Pathophysiology, Microcirculation

Microcirculation is most important part of general hemodynamics. The definition of microcirculation is-  blood flow in micro vessels with a diameter less than 100 micrometers.

The quantitative border is not the most essential. Most essential is that microcirculation is tightly related to fluid exchange between the blood and tissues, the microcirculatory dynamics includes microcirculation in blood vessels, flow of lymph and extravasal tissue fluid. We need to emphasize that microvasculature represents a kind of frame or if you want, tissue "plumbing and sanitation system". But the tubes of this plumbing and sanitation system are embedded, built into the walls of a house. And microcirculation is closely associated with mesenchymal stroma or connective tissue, skeleton of all organs and tissues.

At the stromal and vascular metrics, the parenchymal cells of an organ settle and maintain their differentiated specialized functions. Without stroma and microcirculatory bed, their life is not possible.

Also, the whole construction is known as a structural functional element of an organ or tissue.

And in various organs and tissues, the names for these units are different. In connective tissue, we call them histions; in  the kidney, it is called nephron; in the liver, it is called hepaton - but the principle structure is similar. Stromal skeleton, microcirculatory blood vessels embedded into that stromal walls and differentiated parenchymal cells living on it. Stroma plays a prior part in tissue defense. All typical pathological processes like inflammation, thrombosis, embolism, allergic reaction, they mechanistically take place within the stromal carcasses or on histions, on that microcirculatory bed. It is like in a bank, when robbers attacked the bank, it is not cashier who will defend the bank. It is not director or some other official. For that purpose, you have special custodians, and they are obliged to be the first in protection against aggression. The same thing in organs in tissues.

If for example some organ is involved in inflammation, it is not the whole organ which responds at once. Primarily, stromal cells will be involved, and primarily, stromal elements, including microcirculatory vessels and their content, will establish these kinds of defense, like inflammation and so on.

<h1>The Task of Microcirculation</h1>

The main task of course is local perfusion at the level of minimal metabolic sufficiency, plus it has several additional tasks to save resources of blood flow for surge, for mobilization. And in addition, in specific organs, microcirculator vessels are used for specific tasks like local transport of bioregulators in portal systems between two capillary beds, this existing pituitary, in the liver, in gonads, in kidney, and in all that areas local portal system with two capillary beds, takes the hormones or other bioregulators in one place and transports them for a short distance to second capillary bed. Also, microcirculation is involved in thermal regulation like vessels of skin and mucus membranes. It is involved in erection like in nipples and corpora cavernosa.

You can see here the distribution of blood between microcirculatory bed of various organs: brain, gut and liver, skeletal muscles, skin, kidney, myocardial, bones. And can  see that the level of blood flow in a resting state is quite different in different organs.

Their record highest level of perfusion is observed in kidney, 360 milliliter per minute per 100 gram of kidney tissue, and in adrenal glands, 300 milliliters. And the next place is occupied by thyroid gland, 160 milliliters per minute. And liver occupies the fourth place with 95. And medical students commonly believe that heart and brain are most essential, and they are traditionally perceived as priority organs, but they are significantly inferior to these leading organs with most generous perfusion because the rate of perfusion in brain is 70 milliliters per minute per 100 grams of brain tissue. And in heart, it is about 50. And very, very sparingly supplied with blood, resting muscle, four milliliters, and bones, three milliliters. And that's minimally enough.

<h2>The overall map of typical microcirculatory unit.</h2>

Here is it with arteriole, metarteriole, preferential channel, capillary bed, and venous portion. Commonly in many organs, we have one supplying arteriole and two venules, in every microcirculatory unit, but there are exclusions. For example, in the penis. For better establishment of erection, the construction of microcirculatory units is opposite to arterioles, a supply in-flow, and only one venula in every unit is responsible for out-flow, to make it easy to create local venous congestion.

Pathophysiology, Microcirculation

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