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Minimum-Effort-Name t1_ithurph wrote

Diffusion allows for nutrients and O2 to be between adjacent cells. No need to be directly next the capillary, if your neighbor, or neighbor's neighbor is.

Many forms of multicellular life have no ciculatory system, for example jelly fish. Our advanced ciculatory system allows for more efficient transfer over our large volume, but especially in less active tissue, cells may be far from a capillary.

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cryptotope t1_itikjdz wrote

Oxygen will diffuse from capillaries through tissues: through cells, dissolved in interstitial fluids, etc. The local amount of oxygen in tissue does fall off with distance from the nearest blood vessel(s). By and large, your body is normally quite good at building a capillary network that ensures adequate oxygenation.

This issue most often comes up these days in the context of cancer biology and solid tumors. Tumors often don't have well-formed networks of blood vessels, and there is consequently a range of oxygen levels through a section of malignant tissue: a well-perfused and highly-oxygenated exterior, a low-oxygen hypoxic middle layer, and an oxygen starved necrotic (nutrient-and oxygen-starved, dying) core.

These different regions can have very different behaviors, metabolically and biochemically speaking, which can be part of why cancer is so hard to treat.

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j4ckbauer t1_itk1gwr wrote

Fascinating and r/oddlyterrifying explanation, thanks for this.

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Sensitive_Answer2446 t1_ithuwis wrote

Most tissues in the body have blood vessels that supply nutrients, but there are exceptions. An example is cornea tissue. Corneas have no blood vessels so that we can easily see through them. Cornea cells get nutrients from tears and a liquid behind the cornea called aqueous humour.

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Cavemanb0b t1_itjxpss wrote

I burned my cornea recently. I was amazed at how quickly it healed. Not sure how it healed so quick, but given how important vision is to our survival it makes sense that this tissue would have evolved that ability.

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etv123 t1_itk0rvf wrote

Did that hurt as terribly as I would think?

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HauntingSentence6359 t1_itku9mk wrote

I cut my cornea on the sharp edge of a leaf. Initially, it was very painful, but after a couple of days, the pain all but disappeared. Until the pain subsided; my eye constantly teared.

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Cavemanb0b t1_itsya7f wrote

Yeah. Pain peaked two hours after and then diminished as it debrided, peeled, and healed.

There is this stuff called Tetracane, they put it in your eye in the emergency room. Totally worth the trip. 😉

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69SadBoi69 t1_itlmd2a wrote

I wonder why so many people these days have bad vision despite being able to reproduce? Is it something environmental that is recent enough to not be selected out of the population?

I had to get glasses in elementary school, and presumably most of our ancestors in that situation would not have survived long in the distant past without strong social support

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kafkasquared t1_itlty0a wrote

it’s a good question. but having good vision to the extent that we need it today (to read whiteboards, computers, books, etc.) wasn’t necessary for the majority of human evolution. sure, you may be a bit less capable at seeing that buffalo off in the distance, but it needn’t be sharp for you to see it, and as long as whomever you’re with is seeing it, you can still assist with the kill. maybe people with truly horrible vision were selected out, but in general, i don’t see why you couldn’t thrive and reproduce in primitive times with, for instance, typical near-sightedness.

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chronoflect t1_itmo59p wrote

If I recall correctly, we are currently going through a near-sightedness epidemic that is thought to be caused by things like not spending enough time outside in bright sunlight. Basically, our eyes need environmental cues to properly finish growing, and our modern lifestyles are stunting this growth and forcing more people to develop near-sightedness that would have otherwise had normal vision.

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ExecutiveSkiBum t1_itj3lbn wrote

Most tissues of the body are surrounded by interstitial fluid. Nutrients and gas carried by capillaries diffuse across capillary cell membranes into this balanced fluid and then across membranes of tissue cells.

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Liamlah t1_itk8yil wrote

With a few exceptions, almost every cell in almost every tissue is no further than 50-100 micrometers away from a capillary. For scale, the largest human cell is an egg(oocyte) which is 100 micrometers in length. This is because diffusion rapidly declines with distance. Those exceptions to this distance, as mentioned by others, tend to be cells that have very low metabolic activity.

An example of where this limitation causes problems is when people have chronic swelling of a tissue, such as oedema in the legs resulting from right sided heart failure. The swelling of the tissue increases the distance between the capillaries and the tissue, leading to atrophic skin, loss of hair follicles, etc. In left sided heart failure, you get swelling in the lungs, increasing the distance the oxygen has to travel from an alveolus to a capillary, and you can probably infer the consequences of that.

Sources:
https://www.ncbi.nlm.nih.gov/books/NBK26848/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1522213/

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blutigr t1_itk41la wrote

There are a whole range of tissue types which do not have capillaries and so rely on either diffusion from somewhat distant capillaries, or other oxygen sources.

The first tissue type is a bit of a trick answer. Circulating blood does not rely on oxygen diffusing out of capillaries.

The next is also a bit of a trick answer. The lungs. The alveoli are covered in capillaries but the local tissues rely on this capillaries to let CO2 out and get their oxygen from the little sack of air in the alveolus.

The next was already mentioned. The cornea has no capillaries and relies on oxygen diffusing right in from the air. If you wear contact lenses that don’t allow oxygen diffusion for too long the body can react and start emergency measures to ensure the cornea gets enough oxygen. Capillaries can start to grow into the area of the cornea. Keeps the cornea from dying from lack of oxygen but having a triangle of blood vessels growing across your pupil isn’t great for your vision.

The next is probably the first tissue type the OP was really asking about. Cartilage. You will find it in joints, tendons, ears, noses, etc. cartilage does not have a direct blood supply. Capillaries wending their way through the cartilage would, I can only imagine, wreak havoc on the strength and integrity of the tissue. Most cartilage needs to manage huge forces; whether the forces are stretch and shear in tendons or shear and compression in the lining of joints. And so they survive on oxygen and nutrients diffusing in from distant capillaries outside of the tissue proper. The effect of this is to greatly limit how quickly cartilage can grow and repair itself. Take, for example a bone break. Bone is literally full of blood and heals remarkably quickly. Usually 6 weeks and you are fully back on your feet. Even with a giant crack through it. Now take a tendon just near a bone, attached to a muscle, for example. Do a little bit of fraying of this and you could easily be facing 12 weeks to six months of healing. Transect it like many bone breaks do and without surgery the tendon may never repair.

Another tissue type lots of people might not consider is the epidermis. There are no blood vessels here and again distant diffusion is needed. Of course the very outer layer of the epidermis called the stratum corneum is made up of keratinocytes which are a little bit like waterproof bricks. They are made and layered from the bottom and shed from the surface. Ablative armouring for the body. By the time the cells are fully fledged keratin armour plates they really have very little if oxygen need.

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