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The Digestive System

Image Credits: Biology (Campbell) 9th edition, copyright Pearson 2011, & The InternetProvided under the terms of a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. Derived from content by David Knuffke
by

Thomas Hattori

on 29 February 2016

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Transcript of The Digestive System

Food moves through the GI tract.

Each organ of the tract is separated from each other by muscular sphincters that prevent mixing of material

Food is digested in two ways:
physical
- large pieces are broken into smaller pieces.
chemical
- enzymes disrupt bonds between monomers
Overview
The Mammalian Digestive System
Mouth
Esophagus
Stomach
Accessory Glands
Large Intestine
Control
Adaptations
Animal nutrition involves four phases:
Ingestion
: Intake of food
Digestion
: Breakdown of food macromolecules
Absorption
: Transfer of nutrients into the body
Elimination
: voiding of undigested materials
Diploblastic animals (cnidarians) utilize a
gastrovascular cavity
for nutrition.

The "Bag" plan: The mouth of the animal is also the anus.
Triploblastic animals (everything else) utilize a
gastrointestinal
("
GI
")
tract
for nutrition.

The "tube" plan: The mouth is at one end of the tract and the anus is at the other end.
The evolution of a gastrointestinal tract allows for compartmentalization of the digestive system and increased efficiency & control of nutrition.

Peristalsis
: involuntary contractions of smooth muscle that line the gastrointestinal tract and move food through the tube.
There are a wide variety of strategies used by animals to accomplish this process
Nutritional Requirements
Animals need to acquire all nutrients from their environment.
This includes macromolecules, vitamins and minerals.
Some can be synthesized from raw materials, others must be consumed "pre-made"
Vitamins
: Organic, non-macromolecular, compounds
Minerals
: inorganic elements
Disorders
Deficiencies in nutritional requirements will have effects on the physiology of the organism
Beri-Beri
was the first vitamin deficiency discovered. Polished rice removed the thiamine.
Bringing citrus fruits aboard ships reduced the prevalence of
Scurvy
Data showing a correlation between consumption of folic acid vitamin supplements and a decreased likelihood of neural tube birth defects in a sample of the female population of Great Britain.
General Points:
The human digestive system is a very typical example of the mammalian digestive system.
Structure
Functions
Teeth, tongue, and salivary glands.
Teeth
: mechanically digest food.

Tongue
: moves food into gastrointestinal tract.

Salivary Glands
: Produce
saliva
which moistens food and contains
amylase
(chemically digests starches)
Structure
Functions
A tube that connects the mouth to the stomach.
Begins transport of food through the GI tract.
Pharynx
: beginning of esophagus.
epiglottis
: Prevents movement of food into the respiratory system
Structure
Functions
A muscular pouch.
Capable of rapid expansion.
Filled with "
gastric juice
" (a mixture of pepsin and hydrochloric acid).

Processes food into
chyme
.
Holds food.
Mixes food.
Gastric Juice:
HCl
- Sanitizes food.
pepsin
- chemically digests protein
Stomach Ultrastructure
The walls of the stomach contain many "gastric pits"
Three cell types:
mucous cells
: make a protective mucous coating for the stomach epithelium.
Parietal cells
: Produce HCl
Chief cells
: Produce the pepsin protein precursor pepsinogen

Pepsinogen
is an example of a "
zymogen
", an inactive protein molecule that is converted into the active form in specific conditions

In the case of pepsinogen, HCl is needed to cleave it in to active pepsin.

Why is the pepsinogen/pepsin system necessary?
Term refers to any organs that make secretions that are introduced into the GI tract.
Liver
Pancreas
Makes
bile
which
emulsifies
fat (physical digestion).

Excess bile is stored in the
gallbladder
.
Makes
pancreatic fluid
, which contains a variety of hydrolytic enzymes for all macromolecules
Carbohydrates-
pancreatic amylase
Lipids-
pancreatic lipase
Nucleic Acids-
pancreatic nucleases
Proteins-
trypsin
,
chymotrypsin
, &
carboxypeptidase
The pancreas also produces
bicarbonate ions
, which raise the pH of the chyme (pancreatic enzymes function in basic pH)
Bile and Pancreatic fluid enter the GI tract through a common duct at the beginning of the small intestine (the "
duodenum
")
Structure:
3 sections:
Duodenum
: Digestive section.
Jejunum
: Primary absorptive section (most nutrients)
Ileum
: Final absorptive section (bile salts and some vitamins).
Small Intestine Ultrastructure:
The epithelium of the small intestine is adapted for absorption of nutrients.
Villi
: projections of epithelial tissue. Each vilus is covered in
microvilli
(the "
brush border
").
A network of
capillaries
(circulatory system) and
lacteals
(lymph vessels) runs through each vilus, separated from the
lumen
(interior) of the small intestine by the brush border.

