Arteriovenous anastomoses (AVAs) are direct vascular connections that allow blood to flow from arterioles directly into venules, bypassing the capillary bed. They are present in almost all organs and act as functional shunts, regulating local hemodynamics.
Main FunctionShunting blood directly from arterioles to venules, bypassing capillaries.
MorphologyCharacterized by a wider lumen, shorter length, and thicker walls compared to capillaries.
VelocityIn true anastomoses, blood flows thousands of times faster than in the capillary bed.
Blood CompositionWhen passing through a true shunt, arterial blood maintains its composition without gas exchange.
Functional Classification: Shunts and Partial Shunts
Based on functional and morphological features, anastomoses are divided into two major groups depending on whether the blood composition changes.
I. True AVAs (Shunts) Characterized by a wide lumen (30 to 500 µm) and a thick wall. Blood passes through them at an extremely high velocity, preventing any significant metabolic exchange with tissues. As a result, pure arterial blood is shunted directly into the venous system.
II. Atypical AVAs (Partial Shunts) These are capillary-type vessels that are wider (up to 30 µm) and shorter than typical capillaries. Blood flow velocity is intermediate. While passing through this short segment, the blood undergoes partial gas exchange and metabolic exchange with the surrounding tissue, resulting in mixed blood entering the venule.
Structure of True Anastomoses
Structurally, true AVAs are divided into vessels without specialized structures and vessels with active regulatory mechanisms.
Simple AVAs: Direct connections without specialized muscular devices. The arteriolar wall transitions abruptly into the venular wall. The lumen is regulated by the smooth myocytes of the arteriole itself.
AVAs with a Sphincteric (Contractile) Device: Contain specialized mechanisms to open and close the lumen. They are divided into two subtypes:
Closing arteriole type. The subendothelial layer features cushions (pads) of longitudinally oriented smooth myocytes. Their contraction completely closes the anastomosis, while relaxation opens it.
Epithelioid type. These possess a reinforced tunica media. The arterial end contains two layers of myocytes, whereas the venous end contains specialized modified oval-shaped myocytes known as E-cells (epithelioid cells). These anastomoses can be simple or complex (glomera). Complex forms consist of an entire group of anastomoses surrounded by a single connective tissue capsule.
Summary Nomenclature
There are 5 specific types of arteriovenous anastomoses:
Simple AVAs.
Closing arteriole-type AVAs.
Simple epithelioid-type AVAs.
Complex (glomus) epithelioid-type AVAs.
Atypical AVAs.
Mnemonic
Remember the increasing structural complexity of true shunts: from "Simple" (abrupt transition, no special mechanisms) to "Cushions" (closing arterioles), and finally to modified "E-cells" (epithelioid type).
Frequently asked questions
In which body regions and organs are complex (glomus) epithelioid-type anastomoses most numerous?
While the exact regional distribution is broad, the following facts are established:
Epithelioid-type AVAs can be simple or complex (glomus);
Glomus-type arteriovenous anastomoses are well-described in the skin as neuromyarterial receptors involved in thermoregulation;
Glomus tumors arising from these anastomoses are predominantly located in the distal phalanges of the fingers and toes, particularly in the subungual space.
What specific functions do arteriovenous anastomoses perform besides regulating local hemodynamics?
In addition to local blood flow regulation, AVAs perform several systemic and specialized functions, including:
Regulation of blood volume and systemic blood pressure;
Prevention of sudden pulmonary trunk pressure spikes via shunting;
Enhancement of venous return to the right side of the heart;
Arterialization of venous blood;
Mobilization of pooled blood reservoirs;
Regulation of tissue fluid drainage into the venous system;
Thermoregulation (characteristic of glomus-type anastomoses).
Does blood composition change when passing through a true anastomosis?
No. Due to the extremely high blood flow velocity, metabolic exchange does not occur, and pure arterial blood enters the venule.
What are atypical AVAs and how do they work?
They are capillary-type partial shunts (up to 30 µm). Flow velocity is lower, allowing for partial metabolic exchange, which results in mixed blood entering the venous system.
How do closing arteriole-type AVAs function?
They feature cushions of longitudinal myocytes in the subendothelial layer. Contraction of these cushions closes the lumen of the anastomosis, while relaxation opens it.
Go deeper
Histological features of modified E-cells (epithelioid myocytes).
Structure of the connective tissue capsule in complex glomus AVAs.
Comparison of hemodynamic parameters between shunts and capillaries.
Mechanisms regulating the muscular layer in simple AVAs.