As a part of my breakfast on Day 1, I ate a peanut butter and jelly sandwich. The creamy peanut butter provided both protein and fat for my diet while the multigrain bread provided me with carbohydrates. As I take a bite of the peanut butter and jelly sandwich I crush all three of the macronutrients with my teeth. The role of saliva helps moistens the food while a digestive enzyme in the saliva called salivary amylase (ptyalin) helps breaks down the complex carbohydrates mostly contained in the bread into simple sugars.
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Once the food has been sufficiently chewed, the tongue rolls it into a ball known as a bolus and pushes it into the pharynx. In a combination of voluntary and automatic movements, swallowing propels the food downwards into the upper esophagus using a peristaltic contraction. The epiglottis blocks the trachea (windpope) to prevent food particles from entering the trachea and interfering with respiration. The esophagus is then able to move the bolus downwards. At the bottom of the esophagus there is the cardiac sphincter which then opens to food can enter the stomach.
The sphincter then recloses to prevent regurgitation of stomach contents into the esophagus. The bolus of food stimulates the secretion of gastrin by the cells of the stomach wall. The gastrin cells secrete gastrin which stimulates HCl production in the parietal cells of the gastric glands, gastrin also stimulates muscular contractions fo the stomach to further churn the food and produce an acidic, semi-fluid, partially digested of my peanut butter and jelly sandwich called chyme.
Mucous cells in the stomach secrete mucus to protect the stomach lining from acidic pH of the stomach (pH 2). The chief cells secrete pepsinogen, the precursor to pepsin. The parietal cells of the stomach secrete hydrochloric acid which kills bacteria in the stomach and helps break down intercellular links in food tissues and aids in the conversion of pepsinogen to pepsin; they also secerete intrinsic factor which is necessary for the absorption of vitamin B12.
Between the stomach and the small intestine is the pyloric sphincter which controls the rate of movement of food from the stomach into the small intestive through alternaticing contractions and relaxations. The small intestive is responsible for a large part of digestion and absorption. The small intestine is broken down into three regioins which are respobsible both digestion and absorption. Most digestion occurs in the duodenum while the jejunum and ileum have a greater role in the absorptive function of the small intestine.
Since peanut butter, contains a pretty high fat content, the duodenum releases the hormone enterogastrone, which inhibits stomach perilstalsis, thus slowing down the release of chime into the small intestine. Chyme is mixed with additional diguestive juices including bile from the liver and pangreatic juice and amylase from the pancreas, as well as other intestinal enzymes such as maltase, lactase, and sucrose to break down the chime and assist in nutrient absorption.
Absorbed nutrients flow in the blood stream to the liver where they are further metabolized and then either stroed or sent to cells in other parts of the body. Bile breaks down fat particles into smaller droplets, while pancreatic juice contains enzymes that convert fats into fatty acids and glycerol, plus sodium bicarbonate to neutralize stomach acid. The chyme then moves into the second part of the small intestine called the jejunum wehre chemical breakdown is completed. Pancreatic enzymes, along with enzymes produced in the jejunum wall, finalse the food digestion process.
The ileum is the final section of the small intestine, linked to the large intestine by the ileocecal valve. The main function of the ileum is to absorb nutrients. Bile is also absorbed here and retunrs to the liver through blood vessels in the intestinal walls. The unabsorbed watery remains of the food chyme now pass into the large intestine for water-removal and final processing, before being expelled from the body. After all nutrients have been absorbed from ingested food during its passage through the small intestine, the watery waste passes into the large intestine.
It is the final section of the gastrointestinal tract and its main function is to remove water (plus any remaining minerals) from the food waste and compress it into a form for easy expulsion from the body. As the chyme passes through the large intestine, the water is removed and the chyme is combined with mucus and bacteria (gut flora), and is converted into feces. Feces are stored in the recume until they pass through the two sphincters that regulate elimination and are expelled through the anus.