Digestion after the Stomach
Small intestine:








Most digestion and nutrient absorption occurs in the small intestine.
The small intestine is a long tube that is only about 2.5cm in diameter and approximately 7 m
long. Whereas the large intestine is around 7.6cm in diameter and 1.5m in length.
Lipids, carbohydrates, and remaining proteins are digested in the small intestine.
It is made up of 3 sections:
o Duodenum, jejunum, and the ileum
DUODENUM: the first 25 to 30 cm. where most enzymes are added and where digestion in the
small intestine begins.
JEJUNUM: digestion continues and some nutrients are absorbed.
ILEUM: where most nutrients are absorbed. Can be distinguished from the jejunum based on
shape and size of epithelial cells.
Surface Area: the inner surface of the small intestine is adapted to provide the maximum
amount of surface area for efficient nutrient absorption.
o Inner layer is folded into ridges and has many finger like projections called villi (villus)
o Each villus has even smaller, microscopic projections called microvilli
o Each villus has a network of blood vessels called capillaries
o All nutrients except digested fats, enter the blood stream through the capillaries
o Digested fats transfer through small vessels called lacteals and transported through the
lymphatic system
Chemical Digestion in the Small Intestine:





Chyme is slowly released into the small intestine at a pH of 2.5.
Cholecytokinin (CCK): a hormone secreted by cells in the mucosa of the duodenum and is
released into the blood stream.
 Signals the pancreas to secrete substances to control pH and enzymes to digest
carbohydrates, lipids and proteins.
 Stimulates the release of bile
 CCK signals the stomach to slow digestion
PANCREAS: a long flat gland nestled between the stomach and the duodenum. It secretes
enzymes that are critical to the digestive process.
o Amylase: excreted by the pancreas into the duodenum to break down carbohydrates
o Insulin: controls glucose levels
o Lipase: the pancreas also secretes these enzymes that break down fats. Breaks down
lipids into shorter chains and into individual fatty acid chains at the surface of fat
globules
o Trypsinogen-Trypsin: the pancreas releases trypsinogen, which is the inactive form of
the protein digesting enzyme trypsin (further digests proteins broken down by pepsin).
Once in the duodenum, the enzyme called enterokinase converts trypsinogen into
active trypsin.
Prosecretin-Secretin:
o Prosecretin is secreted by the lining of the small intestine and is converted to secretin
when mixed with the acidic chyme.
o Secretin: active form, which stimulates pancreatic (bicarbonate ions; HCO3-) and bile
secretions.
 The bicarbonate ions neutralize the hydrochloric acid to a pH of 9
 Pepsin is inactivated in the basic pH
 Acts a digestive rate regulator and prevents more food from entering from the
stomach until digested
Carboxypeptidase and Erepsin: other protein digesting enzymes that help break down of small
protein chains into single amino acids
The Liver and Gall bladder- secrete to the duodenum



The liver in its digestive functions is considered a gland because it produces and secretes
bile
o Bile emulsifies fats, breaking down large fat globules into micelles. This provides a
larger surface area for lipases to digest
Bile is continuously made, but stored in the gall bladder. Fat entering the duodenum
stimulates the gall bladder to contract, squeezing the bile through the bile duct and into the
duodenum
Blood passing through the capillaries of the intestines pass through the liver which also
filters toxins such as alcohol. It also stores glycogen and fat soluble vitamins. Diseases of this
organ include hepatitis, jaundice, sclerosis and cancer.
Absorption

Passive transport: the movement of materials across a cell membrane without any expenditure
of the cell’s energy
o
o
o

Diffusion: the movement of molecules from an area of higher concentration to a lower
concentration ( move from the small intestines to capillaries, e.g. amino acids)
Osmosis: the movement of water from an area of higher concentration to a lower
concentration across a selectively permeable membrane (some water content in food is
absorbed by osmosis in the stomach and small/large intestine)
Facilitated diffusion: the diffusion of molecules across a cell membrane via a transport
protein. Each protein has a unique size and shape and only let specific molecules
through. (Many of the monosaccharide’s and disaccharide’s move to the capillaries via
facilitated diffusion)
Active transport: materials are moved across a cell membrane from a lower concentration to a
higher concentration using energy provided by the cell. Special protein carriers are embedded in
the membrane to carry large molecules and ions across
Diffusion
Large Intestine






1.5m in length and 7.6cm diameter
Most of our food has been digested and absorbed except for indigestible material like cellulose.
They provide bulk and help us maintain a full feeling for longer. Helps retain water, which helps
in elimination of wastes.
Water from the leftover matter is absorbed via osmosis as it passes through the colon.
o When water is removed, feces, the undigested waste, is eliminated from the body
o The removal of too much water= constipation
o The removal of too little water= diarrhea
Vitamins B and K, sodium Na+, Cl-, are absorbed
Takes from 4hrs to 72hrs for undigested material to move through the large intestine
There are more than 500 species of bacteria that inhabit the large intestine
o E.coli: produce vitamins K and B
o Bi-product of the bacteria is the Production of gas (carbon dioxide, methane and
hydrogen sulphide
Composed of the CECUM, COLON, RECTUM, ANUS



