Human Anatomy and Physiology (with Age-Related Features of the Child's Body) - Sapin M.R., Sivoglazov V.I. 2002

Splanchnology (The Study of Internal Organs)
Angiology (The Study of the Vascular System)
The Heart and Blood Vessels

The Heart and Blood Vessels form a closed system (Fig. 70, see color insert) through which blood flows due to the rhythmic contractions of The cardiac Muscle. Blood vessels are represented by Arteries, which carry blood away from the heart, Veins, through which blood flows toward the heart, and the microcirculatory bed.

Blood vessels are named after the organ they supply (renal artery, splenic vein), their site of origin from a larger vessel (superior mesenteric artery, inferior mesenteric artery), the bone they lie adjacent to (ulnar artery), their direction (medial circumflex femoral artery), or their depth (superficial or deep artery). Many small arteries are called branches, and small veins are called tributaries.

Depending on their branching area, arteries are divided into parietal, which supply the body walls, and visceral, which supply the Internal Organs. Before entering an organ, an artery is called extra-organ; once inside, it is called intra-organ. Branching within the organ, the artery supplies its parts and structural elements with its branches.

Each artery branches into smaller vessels. In the main-trunk (magistral) type of branching, lateral branches gradually depart from the main trunk—the magistral artery. The diameter of the magistral artery decreases As a result. In the arborizing (dendritic) type of branching, the artery divides into two or more smaller branches immediately after branching off from a larger vessel, resembling the crown of a tree.

The walls of arteries consist of three tunics: internal, middle, and external (Fig. 71). The internal tunic is formed by a layer of endothelial Cells, a subendothelial layer, and an internal elastic membrane.

Endothelial cells line the lumen of the vessel. The subendothelial layer consists of thin elastic and Collagen fibers and poorly differentiated Connective Tissue cells. Externally lies the internal elastic membrane. The middle tunic of the artery consists of spirally arranged myocytes, between which are a small number of collagen and elastic fibers, and an external elastic membrane. The external tunic consists of loose Fibrous connective tissue containing elastic and collagen fibers.

Class="center">

Fig. 71. Diagram of The Structure of the walls of a medium-caliber muscular artery (A) and vein (B):

1 — endothelium, 2 — basement membrane, 3 — subendothelial layer, 4 — internal elastic membrane, 5 — myocytes, 6 — elastic fibers, 7 — collagen fibers, 8 — external elastic membrane, 9 — loose fibrous connective tissue, 10 — blood vessels

Based on the structure of their walls, arteries are subdivided into muscular, mixed (musculoelastic), and elastic types. In the walls of small-diameter muscular arteries, the middle muscular tunic is well developed. The contractions of myocytes in the middle tunic of muscular arteries regulate blood flow to organs and Tissues. As the diameter of the arteries decreases, all layers of their walls become thinner.

The thinnest muscular arteries—arterioles—have a diameter of 30—50 µm and transition into capillaries. Arterioles regulate blood flow into the capillary system.

Mixed-type arteries include large-diameter blood vessels such as the carotid and subclavian arteries, whose middle tunic contains approximately equal amounts of elastic fibers and myocytes. The internal elastic membrane in these arteries is thick and strong.

Elastic arteries include the aorta and the pulmonary trunk, into which blood from the heart enters under high pressure and at high velocity. The middle tunic of these vessels is formed by elastic fibers with myocytes interspersed between them. The external tunic is thin.

The microcirculatory bed, which mediates the interaction between blood and tissues, begins with the smallest arterial vessel—the arteriole—and ends with the venule (Fig. 72). The walls of arterioles, which have a diameter of 16—30 µm, contain only a single layer of myocytes. Precapillaries (precapillary arterioles) branch off from the arteriole, with smooth muscle precapillary sphincters regulating blood flow at their origin. Unlike capillaries, the walls of precapillaries contain isolated myocytes overlying the endothelium. Precapillaries continue into true capillaries (diameter 3—11 µm). True capillaries drain into postcapillaries (postcapillary venules). As postcapillaries merge, they form venules, which empty into veins. Within the microcirculatory bed, there are vessels for the direct passage of blood from an arteriole to a venule—arteriolovenular anastomoses, whose walls contain myocytes that regulate BLOOD FLOW IN the microcirculatory bed.

