Digestive System

Most animals have one of three main types of diets: they are either carnivores, herbivores, or omnivores. Most humans fall into the last category. In humans, herbivores are usually called vegetarians. Strict vegetarians who consume no animal products, whatsoever, are called vegans. Some people are lacto-ovo-vegetarians, meaning that they also eat dairy products and eggs.

Digestion is the process of breaking down food into molecules small enough for the body to absorb. Proteins, carbohydrates, and fat in our diets must be broken down and later, reassembled in forms useful to our body.

The path of food through the human digestive system includes the following organs and structures:
(clipart edited from Corel Presentations 8)

1. the mouth, which includes:
   • the teeth, which grind food to increase the surface area
   • the saliva, which includes mucin, a lubricant; buffers to neutralize acidic foods, antibacterial agents, and amylase, which converts starch to maltose
   • the tongue which tastes and manipulates the food
2. the pharynx, which in humans, leads to both the trachea and the esophagus. While food is being swallowed, the epiglottis blocks the trachea and the uvula blocks off the nose.
3. the esophagus, which is the tube from the pharynx to the stomach. Food is moved along the esophagus by peristalsis, wave-like contractions of the muscles in the walls of the esophagus. The lining of the esophagus secretes mucus to lubricate the ball of food. There are sphincter muscles (rings of constricting muscles) at the top and bottom of the esophagus.
4. the stomach is a J-shaped, expandable sack, normally on the left side of the upper abdomen. Several muscle layers surround the stomach, serving to churn food. The stomach can expand to hold about 2 L of food (= 1/2 gal). The stomach contains hydrochloric acid (HCl) strong enough to dissolve metal (pH about 1.5 to 3, usually around 2), which kills bacteria and helps denature the proteins in our food, making them more vulnerable to attack by pepsin. The stomach secretes mucus to protect itself from being digested by its own acid and enzymes. The stomach also manufactures pepsin, an enzyme to digest protein. The average person secretes about 400 mL of gastric juice per meal, containing 50 to 300 µg pepsin/mL. For an average of around 200 µg/mL × 400 mL of gastric juice, this would be 80 mg (or 0.080 g) pepsin/meal. For HCl with a concentration of around 6.08 g/L × 400 mL, this would be 2.4 g/meal. Consumption of antacids does just what their name suggests: they drastically change the pH of the stomach contents, interfering with pepsin’s ability to digest protein. Here is more background on pepsin and the effects of antacids. Here is a photograph of results of an experiment we perform to study pepsin’s ability to digest the protein in egg white, and a summary of the contents of each of these test tubes.
   

   Tube No.	Distilled Water	0.5% Baking Soda	0.8% Hydrochloric Acid	1% Pepsin Solution	Other Info/Ingredients (@ body temp unless noted)
   1	10 mL
   2	5 mL			5 mL
   3	5 mL	5 mL
   4	5 mL		5 mL
   5		5 mL		5 mL
   6			5 mL	5 mL
   7			5 mL	5 mL*	*boiled pepsin soln.
   8			5 mL	5 mL	@ room temp
   9				5 mL	§ 5 mL mixture w/ Tums
   10				5 mL	§ 5 mL mixture w/ Rolaids

   § For # 9 and 10, 100 mL of HCl was mixed with one “dose” of antacid. 5 mL of that was used.
   Conclusion: OTC antacids interfere with pepsin’s ability to digest protein.

5. the cardiac sphincter (which, officially is not considered to be a true sphincter), which closes off the top end of the stomach and the pyloric sphincter, which closes off the bottom
6. the small intestine, which has a length of about 6 m. The surface of the small intestine is wrinkled and convoluted to produce a greater surface area for absorption. The total surface area is about 600 m² (about the size of baseball diamond). Most enzymatic digestion occurs here. The secretions of the small intestine include amylase maltase, sucrase, lactase, etc. to digest carbohydrates and lipase to digest fats. Several other associated organs secrete chemicals into the small intestine to aid in digestion: the pancreas secretes enzymes like trypsin, chymotrypsin, and alkali solutions like bicarbonate as buffers and the liver and gall bladder make and secrete bile. Bile contains no enzymes, but salts to emulsify fat so it can be digested.
   the sections of the small intestine include:
   • the duodenum, the first portion
   • the jejunum, the second portion
   • the ileum, the third portion
7. the valve between the small and large intestines, which is the ileocecal valve, a sphincter that separates the two
8. the large intestine or colon, which begins with a blind pouch called the cecum. In humans, this terminates in the appendix, a finger-like extension which may function in the immune system. The large intestine functions to re-absorb (resorb) water and in the further absorption of nutrients. The bacterial flora of the large intestine includes such things as Escherichia coli, Acidophilus spp., and other bacteria, as well as Candida yeast (a fungus). These bacteria produce methane (CH₄), hydrogen sulfide (H₂S), and other gases as they ferment their food. Occasionally, some of this gas is released as flatus. As these bacteria digest/ferment left-over food, they secrete beneficial chemicals such as vitamin K, biotin (a B vitamin), and some amino acids, and are our main source of some of these nutrients.
9. the rectum is the terminal portion of the large intestine and functions for storage of the feces, the wastes of the digestive tract, until these are eliminated. The external opening at the end of the rectum is called the anus. The anus has two sphincters, one voluntary and one involuntary. The pressure of the feces on the involuntary sphincter causes the urge to defecate and the voluntary sphincter controls whether a person defecates or not.

Belching is when swallowed gas moves up the esophagus and is released from the mouth and/or nose. Some people, whose larynx had to be removed, have learned to purposely swallow air and control its release to enable them to talk.

Vomiting is an important reflex to protect from harmful substances. Illnesses like flu, extreme pain (anywhere in the body: migraine, kidney stones. . .), and other stressful conditions can trigger the emptying of the stomach contents.

Hiatal hernia is caused in part by failure of the cardiac sphincter to close properly allowing stomach acid to enter and burn the esophagus.

An ulcer is when the gastric secretions eat through stomach (gastric ulcer) or intestinal wall (duodenal ulcer).

Diarrhea is having very loose, watery feces and constipation is having larger, harder, nearly dry feces. Getting enough fiber is importance to proper intestinal functioning because it holds water in the feces. If feces are too dry and hard, they will pass through the digestive tract with difficulty, possibly leading to diverticulosis or diverticulitis. Also, due to the increased transit time, there is more time for bacteria to ferment the left-overs and secrete increased amounts of carcinogenic byproducts, thereby increasing the person’s chances of colon cancer.

Here’s an interesting wordstem “trivia”: carni is also the rootword for carnival. Many people in the Catholic church traditionally fast during Lent, which begins on Ash Wednesday. Thus, the custom developed of eating up any meat and meat products such as lard left in the house on the Tuesday before, called Shrove Tuesday in English, but in French, Mardi Gras, or “Fat Tuesday”. For many people, this developed into a time to get together with family and friends for one last party before the somber season of Lent. In some places, such as New Orleans, this meat-eating party was expanded into a grand celebration--carnival--one last time to eat meat (and do other things) for a while.