Nutrients diffuse through the brush border, into the circulatory/lymphatic system.
Carbohydrates, amino acids, and other water-soluble molecules are absorbed into the circulatory system, while lipids are absorbed in to the lymphatic system.

Blood and lymph flow from the small intestine to the liver for detoxification of absorbed molecules.
Function:
Final stages of digestion and main area of absorption of nutrients
Structure:
A tube that holds undigested, unabsorbed digestive material.

Also holds a massive colony (~90 trillion cells) of symbiotic bacteria.
Function:
Small Intestine
Reabsorption of water.

Production & absorption of vitamins by bacteria.

Storage and (usually) voluntary elimination of undigested food ("feces")
Digestive Hormones
There are several hormones involved in the digestive system.
The GI tract is under the control of an entire division of the autonomic nervous system (the "
enteric
" division), and is subject to several different regulatory feedback loops
2. Regulation of Blood-Glucose Level
Controlled by 2 pancreatic hormones:
Insulin
- removes excess glucose from the blood via storage in body cells and conversion to
glycogen
in the liver.
Glucagon
- increases glucose level in the blood via glycogen breakdown and glucose release from the liver.
1. Digestion Control
1. The expansion of the stomach to accommodate incoming food triggers the release of
gastrin
, which stimulates production of gastric juice.

2. As chyme moves in to the duodenum, the presence of amino acids and fatty acids triggers the release of
CCK
by duodenal cells, which causes the liver and pancreas to release secretions into the GI tract. The hormone
secretin
is also released by the duodenum which causes the pancreas to release bicarbonate ions to neutralize the acidic chyme.

3. Very fatty foods will cause a large amount of secretin and CCK to be released, which has an inhibitory effect on peristalsis and slows down the digestive process.
3. Satiety
Satiety
is the feeling of being "full".

Under the control of the hypothalamus in the brain.

Leptin
,
Insulin
, and
PYY
are three hormones that decrease the hunger sensation.

Ghrelin
increases the hunger sensation and decreases saitety.

These hormones work in an antagonistic fashion, similarly to insulin and glucagon.
While humans have a fairly typical digestive system, other mammals demonstrate various adaptations.
Dentition
Variation in tooth structure allows animals to adapt to particular sources of food.
GI Tract Adaptations
Many herbivores have highly adapted, elongated appendix structures ("
Cecum
"), which serve as locations for colonies of bacteria that can aid in cellulose digestion.

Carnivores do not have these structures.
Ruminants
Herbivores that possess highly adapted, expanded upper GI tracts.

Allow for maximized
mastication
of vegetable matter and prolonged exposure to symbiotic bacteria.
Obesity
Temporary obesity is frequently seen in animals to deal with environmental fluctuations and nutritional requirements of specific life stages.
Disruptions in food cycles that animals have adapted to can lead to unintended obesity.

This is perhaps the major reason why there is currently an obesity "epidemic" in the United States.
There are many disorders of the digestive system. Here are a few examples:
"Upper GI" Disorders:
"Lower GI" Disorders:
Ulcers
Acid Reflux
Diarrhea/Constipation
Inflammatory Bowel Disease
Cause:
Symptoms:
Cause:
Symptoms:
Cause:
Symptoms:
Cause:
Symptoms:
A disruption of the protective mucus lining of the stomach.

Often caused by the Helicobacter pylori bacterium or NSAIDs.
Persistent "Burning sensation" in upper abdomen.
Treatment:
Antibiotics, diet modification, possible surgery.
Movement of stomach contents through the lower esophageal sphincter.
Transient "Burning sensation" in upper abdomen, particularly following meals.
Treatment:
Antacids, medications that reduce HCl production.
Diarrhea: too much water in feces (decreased absorption by large intestine).

Constipation: too little water in feces (feces remains in large intestine longer than normal).
Cramping (Diarrhea),
Tenesmus
(constipation)
Treatment:
Immediate: laxatives, indigestion aids (e.g. pepto bismol).

Long-term: Diet modification. Increased fiber intake. Decreased intake of fatty, sugary foods.
Autoimmune attack on the large and/or small intestine.
Abdominal pain, diarrhea, weight loss, fatigue, fever
Treatment:
Medication and/or surgery.
Chemical Digestion: A summary
Full transcript