Composition of the microcirculatory bed
The morphofunctional complex of the microcirculatory bed integrates lymphatic capillaries and four types of blood vessels:
- Arterioles (afferent link);
- Capillaries (exchange link);
- Venules (efferent link);
- Arteriovenous anastomoses (AVAs) (shunting link).
A key feature of all listed structures is the complete absence of vessels within their walls (vasa vasorum). The nutrition of the wall tissues is carried out directly by the blood flowing in the lumen.
Histological structure of arterioles
Arterioles retain the classic three-layered structure characteristic of arteries, although all tunics are extremely weakly expressed.
- Inner tunic (t. intima)
Formed by a layer of endothelium resting on the basement membrane and an internal elastic membrane (very thin and discontinuous). The ultrastructure of endothelial cells is distinguished by a large number of pinocytotic vesicles and mitochondria in the cytoplasm.
- Middle tunic (t. 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.
- Outer tunic (t. externa)
Represented by a thin layer of loose fibrous connective tissue.
The terminal segments of arterioles immediately preceding the transition into the capillary network are called precapillaries (precapillary arterioles). Smooth muscle sphincters are located at their branching sites into capillaries.
Functional features and regulation
The main function of arterioles is to regulate the vessel lumen (via constriction or dilation). This allows changing peripheral resistance to blood flow. Unlike large arteries, which maintain the kinetic energy of blood, arterioles regulate its distribution among organs and tissues.
The regulatory mechanisms of arterioles are divided into two types:
- Nervous 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 biologically active substances (histamine, angiotensin).
Hormonal regulation of vascular tone
Humoral control of arterioles is a complex multi-step process. Hormones and active substances cannot affect the muscle layer directly.
Stages 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 pass through the endothelial basement membrane and the internal elastic membrane (facilitated by perforations in the latter).
- Final effect: upon reaching smooth myocytes, mediators stimulate or inhibit their contraction.
Example: vasodilation during physical exertion During intensive work of skeletal muscles, adrenaline is released. It binds to $\alpha_2$-adrenergic receptors on the surface of endothelial cells. In response, the endothelium releases a potent mediator — nitric oxide (NO). NO diffuses to the muscle layer and causes relaxation of smooth myocytes. As a result, arterioles dilate, increasing blood supply to the working muscle.
Briefly: answers to topical questions
What types of capillaries exist according to the structure of the endothelium and basement membrane?
Based on the structure of the endothelium and basement membrane, three types of capillaries are distinguished, differing in permeability.
- Somatic (regular) — have a continuous endothelial lining and a continuous basement membrane.
- Fenestrated (visceral) — characterized by the presence of local thinnings (fenestrae) in the cytoplasm of endothelial cells.
- Perforated (sinusoidal) — differ by the presence of large transmural openings (gaps) in both the endothelium and the basement membrane.
What is the histological structure of arteriovenous anastomoses (AVAs)?
The histological structure of arteriovenous anastomoses depends on their type.
- True AVAs — have a thick wall and a wide lumen (30–500 µm).
- Simple AVAs — the wall structure of the arteriole abruptly transitions into the wall of the venule without special muscular devices.
- Closing arteriole-type AVAs — cushions of longitudinally arranged myocytes are present in the subendothelial layer.
- Epithelioid-type AVAs — distinguished by an enhanced middle tunic, where two layers of myocytes are located at the arterial end, and oval epithelioid cells (E-cells) are located at the venous end.
- Atypical AVAs — represent short capillary-type vessels with a lumen of up to 30 µm.
For quick memorization of the composition of the vascular part of the microcirculatory bed, use the acronym ACAM: Arterioles, Capillaries, Anastomoses (AVAs), Venules.
What else to study on this topic
- Ultrastructure of endothelial cells (the role of mitochondria and pinocytotic vesicles)
- Histology and function of precapillary sphincters
- Mechanism of nitric oxide (NO) diffusion through the vascular wall
- Autonomic innervation of smooth myocytes (sympathetic and parasympathetic synapses)
- Differences in the structure of the internal elastic membrane of arteries and arterioles
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