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

Cardiovascular System
The Role of the Cardiovascular System in the Human Body

An Organism can survive only if nutrients, oxygen, and Water are continuously supplied from the external environment to the body Tissues (via the gastrointestinal tract and Lungs) and Metabolic waste products are eliminated (such as carbon dioxide and urea via the excretory Organs, including the Kidneys, lungs, and Skin).

As Blood circulates through the vessels, substances taken into the body or removed from it are transported between various organs. Blood delivers Hormones (derived from the Greek hormao, meaning "to excite") to Cells and Tissues, which are essential for regulating their physiological activity; it also carries Antibodies, immunocompetent cells, and phagocytes that neutralize foreign substances, microbes, and Viruses. The total blood volume in an adult human accounts for 7% of body weight, amounting to 5–6 L. One of the primary Functions of the Circulatory system is thermal regulation within the body. Heat is redistributed between organs with significant heat production and those prone to cooling (such as the skin and respiratory organs).

The Cardiovascular system plays a vital role in maintaining the continuous Circulation of blood through a closed network of vessels. Blood Cells (erythrocytes, leukocytes, and Blood Platelets) are produced in Hematopoietic organs, namely the Cytology/practical/86.html">Red Bone Marrow, Thymus, Spleen, and Lymph Nodes. This process, known as hematopoiesis, ensures the physiological regeneration of blood by replacing Aging and dying cells with new ones. The majority of blood cells are formed in the red bone marrow, which has a total volume of 1500 cm³ in adults. It fills the spaces between the bony trabeculae of spongy bone throughout the Skeletal System. B-lymphocytes multiply in the bone marrow, though their differentiation takes place in lymphoid tissue, whereas T-lymphocytes are produced in the thymus.

Yellow bone marrow, which consists predominantly of fat cells, acts as a reserve hematopoietic organ: following major blood loss or under certain pathological conditions, it can temporarily transform back into red bone marrow and resume blood Cell production.

In the fetus, the Liver serves as the primary hematopoietic organ, beginning to produce blood cells as early as the 6th week of development. The red bone marrow begins functioning by the 12th week. Gradually, hematopoiesis in the liver ceases, stopping completely by birth. In infants, all bone marrow is red, and its replacement by yellow bone marrow within the medullary cavities of the bones occurs progressively, completing only by the age of 20. At birth, blood mass accounts for 15% of body weight, with a volume of approximately 0.5 L. With age, blood volume increases while its relative amount decreases, approaching adult levels by age 12, with a slight surge during Puberty. The relatively larger blood volume in children compared to adults is associated with supporting a higher metabolic rate.

The circulatory system consists of The Heart and a closed network of Blood Vessels—Arteries, Veins, and capillaries—that permeate all bodily tissues and organs (Atlas Fig. 50). Vessels are absent only in Epithelial Tissue, hyaline Cartilage, the lens and Cornea of the eye, tooth enamel and dentin, and keratinized skin derivatives such as Hair and Nails—that is, in areas of the body characterized by a reduced metabolic rate.

Arteries are thick-walled vessels through which blood flows under high pressure away from the heart. They branch repeatedly and terminate in arterioles, which are small vessels with narrow lumens that transition into thin-walled capillaries. Through the capillary walls, gases and other substances are transported from the blood to cells and tissues, while metabolic waste products are returned to the blood. From the capillary bed, blood flows first into venules and then into veins, through which it returns to the heart.

The movement of blood through The Vascular System is driven primarily by the action of the heart. The heart is a hollow muscular organ comprising right and left halves, each of which is transversely divided into an atrium and a ventricle. Through rhythmic contractions, the heart pumps blood into the arteries, and during the subsequent relaxation phase, it draws blood back from the veins.

The left atrium receives arterial blood—that is, oxygen-rich blood—from the lungs and pushes it into the left ventricle. When the ventricle contracts, blood is forced into the aorta, the body's largest artery, from which it spreads via branching arteries to all bodily organs. Small arteries (arterioles) transition into capillaries, where blood changes from arterial to venous, becoming depleted of oxygen and enriched with carbon dioxide. From here, blood collects into small and subsequently large veins, which empty via two large veins (the superior and INFERIOR VENA CAVA) into the right atrium. This pathway is known as the systemic, or greater, circulation.

The right half of the heart drives circulation through the pulmonary, or lesser, circulation loop. Deoxygenated venous blood passes from the right atrium into the right ventricle, from which it is pumped into the pulmonary artery and directed to the lungs. Following Gas Exchange in the lungs, the blood becomes arterial once again—enriching itself with oxygen and releasing carbon dioxide—before returning via the Pulmonary veins to the left atrium.

As a rule, blood passes through only a single capillary network when traveling from arteries to veins. An exception is found in the kidneys, which feature an additional capillary network within the glomeruli of the renal corpuscles. In this case, blood passes through capillaries twice within a single organ. Venous blood draining from the capillaries of The Stomach and intestinal walls (except for the rectum), as well as the spleen, collects into the portal vein, which empties into the liver. Here, the blood passes through a second capillary network, undergoing significant chemical modification and being cleared of harmful substances absorbed from the intestine.

Between certain small arteries and veins in various organs—both external (such as the skin of the fingertips, Nose, and auricle) and internal (the heart, Brain, kidneys, spleen, lungs, and reproductive organs)—there exist arteriovenous anastomoses (shunts). These allow a portion of the blood to bypass the capillaries and flow directly from arteries into veins. Such anastomoses play a crucial role in regulating Regional Blood Flow and local Temperature.

In addition to the cardiovascular system, the vascular network also includes The Lymphatic system.