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Word Count: 998
Circulatory system cardiac location and structures the heart is the driving force of the circulatory system contracting about 70 timesminute to pump an adequate volume of blood with sufficient pressure to perfuse all body organs The muscular organ weights from 300 to 400g its located within the mediastinum of the thoratic cavity Living things must be capable of transporting nutrients wastes and gases to and from cells Single-celled organisms use their cell surface as a point of exchange with outside environment Multicellular organisms have developed transport and circulatory systems to deliver oxygen and food to cells and remove carbon dioxide and metabolic wastes Sponges are the simplest animals yet even they have a transport system Seawater is the medium of transport and is propelled in and out of the songe by ciliary action Simple animals such as the hydra and planaria lack specialized organs such as hearts and blood vessels instead using their skin as exchange points for material This however limits the size an animal can attain To become larger they need specialized organs and organs system The heart has three tissue layers the epicardium outer layer the myocardium middle layerand the endocardium inner layer The epicardium is the thin inner layer of the pericardium The myocardium thickest of the three tissue layers is composed of muscle fibers that contract creating the pumping effect of cardiac activity The endcardium a smooth membranous layer that lines all cardiac chambers and value leaflets is continuous with the intima or lining of the aorta and arteries The hearts four chambers-the right and left atria ventricles-are separated by the interatrial and interventricular septathe atria are thin walled low-pressure chambers that serve primarily as reservoirs for blood flow into the ventricles The ventricles are formed by muscle fibers that contract to eject blood to the pulmonary vasculature right and systemic circulation left Because the left ventricle must achieve the high pressure needed for systemic circulation it is much thicker
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