THE
GASTROINTESTINAL TRACT
Functions of
GI tract
Ingestion -
digestion - absorption
- elimination
Ingestion
The taking in
of food
Digestion
This is in two
parts
a. Mechanical digestion, increases surface
area
b. Chemical digestion, breaks up complex
molecules into simple ones
Absorption
Passage from
the gut in to the blood or lymphatic systems
Elimination
Waste products
remain in the bowel after absorption, these are expelled as faeces
Components of
the gastrointestinal tract
Organs and
structures
Lips, Tongue,
Teeth, Pharynx, Epiglottis,
Oesophagus, Cardiac sphincter,
Stomach, Pyloric sphincter, Small Intestine, (Duodenum,
Jejunum, Ileum), Ileocaecal
valve, Colon, (Ascending,
Transverse, Descending), Appendix, Caecum, Rectum, Anus,
Accessory
organs
3 pairs of
salivary glands Pancreas Liver, Gall Bladder
Function of
components
Mouth and Nose
Taste and
smell
Mechanical
processes - mastication
Saliva - cleans lubricates
digests (an enzyme)
Optimum pH 6.8
Salivary
amylase Starch --- maltose
Tongue
involved in swallowing
Epiglottis
Guards
entrance to airways
Oesophagus
Passage of
food bolus to stomach
Peristalsis
Longitudinal
muscle – outside Circular
muscle - inside
Stomach
Greater and
lesser curvature
Fundus, body
and pyloric regions
Has an oblique
inner muscular layer
Reservoir Mixes food, (chyme) Absorption Intrinsic
factor
Gastric juice
Secreted by
glands in the mucosa Hydrochloric acid Pepsinogen
Pepsin starts
protein digestion Optimum
pH is 1.5 - 3.5
Small
intestine
About 5 metres
long
Duodenum
Midpoint
sphincter of Oddi (the billary tract)
Bile
Excretion of
bilirubin Emulsify fats Colours and deodorises faeces
Pancreatic
juice
Optimum pH 6 -
8
Proteolytic
activity
Trypsinogen
--- trypsin
Chymotrypsinogen
--- Chymotrypsin
Converted by
enteropeptidase in intestinal juice
Carbohydrate
digesting activity
Pancreatic
amylase
Polysaccharides -----disaccharides
Fat digesting
activity
Lipase
Fats --- fatty
acids and glycerol
Jejunum and
Ileum
Intestinal
juice
Succus
entericus enteropeptidase
Surface area
inside small intestine
1. Circular
folds 2. Villi 3. Microvilli
Total
absorptive area - 250 square meters
Enterocytes in
walls of villi
Peptidases Lipase Sucrose, maltase, lactase
Final
digestion mostly in cells of villi
Absorption
Proteins
Carbohydrates
Fats
Mineral salts
Vitamins
Water
Fibre
Amino acids
--- capillaries of villi
Fatty acids
and glycerol ---- lacteal
Diffusion
Active
transport
Ileum joins
colon via ileocaecal valve
Colon (Large
Intestine)
Caecum,
appendix, ascending colon, hepatic flexure, transverse colon, splenic flexure,
descending, rectum, anus.
Absorption/faeces
formation
0.5 - 1.0
litres of fluid enter the colon per day, about 100 mls leave
Faeces is
semi-solid
Mineral salts
and vitamins are also absorbed
Microbial
activity
Natural
bacterial flora symbolically produce vitamin K and folic acid
Coliforms are pathogenic
in other places
Gases produced
pass out as flatus
Defecation
Peristalsis is
more intermittent than small bowel
Mass movement
The
gastro-colonic reflex
Faecal
material in the rectum stimulates stretch receptors
Anus relaxes
under voluntary control
Rectum
contracts under autonomic control
Diaphragm goes
down and abdominal muscles contract