Thanks to the process of natural selection, life on our planet has evolved. Natural selection dictated that in order for us, as living organisms, to survive, we had to adapt and coexist. In today's world, six different kingdoms of life coexist. In many cases, our relationships are based on symbiosis. Animals and plants are interdependent. As animals we need nutrients and oxygen, plants provide us with these nutrients and oxygen. In exchange, we give them carbon dioxide, which they reduce to glucose. We harvest the aforementioned chemicals from the surrounding environment through two organ systems: the digestive system and the respiratory system. This is called “environmental exchange”. Say no to plagiarism. Get a tailor-made essay on “Why Violent Video Games Should Not Be Banned”? Get the original essay Every living creature must acquire nutrients from its environment to survive. A large and complex organism, like humans, requires a large and complex system to break down and absorb nutrients from the foods it eats. We need a digestive system. The digestive system may not have the elegance of the nervous system or the strength of the skeletal system, but it is certainly just as important. The digestive system includes the gastrointestinal tract and various accessory organs. The digestive tract is essentially a long muscular tube, through which food passes, which is broken down, absorbed and becomes waste. In the process, six integrated steps take place: ingestion, mechanical processing, chemical processing (digestion), secretion, absorption and excretion. For example: if a trained marathon runner decides to eat a large bowl of oatmeal, 2 bananas and follow it with a glass of orange juice, the digestive system will try to absorb as many carbohydrates as possible. Food is chewed by the action of the teeth and jaw muscles (mechanical processing) in preparation for swallowing. In the mouth, food is mixed with secretions released by the salivary, parotid, submandibular and sublingual glands. These secretions contain water, electrolytes and enzymes, such as salivary alpha amylase since we are talking about carbohydrates. When food mixes with saliva it becomes a bolus, chemical treatment begins. The bolus passes through the pharynx to the esophagus by swallowing. In the esophagus, striated and smooth muscles are stimulated by the nervous system, resulting in peristalsis that carries the bolus into the stomach. The stomach is a J-shaped organ that contains four main regions: the cardia, fundus, body, and antrum. The body of the stomach is the main site of gastric juice production. Gastric juice consists of water, hydrochloric acid, enzymes, mucus and intrinsic factor. The enzymes, pepsin, alpha amylase and lipase continue chemical digestion. The antrum mixes the bolus with gastric juices, forming chyme. The antrum also stimulates gastric emptying into the small intestine through peristalsis. Once ready, the chyme enters the small intestine, where most of the digestion and absorption of chemical nutrients occurs due to its enormous surface area. The small intestines are covered with a brush border, consisting of villi and microvilli of enterocytes. Enterocytes produce a significant amount of enzymes, including alpha-dextrinase, glucoamylase, glucosidase, sucrase, trehalase, and disaccharidase. Something else is happening in the small intestine, three organsaccessories, the pancreas, liver and gallbladder, facilitate digestive and absorption processes. The pancreas produces more digestive enzymes, such as pancreatic alpha-amylase, among many other substances, and secretes them into the main pancreatic duct. Additionally, the common pancreatic duct joins the common bile duct to form the pancreatic bile duct, which empties into the duodenum. The gallbladder concentrates and stores bile produced by the liver. When needed, bile is secreted into the pancreatic bile duct and also drains into the duodenum. The liver stores, excretes, and converts nutrients absorbed from the small intestine. Enzymes, secreted by the stomach and pancreas, hydrolyze already partially digested nutrients, particularly carbohydrates. As chyme moves through the small intestine, passing through the duodenum, jejunum, and ilium, nutrient polymers are cleaved into dimers and monomers, such as disaccharides and monosaccharides, and are ready to be absorbed into the blood. The newly released carbohydrates are easily absorbed into the intestinal mucosa through active and facilitated transport. Once in the blood, carbohydrates are transported to where they are needed, whether that is skeletal muscle, the brain, or the liver. Nutrients that have not been absorbed continue to circulate through the digestive tract to the large intestine. Once in the large intestine, they pass through the cecum, colon and rectum. They mix with waste, become dehydrated and are stored to be excreted later through the anal canal. The glucose, which our trained marathoner ate the day before his race, will be distributed throughout the body. Since his body is trained for endurance exercise, some of this glucose will be stored in the liver and skeletal muscles as glycogen, some will be transported to the brain for brain function, and some will be converted to fat in the liver via de novo lipogenesis. When marathon running begins, the body begins to “adapt” to the rapidly changing internal environment. Skeletal muscle consumes glucose, lipids and oxygen at a higher rate, releasing carbon dioxide, which lowers blood pH, disrupting homeostasis. During the first 15 to 20 minutes of running, the muscles will use stored glycogen and glucose as their primary source of cellular energy. Glucose will undergo aerobic cellular respiration, which requires oxygen for its oxidative properties. We acquire oxygen through the respiratory system. The respiratory system includes the oral and nasal cavities, pharynx, larynx, trachea, bronchi and lungs. It works in conjunction with other organ systems to maintain homeostasis by exchanging gases with the environment and regulating blood pH. The process of gas exchange between the internal and external environments is called respiration. As our marathon runner continues to run, his blood oxygen levels drop and carbon dioxide levels rise. Central chemoreceptors, located in the chemosensitive areas of the medulla (the brainstem), detect changes and signal the respiratory areas (also the medulla) to increase respiratory rate and tidal volume. As a result, the muscles associated with inspiration (diaphragm, intercostals and abdominals) are stimulated, pulmonary ventilation (breathing) increases, more carbon dioxide is exhaled, more oxygen is inhaled, blood pH increases to normal and thus homeostasis is maintained. In the alveoli of the lungs, oxygen diffuses into the blood across the respiratory membrane, while carbon dioxide diffuses