On the research of plasma circulation
There are all manner of speculation on how the flow of blood through capillaries is regulated and how flow of plasma occurs across capillary walls. The below explanation on plasma flow is a very simple and logical one that does not rely on millions of hypothetical pre-capillary sphinkter valves at the arterial end of capillaries or the magical release of ATP by red blood cells when under shear-stress. In my estimation nature does not resort to complicated ways and always has found the most logical and simplest methods with which to accomplish physical health and the proper functioning of all the cells in all parts of the body.
The movement of plasma back and forth through the capillary walls and also into the lymph system is vital to the delivery of vital supplies for cells, the removal of metabolic waste products, the transportation and distribution of chemical substances manufactured by cells and the delivery of white blood cells to all cell tissues and their function of killing illness causing micro-organisms and the killing and elimination of cancer cells or otherwise damaged or mutated and sick cells. All illness is at the cellular level. When cells get ill then the cell tissue they belong to gets ill. Cells are able to heal themselves if given the right support. There is no cure for illness because cure is
The walls of capillaries are a single layer of so called epithelial cells. These epithelial cells form a smooth capillary wall. These epithelial cells have small gaps between them where the cells join.
Gaps between epithelial cells
The gaps between these cells allow plasma to flow back and forth through the capillary walls. The capillary walls function like a siv that has a mesh size that allows only plasma to flow back and forth through the capillary wall with only the substances diluted in the plasma and items suspended in plasma that are small enough to flow with the plasma through the "mesh size" of the epithelial cellwall "siv". The plasma transports nutrients and oxygen to all the cell tissues in the body and carries away waste products from the cell tissues. The capillaries run through all the cell tissues in the body including vital organs, muscles, brain etc. A slightly different plasma exchange takes place to feed the living cell tissue of cartilage in joints. Discussed seperately on this website.
Capillaries run through cell tissues inside what is called the interstitial space that is filled with interstetial fluid that cells live in and are surrounded by. Interstetial fluid is the same plasma fluid that filters through the capillary walls from the inside of capillaries to the outside of capillaries. This plasma that flows into the interstitial space goes now by the name interstitial fluid. This plasma carries with it nutrients, oxygen and various forms of white blood cells that perform different immune functions in the interstitial space (lymphocytes, monocytes, macrophages). These different white blood cells fulfill different immune functions such as killing virus infected and cancerous cells, clean up cellular debris from dead cells and clean up pathogens (infectious micro organisms).
Replacing billions of cells per day
Cell tissues need supplies (nutrients) and oxygen to produce energy with which they fulfill their functions and they need building materials with which they will replace the 20 billion cells that die from old age every day. New replacement cells are created by the process of cell-division by the process of cell division (mitosis and meiosis). New cells have to be created to replace the 20 billion cells that die every day (give or take a few billion). Different cells in the body have different life spans. Some live hours and some live up to 25 years (some bone cells). Dead cells create debris in the interstitial fluid. Also the cell metabolism creates waste products that are dumped by the cells into the interstitial fluid. The metabolic waste products go back through the capillary wall into the veins and they are transported to the body's waste disposal sites (liver, kidney, colon, skin, lungs). The larger debris from dead cells is too large to go back through the capillary walls and it exits the interstetial space via the lymph system that starts in the interstetial spaces and continnues through lymphnodes wher the larger debris is broken down into smaller pieces that flow up to the region of the neck where the lymp with the smaller debris joins the blood circulation again. So the total circulation of the lymph started as plasma in the blood that filtered through the walls of the capillaries into the interstetial space when it was renamed interstetial fluid and from there 90% of that interstetial fluid goes back through the venous half of the capillary wall into the veinoud
Plasma flow back and forth, but how?
Plasma brings supplies into the interstitial fluid for cell metabolism and to create new cells that are the replacements for the old cells that have died. The cells will absorb these supplies out of the interstitial fluid and create energy to perform their specific functions in the body. The cells in turn dump their metabolic waste products back into the interstitial fluid and the interstetial fluid with the waste products flows through the capillary walls back into the blood stream and is called plasma again. The blood then carries the waste products to the places where the body gets rid of waste products (liver, kidneys, skin etc). So plasma with supplies flows from the capillaries into the interstitial space. Interstitial fluid with waste products and dead cell debris flows at the same time from the interstitial space back into the capillaries. But how is that possible? And what happens to the dead cell debris that is too large in size to fit through the "mesh" of the epithelial "siv" that forms the capillary walls?
There must be a pressure differential
For plasma to start flowing from the inside of the capillaries into the interstitial space there must be first of all blood flow through the capillaries and there must be larger fluid pressure on the inside of the capillaries than in the interstitial space outside the capillaries so that the plasma with all its supplies and white blood cels will be pushed by the higher fluid pressure through the wall of the capillaries into the interstitial space (through the fine mesh siv that forms the wall of the capillaries). How then can the interstitial fluid with all the waste products and dead cell debris flow back into the capillaries at the same time if the fluid pressure in the capillaries is graeter than the fluid pressure in the interstitial fluid? Very simple, as we will learn.
Bernoulli with his "Hydrodymamica" to the rescue
We need to understand some basics about fluid dynamics as first published by the Dutch-Swiss mathematician Daniel Bernoulli in 1738 in his book "Hydrodynamica". The here most applicable insight from this book is that when a fluid flows through a hose that the pressure of the fluid in the hose gradually drops with the distant traveled and it drops exponentially when the diameter of the hose is very small. This then explains why a person with a 100 foot long 1/2" diameter garden hose will be very disappointed to have almost no pressure and only a trickle of water coming out of the end of the hose. This is the reason why fire hoses of great length must be of very large diameter and with a small opening at the end so that there is enough pressure and water delivery at the end of the hose.
