We are equipped with maxi-shunts and mini-shunts throughout our bodies. Every skeletal muscle has shunts in the interfascicular spaces – between the fascicles, as well as in the interfibril spaces – between the fibrils.

The arterial network, the venous network, and the pulmonary network make up the circulatory system. Gray’s Anatomy refers to the venous network as the “venous system”, but a system is a complex. It is properly called a venous network, and a circulatory system. The aorta and vena cava are in the body cavity, as is the lymph cistern. The lymphatic system is actually a dual system; it includes the lymphatic duct network, which is a part of the circulatory system, plus the lymph nodes, which are a part of the immune system. People tend to mix them up, not understanding that they’re two different systems operating in the same unit. 

Large arteries and 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 small arteries and veins. At this same level are the capillary twigs, muscle spindles, and maxi-shunts. Then inside the fascicles, between individual muscle cells, mini-shunts pass between arterioles and venules. Because the heart is a pump, it requires a certain amount of blood flowing into the vena cava in order to maintain its “suck”, so it won’t lose its prime. Having a shunt system in place is the only way the heart can keep the amount of blood it requires returning to it during sleep.

Our skeletal muscles don’t require much blood when they’re not active, so when we are asleep, we need very little circulation in the muscles. In fact, most of the capillaries in skeletal muscles close down during sleep, allowing two-thirds of the arterial blood to shunt straight across to the venal blood without going through the capillaries. Two-thirds of the circulating blood passes through these maxi shunts and mini-shunts, while only one-third actually goes through capillaries. The maxi shunts connect the arteries directly to the veins, and mini-shunts, also called “thoroughfare channels”, connect the arterioles directly to the venules. Both bypass the capillaries and in this way, the blood bypasses the tissues.

Of course there is no pumping of the muscles to get the blood back up when we’re asleep, which is another reason why the shunts are very important; they are a low-resistance way to return the blood.  Again, two-thirds of the blood comes back through the shunts when sleeping; 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 as it is at basal.  Yet the oxygen uptake is sixty times as great at maximum, because then all of the blood is going through the capillaries, while at basal metabolism, two-thirds of the blood is by-passing the 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 to the veins without going through the capillaries, where internal respiration occurs. Therefore, because most of our blood returns to the heart already oxygenated, we don’t need to breathe as much when we’re asleep. This system also helps reduce radical free ions which would result from unneeded oxygen flowing through inactive tissue.

In addition to reducing blood flow in skeletal muscles when we are asleep, the mini-shunts serve an another function, one similar to that of an air bladder in a standing pipe behind a water spigot. If water running out of a spigot is suddenly shut off, the inertia of the water in motion can blow the spigot right off the wall without captured air and the stand-pipe.

When we move into action, our blood flow increases and the capillaries open; blood already in motion is diverted from the shunts to the capillary. Then, if the activity level in that tissue suddenly shuts down, the capillary also shuts suddenly (capillaries are equipped with very tiny sphincter muscles which are either contracted or not contracted. Muscle contraction is all or none, and therefore, all sphincter muscles are open or closed). Without these alternate thoroughfare channel routes, the blood would slam against capillary sphincters, breaking the capillaries. Instead, our bodies divert the fluid flow into mini-shunts (thoroughfare channels).

One last thing on the circulation: although you’ve probably heard about only 1 tibial vein, there are several – 1 very large one and 6 to 8 smaller, running directly through the soleus muscle in the calf.  Although they are rarely depicted in drawings they are nevertheless present within this muscle. Normally the main arteries and veins are located in the septum between the muscles, with only very small arterioles and venules lying between the fascicles within a muscle. These full-blown tibial veins lying between the fascicles within the soleus are the only exceptions.

Because of this, the soleus acts as an auxiliary heart pump during walking; it is actually referred to as a second heart. Therefore I want to reiterate the importance of the soleus muscle, and stress that all therapists must always take special care with this muscle. This means both treating it for any hypertonus, as well as handling it very gently and consciously. Think of it as a very delicate area, and never allow anyone to massage your soleus deeply or along the belly of the muscle.

(As an interesting aside, liver metabolism also changes when we’re asleep. We produce prostaglandins in the morning, which slightly elevate our muscle tension for the daytime activity. While sleeping, the liver gets rid of the day’s prostaglandins, and we produce more when we arise again. 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.)