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Through the process of natural selection, the life on our planet 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 are living together. In many cases, our relationships are based on symbiosis. Animals and plants are interdependent. We, as animals, require nutrients and oxygen, plants provide us with those nutrients and oxygen. In return 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 termed “environmental exchange”.
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Every living creature must acquire nutrients from their surrounding environment to sustain life. A large and complex organism, such as human, requires a large and complex system to break down and absorb the nutrients from the food that it eats. We require a digestive system. The digestive system may not have the elegance of the nervous system or the strength of the skeletomuscular system, but it is certainly just as important. The digestive system consists of the gastrointestinal tract and various accessory organs. The digestive tract essentially is a long, muscular tube, through which the food passes through getting broken down, absorbed and becoming 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 chase it down with a glass of orange juice, the digestive system will attempt to absorb as much carbohydrates as possible.
The food is chewed by the action of the teeth and the jaw muscles (mechanical processing) in preparation for swallowing. In the mouth the food is mixed with secretions released from the salivary glands, parotid, submandibular, and sublingual. Those secretions contain water, electrolytes and enzymes, such as salivary alpha amylase since we are talking about carbohydrates. When the food mixes with saliva, it becomes bolus, the chemical processing begins. The bolus passes through the pharynx into the esophagus via swallowing. In the esophagus both the striated and the smooth muscles are stimulated by the nervous system, resulting in peristalsis, which carries the bolus into the stomach. The stomach is a J-shaped organ that contains four main regions: cardia, fundus, body and antrum. The body of the stomach is the main production site for gastric juice. Gastric juice consists of water, hydrochloric acid, enzymes, mucus, and intrinsic factor. The enzymes, pepsin, alpha amylase, and lipase continue the chemical digestion. The antrum mixes the bolus with the gastric juices, forming chyme. The antrum also stimulates gastric emptying into the small intestines through peristalsis. Once ready, the chyme enters the small intestines, where most of the nutrient chemical digestion and absorptions occurs due to its enormous surface area. The small intestines are covered with the brush border, which consists of villi and microvilli of enterocytes. The enterocytes produce a substantial amount of enzymes, including alpha dextrinase, glucoamylase, glucosidase, sucrase, trehalase, and disaccharidase.
Something else happens in the small intestines, three accessory organs, pancreas, liver, and gallbladder facilitate the digestive and absorptive processes. The pancreas produces more digestive enzymes, such as pancreatic alpha amylase, among many other substances, and secretes them into the main pancreatic duct. Further the main pancreatic duct joins common bile duct to form bile pancreatic duct, which empties into duodenum. The gallbladder concentrates and stores bile produced by the liver. When needed, the bile secretes into the bile pancreatic duct, and empties into the duodenum as well. The liver stores, excretes, and converts nutrients absorbed from the small intestines.
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The enzymes, secreted by the stomach and pancreas, hydrolyze already partially digested nutrients, including carbohydrates. As the chyme moves through the small intestines, through duodenum, jejunum and ilium, polymers of nutrients are cleaved into dimers and monomers, such as disaccharides and monosaccharides, and are ready to be absorbed into the blood. The newly liberated carbohydrates are readily absorbed into the intestinal mucosa through active and facilitated transport. Once in the blood, carbohydrates are carried to where they are needed, whether it is a skeletal muscle, the brain, or the liver.
The nutrients, that were not absorbed, continue moving through the digestive tract to the large intestine. Once in the large intestine, they go through cecum, colon and rectum. They get mixed with waste products, become dehydrated, and are stored to be excreted later through the anal canal.
The glucose, that our trained marathon runner ate the night before his race, will be distributed throughout the body. Since his body is trained for endurance exercise, a portion of that glucose will be stored in the liver and skeletal muscle as glycogen, a portion will be transported to the brain for brain function, and some will be converted to fat in the liver via de novo lipogenesis. When the marathon race begins, the body starts “adapting” to the rapidly changing internal environment. The skeletal muscle consumes glucose, lipids and oxygen at a higher rate, releasing carbon dioxide, which lowers blood pH, thus homeostasis is disrupting. During the initial 15-20 minutes of the run, the muscles will utilize the stored glycogen and glucose as a primary source of cellular energy. The glucose will undergo aerobic cellular respiration, which requires oxygen for its oxidative properties. We acquire oxygen through the respiratory system.
The respiratory system consists of oral and nasal cavities, pharynx, larynx, trachea, bronchi and lungs. It works in collaboration with other organ systems to maintain homeostasis by exchanging gases with the environment and regulating blood pH. The process of gas exchange between internal and external environments is termed respiration.
As our marathon runner continues running, his blood oxygen levels will drop, and carbon dioxide levels will increase. Central chemoreceptors, located in the chemosensitive areas of medulla oblangata (the brainstem), sense the changes and will signal the respiratory areas (also medulla oblangata) to increase respiratory rate and tidal volume. As a result, 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 through the respiratory membrane, while carbon dioxide diffuses out of the blood into the alveoli. The gas exchange occurs by simple diffusion, where each gas diffuses from a region of higher concentration to a region of lower concentration. The gas exchange in the lungs is termed external respiration, however there is also an internal respiration. The internal respiration occurs at the metabolizing tissue: oxygen diffuses into the tissue, while carbon dioxide diffuses out of the tissue.
As we have noticed already, our runner began breathing slightly heavier during the race, than before the race. This is due to the fact that the oxygen demand and CO2 production increased, and thus the respiratory system adapted. When the oxygen demand increases, your body can adapt in two ways: increase the respiratory rate or increase the tidal volume. Using complicated mathematical equations involving the two values (tidal volume and respiratory rate) a medical professional can diagnose and determine problems with pulmonary ventilation, such as COPD.
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