Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Cardiovascular System
General patterns of distribution of arteries and veins in the body

The distribution of Arteries and Veins in the body, along with their branching, topography, and caliber, defines the angioarchitecture of an organ. This architecture is determined by the organ's Functions, as well as its individual and evolutionary development.

In mammals and humans, despite the profound complexity of organizational changes, the distribution of Blood Vessels retains features inherited from primitive aquatic ancestors: the longitudinal position of the aorta, the metameric origin of its paired parietal branches supplying body segments, and branches directed to paired Organs (such as Kidneys and Gonads) (Fig. 2.10). Metamerism is particularly evident in the Topography of the intercostal and lumbar arteries and veins. The unpaired Nature of the branches arising from the descending aorta and supplying unpaired organs (the intestinal tube and its derivatives) is a secondary phenomenon, as these branches are paired during early embryonic stages.

A clear parallelism is evident between the skeletal and vascular systems. Arteries branch and course in accordance with the bone framework of the limbs: a single main artery extends along the arm and thigh, which each contain a single bone; two arteries run along the forearm and leg, whose skeletons consist of two bones; and five pairs of arteries supply the FOOT and hand, corresponding to the five digits. In the periphery, various arteries interconnect through anastomoses.

Main arteries invariably travel to their supplied organs via the shortest possible pathway, thereby conserving the energy expended by The Heart to propel blood and accelerating its delivery. Large vessels consistently lie along the flexor aspect of the trunk or limbs, where they are better sheltered and protected. In these locations, vessels are less vulnerable to injury—a critical factor, given that arterial hemorrhages can be fatal. Furthermore, the flexor side provides a shorter route. On the digits, it is the lateral surfaces rather than the flexor surfaces that enjoy greater protection; consequently, the digital arteries run precisely along these areas.

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Fig. 2.10. Vascular segment:

1 — coelom; 2 — intestinal tube; 3 — Kidney; 4 — aorta; 5 — segmental parietal vessel with ventral (6) and dorsal (7) branches, a branch to the Spinal Cord (8), and lateral (9) and ventral (10) cutaneous branches; 11 — vascular branches to paired and 12 — to unpaired Internal Organs

In joint regions characterized by a wide range of motion, collateral pathways and vascular networks are invariably well-developed. These structures prevent excessive stretching of the arteries and avert circulatory arrest during their compression or injury. The Development of the vascular network corresponds to the degree of joint mobility: the greater the range of motion, the more highly developed this network is.

Arteries may branch in a stem (main) or dispersed pattern. Typically, vessels diverge from main trunks at the level of the organs they supply. If an organ shifts during ontogenesis from its initial developmental site—such as the Diaphragm or gonads—the vessel maintains its original site of origin from the main trunk and thus stretches over a considerable distance to reach the organ. The specific course and branching patterns of arteries are governed by the hydrodynamics of the vascular bed. Vessel curvature minimizes wall resistance to blood flow. For instance, branches originating from the aortic arch arise in the region where a zone of elevated pressure forms due to the changing direction of the blood stream. The angle at which an artery branches off the main trunk is also of great significance: the larger this angle, the slower the blood flow within the artery.

Arteries generally run deep between Muscles, yet in locations where they experience the least muscular pressure. Nevertheless, over a short segment of their course, arteries may also run superficially, allowing the pulse to be easily felt and counted. Knowledge of such sites is essential in first aid, as Hemorrhage can be controlled by compressing the injured artery (Fig. 2.11). In the limbs, this is also achieved by applying a tourniquet, though for no longer than 2 hours to prevent tissue necrosis.

As a rule, vital organs receive blood from two or even several arteries, one of which is the primary and largest vessel, while the others are collateral. The main artery enters the organ through its hilum. Within or near organs, individual small arteries interconnect via branches known as arterial anastomoses. Such collateral connections are crucial for ensuring uninterrupted Blood supply to organs (such as the intestines and muscles) when increased blood delivery is required due to heightened activity, or during various physiological hindrances to blood inflow through the primary artery. If the main artery is obstructed (due to injury, surgical ligation, thrombosis, etc.), anastomoses of collateral branches can serve as pathways for collateral Circulation. In such cases, secondary vessels gradually increase in diameter and fully compensate for the main artery.

Fig. 2.11. Sites of arterial compression during hemorrhage:

1 — superficial temporal; 2 — external maxillary (facial); 3 — common carotid; 4 — subclavian; 5 — axillary; 6 — brachial; 7 — radial; 8 — ulnar; 9 — femoral; 10 — anterior tibial; 11 — dorsal artery of the foot

In the systemic circulation, superficial and deep veins are distinguished. Superficial veins lie within the subcutaneous adipose tissue and, when fat is sparse, are clearly visible through the Skin as bluish trunks or networks. They are particularly well-developed in the neck and limbs and are more prominent in individuals engaged in heavy physical labor. The largest of these veins are used for intravenous drug administration and blood sampling. Superficial veins communicate with deep veins via anastomoses, ensuring optimal venous drainage even when hampered by certain awkward postures or pathological alterations.

Deep veins are primarily located along the course of arteries, and consequently share the same names as those arteries. Together with major arterial trunks and nerve bundles, veins form neurovascular bundles. Medium- and small-caliber arteries are typically accompanied by two venae comitantes (satellite veins) that anastomose repeatedly with one another. As a result, the total capacity of the Venous system can exceed the volume of the Arterial System by a factor of 2–3 or more. Veins of visceral (cavitary) organs and all major veins are single. Variations in branching within the venous system are encountered much more frequently than in the arterial system.

The condition of the vascular bed, particularly the arteries, plays a major role in the development of age-related changes in organs and Tissues. Alterations in the mechanical strength of the vascular wall lead to an increase in the capacity of arterial vessels. The tunica intima (inner layer) thickens, and endothelial integrity is disrupted, becoming less smooth, which can promote thrombus formation within the vascular lumen. This results in impaired nourishment of the vessel wall and its partial destruction. The Connective Tissue of the tunica adventitia (outer layer) also undergoes changes, characterized by uneven Collagen deposition and a reduced Cell count. Cholesterol is deposited within the thickness of the wall. The cumulative result of these described changes is luminal narrowing and a decrease in vascular elasticity. Compromised strength and thinning of the venous wall, along with uneven thickening and sclerosis—particularly in the valve regions—lead to localized dilations alternating with narrowed segments (varicose veins) and impaired venous outflow.

Review Questions

1. Characterize the circulations (circles of Blood Circulation) and describe their function.

2. What types of blood vessels do you know? Describe The Structure of the wall in Different types of vessels. What is the Functional Significance of the layers of the vessel wall?

3. Describe the Structure and Functional Significance of the vessels that form the microvasculature (microcirculatory bed).

4. Characterize the Structural and functional Features of the human venous system.

5. What are the principal regularities governing the distribution of arteries and veins in The Human Body?