Download Lecture 22 - Taft College

22 Digestive
System
Taft College
Human Physiology
Functions of the Digestive System
(4 Basic Functions of Digestion)
1. Ingestion - food intake.
2. Digestion - process of reducing
foods to their constituent building
blocks from larger molecules to
smaller ones that may be absorbed
and used by the body.
– a. mechanical or physical
digestion - ex. - chewing
– b. chemical digestion – ex.
Digestive enzymes (catalysts)
3. Absorption - taking in the nutrients
that have been broken down.
4. Defecation or egestion elimination of unabsorbed residues
of the digestive process.
Components of the Digestive System
1. Alimentary Tract - tube with various
modifications from mouth to anus.
– All digestion occurs in the tube.
– All absorption occurs from this tube.
– Within wall of tract is smooth muscle that
serves to propel foods = peristalsis
2. Accessory organs - structures that lie
outside of the alimentary tract.
– These organs empty secretions into the
digestive tract by way of ducts.
– The secretions are necessary for digestion.
Control of Feeding (Appetite, Hunger)
Feeding is controlled by the hypothalamus.
(remember homeostasis = hypothalamus).
How the hypothalamus determines we are hungry is
not sure, but here are some hypotheses:
1. Gastric sensation hypothesis - filling of stomach
or stretching of abdominal wall.
• Some diet aids attempt to work on this mannerlike a sponge that swells.
2. Glucostat hypothesis - body monitors glucose
levels. Glucose uptake by liver cells influence
vagus nerve and information is transmitted to
lateral hypothalamus (Glucose Receptors).
3. Thermostat hypothesis - intake of food associated
with rise in metabolic activity. Therefore,
increase in heat. The increase in temperature
reduces appetite.
• Closely tied to caloric need. In cooler
temperatures- the more food we eat.
4. General nutritional state hypothesis - body can
monitor usage of a particular nutrient
(substance, vitamin, minerals).
Control of Feeding (Appetite, Hunger)
5. Intake of specific foods hypothesis - body may be able to monitor presence of certain
foods or food groups. It is unlikely that a single hypothesis will explain appetite for
food by itself. More than likely your interest in food is due to a number of factors.
Interesting- tend to have appetite (craving) for different things a different times.
Apparently to satisfy nutritional need at that time.
6. Psychological status hypothesis - sometimes we eat out of habit or boredom.
• Pleasure: food preferences, eat more in cafeteria, appetizers, dessert, Pleasure
Centers in Hypothalamus.
• Social factors: cultural demands to be thin...anxiety.
7. Neurotransmitter and hormones
• Research has shown that neurotransmitters (serotonin) and hormones (leptin from
fat cells and PYY from intestine) are involved in a feeling of fulfillment following
eating.
• Carbohydrates increase serotonin levels in the brain, so one may eat carbohydrates
when depressed.
• Use of drugs such as fenfluramine (FenPhen is a combo drug.) Biological Basis:
fenfluramine increases serotoin levels, while phentermine is an amphetamine mimic
that promotes a sympathetic response. Both curb appetite.) Taken off the market as
heart failure occurred.
• Research in these areas may be very important in in reducing the alarming increase
in childhood obesity in the U.S.
Function and Control of Specific
Digestive Organs
(in order passed by bolus of food)
I.
Mouth
1. Ingestion begins
2. Digestion
a. Mechanical digestion - chewing = mastication.
b. Chemical digestion
Saliva- produced by the salivary glands, is important as it:
1. Begins the digestion of carbohydrates. Starches are
broken down to disaccharides by amylase in the saliva.
Only 3-5%, since food spends such short time in mouth.
2. Dissolves soluble foods.
3. Moistens food or lubricates food for swallowing.
4. Cleanses and moistens mouth- helps to prevent tooth
decay.
Saliva washes away acidic waste of bacteria that promotes
tooth decay.
3. Swallowing
Movement of bolus of food. A voluntary act using skeletal muscles.
Tongue forces food against hard palate (roof of mouth).
Function and Control of Specific
Digestive Organs
(in order passed by bolus of food)
II. Pharynx
•
The food/drink moves into the oropharynx and stimulates receptors that
go to the medulla oblongata. Involuntary impulses cause the soft palate
and uvula to be forced up and close off opening from nose to pharynx.
•
The larynx moves up under the tongue and the vocal cords come
together which seals off the respiratory passage.
•
The bolus passes through the laryngopharynx and enters the esophagus
in 1-2 seconds.
Function and Control of Specific
Digestive Organs
(in order passed by bolus of food)
III. Esophagus
1. Transportation of foods to stomach by peristalsis - wave like contraction of smooth
muscle.
•
Peristalsis is controlled by the medulla oblongata so is involuntary response.
•
Takes about 4-8 seconds for bolus, 1 second for liquid to reach stomach.
2. Secretion of mucus by glandular cells
acts as a lubricant of the bolus.
•
No digestion or absorption occurs in
the esophagus.
