CHAPTER
23
DIGESTIVE SYSTEMS AND NUTRITION
I. Overview:
The three phases of digestion:
The cephalic phase: Starts before food entering the mouth, Mechanical and some
chemical digestion for starch
The gastric phase: Mechanical and some chemical for proteins
The intestinal phase: Most chemical and all of the absorptions
Function: Digestion; Absorption; Elimination
II. Digestive Tracts – The mechanical Digestion:
1. The human digestive tract is a complete tube-within-a-tube system. Each part has a
specific function.
2. Food is never found within the accessory glands, only within the tract itself.
3. The digestion of food in humans is an extracellular process.
4. Enzymes are secreted into the digestive tract by nearby glands which never contain
food themselves.
5. Nutritional homeostasis is under hormonal and nervous regulations.
A. Mouth
1. Three pairs of salivary glands secrete saliva by way of ducts into the mouth.
2. Salivary contains amylase, protease and lipase.
3. Food is chewed and mixed with saliva to form a bolus in preparation for
swallowing.
B. Esophagus
1. The digestive and respiratory passages come together in the pharynx, and then
separate.
2. The esophagus is a muscular tube that moves swallowed food to the stomach by
peristalsis, a rhythmical contraction that moves the contents along in tubular organs.
C. Stomach
1. The stomach stores liters of partially digested food, freeing humans from continual
eating.
c. Walls of the stomach contract vigorously and mix food with juices secreted when
the food enters.
d. The gastric juice contains hydrochloric acid and another digestive substance,
pepsin, produced by cells of gastric glands.
2. Hydrochloric acid (HCl) lowers pH of the gastric contents to about 2.
a. Activating the pepsinogen
b. Kills most bacteria
3. Why the stomach is not digested?
a. the mucus membrane
b. Pepsinogen is activated to pepsin at upper pits
c. Ulcers develop when the lining is exposed to digestive action; recent research
indicates this is usually due to infection by Helicobacter pylori bacteria.
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D. The Small Intestine
1. About three meters long.
2. Additional digestion is aided by secretions from the liver and the pancreas (below).
Bile is a secretion of the liver temporarily stored in the gallbladder before being
sent to duodenum; bile emulsifies fat (allows fat droplets to disperse in water).
3. The lining of the small intestine has villi and microvilli, which increase the surface
area to a ball field.
4. It is also the biggest immune organ.
E. Large Intestine and appendix
1. About 1.5 liters of water enter the digestive tract daily from ingestion and another 8.5
liters enter from various secretions.
a. About 95% of this total liquid is reabsorbed by the small intestine; most of the
remainder is absorbed by cells of the colon.
b. If not reabsorbed, it causes diarrhea which can cause a serious dehydration and
ion loss.
III.
Accessory Organs – The chemical Digestion:
1. The Pancreas
Proteinase; Lipase; Amylase; NaHCO3
2. The Liver
a. The liver has numerous functions:
1) It detoxifies blood by removing and metabolizing poisonous substances.
2) It makes plasma proteins including albumin and fibrinogen.
3) It destroys old red blood cells and converts hemoglobin to bilirubin and
biliverdin in bile.
4) It produces bile stored in the gallbladder before it enters the duodenum to
emulsify fats.
5) It stores glucose as glycogen and breaks down glycogen to maintain a constant
blood glucose concentration.
6) The liver produces urea from amino groups and ammonia.
IV. Nutrition: Please read the book
Review Qs:
1. Along with the mineral calcium, what vitamin is necessary for healthy bones and teeth?
2. Except for partnering with muscle in motion, what other functions do bones have?
3. What are contained in gastric juice? Their functions?
4. Functions of the digestive system? When does digestion start?
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5. What are the protection mechanisms against stomach ulcer?
6. What are the functions of the salivary gland? Endocrine or exocrine?
7. What are the functions of the liver? Endocrine or exocrine?
8. What are the common liver diseases? Stomach diseases? Diseases involve intestines and
colons?
9. What are contained in pancreatic juice? What are the functions of the pancrea? Endocrine or
exocrine?
10. Which organ has food for the least time and participates least in digestion?
11. What are the roles of HCl in the stomach?
12. what is the form glucose stored in liver and muscle?
13. Starch is digested in ____________________. Protein is digested in _________________.
Fat is digested in ___________. Function of bile?
14. What is the most important section of intestine? Why?
15. Where does the absorption happen? Function of villi? Except for digestion, absorption, what
are the other function of the digestive tract?
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Critical Thinking
Question 1. Sometimes a section of intestine must be removed when it is found to be
cancerous. Which would most adversely affect the functioning of the total digestive system: the
loss of one foot of duodenum, one foot of the lower small intestine, or one foot of the large
intestine? Why?
Answer:
There would be substantial portions of small and large intestine left over to perform
the functions of nutrient and water absorption, respectively. However, one foot of duodenum
would constitute most of this section. In addition, it is the region that must accommodate the
influx of stomach contents, bile salts, and pancreatic juices.
Question 2. Although we have developed artificial kidneys and hearts, there really is no
machine that can function as an artificial liver. Why is it so difficult to replicate all of the
functions of the liver?
Answer:
Unlike the heart that has the single responsibility to pump blood, or the kidney that
mainly filters blood, the liver has six major and distinct functions: detoxifying blood of
metabolic poisons, making blood proteins, destroying old red blood cells and converting
hemoglobin to bile products, producing bile to emulsify fats, storing and releasing
glucose/glycogen, and producing urea from amino groups. An artificial liver would have to
consist of six complex machines.
Question 3. There are a few populations of people on the globe who have a long tradition of
many more individuals living to great old age (over 100). When emigrants from these regions
live in modern settings with ample diets, their survivorship is lower and near average levels. The
one common factor appears to be a much lower caloric intake, perhaps 70% of the average daily
allowance considered “normal.” Discuss this in the light of “antioxidants.”
Answer:
A person who lives on less food would produce fewer free radicals and suffer less
cellular damage. However, they would risk malnutrition by living nearer the minimal threshold
for vital nutrients and vitamins.
Question 4. If the food product is appetizing to a person, why is it unwise to focus on a diet of
just this one food item, even if it is high in protein like soybeans?
Answer: A food can be high in protein but have a limited number of the essential acids that
cannot be formed by body metabolism and must be in the diet. The amino acids provided by soy
products would not include all of these amino acids and another source would be necessary to
prevent deficiency diseases.
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