The Process of Digestion
DIGESTION: The breakdown of food into simpler molecules that can
be absorbed by the body. Gastrointestinal tract (GI tract)
Includes: mouth, pharynx, esophagus, stomach, small intestine, &
large intestine. Glands include: salivary, pancreas, & liver.
Three tasks: 1) break down food into a fine pulp 2) chemically act on
the food, breaking down molecular structure 3) absorption into blood
stream
The Mouth: Prepares food for entry into GI tract.
Mechanically: teeth, cheeks, tongue (taste buds)
Types of teeth: canines, molars, inscisors
Fig. 39-15
Teeth: enamel and dentin, no nerve cells, hardest material in body
Anchored by roots extend into jaw and are held by periodontal
membrane.
Tooth ache: Pulp cavity w/ blood vessels and nerve cells is disturbed
THE MOUTH
Salivary Glands: secretes the first digestive enzymes of the GI tract
Three impt. Functions: 1) Dissolves some food and speeds mov’t.
2) Contains enzymes that attack many potentially dangerous
microorganisms entering the mouth.
3) Contains the enzyme salivary amylase, breaks down starch
molecules into maltose.
Bolus: Gathering food in the mouth into a small ball for swallowing.
Pharynx: The GI tract and Respiratory tract cross each other here.
Epiglottis: Flap of cartilage that covers the respiratory tract when
swallowing.
THE ESOPHAGUS
25cm in length, connecting the pharynx with the stomach
Peristalsis: Contraction of muscles in the esophagus pushing the food
Sphincter: Thick ring of muscle where esophagus connects stomach
THE STOMACH
Stomach: Thick muscular sac below the abdomen to store
food
Has 3 sets of glands: 1) makes mucus to lubricate & protect
2) secretes HCL to break down food
3) secretes pepsin which works w/HCL to begin protein
digestion forming smaller polypeptides
Chyme: pasty mixture in stomach after a few hours of
churning
Pyloric valve: Peristalsis forces mixture past this valve into
small intestine.
Most proteins, some starch broken down by the time of
departure.
Sugars and fats are not affected as of yet.
THE SMALL INTESTINE
Duodenum: First part of small intestine where food is flooded with
enzymes
Enzymes come from three sources: intestine itself, pancreas, & liver
Duodenum: peptidases-proteins; maltase-maltose; lactase-lactose
Pancreas: Long organ located behind the stomach consisting of glands
pancreatic fluid: digestion of carbos, fats, & proteins
amylases and protease breaks down carbos and proteins
lipase breaks down fats into glycerol & fatty acids
sodium bicarbonate- neutralizes HCL in tummy: pH of 7-8 ideal
Liver: large brownish organ above the tummy (1500g)
Secretes bile which is stored in the gallbladder
the bile salts help lipase emulsify fats
Very long (7m); peristalsis pushes it through in several hours
After leaving duodenum, food goes through jejunum and illium, where
digestion is completed and absorbed into blood stream
SMALL INTESTINE
Small intestine has folds to increase the surface area.
Villi: fingerlike projections in the small intestine w/microvilli folds.
Richly supplied with blood vessels to absorb and carry nutrients.
Carbos and proteins absorbed into capillaries of villi
Undigested fat and some fatty acids enter lymph directly
When leaving the food is nutrient free; water, cellulose & ? Behind
As it leaves it passes the appendix (vestigial organ)
THE LARGE INTESTINE (COLON)
Remove water from undigested materials passing through it
Result is solid waste (feces) removed by peristalsis to rectum.
Valvelike anal sphincter muscles prevent early departure.
When problems occur: Not enough water absorbed=diarrhea; Too
much water=constipation;
Contains bacteria aiding in final digestion and giving vitamin K