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Digestive System
Biology 20

The digestive system is the only body system that
provides two points of contact (mouth and anus)
between the internal and external environment of
your body
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5-9 meters long in adults
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Mainly lipids, proteins, and carbs
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Lipids - glycerol & fatty acids, carbs - monosaccharides, proteins - amino acids
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○ Chemical: use of ​enzymes​ and chemicals to increase food surface area
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Increasing surface area to breaking things down
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Assimilation: storage of nutrients or their incorporation into tissues
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Food is broken down small enough to pass through the ​cell membrane​ of the ​small
intestine​ into the ​bloodstream​
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Mouth/Teeth:
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1

Teeth are important structures for physical digestion
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Physical digestion breaks food down into smaller pieces, increasing SA for
chemical digestion
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Tongue:
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Taste buds on tongue allows us to enjoy or dislike our food
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Salivary Glands:
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3 salivary glands on each side (6 in total)
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Saliva contains ​water​, ​mucous​, ​amylase​ (enzyme - ends in “ase”)
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Amylase​: digests ​starch​ into maltose and then glucose
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*This
soft mass of food is now ready for swallowing*
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Dives into 2
tubes: esophagus (food tube) and trachea
(windpipe)
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Helps to direct the food down the right path
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Epiglottis:
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2

Valve that covers/blocks the opening/entrance to the trachea (lungs) during swallowing
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Uses
muscular contractions called peristalsis
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Stomach:
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3

J-shaped muscular sac-like organ​ with
three important functions:
○ Storage (about 1
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Entrance and exit are controlled by sphincters (like a gate):
○ Cardiac sphincter: between esophagus and stomach (top of stomach)
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3 Layers of tissue:
○ Outer (connective, tough)
○ Middle (smooth muscle, contract to mix and move food)
○ Inner (mucosa)
Stomach digestion:
○ Physical (occurs as the stomach contents are churned with the muscular
contractions of ​peristalsis​)
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This digestion
is facilitated by ​gastric secretion​)
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The ​inner walls of the stomach​ (mucosa tissue) is folded into many ridges
called ​rugae​ (increase SA)
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■ Gastric pits​ have cells at the bottom that secrete (into organ) gastric juices
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Hydrochloric Acid:
■ Changes the pH of the stomach to allow enzymes to work efficiently
(1-2)
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Mechanically digest food
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■ Pepsin: breaks down large protein chains into shorter chains of
amino acids (active), called ​polypeptides​ or ​peptones​
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2) Proteins + Pepsin -> Polypeptides (Peptides)
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■ Digestion of milk protein: 1) Prorennin + HCl -> Renin
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The storage of highly corrosive acid (HCl) and a protein digesting enzyme
(pepsin) in the stomach could cause some major problems
...
When the protective mucus
lining of the stomach breaks down, the cell membrane is exposed to
the corrosive acid and protein digesting enzymes and may start to
digest the stomach lining itself (may be due to bacteria in stomach)
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○ Stomach stapling​: surgical treatment for extreme obesity (weight control)
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Control of stomach secretions:
○ Entry of protein into the stomach causes the stomach wall to secrete
gastrin (hormone) - act as a messenger​
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Can cause sleepiness (especially after a large meal)
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○ Hormonal Control: when food passes the cardiac sphincter and enters the
stomach, a hormone is released from the stomach wall
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It travels via the bloodstream and arrives at the gastric
glands of the stomach wall, increasing gastric secretions
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Most chemical digestion takes place
in the small intestine
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3 parts: ​duodenum​ (majority of digestion occurs here),​ jejunum​ (absorption of soluble
molecules), ​ileum​ (digestion is completed here, most food is absorbed here)
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○ Each villus contains: ​capillary bed ​[absorbs sugars and amino acids] and
lacteal​ (lymph vessel: carries substances like fat throughout your body)
[absorbs fat]
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Intestinal enzymes:
○ Trypsinogen​ (inactive): activated by a small intestinal enzyme to become
trypsin (protein enzyme)-breaks long chain peptides (stomach already
broke down proteins to peptides) into shorter chains
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erepsin): breaks peptides into amino acids
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○ Proteins -> Peptides -> Amino acids
○ Carbohydrase:
■ Enzymes: sucrase, maltase, lactase
■ Substrate (food) digested: sucrose, maltose, lactose
■ Breaks disaccharides -> monosaccharides (glucose, fructose)
○ Amylase:​ breaks starch -> maltose (di) to glucose (mono)
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Bile emulsifies fats:​ mechanically digests into fat goblets
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Emulsifies​ - making a mixture of liquids that are normally immiscible, miscible (oil
and water)
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Secretions that enter the ​lumen​ (space inside the tubular structure) of the
duodenum come from accessory organs such as the pancreas and gallbladder as
well as from the lining of the duodenum itself
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1
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2
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Pancreas:
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6

