Functional classification: shunts and semi-shunts
According to their functional and morphological features, anastomoses are divided into two large groups depending on changes in blood composition.
I. True AVAs (shunts) Characterized by a wide lumen (from 30 to 500 µm) and a thick wall. Blood flows through them at a tremendous speed, so metabolism with tissues has absolutely no time to occur. As a result, pure arterial blood is dumped into the venous bed.
II. Atypical AVAs (semi-shunts) These are capillary-type vessels, but they are wider (up to 30 µm) and shorter than regular capillaries. Blood flow velocity in them is intermediate. Passing through this short segment, blood manages to undergo partial gas exchange and metabolism with the surrounding tissue. Already mixed blood enters the venule.
Structure of true anastomoses
Based on wall structure, true AVAs are divided into vessels without special structures and vessels with active regulatory mechanisms.
- Simple AVAs: Direct connection without special muscular devices. The arteriole wall transitions abruptly into the venule wall. The lumen is regulated by the contraction of smooth myocytes of the arteriole itself.
- AVAs with a closing (contractile) device: Contain a special mechanism for opening and closing the lumen. Divided into two subtypes:
- Closing arteriole type. In the subendothelial layer, there are cushions of longitudinally oriented smooth myocytes. Their contraction completely blocks (closes) the anastomosis, and relaxation opens it.
- Epithelioid type. Have an "enhanced" tunica media. The arterial end has two layers of myocytes, while the venous end contains specific modified oval-shaped myocytes, the so-called E-cells (epithelioid cells). These anastomoses can be simple or complex (glomerular). Complex forms represent a whole group of anastomoses surrounded by a single connective tissue capsule.
Final nomenclature
A total of 5 specific varieties of arteriovenous anastomoses are distinguished:
- Simple AVAs.
- Closing arteriole-type AVAs.
- Simple epithelioid-type AVAs.
- Complex (glomerular) epithelioid-type AVAs.
- Atypical AVAs.
Briefly: answers to topical questions
In which body parts and organs are complex (glomerular) epithelioid-type anastomoses most numerous?
The provided sources do not specify in which body parts and organs complex (glomerular) epithelioid-type anastomoses are most numerous.
Only the following has been confirmed:
- epithelioid-type AVAs can be simple or complex (glomerular);
- glomus-type arteriovenous anastomoses are described in the skin as neuromyoarterial receptors involved in thermothergulation;
- glomus tumor originating from such anastomoses is predominantly localized in the distal phalanges of the fingers and toes and in the subungual space.
What specific functions do arteriovenous anastomoses perform in addition to regulating local hemodynamics?
In addition to influencing local blood flow, arteriovenous anastomoses perform a number of systemic and specialized functions. These include:
- regulation of blood filling and general blood pressure;
- mitigation of a sharp increase in pressure in the pulmonary trunk through shunting;
- stimulation of venous blood flow toward the right heart;
- arterialization of venous blood;
- mobilization of pooled blood;
- regulation of tissue fluid flow into the venous bed;
- thermoregulation (characteristic of glomus-type anastomoses).
The development of the true shunt wall is easy to remember by increasing complexity: from "Simple" (abrupt transition, no mechanisms) to "Cushions" (closing arterioles) and up to modified "E-cells" (epithelioid type).
Class and exam questions
Does blood composition change when passing through a true anastomosis?
No. Due to the huge blood flow velocity, metabolism does not have time to occur, and pure arterial blood enters the venule.
What are atypical AVAs and how do they work?
These are capillary-type semi-shunts (up to 30 µm). The flow velocity in them is lower, so partial metabolism is possible, resulting in mixed blood being dumped into the venous bed.
How do closing arteriole-type AVAs function?
They feature cushions of longitudinal myocytes in the subendothelial layer. By contracting, these cushions block the lumen of the anastomosis, and upon relaxation, they open it.
What else to study on this topic
- Histological features of modified E-cells (epithelioid myocytes).
- Structure of the connective tissue capsule of complex glomerular AVAs.
- Comparison of hemodynamic parameters of shunts and capillaries.
- Mechanism of muscular layer regulation in simple AVAs.
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