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DIGESTION continued 2
11) The cardiac sphincter relaxes, allowing the bolus to enter the stomach
- Stomach = J-shaped stretchable organ that can hold approximately 1.5 L of food
 The stomach acts as a food reservoir; it receives food from meals at
once, then releases it into the intestine at intervals
12) The stomach’s muscular walls churn and squeeze the food using the thick layers of
smooth muscles
- This is another form of mechanical digestion
13) Gastric juices are added from cells of the gastric glands and cells that line the inner
wall of the stomach
- This aids the mechanical digestion in the stomach and is a form of chemical
digestion
 Approx. 500 mL of gastric juice produced / large meal
 Approx. 1500 mL of gastric juice produced / day
- The gastric fluid, or gastric juice, includes:
 Hydrochloric acid (HCl – pH=1-2) – kills harmful substances that are
ingested with food, and converts pepsinogen into its active form,
pepsin
 Pepsin – a protein-digesting enzyme (protease)
 Mucus – provides a protective coating (because of HCl and Pepsin)
- If the protective mucous lining of the stomach breaks down, the cell membrane is
exposed to HCl and pepsin, and eventually blood vessels can be broken down as
well
 This destruction of the cell membrane is known as an ulcer
14) At this point, the food bolus is partially digested and is in the form of a liquefied
paste called chyme
15) The stomach’s muscular contractions propel the chime through the pyloric
sphincter
 Pyloric sphincter = ring of smooth muscles that opens to allow small
amounts of chyme to pass with each wave of gastric peristalsis
16) Chyme enters into the duodenum section of the small intestine
- Small intestine: the major site of digestion and absorption
 It is called the small intestine because of its diameter. It is actually
much longer than the large intestine (7m vs 1.5m) in humans.
 The small intestine consists of three sections:
 Duodenum,
 Jejunum,
 Ileum
- How are these cells in the small intestine protected from the HCl and pepsin
soaked chime?
When acids enter the small intestine,
the chemical prosecretin is
converted into secretin
Secretin is absorbed into the
bloodstream  it is carried to
the pancreas, where it signals
the release of a solution of
bicarbonate ions  this
solution of bicarbonate ions is
then carried to the small
intestines to neutralize the
gastric fluid (from pH 2.5acidic to pH 9-basic)
17) In the duodenum, chyme is digested even further, breaking down the proteins,
carbohydrates, and lipids into their simplest forms
PROTEIN DIGESTION:
- The pancreas releases trypsinogen, which is converted to its active form
trypsin once it reaches the small intestine.
 Trypsin breaks down long-chain polypeptides into shorter-chain
peptides
- The pancreas and small intestine release erepsins to complete the protein
digestion, breaking down the short-chain peptides into individual amino acids.
CARBOHYDRATE DIGESTION:
- The pancreas releases amylase enzymes to continue the carbohydrate
digestion started in the mouth, and disaccharidases (disaccharide enzymes)
help complete the digestion of carbohydrates
LIPID (FAT) DIGESTION:
- The pancreas releases lipases to break down lipids into fatty acids and glycerol
- The liver continually produces bile and stores it in the gall bladder when the
stomach is empty. Bile contains bile salts, which speed up fat digestion
(emulsifying the fats into smaller droplets, with larger surface area for other
fat digesting enzymes to finish the job).
18) Nutrients are absorbed through the walls of the jejunum and ileum
- Approximately 80% of all absorption occurs in the small intestine
 The small intestine lining has finger-like extensions called villi – this
shape allows them to greatly increase the surface area for absorption
- Each villus has blood
capillaries and lymph vessels
(called lacteals) to help absorb
and transport nutrients
- The cells that line each
villus have their own microvilli
to increase the surface area
available for absorption even
more.
19) Chemical digestion is done by the time food/chyme reaches the large intestine
20) The colon, the largest part of the large intestine, must store wastes long enough to
reabsorb water out of the wastes
- Some inorganic salts, minerals, and vitamins are also absorbed with the water
- The large intestine also houses bacteria, like E. coli which process our waste and
synthesize vitamins B and K.
- Cellulose cannot be broken down by humans, but this roughage/fibre provides
the bulk of the waste build-up, and in turn, prompts bowel movements.
21) Left over solid waste, called feces, passes through the rectum
22) Feces exits the body through the anus (Defecation)