Fig. 72. Microcirculatory bed:

1 — capillary network (capillaries), 2 — postcapillary (postcapillary venule), 3 — arteriolovenular anastomosis, 4 — venule, 5 — arteriole, 6 — precapillary (precapillary arteriole).

Arrows from capillaries — delivery of nutrients to tissues, arrows to capillaries — removal of Metabolic waste products from tissues

True blood capillaries (hemocapillaries) have walls formed by a single layer of flattened endothelial cells (endotheliocytes), a continuous or discontinuous basement membrane, and occasional pericapillary cells (pericytes). Pericytes (Rouget cells) are elongated, multi-processed cells located external to the basement membrane. Each endothelial Cell is in contact with the processes of pericytes. In turn, the axon terminal of a sympathetic neuron approaches each pericyte. The pericyte transmits impulses to the endothelial cell, causing the endothelial cell to swell or lose fluid. This leads to periodic Changes in the capillary lumen.

Depending on the STRUCTURE OF THE endothelium and the basement membrane, Three types of hemocapillaries are distinguished (Fig. 73). These are capillaries with a continuous endothelium and basement membrane. Such capillaries are found in the Skin, skeletal and smooth Muscles, and the Cerebral Cortex. The second type consists of fenestrated capillaries, in which the endothelial cells have numerous rounded fenestrae 60—120 nm in diameter, closed by a thin Diaphragm (membrane), and a continuous basement membrane. These capillaries are located in organs with high rates of secretion or absorption, such as the villi of the Small Intestine, renal glomeruli, and digestive and Endocrine glands. The third type comprises sinusoid (sinusoidal) capillaries, which have a large lumen of up to 25—40 µm. In these capillaries, the endothelial cells have pores. Their basement membrane is partially absent or discontinuous. Such capillaries are located in the Liver, Spleen, Bone Marrow, and glands.

Postcapillaries (postcapillary venules) with a diameter of 8— 30 µm drain into venules with a diameter of 30—50 µm, which gradually merge and increase in size. Venules develop an external tunic formed by collagen fibers and fibroblasts. The middle tunic of larger venules contains 1—2 layers of smooth muscle cells.

Veins. The walls of veins, like those of arteries, consist of three tunics (see Fig. 71). Two Types of veins are distinguished: non-muscular and muscular. In non-muscular veins, a basement membrane lies external to the endothelium, followed by a thin layer of loose fibrous connective tissue. Non-muscular veins include the VEINS OF THE Meninges, retina, bones, spleen, and Placenta.

Muscular veins have a well-defined muscular (middle) tunic formed by circularly arranged bundles of myocytes.

The internal tunic of most medium and some large veins forms pocket-like folds—Valves (Fig. 74). The SUPERIOR VENA CAVA, brachiocephalic, common and internal iliac veins, as well as the Veins of the Heart, Lungs, Brain, and some other organs, do not have valves. The valves are oriented to allow blood flow in only one direction—from organs and tissues to the heart. The valves prevent the backflow of blood.

Fig. 73. Structure of three types of capillaries:

I — blood capillary with continuous endothelial cells and basement membrane, II — blood capillary with fenestrated endothelium and continuous basement membrane, III — sinusoidal capillary with slit-like Pores in the endothelium and discontinuous basement membrane; 1 — endothelial cell, 2 — basement membrane, 3 — pericyte, 4 — pericyte-endothelial contact, 5 — nerve fiber ending, 6 — adventitial cell, 7 — fenestrae, 8 — slits (pores)

Fig. 74. Venous valves. The vein is cut longitudinally and opened:

1 — lumen of the vein, 2 — cusps of venous valves

The total number of veins is greater than that of arteries, and the total capacity of the venous bed exceeds that of the arterial bed. The velocity of blood flow is lower in veins than in arteries.

Veins are classified into superficial (subcutaneous) and deep. In the limbs, deep veins run alongside the arteries. These deep veins typically accompany the arteries in pairs and are therefore called companion veins (venae comitantes). The names of these deep veins correspond to those of the adjacent arteries. Most veins located in Body Cavities, as well as the large veins of the limbs, are unpaired and solitary. These include the axillary, subclavian, popliteal, and femoral veins, as well as the splenic, superior and inferior mesenteric, and other veins. Superficial veins connect with deep veins via perforating veins, which act as venous anastomoses. Adjacent veins are also interconnected by numerous anastomoses, which collectively form venous plexuses. Such plexuses are particularly well-developed On the surface or within the walls of certain internal organs (such as the Urinary Bladder, Esophagus, and rectum) that periodically change in volume, filling and emptying.

