We are equipped with maxi-shunts and mini-shunts throughout our bodies. Every skeletal muscle has shunts in the interfascicular spaces between the fascicles.
A system is a complex. The arterial network, the venous network, and the pulmonary network make up the circulatory system. The aorta and vena cava are located in the body cavity, as is the lymph cistern. The lymphatic system is actually a dual system which includes the lymphatic duct network (part of the circulatory system), and the lymph nodes (part of the immune system). They are actually two different systems operating in the same unit.
Arteries and small veins are found in the inter-epimysial, or inter-muscular level (remember the epimysium is the outer covering of the muscle, so ‘between epimysium’ also means ‘between individual muscles’). The next level down, i.e. at the ‘inter-fascicular’ level, or between perimysial spaces, the muscle is actually penetrated by the venules and arterioles. At this same level are the shunts.
Because skeletal muscles don’t require much blood when they’re not active, they need very little circulation when we are sleeping and most of the capillaries in the muscles close down. A full two-thirds of the arterial blood is shunted straight across to the venal blood without going through the capillaries. Maxi shunts connect to arteries to veins and mini-shunts (also called “thoroughfare channels”) connect arterioles to venules, bypassing capillaries. In this way, the blood bypasses the tissues, leaving most of the blood in the circulatory system.
The heart is a pump requiring a certain amount of blood flowing into the vena cava in order not to lose its prime. Because the muscles are not pumping blood back up the veins during sleep, the shunt system is a low-resistance method to keep the required amount of blood consistently returning to the heart to keep its ‘suck’.
Again, during sleep two-thirds of the circulating blood passes through maxi and mini-shunts, while only one-third actually goes through capillaries; therefore, when comparing the amount of blood flowing through the muscles of a person at maximum muscle use with that of a sleeping person, the blood flow is only twenty times as great at maximum than at basal. Yet the oxygen uptake is sixty times as great at maximum, because then all of the blood is going through the capillaries. At basal metabolism, two-thirds of the blood is by-passing your capillaries; twenty times the blood flow equals sixty times the oxygenation, creating that three-times factor. Although mini-shunts are much smaller than maxi shunts, there are many more of them in number; therefore, most of that two-thirds bypass blood actually uses the mini-shunts.
Remember, this blood is going back oxygenated, even though it hasn’t traveled to the lungs; it’s oxygenated because it’s been shunted directly from the arteries into the veins without going through the capillaries, where internal respiration occurs. And because most of our blood returns to the heart already oxygenated, we don’t need to breathe that much while we’re asleep.
We know the body does most of its repair work during sleep; how can those repairs occur without capillary flow? The answer is that when we are sleeping our muscles are not competing for the blood; the only work they’re doing is to maintain minimal muscle tone. Skeletal muscles make up to 50% of our body weight and when they’re out of the picture, 40% of the capillaries don’t need to function, again closing in on the one-third factor of blood passing through the capillaries during basal.
Liver metabolism also changes when we’re asleep. We produce prostaglandins in the morning, which slightly elevate our muscle tension for the daytime activity. (If we miss a night’s sleep, we tend to retain the previous day’s prostaglandins and add them to those produced the following day, which explains why the longer we go without sleep, the more tense we become.) During sleep the liver gets rid of the day’s accumulation of prostaglandins, so needs more blood at night for this additional function, provided by the shunt system.
All sphincter muscles, including the capillary sphincter muscles, are either open or closed, and capillaries slam shut when not needed. When the body moves into action, the capillaries are open and blood is flowing through them; but if the activity level in that tissue suddenly shuts down, the capillary also shuts suddenly. Here mini-shunts serve the additional function of providing alternate thoroughfare channel routes for the blood to travel, keeping the blood from slamming against the capillary sphincter and breaking the capillaries.
This design is very much like a water spigot with a standing tube and air bladder; if water is running out of a spigot and then is suddenly shut off, the inertia of the water in motion would blow the spigot right off the wall if there weren’t that stand-pipe with captured air. The mini-shunts are similar, but instead of using an air pocket, the fluid flow is diverted into the mini-shunt to avoid breaking the capillaries every time one shuts. The very tiny capillary sphincter muscles are most particularly either off or on, preventing the formation of radical free ions which would occur if they remained open all the time, allowing oxygen to flow whether needed or not.