Human Physiology ❮ 181Digestive System
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 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
issaliva,which contains enzymes such as amylasethat 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, we 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 quitean 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 that 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 the cilia 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 pan-
creasand the local glands. (Important note to remember:hormones are vital to the turning
on and off of the digestive glands.)
Those of you on the left side of the bus have a good view of the pancreatic duct as it expels
lipase, amylase, trypsin,andchymotrypsin.Lipase is the major fat-digesting enzyme of theBIG IDEA 2.A.3
Villi create high
surface area, help-
ing in exchange of
nutrients with the
environment.
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