Venous anastomoses and venous plexuses serve as pathways for collateral blood flow from organs and tissues, bypassing the main routes.

A distinction is made between intrasystemic venous anastomoses, which connect the tributaries of a single major vein, and intersystemic anastomoses, which connect the tributaries of different major veins (the superior and inferior venae cavae, and the portal vein). Given the Latin names of the superior and inferior venae cavae (vena cava superior, vena cava inferior) and the portal vein (vena portae), the large and highly important intersystemic anastomoses between the tributaries of these veins are referred to as cavocaval and portocaval (cavoportal) anastomoses.

Venous blood drains from organs and body parts into two large venous vessels — the superior and inferior venae cavae, which empty into the right atrium.

The course of arteries and veins and the Blood supply to various organs depend on the structural, functional, and developmental characteristics of these organs. Large arteries are positioned in accordance with the Skeletal structure. For instance, the aorta and the INFERIOR VENA CAVA run along THE Vertebral Column. In the limbs, the number of major arteries and veins corresponds to the number of bones forming their Skeleton. For example, the brachial artery and its companion vein run along the humerus. The radial and ulnar arteries and veins lie along the radius and ulna. In line with bilateral Symmetry and the segmental body plan of humans, most arteries and veins are paired.

Arteries take the shortest route to their respective organs. Consequently, each artery supplies the nearest organ. If an organ migrates during prenatal development, the supplying artery elongates and follows it to its final Location (e.g., the diaphragm, Ovary, or Testis). Arteries are located on the flexor surfaces of the body, which protects these blood vessels from overstretching. To ensure uninterrupted blood flow during flexion, an articular arterial network forms around joints, providing collateral Circulation. The protection of arteries (and veins as well) from compression is provided by the skeletal bones, as well as various grooves and canals formed by bones, muscles, and fasciae.

Arteries enter organs through the hilum, located on their concave medial or internal surface facing the source of blood supply (the aorta or a major artery). The diameter of the arteries and their branching pattern depend on the shape, size, and function of the organ. In tubular organs, arteries branch in a circular or longitudinal fashion. In organs with a fibrous structure (muscles, ligaments, nerves), arteries enter at several points and branch along the course of the fibers that make up these organs.

In the arterial blood supply of organs and body parts, an important role is played by collateral circulation through anastomoses and detour pathways, bypassing the main routes of blood flow.

Age-related features of blood vessels

By the time of birth, blood vessels are well developed, with arteries being more formed than veins. After birth, the length, diameter, cross-sectional area, and wall thickness of the vessels increase. The anatomical relationships between blood vessels and organs, which also grow and increase in volume, undergo changes. The level of origin of arteries from major trunks, the branching angles of arteries, and the levels of venous confluence also change.

The microscopic Structure of Blood vessels changes most intensively during early childhood (from 1 to 3 years of age). During this period, the tunica media develops rapidly within the vessel walls. The final dimensions and shape of blood vessels are established by 14—18 years of age.

Starting from 40—45 years of age, the tunica intima of arteries thickens, the structure of endothelial cells changes, lipid-like substances are deposited within them, atherosclerotic plaques appear, the arterial walls undergo sclerosis, and the vessel lumen narrows. These changes largely depend on a person's diet and lifestyle. For instance, physical inactivity, consumption of large amounts of animal fats, and table salt contribute to The Development of sclerotic changes. A proper, regular diet and systematic Physical Exercise or sports slow down this process.

Review and Self-Assessment Questions:

1. Provide a General Overview of the Circulatory system. Which organs make up this system?

2. Name the vessels that make up the microcirculatory bed.

3. List the types of arteries (based on their wall structure and branching pattern).

4. Name the types of veins (based on their wall structure). How does the structure of vein walls differ from that of arterial walls?

5. Which vessels are called collateral, and why?

6. What types of arterial and venous anastomoses do you know? Which anastomoses are called intersystemic, and which are intrasystemic? Provide Examples.