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Summary Course BBS2041 Human Intermediary Metabolism
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CASES BBS2041Summary of articles and lectures
, Case 01 BBS2041
GENERAL ANATOMY OF THE GI TRACT
Digestive system:
1. The alimentary canal
a. The alimentary canal, also called the gastrointestinal (GI) tract or gut, is the
continuous muscular tube that winds through the body from the mouth to the anus.
It digests food and absorbs the digested fragments through its lining into the blood.
The organs of the alimentary canal are:
i. The mouth
ii. Pharynx
iii. Oesophagus
iv. Stomach
v. Small intestine
vi. Large intestine.
2. Accessory digestive organs:
a. The accessory digestive organs are:
i. Oral cavity
1. Tooth
2. Tongue
ii. Gallbladder
iii. Large digestive glands, the salivary glands, liver, and pancreas.
,GENERAL FUNCTIONS OF THE GI TRACT:
1. To move nutrients, water, and electrolytes from the external environment into the body’s
internal environment:
a. Food is mostly in the form of macromolecules, such as proteins and complex
carbohydrates, so the digestive systems must secrete powerful enzymes to digest
food into molecules that are small enough to be absorbed into the body.
2. These enzymes must not digest the cells of the GI tract itself (autodigestion):
a. If protective mechanisms against autodigestion fail, we may develop raw patches
known as peptic ulcers on the walls of the GI tract.
3. Matching fluid input with output:
a. People ingest about 2 litres of fluid a day and exocrine glands and cells secrete 7
litres or so of enzymes, mucus, electrolytes, and water into the lumen of the GI
tract. That volume of secreted fluid is the equivalent of one-sixth of the body’s total
body water and must be reabsorbed or the body would rapidly dehydrate. Normally
reabsorption is very efficient, and only about 100 mL of fluid is lost in the feces.
b. Vomiting and diarrhoea (excessively watery feces) can become an emergency when
GI secretions that would normally be reabsorbed are lost to the environment.
c. In severe cases, this fluid loss can deplete extracellular fluid volume to the point that
the circulatory system is unable to maintain adequate blood pressure.
4. Repelling foreign invaders:
a. The largest area of contact between the internal environment and the outside world
is in the lumen of the digestive system. The GI tract faces daily conflict between the
need to absorb water and nutrients, and the need to keep bacteria, viruses, and
other pathogens from entering the body.
b. The transporting epithelium of the GI tract is assisted by an array of physiological
defence mechanisms, including mucus, digestive enzymes, acid, and the largest
collection of lymphoid tissue in the body, the gut-associated lymphoid tissue (GALT).
80% of all lymphocytes in the body are found in the small intestine.
HISTOLOGY OF THE ALIMENTARY CANAL
From the esophagus to the anal canal, the walls of the alimentary canal have the same four
basic layers:
1. The mucosa or mucous membrane:
a. The innermost layer is a moist epithelial membrane that lines the alimentary canal
lumen from mouth to anus.
b. Its major functions are to:
i. Secrete mucus, digestive enzymes, hormones.
ii. Absorb the end products of digestion into the blood.
iii. Protect against infectious disease
c. The typical digestive mucosa consists of three sublayers:
i. A lining epithelium
1. Except for that of the mouth, esophagus, and anus where it is
stratified squamous, the epithelium of the mucosa is a simple
columnar epithelium rich in mucus-secreting cells.
2. The slippery mucus protects certain digestive organs from being
digested by enzymes working within their cavities and eases food
passage along the tract.
, 3. In the stomach and small intestine, the mucosa also contains both
enzyme-synthesizing and hormone-secreting cells.
ii. Lamina propria
1. The lamina propria which underlies the epithelium, is loose areolar
connective tissue. Its capillaries nourish the epithelium and absorb
digested nutrients. Its isolated lymphoid follicles, part of MALT (the
mucosa-associated lymphoid tissue) help defend us against bacteria
and other pathogens, which have rather free access to our digestive
tract. Particularly large collections of lymphoid follicles occur within
the pharynx (as the tonsils) and in the appendix.
iii. Muscularis mucosae
1. External to the lamina propria is the muscularis mucosae, a scant
layer of smooth muscle cells that produces local movements of the
mucosa. In the small intestine, this muscle layer’s tone throws the
mucosa into a series of small folds that immensely increase its
surface area.