Back to capillaries
Capillaries have an extremely small flow diameter and that causes a large drop of pressure between the arterial end of the capillary and the venous side of the capillary. Hydrodynamic blood pressure on the arterial side of capillaries is 35 mm HG (35mm of mercury column) and is only 10 mm HG on the end of the capillary, the venous side. It reasonable now to conclude that the pressure of the interstitial fluid in the interstitial space is somewhere between these two values at a more or less constant 22.5 mm HG. It can now safely be concluded that the hydrodinamic pressure in the middle of the capillary will be about this same 22.5 mm HG. That then makes for a higher pressure of the capillary fluid of 12.5 mm HG over the 22.5 mm HG pressure of the interstitial fluid at the beginning of the capillary and gradually declining toward the middle of the capillary to 0 mm HG pressure differential for an average of 6.25 mm HG higher fluid pressure in the capillary than in the interstitial fluid. This higher pressure in the capillary pushes then the plasma with supplies and oxygen and white blood cells through the capillary wals into the interstitial space.Then from the middle of the capillary the capillary fluid pressure gradually decreases from being equal to the 22.5 mm hg of the interstitial fluid to at 10 mm HG being 12.5 mm HG below the fluid pressure of the interstitial fluid. That results in an average 6.25 mm HG lower pressure of the blood pressure over the half of the capillary length at the venous end of the capillary. That higher pressure in the intersteitial fluid pushes the CO2 and waste product laden interstitial fluid back into the capillary.
So what makes capillary diameters increase and decrease?
A greater concentration of CO2 in contact with capillary walls increases the flow diameter of capillaries and a decrease of CO2 near capillary walls decreases the capillary diameter. With the smaller diameters the red blood cells get stuck in the capillary and pile up in "a roll of coins" to form the "cork" that will close the capillary. With the capillary closed the flow of plasma with oxygen to the interstitial space will stop and the flow of interstitial fluid with carbon dioxide back into the capillary will also stop. The cells will continnue to use oxygen and to produce carbon di-oxide CO2. This will continnue until the oxygen is depleted and CO2 in the interstetial fluid is at high concentration. The high concentration of CO2 around the capillary with cause the flow diameter of the capillary to increase and the bloodpressure on the arterial side of the capillary will push the red bloodcell "roll of coins cork" out of the capillary and restore capillary circulation which in turn will restore plasma flow with oxygen into the interstitial space and return flow at the same time of interstitial fluid with carbon dioxide back into the capillary.
That still leaves two questions
When the plasma from the capillary with all the fresh nutrients and oxygen and white blood cells is pushed by the blood pressure into the interstitial space it will mix with the "dirty" interstitial fluid that is laden with CO2 and waste products and debris from dead cells. It seems now unavoidable that much of this new mixture of clean and dirty will be pushed out with the interstitial fluid back into the capillary at the same time as the new fresh plasma is coming in. That seems not very efificien. Is that actually happening? And what happens with the larger dead cell debris that is too large to be pushed by the interstetial fluid back into the capillary?
Yes that is indeed happening. Seems like pouring a half bottle of good wine into a half bottle of bad wine and wind up with a full bottle of bad wine. Slightly different system here. The flow of new fresh plasma and the pushing of dirty mixture interstitial fluid back into the capillary will continnue as long as the concentration of CO2 in the interstetial fluid has not dropped to the level when it is no longer in a concentration strong enough to stretch the flow diameter of the capillary above the 7 micron that will keep the capillary open. That will then keep the plasma flow going untill the CO2 concentration in the interstitial fluid drops to a low level whery the capillary diameter will decrease to where the red blood cells will plug the capillary again.
Obviously a similar amount of fluid that comes into the interstitial space must also leave that spce again. Only about 90% of the volume of new plasma coming into the interstial space will leave again as interstitial fluid back into the capillary. The remaing 10% leaves as lymph through the lymphatic capillaries. The lymphatic capillaries have their beginnings inside the interstitial space. Lumphatic capillaries also have a wall of endothilial cells but thes lymph cell walls have larger openings in them for fluid to flow into with larger debris being able to be pushed into the lymph capillaries. The endothilial walls of lymph capillaries are differen in that they allow fluid only to enter into the lymph capillaries but do not allow any flow into the other direction back into the interstitial space. Instead of adjoining endothilial cells like blood capillary cells, the lymph capillaries have overlapping endothelial cells in which the overlaps are pushed open like a one way swinging door by the fluid pressure of the interstitial fluid that is higher than the pressure in the lymph capillaries. Any flow in the opposite direction is impossible because the one way swinging door will slam shut. The door openings of the one way doors are quite a bit larger than the small openings in blood capillaries that allow plasma to flow i\out of the blood capillaries and interstitial fluid back into the blood capillaries. These so much larger "door openings" will allow the largest of dead cell debris and other unwanted matter such as bacteria and viruses into the lymph capillaries from where they are transported to the so-called lymph nodes. The lymph nodes are little "factories" that break down large pieces of debris and other unwanted matter into smaller pieces that the body can easier dispose of. The lymph system has one way valves that transport the lymph fluid and all foreign matter in one direction through the lymph nodes and finally back into the blood circulation into the veins via the right lymphatic duct and thoracic duct.
There the lymph joins the plasma in the blood again and starts its next journey through the cycle of plasma circulation.