is Digestion followed by
Absorption, Transport & Elimination
9/07/11; S.E. Fleming
Recommended reading (same as Lecture 2):
Smolin & Grosvenor, Chapter 3.
Lecture 3
1
Lecture Outline
Review
Overview & basic concepts important for todays
lecture
From mouth to stomach
Role of stomach
Role of small intestine
Movement of nutrients to other organs and cells
Summary
2
Digestion
From food
Nutrients
proteins
carbohydrates
lipids
minerals
vitamins
water
3
Food is digested into its components
(atoms, molecules) which can be used
(assimilated and metabolized) by the body
4
Digestion/absorption happens in the GI tract
5
Lecture Outline
Review
Overview & basic concepts important for todays
lecture
From mouth to stomach
Role of stomach
Role of small intestine
Movement of nutrients to other organs and cells
Summary
6
The Big Picture
Food
Components
of food
Transport
Absorbed
into body
Transport
Fermented within
lumen of GIT
Transport
Eliminated
in feces
Transport
Metabolized by
various tissues
7
Recall from Lecture 2 Vocabulary
Transport The process by which a molecule
moves across a barrier (eg. intestinal wall,
membrane of cell)
8
Basic Biology: What is a Cell?
9
Structure of the Gut Wall
10
Absorption is favored by high
surface area created by:
1. folds
2. villi
3. microvilli
11
Analogy to Surface of Small Intestine
Rolling foothills = Large circular folds
Mountain = Villi
Trees on mountain = Microvilli
12
Absorption analogy
Within the mountains (Villi)
are pipes (arteries, veins, lymph vessels)
that run from ground surface (epithelial cell layer)
into the interior of the earth (body)
13
Absorption requires transport of
molecules ACROSS cell membranes
Some molecules can cross
membranes on their own.
Simple diffusion
eg. alcohol
Osmosis
eg. water
14
Other molecules need help
from carrier molecules
(proteins) and energy to
membranes.
Facilitated diffusion
eg. fructose
Active transport
eg. glucose
15
Lecture Outline
Review
Overview & basic concepts important for todays
lecture
From mouth to stomach
Role of stomach
Role of small intestine
Movement of nutrients to other organs and cells
Summary
16
Mouth
Digestion is favored by actions that occur in
the mouth (chewing, emulsification,
solubilization) and some hydrolysis of
starch
BUT
No Absorption occurs from the mouth
17
Role of Pharnyx and Epiglottis in
Digestion and Absorption
Pharnyx is a common passageway for food and air, connecting
the nasal passages and mouth to the respiratory passages and esophagus
Neither the
pharnyx nor
epiglottis
contribute to
digestion or
absorption
18
The Esophagus Facilitates Transit of
Bolus of Food from Mouth to Stomach
Opening and closing of sphincter allows food to enter stomach and prevents
stomach contents from re-entering esophagus and mouth
19
Lecture Outline
Review
Overview & basic concepts important for todays
lecture
From mouth to stomach
Stomach
Small intestine
Movement of nutrients to other organs and cells
Summary
20
Roles of the Stomach
1. Mixing
2. Secretions:
Acid - plays several roles:
o Protects against infections by killing bacteria
consumed with food
o Alters the chemical structure of proteins,
making them more easily digested
Pepsinogen - converted to pepsin, that an
enzyme helps to hydrolyze proteins
3. Absorption is limited to very small molecules
including alcohol.
21
Cells line the inner mucosal surface of the
stomach and each cell type has a unique role
Parietal cells secrete acid
Chief cells secrete
pepsinogen
22
Antacids can reduce discomforts
(eg. Heartburn) due to hyperacidity
23
Muscles
Muscles are composed
of many elongated cells
and are responsible for
mixing and movement
of contents Rate of
stomach emptying is
regulated to maximize
utilization of nutrients.
24
Stomach contents enter duodenum through
a sphincter that controls rate of stomach
emptying
Rate of stomach emptying
is regulated to maximize
utilization of nutrients.
By slowly releasing
nutrients into small
intestine, nutrients will be
most efficiently absorbed.
Thus, a meal containing
1000 kcals will take twice
as long to empty as a meal
of 500 kcals.
25
Lecture Outline
Review
Overview & basic concepts important for todays
lecture
From mouth to stomach
Role of stomach
Role of small intestine
Movement of nutrients to other organs and cells
Summary
26
Structure of small intestine maximizes
absorption
Massive absorptive surface!
27
Digestive Secretions
Digestion inside the GI tract is assisted by
digestive secretions.
Mucus: viscous material produced by
goblet cells; mucus moistens, lubricates
and protects the GI tract
Enzymes: protein molecules that speed
up chemical reactions; enzymes are not
changed during reactions
28
Role of GI Tract in Immune
Function
The GI tract plays an important role in
protecting the body from infection.
The GI tract limits the absorption of toxins
and disease-causing organisms.
Some cells of the immune system are
present in the GI tract:
Phagocytes
Lymphocytes
Antibodies
29
Crohns disease is due to inflammation
of the small intestinal mucosa
Due to inflammation of
small intestinal mucosa
Causes pain, and
malabsorption of nutrients
resulting in diarrhea and
weight loss
30
Lecture Outline
Review
Overview & basic concepts important for todays
lecture
From mouth to stomach
Role of stomach
Role of small intestine
Movement of nutrients to other organs and cells
Summary
31
Following absorption, nutrients travel
around the body in blood and lymph
32
Blood and lymph transport nutrients and wastes
Most nutrients
are transported
by the blood
Lipids and fat
soluble vitamins
are transported
by the lymph
Blood picks up
waste which are
filtered and
excreted by the
kidney
33
Lecture Outline
Review
Overview & basic concepts important for todays
lecture
From mouth to stomach
Role of stomach
Role of small intestine
Movement of nutrients to other organs and cells
Summary
34
Summary
Components in food must be converted to smaller
molecules that can be absorbed into the and used for
essential functions. To be absorbed, molecules must
cross cell membranes. To do this, most molecules
need help from carrier molecules, and many molecules
also need the boost provided by energy. Once
molecules leave the intestine, they are transported in
lymph and/or blood to organs and cells. Nutrient
utilization is maximized by slowing the rate of gastric
emptying, by maximizing mucosal surface area, and
by making efficient use of enzymes and other
substances such as acids.
35