3. Vomiting - reverse peristalsis. Usually
stimulated by irritation and distention
of the stomach. Medulla oblongata
vomit center causes abdominal wall
muscles and diaphragm to squeeze
the stomach and expel its contents
through the open esophageal
sphincters.
•
Heart burn is due to HCl from the
stomach contents entering the
inferior part of the esophagus causing
burning and irritation. Don’t lie down
after a meal. Prolonged exposure can
lead to esophageal cancer.
Function and Control of Specific
Digestive Organs
(in order passed by bolus of food)
IV. Stomach
Major functions:
1. Storage (reservoir)
2. Chyme formation = combination of
saliva, gastric juice, food, and drink.
3. Protein digestion by HCl and pepsin.
(some lipase activity)
4. Little absorption of food. Water, ions,
some drugs (alcohol , aspirin).
Function and Control of Specific
Digestive Organs
(in order passed by bolus of food)
Digestion in the Stomach
•
The stomach contains many gastric glands that produce
gastric juice.
•
Gastric juice = watery solution of HCl and pepsin, lipase,
gastrin, and intrinsic factor for B12 absorption.
•
3 important types of cells in the stomach, each with
different functions:
1. Parietal (oxyntic) cells- secrete HCl, hydrochloric acid
which:
•
a. Kills microbes in food.
•
b. Denatures proteins.
•
c. Converts pepsinogen to pepsin.
2. Chief (zymogenic) cells- secrete pepsinogen (inactive
form) which is converted to pepsin (active form) in the
presence of HCl. Breaks certain peptide bonds in
proteins.
•
Secrete gastric lipase (breaks down triglycerides).
3. (Mucous) Neck cells- secrete mucus that forms a
protective layer barrier that prevents digestion of
stomach wall (and absorption of materials).
Function and Control of Specific
Digestive Organs
(in order passed by bolus of food)
The Small Intestine – Structures
1. 3 sections of small intestine, in order: Duodenum, Jejunum & Ileum
2. Folds in small Intestine - All these folds dramatically increase surface area for
absorption.
•
a. Plicae of small intestine
•
b. Villi- finger like projections – Villi contain blood and lymph capillaries that
will carry the nutrients away.
•
c. Microvilli - absorptive
simple columnar epithelium
cell surface (w/ 1700
microvilli per cell, which
makes for a massive area for
absorption.
3. Structures (digestive glands)
associated with the small
intestine.
•
Pancreas, liver, and
gallbladder – All produce
digestive secretions
Function and Control of Specific
Digestive Organs
(in order passed by bolus of food)
The Small Intestine-Function
1. Absorption of nutrients (microvilli is
absorption unit). 90% of nutrients
absorbed here
2. Digestion - Many digestive enzymes are
present.
3. Secretion - of intestinal juice about 1- 2
L/day. Slightly alkaline (7.6) mix of
water and mucus. Provides a vehicle
for chyme to move through the
intestine to be absorbed.
4. Protection - lymphoid tissue (Peyer’s
patches) especially towards the end
(ileum) of the small intestine. The colon
contains huge numbers of bacteria that
must be kept in check.
Function and Control of Specific
Digestive Organs
(in order passed by bolus of food)
The Small Intestine Function
Digestion in the Small Intestine
• We shall take a look at digestion
of the basic food groups:
• Carbohydrates, Proteins, & Fats
We will see that the pancreas is:
1. The primary source of digestive
enzymes that act in the intestine.
• Pancreatic juice is ducted to
duodenum – also exocrine gland.
2. Provides the alkalinity
(bicarbonate) necessary for
creating optimum pH for enzyme
activity in the small intestine.
Pancreas
Digestion of Basic Food Groups
End products
Note: -ase on end indicates an enzyme that breaks down chemical.
Note: significance of pancreas in all food groups.
Bile
• Bile (1 L/d) is produced in liver and
stored in gallbladder. Greenish to
brownish consisting of water, bile acids,
phospholipids, bile pigments.
• Gives stool & urine its color.
• Bile serves to emulsify fats. It breaks
down lipids so they can be absorbed.
Emulsifies large lipid globules into 1mm
droplets.
• If bile backs up due to liver injury or
obstruction, bilirubin (from breakdown of
heme group in rbc) will back up and
deposit in body producing a yellow color
= jaundice.
Function and Control of Specific
Digestive Organs
(in order passed by bolus of food)
The Small Intestine – Function
Absorption in Small Intestine
•
•
Long chain
fatty acid
Absorption of the end products of
digestion takes place through
epithelial lining of small intestine.
Microvilli offers increased surface
area.
Most end products of digestion,
monosaccharides, amino acids,
plus water, vitamins, and ions are
absorbed into blood capillaries of
the villi. Most dietary fatty acids
(long chain) differ in that they are
absorbed by the much leakier
lacteals of the lymphatic system.
Note - Any disruption of this lining may lead to discomfort, diarrhea, and
malabsorption. The lining of the small intestine may be affected by
chemotherapy, radiation therapy, toxins, or infection.
