[scientizscht (OP)]

Hello

Red blood cells have 6-8μm diameter. Other immune cells in blood are larger.

The capillaries which is the end of the arterial circulation, have diameter of 4μm.

Why don't the cells in blood block capillaries or they don't just gather at the smallest point of the circulation since the blood continually flows towards smaller and smaller diameter vessels?

I think red cells are flexible and they get squeezed to pass through capillaries but what about neutrophils who are triple their size?

[chris]

Cell membranes are "fluid" - hence Davson and Danielli "fluid mosaic model" of the plasma membrane - meaning that, unconstrained by the cell wall that is a feature of a plant cell, animal cells can deform and change shape when external forces are applied to them. Consequently, the cells "squeeze and ease" their way through capillary beds, contorting and flexing themselves as they go, a bit like a potholer negotiating tight squeezes underground

This arrangement is actually intentional: by restricting the diameter of the vessel, the distance for diffusion - and hence gaseous exchange - is reduced, which improves the rate and efficiency of this process.

For immune cells, including neutrophils, it is also beneficial. Their job is to track down and migrate to sites of inflammation; the narrow confines of the capillary encourage interaction with the vessel wall, enabling the engagement of receptors that promote diapedesis - the movement of cells from the lumen into the extra-vascular space.

Usually, the numbers of polymorph (neutrophil) cells is quite low in circulation relative to the red cells, so traffic jams don't usually form. And when cell numbers through a tissue do begin to rise - as in during inflammation and infection - the risk of congestion is reduced by causing vasodilatation: opening up the vascular bed keeps blood flow slow but patent, but crucially enables far more white cells to pass through to "inspect" the area without a blockage.

That said, cells can clog vascular beds, and people with deformed red blood cells caused by spherocytosis and sickle cell disease know the consequences of this only too well...

Addendum: Immunologist Claire Bryant discussed some of these points in a recent interview on the Naked Scientists about how fever also helps you fight infection.

[scientizscht (OP)]

Thanks for the insight.

I understand that cells are flexible but I assume changing their shape to pass through a tiny tube will create some tension given that their perimeter is not flexible I suppose. It's interesting how this tension on the walls is surpassed by the hydrostatic pressure in the vessel and that there is no damage etc. Just some thoughts.

[evan_au]

cells are flexible but ... their perimeter is not flexible

Red blood cells are shaped like a cross between a dish and a child's life preserver pool toy.

When they enter a capillary, they bend almost double, which dramatically changes their perimeter.

As Chris mentioned, sickle cell disease changes this flexible folding behavior, and causes severe circulatory problems.
See: https://en.wikipedia.org/wiki/Sickle_cell_disease

[Bored chemist]

The simple answer is that tehy get bent out of shape

The problem for anyone trying to video it is that red blood cells are pretty near the limit of what you can see with visible light. If they were 10 times smaller then they would just be dots.

[chris]

Lovely footage to watch! Thats @Bored chemist

[scientizscht (OP)]

Yeah nice video, it's strange that at some point the flow goes backwards.

[chris]

Yeah nice video, it's strange that at some point the flow goes backwards.

Not really; if a tissue through which the blood is passing "moves" - such as a muscle contracting - then the pressure gradient in the vessel, which is driving the flow from arterial end towards venous end, may temporarily reverse; there are no valves in capillaries, which are also densely interconnected, so the blood feeling a temporarily higher pressure in one location will reverse its flow along a collateral route, wherever one exists.