The Circulatory System

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a) Diagram of the circulatory system. Blood from the lower part of the body travels to the heart in the inferior vena cava. Blood from the upper part of the body travels to the heart in the superior vena cava. The pulmonary artery travels from the heart to the lungs and the pulmonary vein travels from the lungs to the heart. B) A diagram of the heart showing the flow of blood. Beginning in the superior and inferior vena cavas, blood travels to the right atrium, through the tricuspid valve, to the right ventricle and out the pulmonary artery. The image does not follow the pulmonary artery. From the pulmonary vein blood travels to the left atrium, through the mitral valve, to the left ventricle and out the aorta.  C) The layers of the heart. The Pericardium is the outer layer. The myocardium is the thick middle layer. The endocardium is the inner layer.
The major components of the human circulatory system include the heart, arteries, veins, and capillaries. This network delivers blood to the body’s organs and tissues. (credit top left: modification of work by Mariana Ruiz Villareal; credit bottom right: modification of work by Bruce Blaus)

OpenStax Microbiology

The circulatory (or cardiovascular) system is a closed network of organs and vessels that moves blood around the body. The primary purposes of the circulatory system are to deliver nutrients, immune factors, and oxygen to tissues and to carry away waste products for elimination. The heart is a four-chambered pump that propels the blood throughout the body. Deoxygenated blood enters the right atrium through the superior vena cava and the inferior vena cava after returning from the body. The blood next passes through the tricuspid valve to enter the right ventricle. When the heart contracts, the blood from the right ventricle is pumped through the pulmonary arteries to the lungs. There, the blood is oxygenated at the alveoli and returns to the heart through the pulmonary veins. The oxygenated blood is received at the left atrium and proceeds through the mitral valve to the left ventricle. When the heart contracts, the oxygenated blood is pumped throughout the body via a series of thick-walled vessels called arteries. The first and largest artery is called the aorta. The arteries sequentially branch and decrease in size (and are called arterioles) until they end in a network of smaller vessels called capillaries. The capillary beds are located in the interstitial spaces within tissues and release nutrients, immune factors, and oxygen to those tissues. The capillaries connect to a series of vessels called venules, which increase in size to form the veins. The veins join together into larger vessels as they transfer blood back to the heart. The largest veins, the superior and inferior vena cava, return the blood to the right atrium.

Other organs play important roles in the circulatory system as well. The kidneys filter the blood, removing waste products and eliminating them in the urine. The liver also filters the blood and removes damaged or defective red blood cells. The spleen filters and stores blood, removes damaged red blood cells, and is a reservoir for immune factors. All of these filtering structures serve as sites for entrapment of microorganisms and help maintain an environment free of microorganisms in the blood.


Parker, N., Schneegurt, M., Thi Tu, A.-H., Forster, B. M., & Lister, P. (n.d.). Microbiology. Houston, Texas: OpenStax. Access for free at: