MEDICAL BIOLOGY, ANATOMY, HUMAN PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedoniuk 2010

ANATOMY, PHYSIOLOGY, PATHOLOGY

CHAPTER 5. BLOOD AND LYMPH CIRCULATION PROCESSES AND THEIR PATHOLOGY

GENERAL ISSUES OF CARDIOVASCULAR SYSTEM ANATOMY AND PHYSIOLOGY

The Cardiovascular system comprises The Heart—the central organ of Blood Circulation, whose rhythmic contractions drive the flow of blood—and Blood Vessels, a system of tubular conduits through which the fluid moves. Depending on The Nature of the fluid, blood vessels and lymphatic vessels are distinguished.

Blood vessels include Arteries, arterioles, capillaries, venules, and Veins. Arteries transport blood from the heart to the periphery (Tissues), whereas veins carry blood from the periphery (tissues) back to the heart. Following the direction of blood flow, arteries branch into progressively smaller vessels down to arterioles, which break up into a network of microscopic vessels known as capillaries. Capillaries have a lumen diameter of about 8 µm, which roughly corresponds to the diameter of red Blood Cells. Small veins (venules) originate from the capillaries and gradually merge to form larger vessels, with blood returning to the heart through the largest veins.

Arterioles regulate the volume of blood flowing through an organ. I.M. Sechenov referred to these vessels as the "taps of the Circulatory system." They possess a relatively well-developed muscular layer, allowing their lumen to dilate and constrict According to the metabolic demands of the organ. The Capillary Wall consists of a single layer of flat endothelial cells, which readily permits the passage of various substances dissolved in the fluid as well as gases, meaning these vessels perform an exchange function.

Arteries are cylindrical tubes that transport blood from the heart to Organs and tissues. Their walls are quite thick and consist of three layers: the outer layer (adventitia), which is a Connective Tissue sheath; the middle layer (media), a muscular tunic containing smooth Muscle fibers; and the inner layer (intima), an endothelial lining that in certain vessels also features an internal elastic membrane, providing the walls with strength and elasticity.

Capillaries are microscopic vessels located within tissues that connect arteries to veins. It is precisely here that metabolic exchange takes place. Capillaries are present in almost all organs and tissues, with the exception of the epidermis, cornea, lens, Hair, Nails, and the enamel and dentin of Teeth. Their wall thickness is approximately 1 µm, and their length ranges from 0.2 to 0.7 mm. The total length of all capillaries is roughly 100,000 km, meaning that if placed end-to-end, they could encircle the Earth along the equator 2.5 times.

Veins have a tendency to collapse because their walls are thinner than those of arteries and contain fewer smooth muscle and elastic fibers. However, unlike arteries, veins feature Valves that prevent the backward flow of blood within these vessels.

Connections known as anastomoses exist between arterial and venous vessels. If circulation through the primary (main) vessels is impaired for any reason (such as trauma, tumor, or atherosclerosis), anastomoses take over their function. In such cases, the respective organ receives its blood supply via so-called collateral circulation. The following types of anastomoses are distinguished: 1) transverse, oblique, and arcuate connections between two vessels; 2) arterial and venous arches; 3) arterial and venous plexuses; 4) arterial and venous networks.

The cardiovascular system (circulatory system) facilitates Metabolic exchange between body tissues and the external environment while also maintaining the constancy of the internal environment. The Functions of the circulatory system include transporting nutrients, gases, Hormones, and protective agents to tissues, and carrying cellular Metabolic waste products away from tissues to excretory organs.