2. The Submucosa:
a. The submucosa, just external to the mucosa, is areolar connective tissue containing
a rich supply of blood and lymphatic vessels, lymphoid follicles, and nerve fibers
which supply the surrounding tissues of the GI tract wall. Its abundant elastic fibers
enable the stomach to regain its normal shape after temporarily storing a large meal.
3. The Muscularis Externa:
a. Surrounding the submucosa is the muscularis externa, also simply called the
muscularis. This layer is responsible for segmentation and peristalsis. It typically has
an inner circular layer and an outer longitudinal layer of smooth muscle cells. In
several places along the tract, the circular layer thickens, forming sphincters that act
as valves to control food passage from one organ to the next and prevent backflow.
4. The Serosa:
a. The serosa, the outermost layer of the intraperitoneal organs, is the visceral
peritoneum. In most alimentary canal organs, it is formed of areolar connective
tissue covered with mesothelium, a single layer of squamous epithelial cells.
b. In the oesophagus, which is located in the thoracic instead of the abdominopelvic
cavity, the serosa is replaced by an adventitia ordinary fibrous connective tissue
that binds the oesophagus to surrounding structures.
,ORAL CAVITY
The mouth
1. The mouth is the only part of the alimentary canal involved in ingestion.
2. Most digestive functions associated with the mouth reflect the activity of the related
accessory organs, such as teeth, salivary glands, and tongue.
3. In the mouth the food is chewed and mixed it with saliva containing enzymes that begin the
process of digestion. The mouth also begins the propulsive process of swallowing, which
carries food through the pharynx and oesophagus to the stomach.
4. The walls of the mouth are lined with a thick stratified squamous epithelium which
withstands considerable friction.
5. The epithelium on the gums, hard palate, and dorsum of the tongue is slightly keratinized for
extra protection against abrasion during eating.
6. The mouth ingests, begins mechanical breakdown by chewing, initiates propulsion by
swallowing, and starts the chemical breakdown (digestion) of polysaccharides.
7. Except for a few drugs essentially no absorption occurs in the mouth.
SALIVARY GLANDS
In the oral cavity the first stages of digestion begin with chewing and the secretion of saliva by three
pairs of salivary glands:
1. Sublingual glands under the tongue
a. Contain mostly mucous cells
2. Submandibular glands under the mandible (jawbone)
a. Contain mostly serous cells
3. Parotid glands lying near the centre of the jaw
a. Contain mostly serous cells.
Functions of Saliva:
1. Cleanses the mouth
2. Dissolves food chemicals so they can be tasted
3. Moistens food and helps compact it into a bolus
4. Contains enzymes that begin the chemical breakdown of starchy foods .
, The salivary glands are composed of two types of secretory cells:
1. Serous
a. Serous cells produce a watery secretion containing enzymes, ions, and a tiny bit of
mucin.
2. Mucous
a. Mucous cells produce mucus, a stringy, viscous solution.
Composition of Saliva
1. Saliva is slightly acidic, pH 6.75 to 7.00, but its pH may vary.
2. Contains electrolytes (Na+ K+ Cl-, PO43- and HCO3-)
3. The digestive enzymes salivary amylase and lingual lipase. Lingual lipase favours short- and
medium-chain triglycerides, in contrast to pancreatic lipase, which preferentially hydrolyses
at the sn-2 position of long-chain triglycerides.
4. The proteins mucin, lysozyme, and IgA antibodies.
5. Metabolic wastes (urea and uric acid).
6. Saliva protects against microorganisms because it contains IgA antibodies, lysosomes and
defensins, local antibiotics which also acts as cytokines to call defensive cells into the mouth
for battle.
7. The friendly bacteria that live on the back of the tongue promote the conversion of food-
derived nitrates in saliva into nitric oxide (NO) in an acid environment. This transformation
occurs around the gums, where acid-producing bacteria tend to cluster, and in the
hydrochloric acid–rich secretions of the stomach. The highly toxic nitric oxide is believed to
be bactericidal in these locations.
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