AP Assessment #2

(Part 4)  Trace the path of “the wings” from the mouth to the anus, listing the path in as much detail as possible.  Again, be very specific and very detailed including the absorption/elimination of nutrients/wastes.

 First, the sight and smell of the wings activates neural centers in the brain that stimulate the facial (VII), glossopharyngeal (IX) and vagus (X) nerves. These nerves stimulate the salivary glands to secrete saliva and the gastric glands to secrete gastric juices.
  Once the wings enter the mouth mechanical digestion begins as the teeth physically break down the wings. The tongue will move the food in the mouth to mix it with saliva which helps form the bolus. In the mouth salivary amylase helps break down starches by breaking down the chemical bonds. Once the bolus leaves the mouth it enters the pharynx where it is propelled by muscular contractions towards the esophagus, which transports the food to the stomach. This process involves three stages: the voluntary stage, pharyngeal stage and the esophageal stage.  The voluntary stage involves the voluntary swallowing of the bolus where the bolus then enters the pharyngeal stage. During the pharyngeal stage breathing is temporarily stopped as the bolus passes the oropharynx once past the oropharynx breathing resumes. After the bolus passes the oropharynx the esophageal sphincter relaxes allowing food to enter the esophagus. Once in the esophagus, the esophageal stage begins with a process called peristalsis. Peristalsis is a process where the bolus is moved through the esophagus by circular muscle fibers that contract above the bolus and squeeze the bolus down to the lower esophageal sphincter which, once relaxed will allow the food to enter the stomach.
  The bolus will eventually stretch the walls of the stomach and change the pH level. This triggers the stomach to release gastric juices and movement of the muscularis called “mixing waves”. Gastric juices include pepsinogen (which is ultimately converted to the digestive enzyme pepsin which breaks down the peptide bonds in amino acids of proteins), hydrochloric acid, intrinsic factor and mucous. Mixing waves are the stomachs way of mixing the bolus with the gastric juices to eventually form chyme. The chyme slowly passes through the pyloric sphincter into the duodenum by a process called gastric emptying.
  Once the stomach has turned the bolus into chyme it enters the small intestine. Here, pancreatic juice will enter the small intestine through the pancreatic duct. Pancreatic juice includes pancreatic amylase, a starch digesting enzyme. Trypsin, chymotrypsin, and carboxypeptidase which are protein-digesting enzymes. Pancreatic lipase, which is a triglyceride-digesting enzyme. Ribonuclease and deoxyribonuclease, which are nucleic acid-digesting enzymes. Bile from the gallbladder will also enter the small intestine through the common bile duct the common bile duct and the pancreatic duct will combine into the common duct to the duodenum.
The mucosa (inner most layer of the small intestine) contain simple columnar epithelium which absorb nutrients through the microvilli. The small intestine will slosh the chyme back and forth over the mucosa (more importantly the absorptive microvilli) to increase digestion and absorption. 90% of absorption takes place in the small intestine. Monosaccharides are transported from the epithelial cells into blood capillaries via facilitated diffusion, which lead to the liver and then to the heart. Inside epithelial cells peptides are digested into amino acids which then enter blood capillaries via diffusion. From there, the amino acids go to the liver for absorption or to the heart to circulate in the body. As these nutrients enter the epithelial cells the “pull” water through with them through osmosis. Most of the water we drink or contained in food is absorbed in the small intestine. Major ions such as sodium, potassium, calcium, iron, magnesium, chloride, nitrate and iodide are also absorbed through the small intestine. Most vitamins are absorbed through simple diffusion. Vitamin B12 must be combined by intrinsic factor (discussed earlier, produced in the stomach) for absorption.
  Once through the small intestine the chyme moves into the cecum of the large intestine. In the large intestine bacteria break down what is left of the carbohydrates and proteins in the chyme. The large intestine will absorb some water, ions and vitamins and eventually the chyme will reach a semi solid state called feces.  Feces are eliminated from the body through defecation reflex. 

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