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Krail t1_j9hlwx4 wrote

Reply to comment by Georgie___Best in when a limb gets amputated, how do they stop the flow of blood? by EnchantedCatto

This is making me realize a question I didn't know I had about the circulatory system.

How do the "delivery" vessels connect back to the "return to the heart" vessels? Does blood come out from the capillaries and then get taken up by other capillaries?

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dcs1289 t1_j9huya2 wrote

The blood cells don't actually leave the capillaries if everything is going fine. The capillary bed is basically a mesh of VERY tiny blood vessels - picture two sets of tree roots with the bottom portion of the roots aiming towards each other and connected - arteries (tree trunk on the left of your mental image) diverge into smaller arterioles which diverge more into capillaries.. which merge back together into venules, which merge into veins (tree trunk on the right).

When the blood is moving through the capillaries, exchange of oxygen and nutrients is able to occur through small pores in the cells (or straight across the cells in some cases). The blood cells don't leave the circulation, but the plasma that they float around in can move back and forth through the tiny gaps.

Here's a good picture to describe the mental image I'm trying to verbalize above.

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Krail t1_j9i6e4e wrote

Ahh, okay.

I think I have one more follow up question, about how the nutrients actually get from blood to the cells. I guess it's... capillaries have pretty broad coverage throughout the whole body, right? What's the furthest a living cell is likely to be from any capillaries?

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dcs1289 t1_j9i74cj wrote

Capillaries are everywhere. It's the end point of the circulatory system everywhere, so there are beds of capillaries in basically any living tissue.

Probably the most well-known tissue with poor/limited blood supply is tendons/ligaments - these are connective tissues with a lot of intracellular matrix made up of collagen, and blood supply to these tissues is poor the further you get from the source blood vessel as there is often very little collateral circulation (what it's called when an area has multiple arteries that feed the capillary beds).

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Georgie___Best t1_j9j3r5b wrote

Capillaries are very small. We are talking of a scale where red blood cells might only fit through single file. At this microscale, diffusion/osmosis are what predominantly facilitates the movement of useful stuff out of the blood and into the extracellular fluid/cells, and waste back into the blood vessels to be taken away.

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seto555 t1_j9l45c2 wrote

To add to the other answers. The process of oxygenation of the cells from the capillaries is called diffusion. Basically the capillary wall is thin enough, that it is leaking Oxygen molecules to the surrounding tissues. The rate of oxygenation depends on partial pressure of oxygen and carbon dioxide (and some other factors) in the surrounding tissues, but usually you can't diffuse farther then a few mm's in a living organism.

Diffusion is mostly used by insects as the main oxygenation process since they are small enough to just have some air filled pipes (tracheas) inside their bodies where the oxygen is directly absorbed from the air.

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