Accessory organ
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Responsible for
the ​secretions of many hormones and
enzymes​
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3 hormones secreted:
○ Secretin​: carried from the ​duodenum
(lining senses acid) ​to the pancreas​ via
the ​bloodstream​ and promotes the
secretion of ​bicarbonate ions​ (buffers)
into the ​pancreatic duct​ which is
connected to the duodenum
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○ Enterogastrone​ (”​GIP​”): travels through the bloodstream and targets
both the ​stomach and duodenum​
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This helps digest stuff
like ​fat
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○ CCK​: promotes secretions from the gallbladder and the pancreas into the
small intestine
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Secreted from intestinal
walls when fat enters small intestine
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Bile helps in the digestion of fat
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Pancreas Secretions: pancreatic juices to the small intestine contain:
○ Bicarbonate solution (buffers stomach acid: 8-9)
○ Trypsinogen (digests protein in the duodenum into polypeptides)

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Erepsin (complete breakdown of proteins)
Lipase (breaks down lipids into fatty acids and glycerol)
Amylase (continues the breakdown of carbohydrates: starch; maltase
breaks down maltose; lactase breaks down milk sugar)

Liver:
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Largest gland in the body (1
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Located high in the abdominal cavity directly beneath the diaphragm on the right
side (partially overlaps the stomach)
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The amino group is removed from the amino acid and
converted to ammonia
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○ Like detergent of greasy dishes (large blobs of fat are broken down into tiny
droplets-physical digestion)
○ Contains bile salts, bicarbonate ions, bile pigments, cholesterol
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Stores bile​ until fat enters small intestine
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Can become blocked with ​gallstones​ which are solid lumps of cholesterol crystals
that form in the gallbladder
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But if
your liver excretes more cholesterol than your bile can dissolve, the excess
cholesterol may form into crystals and eventually into stones)
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5 m in length
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Colons: ascending colon (up), transverse colon (right), descending colon (down)
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The ​appendix​ is attached to caecum
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○ Humans cannot digest cellulose - caecum is nonfunctional in humans
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Also
produces vitamins B and K
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Water and salt absorption and feces solidifies
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Anal sphincters (external sphincter under voluntary control: you can decide when
to open/close)

Disorders:
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Hemorrhoids:​ veins that squeeze out of the anus
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Food material passes too quickly through
the large intestine
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Usually caused by an infection
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Food wastes spend too much time in the large
intestine
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Usually from a diet high in fat or protein
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Disorders of Accessory Organs:
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Hepatitis:​ inflammation of the liver (A, B, and C- viral)
Cirrhosis:​ chronic liver disease where healthy liver tissue is replaced by fat and
scar tissue (alcohol usually the cause)
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Appendicitis:​ inflammation of the appendix believed to be caused by a blockage
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The small intestine is regulated by the nervous and endocrine systems
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In the stomach, proteins in the food stimulate the production of gastrin, which
stimulates the HCl production
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The movement of chyme into the duodenum inhibits muscular contractions in the
stomach, preventing more chyme from entering the small intestine
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A hormone known as secretin stimulates the pancreas to produce bicarbonate
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CCK and GIP are both released when high-fat food enter the duodenum
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Secretin, CCK and GIP reduce motility in the stomach, preventing more food from
entering the small intestine
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CCK also increases the release of bile from the gallbladder
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Stomach: very limited - only water, alcohol, glucose and some vitamins
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Small Intestine: most food is absorbed here (mostly in the ileum)
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3
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