Anatomy and Physiology of Digestion Paper

Introduction

One lovely Saturday morning I sat down hungrily in Chucks to eat breakfast with my friends. After much thought, I decided to make pancakes and eggs, and to have milk to drink. By having these thoughts, the cephalic phase of digestion began. The pancakes, I thought, would cover the carbohydrate and starch groups, and the eggs and milk would cover the lipids and proteins for this meal. Once I had the food in front of me, I sat there for a moment, enjoying the smell and sight of the soon to be devoured meal. And, once I tasted all of it, I was very happy with my choice of food. The sight, smell, and taste of the food are also components of the cephalic phase, as the body is preparing the stomach for the entrance of food. This preparation involves salivary and gastric secretions, as well as motility.

Mouth

In the mouth, both mechanical and chemical digestion take place. Mechanical digestion involves mastication of the pancakes and eggs into smaller particles. These smaller particles come together to form a food bolus. The chemical digestion in the mouth includes salivary amylase secreted from the salivary glands by way of the parasympathetic system. The salivary amylase helps to begin digesting the starch from the pancakes.  After all this has taken place, the food bolus is swallowed with help from the tongue and the skeletal muscles in the throat, and then travels down the pharynx and esophagus. No absorption takes place in the mouth.

Stomach

The stomach functions as a temporary holding place for the bolus, as well as a mixing area once it travels down the esophagus. The gastric phase of digestion occurs here in the stomach after the bolus has been swallowed. During this phase, stretch receptors have the ability to activate and expand the stomach. When the stomach expands, rugae start to disappear to form smooth muscle. Or, the presence of protein can cause the release gastrin. Gastrin is secreted from the G cells in the stomach, and it also triggers the release of hydrochloric acid from the parietal cells. The hydrochloric acid creates an environment with a lower pH in the stomach. In response to all these triggers, the pyloric sphincter relaxes, and gastric juice continually flows, and the peristaltic waves continue on as well. Most substances cannot be absorbed through the stomach wall. However, some such as electrolytes, water, aspirin and alcohol can permeate the lining. Aspirin and alcohol can be detrimental however, as they can disturb the gastric mucous barrier and cause gastric ulcers. Mechanical digestion consists of mixing the bolus in waves. Chemical digestion primarily concerns pepsin, an enzyme converted by hydrochloric acid from its inactive precursor, pepsinogen. Pepsin comes from chief cells, and the enzyme catabolizes peptide bonds from the protein in the milk and eggs. Pepsin is the only effective enzyme in the stomach’s extremely acidic environment. In addition to pepsin, there is another enzyme, gastric lipase, which works to break down fats. This enzyme is also secreted from the chief cells.  Both the chief cells and the parietal cells are considered exocrine cells.  Mechanical and chemical digestion work together to change the bolus into chyme (consisting of the partially digested pancakes, eggs and milk along with gastric juices) which can then flow out of the stomach through the relaxed pyloric sphincter, and into the duodenum of the small intestine for the next phase of digestion. Most of the food leaves the stomach about two to six hours after it has been ingested. The pancakes would have spent the shortest amount of time in the stomach, and the fatty components of the milk and eggs would have spent the longest time in the stomach.