Function and Control of Specific
Digestive Organs
(in order passed by bolus of food)
Function of the Colon (large intestine)
•
No more digestion by you, but some by
bacteria of indigestible foods. Can be
beneficial (vitamins) or embarrassing
(gas).
Important for:
1. Water absorption.
2. Electrolyte absorption.
3. Vitamin absorption- bacterial products Vit.
K, B complex (B1, B2) .
•
Vitamin K is necessary for formation of
clotting proteins in the liver. If given
wide spectrum antibiotics that kills off
bacteria, give Vit. K injections.
4. Formation of feces (stool, solid waste
(hopefully)).
•
As feces enter and fill the rectum,
stretch receptors initiate a defecation
reflex that empties the rectum to the anal
canal.
•
Once the reflex is triggered, only the
voluntary external sphincter is holding
the feces back so voiding can be
postponed until an appropriate time.
Functions of the Liver
1.
2.
3.
4.
5.
6.
7.
Detoxification: detoxifies
alcohol, drugs, and other
substances or modifies them
so they are less toxic.
Bile formation
Metabolism and storage of
many nutrients, vitamins, and
minerals
Storage reservoir of blood in
veins.
Cleanses blood from gut of
bacteria.
Produces blood in fetus.
Destroys old RBC. The heme
portion is absorbed as bilirubin
that is secreted in the bile.
Hours
a
c
b
c
a
b
e
d
d
e
Intermediary Metabolism
• Intermediary metabolism is all the
thousands of different chemical
reactions in your cells.
• Intermediary metabolism mainly
manages carbohydrates, proteins,
and fats.
• To burn glucose, lipids, proteins, all
use same reactions:
• 1. Glycolysis
• 2. Krebs cycle- CO2 and H20 are
wastes
• 3. Electron transport system (ETS)needs O2, = aerobic respiration
makes most ATP
• If no O2 = anaerobic respiration
occurs = fermentation = lactic acid is
produced
Intermediary Metabolism
•
•
•
•
•
•
•
•
•
•
•
Carbohydrate metabolism
Catabolism = oxidation = cellular respiration = chief source of ATP.
CO2, H20 are wastes.
Makes most of ATP, needs O2 (aerobic respiration). If no O2 =
anaerobic reactions = fermentation = lactic acid.
Anabolism
Some glucose is converted to glycogen for storage. Can be
reconverted to glucose for ATP. A.a., glycerol, lactic acid can be
converted to glucose too.
Triglycerides
Catabolism
Broken down into glycerol and fatty acids. Burned for energy in
Krebs-ETS . Ketone bodies appear in large amounts during a
diabetic crisis.
Anabolism
Synthesis of triglycerides from glucose and fatty acids = lipogenesis.
Stored in adipose tissue.
Intermediary Metabolism
• Protein
• Catabolism
• Ammonia converted to urea in liver, excreted in urine. Can be
converted to glucose, fatty acids, of ketone bodies.
• Anabolism
• Protein synthesis is directed by DNA and carried out by RNA and
ribosomes.
Disorders of Digestive Tract
Diarrhea - defecation of liquid stool.
• Caused by increased motility and decreased absorption of water
by the intestines. Diarrhea can be caused by microbes (toxins
or intestinal wall invasion), certain foods (some fruits increase
motility) (lactose intolerance, enzyme deficiency with increased
osmolarity in lumen), stress (anxiety increases parasympathetic
activity to lower bowel with increased motility).
• Dehydration and electrolyte imbalance may occur if prolonged.
Constipation - infrequent of difficult defecation due to lack of
motility.
• Feces remain in colon too long with excessive water absorption
so feces become dry and hard. Causes include lack of fiber in
diet, improper bowel habits (failure to heed the call), lack of
exercise, emotional upset, lack of adequate fluid intake.
Disorders of Digestive Tract
Ulcers
•
•
PUD = peptic ulcer disease = a crater like lesion in the
gastro/intestinal membrane exposed to acidic gastric juice. Usually
occurs in pyloric region (10%) or first part of duodenum (90%).
Serious bleeding can occur (GI bleed) with anemia.
3 causes:
a. Bacteria = Helicobacter pylori.
b. nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin.
c. hypersecretion of HCl, e.g. a gastrin producing tumor.
Food Poisoning
•
Microbes can cause irritation of the intestines by direct invasion of
the intestinal wall or by secreting toxins. Microbes find their way
into foods where they multiply. Common causes are Salmonella,
Shigella, Campylobacter, Staphylococcus toxin.
Disorders of Digestive Tract
Botulism
• Ingested toxin poisoning that is produced by Clostridium botulinum.
Found in improperly cooked non acidic foods. The toxin inhibits ACh
release so skeletal muscle paralysis results: blurred vision, difficult
breathing, swallowing, speech, general weakness.
Assignment 8
1. What part of the brain controls appetite?
2. Summarize each of the 6 hypotheses
covered in lecture that serve to attempt
to explain how appetite may be
controlled.
3. Based on your own eating behavior,
state which 2 hypotheses you can
specifically relate to and why.