty much devoid of blood supply and mainly consists of heavily myelinated, fatty tissue supporting axonal fiber tracts (Fig. 5).

blood supply
Fig. 5. Arterial innervation of the brain. Source: Human Physiology Academy.

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    $\begingroup$ Nicely illustrated answer :) +1 $\endgroup$ – Remi.b Aug 17 '15 at 0:32
  • $\begingroup$ Nice answer. Can you also add a note on blood-brain barrier and the role of CSF as a medium of metabolite exchange? $\endgroup$ – WYSIWYG Aug 17 '15 at 5:25
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    $\begingroup$ @WYSIWYG - the blood-brain-barrier is not really of relevance for oxygen exchange. And since the question is on oxygen, and not on metabolites, I deliberately left CSF out of the answer. $\endgroup$ – AliceD Aug 17 '15 at 5:30
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    $\begingroup$ A person can be alive and awake with his skull open and brain exposed like that? o.O $\endgroup$ – Abdul Aug 17 '15 at 19:00
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    $\begingroup$ @Abdul Not only can be, it's necessary for them to be, for most forms of brain surgery. Feedback from speaking with the patient is vital to allow the surgeon to find and identify the right bits to cut/prod/etc. $\endgroup$ – Matthew Najmon Aug 18 '15 at 9:17

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