Human Anatomy: Part 2 - K. A. Dyubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2008

Specialized Part
Cardiovascular System (systema cardiovasculare) — General Overview
Arteries — Concept of the Microcirculatory Bed (MCB) — Components of the MCB

Arterioles, arteriolae, are Blood Vessels with a diameter of 50–100 μm that branch off from Arteries and gradually transition into precapillary arterioles (Fig. 3). They constitute the initial link of the microcirculatory bed and function as resistance vessels. The wall of the arterioles contains 1–2 layers of closely packed smooth myocytes with a spiral orientation. The inner surface of the arterioles is formed by the endothelium, which rests on the basement membrane with a thin subendothelial layer and a thin internal elastic membrane.

Precapillaries, or precapillary arterioles, arteriolae praecapillares, are blood vessels with a diameter of 14–16 μm formed by the branching of arterioles. Smooth Muscle Cells in precapillary arterioles are located individually.

A characteristic morphological feature of arterioles is the absence of elastic elements in their wall, while the muscle cells spiraling around the endothelial tube are positioned at a more or less significant distance from one another.

At the sites where precapillaries branch off from arterioles and where precapillary arterioles divide into capillaries, smooth muscle cells form distinct clusters known as precapillary sphincters (see Fig. 3). Precapillary sphincters regulate blood flow into the exchange segments of the microcirculatory bed.

The Significance of muscular elements in the walls of blood microvessels was first highlighted by I. M. Sechenov* (1907): "Muscles act like Valves, letting blood into the capillaries when necessary."

Blood capillaries (hemocapillaries), vasa hemocapillarae, are the main structural unit of the microcirculatory bed, as METABOLISM and gas exchange take place precisely at their level. These are thin-walled vessels ranging from 3–5 to 30–40 μm in diameter, through which blood passes from the arteriolar to the venular segment. Capillaries form capillary networks, vascular glomeruli, and loops. The Structure of the capillary bed depends on the morphofunctional Organization OF THE tissue or organ. According to A. August Krogh** (1927), a view supported by many modern researchers, capillaries are the most vital link in The Vascular System.

*Ivan Mikhailovich Sechenov (1829–1905) was a founder of the physiological school and materialist psychology in Russia, and a member of the St. Petersburg Academy of Sciences. He worked at Novorossiysk, St. Petersburg, and Moscow universities.

**August Krogh (1874–1949) was a Danish physiologist and Nobel laureate (1920). August Krogh was the first to establish the anatomical and PHYSIOLOGICAL CHARACTERISTICS OF the Capillary Wall in various Organs.

Class="center">

Fig. 4. Structural Features of the hemocapillary wall (according to V. G. Eliseev):

I – hemocapillary with a continuous endothelial Cell and basement membrane. II – hemocapillary with fenestrated endothelium and a continuous basement membrane. III – sinusoidal hemocapillary with slit-like openings in the endothelium and basement membrane; 1 – endothelial cell, 2 – basement membrane, 3 – fenestrae, 4 – slits, 5 – pericyte, 6 – adventitial cell, 7 – pericyte contact, 8 – nerve fiber

The wall of blood capillaries is formed by a single layer of endothelial cells, with a membrane containing pericytes located on the outer surface of the wall. The Cytoplasm of some endotheliocytes contains pores, or fenestrae, which facilitate the passage of macromolecular substances through the capillary wall. The basement layer of capillaries can be continuous, discontinuous, or entirely absent.

According to the majority of domestic histologists, the endothelium is a single-layered epithelium of mesenchymal origin that lines blood vessels and The Heart. The basement membrane has a microfibrillar structure comprising an amorphous substance and fine Collagen fibers. The capillary adventitia lies outside the basement membrane and contains poorly differentiated cells, an amorphous substance, and collagen fibers.

Depending on the structural Features of the wall, the following types of capillaries are distinguished (Fig. 4):

1. Somatic capillaries are characterized by a continuous endothelium and a continuous basement membrane, and are found in striated muscles, Lungs, nerves, mucous membranes, and Skin.

2. Capillaries with fenestrated endothelium and a continuous basement membrane are found in the villi of the Small Intestine, renal glomeruli, digestive glands, and Endocrine glands.

3. Sinusoidal capillaries have slit-like openings in the endothelium and a discontinuous basement membrane. They are located in the Liver, Spleen, and pancreatic islets. Individual capillaries interconnect and transition into postcapillaries or postcapillary venules.

Postcapillaries, or postcapillary venules, venulae postcapillares, are blood vessels formed by the merger of two or more true capillaries. They have a diameter of 15–30 μm, which is larger than that of capillaries. Blood flows from them into small collecting venules. The structure of postcapillary venules resembles that of capillary walls. The number of adventitial cells increases around postcapillary venules. The wall of postcapillaries can expand significantly and exhibits high permeability.

Venules, venulae, are blood vessels arising from the convergence of postcapillary venules (Figs. 2, 3). Their diameter ranges between 30 and 50 μm. The venular wall becomes thicker due to Connective Tissue fibers and cells, and isolated muscle cells appear within it. Together with postcapillaries, venules form the labile segment of the microcirculatory bed with a pronounced capacitance function. Venules transition into small and medium-sized Veins; in some organs, venules and small veins form lacunae (venous dilatations) and venous plexuses, rete venularum.

Thus, postcapillaries and venules serve as the roots of the Venous system. The venous segment of the microcirculatory bed performs a drainage function, removing tissue fluid containing Metabolic waste products from Tissues and organs. Leukocyte migration occurs through the walls of venules, much like in capillaries.