To this day one question has remained unanswered:
How does venous blood find its way back to the heart when there is theoretically no muscle activity?
In case of paraplegia, coma or during sleep, muscle activity is supposedly almost nil and the force of aspiration of the right auricle is negligible.
Yet the blood does go back to the heart…
In a flaccid paraplegic (severed nerves) maintained in a standing position, how can the blood going through the feet's capillaries go back up to the groin?
Bearing in mind that, when the blood exits the capillary, the residual venous pressure is very weak, around 1,5 cm Hg and that the length of the limb is close to 100 cm.
Therefore the solution must lie between the capillary and the root of the thigh.
We are well aware of the forces at work during the muscles' active participation
They pump out the blood either in an intrinsic or extrinsic way:
-intrinsic way
The muscle contracts pumping out the blood contained in its veinules. That can be observed when the practitioner compresses the calf during a Doppler test.
-extrinsic way
When in action, the muscle compresses nearby liquid mass, particularly the thoracic diaphragm which, by pressing on the abdomen, facilitates the drainage of the organs underneath: liver, intestines, etc.
Other explanations have been given, more or less successfully. We have commented on them in the article "Venous Circulation, Circulation of fluids, venous blood, L.C.R., lymph", which you can read on this site.
The PMM is the muscle activity when the muscle is theoretically at rest.
Any osteopath can feel it under their hand when it rests on a muscle.
The latter swells and deflates periodically. Indeed, the muscle's volume increases along the three planes of space, and decreases likewise during slight contractions. We have established a protocol of simple palpation to guide those whishing to perceive those rhythms.
This increase in volume during a passive phase is the result of a penetrating element within the muscle. The rhythms are too fast (7 à12 pulsations/minute on average) to allow us to think that it is extracellular liquid; only the blood present in this environment can play this role.
All the elements allowing for such a hypothesis are present in the skeletal muscle :
-the reservoirs : which are the veinules within the muscle
-the engine : which are the slow oxydative fibers
-the control system upon arrival of the blood: the volume of blood arriving in the muscle is controlled by the vasodilatation / vasoconstriction of the arterioles as well as by the presence of glomi which shunt the capillaries. This shows that more blood is brought in than needed at local level, the vasoconstriction of arteries and arterioles regulating the blood volume.
-the captors controlling the muscle's internal pressures : the neuromuscular spindles
-the captors adaptable to external dynamic constraints : the Golgi organs
There are three categories of muscle fibers in the striated muscle
The slow oxydative fibers are best able to ensure "return circulation" when no other type of muscle contraction occurs.
Of course, these contractions are not produced by all muscle fibers. Slow oxydative fibers are best able to play this part, because their contract and they tire slowly. This hypothesis may be strengthened by the fact that the slow oxydative fibers' motoneurons are small and less sensitive at synaptic activity's level to variations in the Na+ exchanges than glycolytic or fast oxidative fibers. But it may also be that these fibers only act sequentially towards each other within the same muscle.
We are presenting here a model for the vascular return system which can function autonomously. Nevertheless the section "Neurological Characteristics of the PMM" shows that control is exerted from the subcortical regions and the PMM can be partially controlled by will .
The PMM has 4 fundamental characteristics :
Adaptability to:
1. local homeostasy
The contraction of the slow oxydative fibers only occurs when the muscle has let its veinules fill up with a certain amount of blood. Lifting or lowering the arm will modify the PMM's rhythm which will go back to a regular pace after some time (a few seconds), in order to balance out the pressures in the muscles.
2. Nearby muscle tensions
The PMM also occurs when the muscle is in a phase of static contraction. When we stand still, the PMM's role is to send the blood back to the heart. The fast oxydative fibers contract isometrically (without leading to movement) to maintain the standing posture. But these isometric contractions cannot take on a vascular function. The latter is played by the slow oxydative fibers while taking into account the internal pressure put on the muscle by the contraction of the previous fibers.
3. Peripheral pressures
When a balloon filled with water is squeezed, internal pressures are altered. The same occurs with a muscle. A muscle empties itself of blood only when the blood's volume has reached a certain level.
4. Local needs
PMM's rhythms increase locally depending on local needs. After running, leg muscles pump faster than arm muscles.
Rhythmicity
There is a series of contractions therefore a certain rhythmicity. The PMM's rhythm ranges from 4 to 15 pulsations per minute, a "standard" pace being between 8 and 12 pulsations per minute. Nonstandard rhythmicities can be due to functional lesions, various pathologies, genetic disorders, certain therapeutic actions or intense physical activity.
Alternation
Two muscle groups are involved in the PMM:
1. The "Inspir"(breathe in) muscle group which the slow oxydative fibers contract during a pulmonary inspiration following an empty-lungs retention.
2. The "Expir" (breathe ou) muscle group which the slow oxydative fibers contract during a pulmonary expiration following a full-lungs retention.
Relation between pulmonary respiration and PMM activity
When the subject is in respiratory apnea, the PMM progressively slows.
There is no perfect parallelism between pulmonary rhythm and PMM's rhythmicity. There is only a pseudo attempt at synchronism.
