The circulatory and digestive systems are two vital and complex systems necessary to maintain a homeostatic state within the human body.
The digestive system consists of the mouth, esophagus, stomach, small intestine, and large intestine. The digestive system is responsible for the breakdown and absorption of food (nutrients) to be stored, and then later utilized as energy to perform functions of the body. The breakdown of food begins in the mouth where food is masticated by the teeth, and "softened" by saliva. Upon swallowing, food is transported down the esophagus via muscular contractions called peristalsis. The next "stop" is the stomach. It is here where gastric fluid (Acidic) continue to breakdown food. At this point, a small potion of nutrients from food are absorbed through the stomach wall. After food is broken down, it will travel to the small intestine where the majority of nutrients are absorbed. The nutrients are absorbed by finger-like structures called villi. Once the nutrients are absorbed in the small intestine, the remaining solids are transported to the large intestine to be excreted through the anus.
All of these structures require oxygenated blood to remain viable. This is where the circulatory system comes into play.
The structures of the circulatory system consist of the heart, arteries, veins, and capillaries. Although part of the respiratory system, the lungs also play a large role in the circulatory system. The function of the circulatory system is to transport oxygenated and de-oxygenated blood throughout the body. De-oxygenated blood is transported to the heart via the vena cava (superior and inferior). From there it enters the right atrium (upper chamber), travels through the tricuspid valve into the right ventricle (lower chamber). From here, blood is transported to the lungs via the pulmonary artery. Blood is re-oxygenated through carbon dioxide/oxygen exchange in the structures of the lungs called alveoli. Once oxygenated, the blood is transported to the heart through the pulmonary vein. Blood then enters the left atrium, travels through the mitral valve into the left ventricle. From here, the blood is transported through the aortic valve into the aorta. The aorta is the largest artery in the body. The circulatory structures now act like a tree to accommodate the size of the structures they must supply blood. The aorta breaks down into arteries, then arterioles. Throughout its travels, blood is slowly "depleted" of its oxygen to structures that require oxygen. This is where a "connector" called the capillaries comes into the picture. Capillaries allow deoxygenated blood to travel into venules, then to veins, and finally the vena cava for transport to the heart.
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