Digestive System for AP Biology
Practice problems for these concepts can be found at: Human Physiology Review Questions for AP Biology
Okay, folks, it is time to take the tour of the digestive system. Hang on tight—we are going to take a shortcut to the mouth as we are exhaled back through the system. Here we go: bronchioles, bronchi, trachea, larynx, pharynx, and mouth!
Here we sit in the oral cavity. This is where the digestion of food begins. Food is, of course, tasted and smelled in the oral cavity, and the teeth that help us chew (masticate) are performing a task called mechanical digestion. The liquid sloshing up against the windows of the bus is saliva, which contains enzymes such as amylase that help dissolve some of the food. Amylase breaks down the starches in our diet into simpler sugars like maltose, which are fully digested further down in the intestines. The saliva also acts as a lubricant to help the food move along the digestive pathway.
We need to carefully avoid the tongue, which functions to move food around while we chew and helps to arrange it into a ball that we swallow called a bolus. The tongue pushes the food toward the crossroad we visited during the tour of the respiratory system. You may notice that this time, as the swallowing occurs, we do not go through the glottis toward the lungs, but instead into the esophagus, which connects the throat to the stomach. The force created by the rhythmical contraction of the smooth muscle of the esophagus (currently pushing us toward the stomach) is called peristalsis.
After passing through the esophageal sphincter, which acts like a valve or trapdoor, food enters into the stomach where more digestion will occur. The sphincter is usually closed in order to keep food from returning back up the esophagus to the mouth. In the stomach, the digestion occurs by a churning action that mixes the food and breaks it into smaller pieces. Folks, I would recommend that you do not step out of the bus here because the pH is way down in the 1.5–2.5 range, which provides quite an acidic environment. If you look closely along the edges of the stomach, you will see many glands. Some of these glands secrete gastric juice, composed of hydrochloric acid (HCl) and digestive enzymes, which helps in digestion and lowers the pH. The major enzyme of the stomach is pepsin, which breaks proteins down into smaller polypeptides, which are handled by the intestines. The glands here secrete pepsinogen—the precursor to pepsin. Pepsinogen is activated into pepsin by HCl. Pepsin is picky and will function only in a particular range of pH values. This is a good thing because if it were active all the time, it would digest things it is not supposed to digest. Other glands secrete mucus to help line the stomach. It is this mucus that helps prevent the wall of the stomach from being digested along with the food.
Now we move on to the small intestine. To get to the small intestine, we need to pass through the Panama Canal of the body: the pyloric sphincter. For those of you interested in useful AP exam trivia, the small intestine is where most of the digestion and absorption occur. The terrain is a bit different in this organ. The walls are arranged into folds and ridges, which have more waving structures, this time called villi, similar to what we saw in the respiratory tract. The walls in the small intestine contain something called a brush border, which is composed of a large amount of microvilli that increases the surface area of the small intestine to improve absorption efficiency. Digested nutrients absorbed in the small intestine are dumped into various veins that merge to form the hepatic portal vessel, which leads to the liver. The liver then gets first crack at the newly absorbed nutrients before they are sent to the rest of the body. As the food moves into the small intestine, it brings with it an acidity that promotes the secretion of numerous enzymes from the pancreas and the local glands. (Important note to remember: Hormones are vital to the turning on and off of the digestive glands.)
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