The Stomach
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J shaped expandable organ located on the left side of the abdominal cavity.
The muscular stomach is involved in chemical (mostly protein) and mechanical digestion, as
well as storage of food (up to 2 litres of food).
The stomach has muscles that cause it to churn the contents, helping to break up food.
This churning usually starts automatically at meal times. This is what is happening when you
hear your stomach "growling."
The churning also mixes the food with Gastric Juices secreted by the epithelial cells and
HCl (pH 1.5-2.5).
Gastric juices are a mixture of digestive juices made by the stomach (2L/day). These
secretions are controlled by the autonomic nervous system (smells, thought, and caffeine)
and by various digestive system hormones.
o It includes:
 PEPSIN an enzyme used to partially hydrolyze protein (start protein
digestion). Pepsin is released in an inactive form PEPSINOGEN from the
cells that line the gastric pits. The pepsinogen reacts with HCl to form pepsin.
 The hormone GASTRIN is secreted by the stomach cells to regulate the
production of gastric juices. Gastrin causes an increase in the secretion of HCl
from the parietal cells and pepsinogen from the chief cells in the stomach.
Gastrin is released by G-cells in the stomach as a response to distenstion and
digestive products.
 The hormone CHOLECYSTOKININ (CCK) decreases gastric emptying and
increases release of pancreatic juice, which is alkaline and neutralizes the
chyme.
 The hormone SECRETIN is produced by the small intestine and can diminish
acid secretion in the stomach.
 MUCUS - protects the lining of the stomach from being digested.
 HCl- a strong acid that is needed for pepsin formation. Also helps to break
down food and kill bacteria in the stomach.
The stomach is closed at its posterior end by the PYLORIC SPHINCTER.
By the time bolus of food leaves the stomach for the small intestine it is a thick liquid called
CHYME.
The cardia, fundus, body, and pyloric (w/antrum) are specialized areas of the stomach.
 Body- Bolus churns with digestive juices.
 Pyloric (w/antrum)- Region of the stomach that connects with the duodenum.
 Cardia- where the contents of the esophagus empty into the stomach.
 Fundus- allows for the accumulation of stomach gases produced by chemical digestion.
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The muscularis externa layer of the stomach wall is unique in that it has three sheets of
muscle (circular, longitudinal, and oblique).
o The circular muscle layer prevents food from travelling backward. The longitudinal
layer shortens the tract and the oblique layer (only seen here) churns and physically
breaks down the food. (Figure 1.).
The stomach can expand greatly because of internal folds called RUGAE (Figure 2.).
Figure 1. Muscle layers of the Stomach Figure 2. Rugae of the Stomach
The Small Intestine
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Most hydrolysis of macromolecules occurs in the small intestine. It is more than 6 meters in
length and 2.5cm in width. It has smaller diameter than that of the large intestine.
The walls are muscular and these muscles move food through the intestine by peristalsis.
Bordered by the pyloric sphincter and the ileocecal valve.
It is divided into 3 sections (Duodenum, Jejunum, and ileum).
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Accessory Organs (Pancreas, Liver, and Gall Bladder), add digestive enzymes, juices and
hormones into the small intestine.
o Pancreas: Secretes bicarbonate to prevent damage to the intestine by the acid from the
stomach.
 Releases digestive juices that digest starch, proteins and fats.
o Liver: Produces bile to emulsify fat.
o Gallbladder: Stores the bile made by the liver.
As the acid chyme enters the duodenum (first 25 cm of the small intestine) a hormone called
SECRETIN is released from the intestinal walls to signal the pancreas to release a
bicarbonate solution which neutralizes the acid.
The hormone CHOLECYSTOKININ (CCK) is released from the intestinal cells causing
the gall bladder to release bile.
o It also causes the pancreas to release its digestive enzymes.
The hormone ENTEROGESTRONE is also secreted to slow down peristalsis.
Protein Digestion:
o Trypsin and Chymotrypsin are enzymes that break bonds next to specific amino
acids. Carboxypeptidase splits off one amino acid at a time. This enzyme works on
the end with the free carboxyl group.
o Aminopeptidase works in the opposite direction.
o All the above enzymes are secreted in an inactive form. They are activated by the
hormone ENTEROKINASE.
Fat Digestion: Bile from the Liver (stored in the gallbladder) emulsifies fat. This creates a
larger surface area for the enzyme lipase to digest it.
Carbohydrate Digestion: Disaccharide digestion is under the control of the enzymes maltase,
lactase, sucrase.
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Once the chyme is digested, it is absorbed into the bloodstream by the walls of the small
intestine.
The small intestine has plicae circularis (circular folds) and is lined with small finger-like
projections called VILLI. These in turn are full of smaller projections called microvilli.
o Each villus contains a net of capillaries and a large lymph vessel called a lacteal. The
nutrients except fat are absorbed into the capillaries, while the fat enters the lacteal.
o The three of these increase surface area.
Macronutrients
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Carbohydrates – contain hydrogen, carbon and oxygen atoms.
o Main energy source for the body. Small and therefore easily enter cells for energy
o Simple sugars: monosaccharide (Glucose, Fructose, Galactose), Disaccharides
(Sucrose, Maltose, Lactose). Monosaccharides form disaccharides and
polysaccharides by dehydration synthesis.
 Glucose + glucose  maltose
 Glucose + fructose  sucrose + H2O
 Glucose + Galactose  Lactose + H2O
o Polysaccharides:
 Many simple sugars, joined together for storage  long chains of
monosaccharide. Plants form cellulose and starch and animals store extra
sugar as glycogen (stored in the liver and muscle).
Lipids (Fats and Oils) - Stores twice as much energy as carbohydrates.
o Used by the body to store excess energy and cushion and insulate our organs
 Allows the dissolving of some fat-soluble vitamins.
o Components of lipids:
 Glycerol and fatty acids
 Glycerol + 3 fatty acids  triglyceride
o Saturated fats – solid at room temperature and come from animals, butter, margarine
(lard), coconut oil.
o Unsaturated fat – liquid at room temperature and come from vegetable sources
Proteins: Complex compounds that are composed of amino acids needed for structures of the
body