Color Atlas of Human Anatomy (eBook)

2.1 Overview

Circulatory System and Lymphatic Vessels

Circulation of blood occurs in a closed system of tubes consisting of blood vessels, with the heart serving as the central pump. The heart can be divided into a right half and a left half, each consisting of an atrium and ventricle. Irrespective of blood oxygen level, vessels that carry blood away from the heart are referred to as arteries and vessels that carry blood to the heart are referred to as veins.

The organization of the human circulatory system demonstrates a high level of differentiation. A distinction is made in postnatal life between pulmonary circulation and systemic circulation. In systemic circulation, arteries carry oxygen-rich blood away from the heart and veins carry deoxygenated blood toward the heart. In terms of function, pulmonary and systemic circulation are consecutive. Human postnatal circulation can be illustrated schematically as a figure-of-eight, with the heart located at its intersection acting as a suction and pressure pump (A). Blood is driven through the circulation by the arterial blood pressure (formula: arterial blood pressure = cardiac output-peripheral resistance).

Pulmonary circulation. Deoxygenated blood from the systemic circulation flows from the right atrium (A1) into the right ventricle (A2) of the heart and from there into the pulmonary circulation. Pulmonary circulation begins with the pulmonary trunk (A3), which bifurcates into right (A4) and left pulmonary arteries (A5). These vessels divide in the lungs (A6), parallel to the branchings of the airways as far as the capillaries, which surround the terminal portions of the airways known as the alveoli. There the blood is enriched with oxygen and carbon dioxide is released into the airways. The oxygenated blood leaves the lungs by the pulmonary veins (A7) and flows to the left atrium (A8).

Systemic circulation. Oxygenated blood from the lung flows from the left atrium (A8) of the heart into the left ventricle (A9). From there it is pumped through the aorta (A10) into the systemic circulation, which consists of numerous separate circuits (A11–A14) supplying individual organs and regions of the body. Large arteries branch off the aorta and pass to the separate circuits, where they divide many times and finally ramify into arterioles. These branch into a network of capillaries, where exchange of gases and metabolic products occurs. At the capillary plexus, the arterial portion of the systemic circulation passes into the venous portion in which deoxygenated blood is collected in venules, which unite closer to the heart, to form veins. Venous blood from the legs and lower half of the trunk is conveyed to the inferior vena cava (A15), and that from the head, arms, and upper half of the trunk to the superior vena cava (A16). The inferior and superior venae cavae empty into the right atrium (A1).

Portal circulation is a special part of the systemic circulation. Venous blood from unpaired abdominal organs (stomach, intestine, pancreas, and spleen) does not flow directly into the vena cava. Instead, substances from these organs are absorbed from the intestine, and carried in the blood by the portal vein (A17), to a capillary bed in the liver. After metabolism in the liver, the blood is collected in the hepatic veins (A18) and conveyed to the inferior vena cava.

Lymphatic system. The lymphatic system (green) (see p. 78) acts within the systemic circulation to shunt lymph to the venous portion of the circulatory system. Unlike the system of blood vessels, the lymph drainage system originates as blind-ended vessels that collect fluid from the extracellular space in the periphery of the body via lymphatic capillaries (A19) and conveys it via larger lymphatic vessels and the main lymphatic trunks, the thoracic duct (A20), and right lymphatic duct to the superior vena cava. Biologic filters known as lymph nodes (A21) are interspersed along the lymph vessels (see p. 94); see also Lymph nodes, thorax, and abdomen (p. 96) and Lymph nodes (p. 424).

A22 Chyle (see p. 78)

Fetal Circulation (A)

During prenatal life, the fetus (unborn offspring from the 9th week after fertilization to birth) receives oxygen and nutrients from the mother’s blood and releases carbon dioxide and metabolic waste products into it. The placenta (A1) serves as the connecting organ for exchange between mother and fetus. Oxygen-rich blood carrying abundant nutrients passes from the placenta to the fetus via the umbilical vein (A2), which initially lies in the umbilical cord. The umbilical vein enters the fetal abdominal cavity at the navel, or umbilicus (A3), and passes to the visceral surface of the liver (A4), where it connects to the left branch of the portal vein (A5). Although some of the blood from the umbilical vein thus enters the portal circulation, most bypasses the liver via a shunt called the ductus venosus (A6) and is carried into the inferior vena cava (A7). Blood from the ductus venosus thus mixes with deoxygenated blood from the inferior vena cava and hepatic veins (A8). Owing to the relatively minimal admixture of deoxygenated blood, it remains well oxygenated and passes via the inferior vena cava to the right atrium (A9). From there, the blood is directed by the valve of the inferior vena cava toward the foramen ovale (A10) that lies in the septum between the right and left atria and connects them. Most of the blood therefore reaches the left atrium (A11), passes from there into the left ventricle (A12), and flows via the branches of the aortic arch (A13) to the heart, head, and upper limbs. Deoxygenated blood from the head and arms of the fetus flows through the superior vena cava (A14) into the right atrium and crosses the bloodstream from the inferior vena cava to reach the right ventricle (A15), passing from there into the pulmonary trunk (A16). A minimal amount of blood passes through the pulmonary arteries (A17) into the lungs, which are not yet aerated, and from there through the pulmonary veins (A18) to the left atrium (A11). Most of the blood from the pulmonary trunk flows directly into the aorta through the ductus arteriosus (A19), a shunt connecting the bifurcation of the pulmonary trunk or left pulmonary artery with the aorta. The branches given off by the portion of the aorta after the connection of the ductus arteriosus thus receive blood with a lower oxygen concentration than those before the connection, which supply the head and upper limbs. A considerable amount of blood from the fetal aorta is returned to the placenta through the paired umbilical arteries (A20).

Circulatory Adjustments at Birth (B)

At birth, the fetal circulation is converted into postnatal circulation. With the first cry of the infant, the lungs are inflated and aerated, reducing resistance in the pulmonary circulation, which in turn increases the volume of blood flowing from the pulmonary trunk into the pulmonary arteries. The blood is oxygenated in the lungs and transported by the pulmonary veins into the left atrium. Backflow of blood from the lungs increases the pressure in the left atrium, causing functional closure of the foramen ovale as the flaps of the opening overlap. The foramen ovale is thus converted into the oval fossa, which is normally completely closed. The shunts, the ductus venosus and ductus arteriosus, are closed off by contraction of the muscle within the vessel walls. After obliteration, the ductus venosus forms the ligamentum venosum (B21) and the ductus arteriosus forms the ligamentum arteriosum (B22). Cutting the umbilical cord disrupts the connection between the placenta and umbilical cord vessels, leading to thrombosis and gradual obliteration of the vessels. The umbilical vein becomes the round ligament of the liver (B23) and the umbilical arteries become the cords of the umbilical arteries (B24).