Composition of the Microcirculation
The morphofunctional complex of the microcirculation combines lymphatic capillaries and four types of blood vessels:
- Arterioles (afferent/inflow limb);
- Capillaries (exchange limb);
- Venules (efferent/drainage limb);
- Arteriovenous anastomoses (AVAs) (shunting limb).
A key feature of all listed structures is the complete absence of vasa vasorum in their walls. The tissue of the vessel wall receives nutrients directly from the blood flowing through the lumen.
Histological Structure of Arterioles
Arterioles retain the classic three-layered structure typical of arteries, although all tunics are extremely thin.
- Tunica intima
Composed of an endothelial layer resting on a basement membrane, and an internal elastic membrane (very thin and discontinuous). The ultrastructure of endothelial cells is characterized by numerous pinocytotic vesicles and mitochondria in the cytoplasm.
- Tunica media
Consists of only 1–2 layers of smooth myocytes. The cells are arranged circularly, allowing them to effectively narrow the vessel lumen upon contraction.
- Tunica externa
Represented by a thin layer of loose connective tissue.
The terminal segments of arterioles immediately preceding the capillary network are called precapillaries (precapillary arterioles). At their branching points into capillaries, smooth muscle sphincters are located.
Functional Features and Regulation
The primary function of arterioles is the regulation of vessel diameter (via constriction or dilation). This alters peripheral vascular resistance. Unlike large arteries, which store kinetic energy from cardiac output, arterioles regulate blood distribution to organs and tissues.
Mechanisms of arteriolar regulation are divided into two types:
- Neural regulation: controlled by the autonomic nervous system. Sympathetic and parasympathetic nerve fibers form synapses directly with smooth myocytes.
- Humoral regulation: response to endocrine signals and bioactive substances (histamine, angiotensin).
Hormonal Regulation of Vascular Tone
Humoral control of arterioles is a complex multi-step process. Hormones and active substances cannot act directly on the muscle layer.
Steps of the humoral response:
- Primary target: circulating substances bind to receptors on the apical surface of endothelial cells.
- Endothelial response: the cell begins producing local mediators.
- Diffusion: mediators cross the endothelial basement membrane and the internal elastic membrane (facilitated by perforations within it).
- Final effect: upon reaching smooth myocytes, mediators stimulate or inhibit their contraction.
Example: Vasodilation during physical exertion During intensive skeletal muscle work, epinephrine is released. It binds to $\alpha_2$-adrenergic receptors on the endothelial surface. In response, the endothelium releases a potent mediator: nitric oxide (NO). NO diffuses to the muscle layer and causes smooth myocytes to relax. As a result, arterioles dilate, increasing blood flow to the working muscle.