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DIGESTION
The human digestive system has two important functions: breaking down large food molecules into smaller, usable molecules and absorbing these smaller molecules. Fats get broken down into glycerol and fatty acids, starch into monosaccharides, nucleic acids into nucleotides, and proteins into amino acids. Vitamins and minerals are small enough to be absorbed without being digested.
The digestive tract is about 30 feet long and made of smooth (involuntary) muscle that pushes the food along the digestive tract by a process called peristalsis. The muscles of the digestive tract are controlled by the autonomic nervous system. Figure 15.1 is a chart detailing the structures and function of the human digestive system.
Mouth
• Mechanical and chemical digestion begins here.
• The enzyme salivary amylase in saliva begins starch digestion.
• The tongue and differently shaped teeth work together to break down food mechanically.
• The type of teeth an animal has is a reflection of its dietary habits. Humans are omnivores and have three different types of teeth: incisors for cutting, canines for tearing, and molars for grinding.
• Digestion
• Gas exchange
• Circulation
• Endocrine system
• Nervous system • Eye and ear
• Excretion
• Muscles

REMEMBER
Starch digestion begins in the mouth.
Esophagus
• No digestion occurs here.
• After swallowing, food is directed into the esophagus and away from the windpipe by the epiglottis, a flap of cartilage in the back of the pharynx (throat).
• The esophagus transports food from the throat to the stomach.
Figure 15.1 Human digestive system
Stomach
• Both mechanical and chemical digestion occur here.
• Protein digestion begins in the stomach.
• The stomach’s thick, muscular wall churns food mechanically and secretes gastric juice, which contains hydrochloric acid and enzymes that digest proteins.
• Hydrochloric acid begins the breakdown of muscle (meat) and activates the inactive enzyme pepsinogen to become pepsin, which digests protein.
• The enzyme rennin aids in the digestion of the protein in milk. • The pH in the stomach is acidic, ranging from 2 to 3.

• The cardiac sphincter at the top of the stomach keeps acidified food in the stomach from backing up into the esophagus and burning it. The pyloric sphincter at the bottom of the stomach keeps the food in the stomach long enough to be digested.
• Excessive acid can cause an ulcer to form in the esophagus, the stomach, or the duodenum (the upper intestine). We now know that a common cause of ulcers is a particular bacterium, Heliobacter pylori, which can be effectively treated with antibiotics.
Small Intestine
• All digestion is completed and nutrients are absorbed here.
• The pH in the small intestine is 8.
• It is 6 meters long.
• All digestion is completed in the duodenum, the first 10 inches of small intestine.
• The intestinal enzymes are amylases, proteases, lipases, and nucleases.
• Pancreatic amylases, which digest starch, are secreted into the small intestine.
• Peptidases, such as trypsin and chymotrypsin, continue to break down proteins.
• Nucleases hydrolyze nucleic acids into nucleotides.
• Lipases break down fats.
• Millions of fingerlike projections called villi line the small intestine and absorb all nutrients that were previously released from digested food.
• Each villus contains capillaries, which absorb amino acids, vitamins, and monosaccharides directly into the bloodstream, and a lacteal, which absorbs fatty acids and glycerol into the lymphatic system.
• Villi have microscopic appendages called microvilli that further enhance the rate of absorption. Figure 15.2 is a drawing of a villus.
REMEMBER
Protein digestion begins in the stomach.
REMEMBER
All digestion is completed in the small intestine.

Figure 15.2 Villus
Liver
• Produces bile that emulsifies fats.
• Bile = pH 11; neutralizes chyme (acidified food from stomach) entering small intestine
• Sends bile to the gallbladder until its release into the small intestine • Has other functions besides digestion

1. Breaks down and recycles red blood cells


2. Detoxifies blood—removes alcohol and drugs


3. Produces cholesterol necessary for structure of cell membranes 4. Produces the nitrogenous waste urea from protein metabolism
   Gallbladder
   • Stores bile that is produced in liver
   • Bile emulsifies fats in small intestine
   • Body can function well without a gallbladder
   REMEMBER
   Bile is not an enzyme but it breaks down fats.

Pancreas
• Secretes peptidases, protein-digesting enzymes, into small intestine
Large Intestine or Colon
• No digestion occurs here
• Has three major digestive functions: egestion, vitamin production, reabsorption of water
• Egestion—removal of undigested waste
• Vitamin production—bacteria symbionts living in the colon produce the B
vitamins, vitamin K, and folic acid • Reabsorption of water—

1. Constipation—too much water is reabsorbed from the intestine into body 2. Diarrhea—an inadequate amount of water is absorbed back into body
   Rectum
   • Egestion—removal of undigested waste
   • Last 7 to 8 inches of the gastrointestinal tract stores feces until their release through the anus