CECUM: the end the large intestine below the opening to the small intestine. A “dead end” that
receives processed material from the small intestine. Also contains an opening to the APPENDIX
COLON: longest part of the large intestine and has four segments; the ASCENDING,
TRANSVERSE, DESCENDING and SIGMOID colon
RECTUM: the last 20cm of the large intestine. Holds waste products of digestion until they are
eliminated through the external opening, the anus.
o Nerves in the wall of the large intestine detect the movement of feces in the rectum and
this stimulates the defecation reflex. We are in control of this reflex to a certain degree.
o The internal( smooth muscle and involuntary) and external(skeletal and voluntary) anal
sphincters control the process of defecation
Nausea, Vomiting and Diarrhea





Nausea: unpleasant feeling associated with the urge to vomit. Your mouth may water, you may
break into a cold sweat, look pale and your stomach may feel upset.
Emesis (vomiting): a protective mechanism for removing toxins or foreign substances from the
body. The expulsion of stomach contents through the esophagus and mouth
o Strong muscular contractions of the diaphragm and abdominal muscles, force the
contents (vomitus) of the stomach up
o The gastroesophageal sphincter relaxes
o Breathing ceases as the larynx and epiglottis close the entrance to the trachea
Causes of vomiting:
o Symptom of an underlying condition
o Occurs in response to a signal from the stomach, intestines, bloodstream, inner ear, or
sensory organs
o Examples:
 Inner ear: motion sickness
 Drugs: chemotherapy, antibiotics, nicotine, alcohol
 Infections: bacterial (E. coli), viral (stomach flu)
 Brain: meningitis, migraines, tumors
 Injury: concussion, hemorrhage
 Diseases: gall bladder disease, cancer, ulcers
o Risks: dehydration, vomitus in respiratory tract causing asphyxiation or infection, tears
in the lining of the esophagus and/or strains in abdominal muscles
Diarrhea: loose or watery feces as a result of inadequate absorption
o Accompanied by upset stomach, stomach pains/cramps and intestinal gas
o Causes: bacteria (salmonella,E.coli), viruses and parasites
o E. coli infections can lead to internal bleeding, severe dehydration, and in severe cases
kidney failure (Walkerton water)
Colitis: is swelling (inflammation) of the large intestine (colon).
o Causes: food poisoning, inflammatory disorders (ulcerative colitis, Crohn's colitis)
irritable bowel syndrome, lack of blood flow (ischemic colitis) or past radiation to the
large intestine
o Symptoms can include: bloating, gas, bleeding, chills, constant urge to defecate,
diarrhea, dehydration, fever and abdominal pain
Feedback Loops
Hormones operate on positive or negative feedback loops:
Homeostasis – state of constancy / maintaining an internal steady state
Negative feedback – a change in a physiological variable triggers a response that
counteracts the initial change
Positive feedback – a change in a physiological variable triggers a response that amplifies
the change
Examples of Positive Feedback
1) Childbirth
 Pressure of baby’s head against sensors near opening of the uterus stimulates uterine
contractions
 Contractions cause greater pressure against the uterine opening which causes stronger
contractions
 Stronger contractions cause even greater pressure
 Eventually allows child to be born
2) Gastrin
 Impulses from brain (sight, smell, taste of food) stimulate stomach wall to produce
gastric juice which begins digestion of food in stomach
 Then the products of digestions (substances in the food) stimulate the stomach wall to
release the hormone gastrin into the circulatory system
 As gastrin recirculates in the bloodstream back to the stomach wall, the hormone
stimulates further release of gastric juice
 Therefore initial burst of gastric juice is followed by a sustained secretion that continues
to add gastric juice
Examples of Negative Feedback
1) Thermostat
 Set thermostat to 20 ºC (set point)
 If temperature falls below 20 ºC, the thermostat switches on the heater
 When thermometer detects that temperature is now above 20 ºC, the thermostat
switches the heater off
 Therefore, change in variable (temperature) triggers mechanism that counteracts the
change (cold) to prevent further change in the same direction (more cold)
2) Body Temperature (sweating)
 Brain detects rise in body temperature
 Sends nerve impulses to sweat glands to increase the production of sweat
 Sweating lowers body temperature by evaporative cooling
(H2O(l)  H2O(g) is an endothermic reaction)

When body temperature drops below set point then brain stops sending signals
3) Gastrin
 Gastric juice is very acidic (HCl)
 If the pH of the stomach becomes too acidic (too much gastric juice) the acid will inhibit
the release of gastrin
 This decreases the production of gastric juice helping the pH to return to normal levels
**Therefore Gastrin is an example of both a positive AND a negative feedback loop!