The circulatory system and red blood cells

Related: Immune

Our circulatory system and lymphatic system is so cool! Reading Immune made me revisit parts of it and I realized I had a lot of things wrong.

Weirdly, I did not realize that the circulatory system is closed in vertebrates! That implies, red blood cells never leave the circulatory system even when they are in the capillaries. Only the white blood cells leave it, that too only under specific conditions (when there is an infection and the walls are dilated slightly).

Once I realized that the circulatory system is closed, it became clear that the point of the red blood cell is to keep the plasma enriched with $O_2$. The capillary is narrow enough that the $O_2$ can diffuse to the cells nearby and $C{O}_2$ can diffuse out. The plasma is the one that circulates the $O_2$ and $C{O}_2$.¹

In that sense, the red blood cells are just tiny batteries that keep the local $O_2$-$C{O}_2$ concentration at a desirable value (high on $O_2$ and low on $C{O}_2$). The blood cannot carry nearly enough $O_2$ for all the cells in the body so it just added a ton of batteries that can carry and release $O_2$ to attain the same effect.

I found the realization very cool! For some reason, I thought red blood cells move around the cells and distribute oxygen. And I come from a background where these basic concepts were not taught clearly and my understanding was muddled.

In terms of diffusion, there should be physical limits that set how far away a capillary should be to ensure proper diffusion of $O_2$. I should try to derive that here.

Footnotes

1. This also made me realize why the lymphatic system exists. ↩