Our Bloodstream: I Never Understood...

Factual Questions
1

Every cell needs the bloodstream to deliver O2 and nutrients while removing waste products, right? Ok, but the bloodstream is a closed loop system. So, this has always bothered me:

Never does the blood dump out to spread like paint to contact every single cell. (Yet, I recall from advanced bio some animals have this ability.) So, then, every single cell would need its own direct line to the bloodstream, but the bloodstream does not spider out in this fashion. Do more distant cells “feed” off of closer cells, like a bucket brigade, on up the chain to those cells closest to a passing blood vessel…via osmosis?

If not, then are nutrients and waste simply dumped in/out of the bloodstream at random with hopes a needy cell will pick up/drop off what it needs to survive? How exactly does this work?

2

The short answer is Capillaries which supply the metabolic needs of your body’s cells.

3

According to this article:

Therapeutic Vasculogenesis,

the capillaries must normally reach within 0.2 mm of a cell in order for diffusion to enable enough Oxygen transport to prevent cell death (with some exceptions, such as epidermal cells). Thus, the cell does not have to be directly in contact with a capillary, but it has to be close.

4

Capillaries have holes, called pores, in 'em (which can be made bigger or smaller by chemicals in your body I won’t go into here.) So the oxygen gets glommed onto a hemoglobin molecule in the red blood cell, and it goes for a ride through your heart and then up your aorta and out to the smaller vessels in the circulatory system.

When it gets to a place where there’s low oxygen (so it’s not like a bucket brigade, but rather nearer places to the heart get served their oxygen first), the oxygen detaches from the hemoglobin, moves out of the red blood cell (through pores in the cell wall), and through one of the holes in the capillary wall.

It doesn’t go directly into the cell from the capillary, but into a space between the capillaries and the cell. This space is called “interstitial space”. From there, it goes into a cell that’s low on oxygen.

Meanwhile, when carbon dioxide builds up in a cell, some of it leaves the cell and goes into the interstitial space, and then when the blood in the capillary has less carbon dioxide than the interstitial space next to it, the carbon dioxide moves into the blood through holes in the capillary, where it’s taken back to the heart, then to the lungs, and the carbon dioxide again moves into the interstitial space of the lungs, then into the alveoli of the lungs, then you can exhale it.

Ditto for all the other nutrients and wastes of the cells. Nutrients move from the digestive tract into the interstitial space, into the capillaries, into different interstitial space and then into cells. Wastes go from cells to interstitial space to capillaries to interstitial space to bladder or bowel. It all has to do with diffusion. Where a concentration is higher on one side of a membrane than the other, the substance will pass through a hole in the membrane.

Interstitial space isn’t generally taught until A&P II, but it makes everything you’ve been learning in chemistry and biology until then suddenly make sense. It’s what makes diffusion (and osmosis) actually happen, because it provides for different concentration gradients in a very close proximity.

5

I think your explanation is correct other than the part I bolded. AFAIK oxygen is a small non-polar molecule which diffuses freely through the capillary endothelium and does not need to pass through capillary fenestrations to reach the interstitial space.

6

Hmmm…you could be right. I’d have to review my notes, and I’m not sure which cardboard box they’re in!

If that’s so, I wonder why peripheral vascular disease (in which the capillaries constrict) leads to deoxygenation in the peripheral tissues. Is it just because constriction decreases the blood flow through the capillaries? I thought it actually restricted the oxygen from getting out of the capillaries.

7

Those interstitial spaces are wet. The cells are in a continual bath. The liquid comes from the blood, and it leaks out through the capillaries to facilitate the movement of the goods between blood vessels and cells. The leaked out stuff then is recycled through the lymphatic system and eventually gets added back into circulation.
~VOW

8

Maybe it’s the CO2 going the other way that limits things. CO2 is polar, right? I think the O’s are not opposite from each other, but at a wide angle instead.

On another subject, about the bloodstream in general, Isaac Asimov wrote an absolutely fantastic book about this. IIRC it’s just called “The Bloodstream”, but it’s been a while.

The point is, this is actually what Asimov was trained in, and was his profession before he became a spectacularly prolific (and sometimes inaccurate) writer on seemingly all topics. I’ve probably read 40 or 50 of his books (yeah, I went through an Asimov phase), and all nonfiction (I hear he wrote science fiction too, that is, stuff he acknowledged as fiction). Of all these books, many of them on topics I had special interest in, the one on the bloodstream was by far the best. Just stellar.

9

FTR, CO[sub]2[/sub] is nonpolar. It’s O=C=O, with no net dipole.

10

Capillaries do not have smooth muscle and therefore are not contractile in the same way that arteries or even veins are. My understanding is that peripheral vascular disease usually refers to atherosclerosis and/or thromboemboli in the arteries supplying the limb, so the issue is of not getting blood to the capillaries.

As a side note, the ability of the blood to “collect” and carry CO2 is limited compared to the amount that is produced; therefore, where CO2 concentrations are high, the ubiquitous enzyme carbonic anhydrase converts CO2 to carbonic acid, H2CO3, which dissociates to bicarbonate (HCO3-) and protons. In the lungs, where CO2 concentrations are low (because it diffuses into the alveoli and is exhaled), the reaction runs backward.

As noted above, O2 and CO2 diffuse through cell membranes.