Download function is storage and digestion

GI Tract: The Basics
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Stages of digestion
Mechanical versus chemical digestion
Composition of GI tract
Exocrine glands
Step by step through the digestive tract
• explore the digestive tract
• UPenn animation
• animation of organs of
digestion
Stages of digestion
Mechanical digestion: Peristalsis
Figure 24.4
Chemical digestion: Acids Bile
Enzymes
Why do we need enzymes for digestion?
The GI tract is composed of four layers
The GI tract has many exocrine gland cells
Exocrine glands (salivary, stomach,
pancreas): acinus, duct, secretory
epithelium: secrete products into GI tract
Pancreatic cells (adapted for secretion)
Cells are
sandwiched
together by tight
junctions
Digestion/Absorption: The stomach
Structure of the stomach
Rugae: rough ridges which grind food
(mechanical digestion)
What does your stomach do?
Main function is storage and digestion
(minimal absorption)
Hydrochloric acid lowers pH to 2, killing
bacteria and denaturing proteins
1. Mechanical breakdown of food through
muscular action and rugae
2. Chemical breakdown of food with acids and
enzymes
(pepsin begins protein digestion)
Gastrin (hormone) increases acid synthesis
and release
Also produces other hormones (Ghrelin, GIP…)
• Produces chyme
Control of exocrine (gastric juice)
secretion in the stomach
Gastric juice secretion
(enzyme -pepsin, acid HCl
and hormone gastrin is
controlled by:
1. Cephalic factors
2. Gastric factors
3. Intestinal factors
Control of gastric function
The Stomach Lining
Figure 24.13c, d
Serendipity and the stomach…
Saliva/ Gastric juice/ pancreatic juice
Site of
action
pH
Enzymes
Function
Saliva
Mouth/
throat
7-8
amylase
Starch - maltose
Gastric
juice
stomach
1-2
pepsinogen
Proteins – polypeptides/ peptides
Gastric
juice
Pancreatic
juice
Duodenu
m/ small
intestine
8
Trypsin
Amylase
lipase
Proteins – peptides
Starch – maltose
Lipids – fatty acids/ glycerol
GI problems 1: Stomach ulcers….
In 1990’s it was discovered that most GI
ulcers are associated with Helicobacter
infection
Helicobacter survives by producing urease
and protease enzymes
It secretes urease which neutralises the gastric acid to lower the acidity of the stomach for
further colonisation
It also secretes proteases (e.g. mucinase) to degrade the mucosal lining of the stomach wall,
allowing it to burrow into this lining
The degradation of this protective lining by H. pylori allows for damage to the stomach wall by
gastric acids (causing ulcers)
The prolonged presence of stomach ulcers may lead to the formation of stomach cancers
There is a correlation between chronic H. pylori infection over a number of years (20 - 30 years)
and the development of stomach cancer
Barry Marshall
was awarded
Nobel prize "for
discovery of the
bacterium
Helicobacter
pylori and its
role in gastritis
and peptic ulcer
disease"
Stomach problems 2 – ‘heartburn’…
‘GERD’ is characterised by acid reflux
into the oesophagus
Contributing factors
causing weakening of
the cardiac sphincter
include:
• Obesity
• Diet – fatty foods,
chocolate (!), alcohol
(!)
• Smoking
• pregnancy
ONE treatment for GERD blocks acid
production by the parietal cells
Proton pumps in the stomach
• Proton pumps are integral
proteins in the membrane of
gastric epithelial (and many
other) cells
• Using ATP, they pump H+ into
the lumen of the stomach in
exchange for K+
Proton pump inhibitors are controversial
drugs for treatment of heartburn