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.
Esophagus
• No digestion occurs here.
• After swallowing, food is directed into the esophagus and away from the windpipe by the epigloixis, a
flap of cartilage in the back of the pharynx (throat).
• The esophagus transports food from the throat to the stomach.
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 pylon, 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.
• Pepsidases, 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 villas contains capillaries, which absorb amino acids, vitamins, and monosaccharide’s 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.
Liver
• Accessory gland
• Produces bile that emulsifies fhts.
• 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
• Accessory gland
• Stores bile that is produced in liver
• Bile emulsifies fats in small intestine
• Gallstones (abnormal cholesterol deposits) block normal function and cause great pain, requiring
gallbladder to be removed
• Body can function well without gallbladder
Pancreas
• Accessory gland
• 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