Small Intestine

The intestinal phase of digestion begins once food begins exiting the stomach and entering the duodenum. While the other phases of digestion work to speed up the digestion process, the intestinal phase works to decelerate the chyme. If this process did not occur, the duodenum could become overloaded with chyme. Occurrences of this phase include both hormonal and neural factors. The neural regulation is sparked by the distension of the duodenum, which then stimulates the enterogastric reflex which keeps the stomach from periodically releasing chyme. The hormonal regulation, on the other hand, is triggered when certain components of the chyme enter the small intestine. If the chyme is comprised of amino acids or fatty acids, CCK cells will release CCK into the duodenum. CCK keeps gastric emptying from occurring, triggers the release of digestive enzymes from the pancreatic acinar cells, causes bile to eject from the gallbladder, and induces a feeling of satiety. Next, the acidity of the chyme sparks the release of secretin from S cells  in the small intestine. Secretin spurs the release of bicarbonate from the liver and pancreas. Bicarbonate ions are discharged into pancreatic juice and bile. Mechanical digestion in the small intestine consists of two parts: segmentation and peristalsis. Segmentation basically “sloshes the chyme mixture back and forth”, and is considered the main movement of the small intestine. It occurs via alternating contractions and relaxations of the circular muscularis of the small intestine. During this sloshing phase, digestive enzymes are being mixed in with the chyme. This part of digestion is where a lot of absorption takes place. Peristalsis starts up directly after segmentation ceases. Unlike segmentation, peristalsis is made up of harmonized muscle contractions. These contractions occur directly behind the bolus, which propels it further along the GI tract with each wave. With each contraction, the bolus gets closer and closer to the rectum. Peristalsis is governed by the enteric nervous system. Chemical digestion in the small intestine occurs with the collective efforts of intestinal juice, brush border enzymes, pancreatic juice and bile. Intestinal juice is secreted by intestinal glands into the lumen of the tract, but it contains no enzymes. Its enzymes are synthesized in the villi and therefore are a part of the brush border. Brush border enzymes include disaccharidases, peptidases, and enterokinase. Disaccharidases break disaccharides down into monosaccharides, which is a part of the breakdown of the pancakes. Peptidases break proteins down into amino acids, so the milk and eggs are now amino acids, and enterokinase converts trypsinogen into its active form, trypsin. Bile aids in the break down of lipids by a process called emulsification, which essentially breaks the fats into smaller pieces. The milk and eggs also contained fat, so, here, bile is increasing the surface area of their fat particles. Bile is created in the hepatocytes of the liver, and can either be transported to the gallbladder for later use, or can be directly transported into the duodenum via the ampulla of Vater. Lastly, the pancreatic juice consists of digestive enzymes and sodium bicarbonate. There are four different pancreatic enzymes that help aid in different types of digestion: pancreatic amylase, pancreatic proteases, pancreatic lipase, and pancreatic nucleases. Pancreatic amylase digests starch, and pancreatic proteases digest protein. Pancreatic lipase digests lipids after bile increases the fats’ surface area, and pancreatic nucleases digest nucleic acids. Sodium bicarbonate helps to increase the activity of all these enzymes. The pancakes, eggs and milk are all digested a little bit more by these enzymes. Once chemical digestion has finished, these end products are ready for absorption. The majority of absorption occurs in the small intestine. Osmosis, simple diffusion, active transport and facilitated diffusion are all processes by which absorption takes place. Villi have a huge role in absorption in the small intestine. Water is absorbed into the blood capillaries via osmosis. Electrolytes and water soluble vitamins are absorbed into the same place, but are absorbed by simple diffusion. Disaccharides are digested into monosaccharides at the brush border, and are then absorbed into the villi by active transport. However, they are not actively transported all the way to the blood capillaries, they have to go through the process of facilitated diffusion from the columnar cells and over to the capillaries. Proteins from the eggs and milk that have been broken down into tripeptides, dipeptides, and amino acids can also be absorbed into the villus by active transport. From the epithelium, the tripeptides and dipeptides are digested further into amino acids, and then they are all transported into the capillaries. Short and medium length chains of lipids are absorbed by simple diffusion in the blood capillaries. However, long lipid chains cannot, and have a special process of absorption. When lipids are emulsified by bile salts, the resulting fat droplets are called micelles, or long-chained fatty acids. Other components of micelles include: fat soluble vitamins (DAKE), cholesterol, monoglycerides and other dietary lipids. Bile keeps micelles soluble until they can be absorbed into the epithelium of the villi. Once micelles are absorbed, they are encased together with a protein shell, and are then known as chylomicrons. Once this happens, they are exocytosed into the lacteal of the villus. In the ileum, bile salts are reabsorbed back into the blood stream to travel to the liver through the hepatic portal system to be used again. Vitamin B12 is absorbed in the ileum, but only in the presence of intrinsic factor, a substance secreted from the parietal cells of the stomach.