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DIGESTIVE SYSTEM NOTES
Digestion is: the enzymatic hydrolysis of macromolecules into monomers that can be absorbed across cell membranes.
The digestive system consists of the alimentary canal and the accessory organs.
 Physical/Mechanical digestion – mastication of food and peristaltic contractions of the
intestines/esophagus/stomach. Does not involve chemical change; however, it facilitates chemical digestion by
increasing the surface area for enzymes to do their job.
 Chemical – glandular/enzymatic involvement; breaks down complex molecules into smaller molecules that the
body can absorb and use.
A. Alimentary Canal – includes oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus.
1. Structures and Functions
a. Oral Cavity – both physical and mechanical digestion
 Salivary glands lubricate food, protect against bacteria, the enzyme amylase breaks down starch
 Tongue and teeth masticate food to prepare it for enzymatic action
 Chewed up wad of food = bolus – is swallowed into the pharynx.
b. Pharynx – leads from the mouth to the esophagus
 Epiglottis closes to prevent food from entering the trachea
c. Esophagus – leads to the stomach; NO digestion occurs here
 Lined with epithelial tissue
 Peristaltic contractions move food toward stomach
 Where it leads to the stomach, it contains a cardiac sphincter – ring of smooth muscle that opens and
closes to allow food to enter the stomach.
d. Stomach – large, muscular organ; functions to store food, initiate protein digestion, and move chyme into the
small intestine.
 Mechanical digestion continues with the churning action of the smooth muscles.
 Cells in the stomach secrete HCl which activates pepsin (gives stomach has a pH of 1-2)
 Pepsin – enzyme activated by HCl that chemically breaks down proteins
 Mixture of food & gastric juices is now called chyme
e. Small Intestine – most enzymatic hydrolysis and nutrient absorption occurs here
 Lined with epithelial tissue that is folded extensively into finger-like projections called villi and microvilli.
These projections increase the surface area for maximum absorption.
 Divided into three sections:
 Duodenum – Complete chemical breakdown of ALL macronutrients occurs here!!
1. Pancreas secretes Na+ bicarbonate into the duodenum to neutralize the acidic chyme.
2. Liver produces bile (stored in the gall bladder), and releases it slowly into this section of the
small intestine. Bile is an emulsifying agent that MECHANICALLY breaks down fat globules
into tiny fat droplets.
3. Pancreas produces and releases the enzymes amylase, trypsin, & lipase to complete the
chemical breakdown of carbohydrates, proteins, & lipids.
 Jejunum – majority of nutrient monomers are absorbed here
 Ileum – water, vitamins, electrolytes are absorbed here
f. Large Intestine – large in diameter; also called the colon; contains a large population of bacteria. Main function
of the large intestine is to absorb water and remove indigestible wastes.
 Caecum – large pocket where the small and large intestines join
 Appendix – tiny projection on the caecum that has no apparent function
 Rectum – last portion of the alimentary canal where feces are stored
 Anus – opening (sphincter) through which feces are removed
B. Accessory Structures – structures that aid in digestion, but food does NOT enter these!
1. Liver – produces bile which is secreted into ducts which drain into the gall bladder and then into the duodenum.

Bile – made up of bile salts, a pigment, phospholipids, cholesterol, and ions. Bile acts as an emulsifier for
fats NOT an enzyme! Fat globules are broken down into smaller droplets so that more surface area is
created to facilitate chemical digestion.
 A second important function of the liver relates to digestion, but is not directly involved in the process.
The liver helps to maintain a constant blood glucose concentration by storing excess glucose as glycogen
after eating, and breaking glycogen down into its glucose monomers between meals.
 The liver also helps in the detoxification of blood. Excess hormones, drugs, toxins are removed
2. Pancreas – major organ of the endocrine system, but is also an accessory organ to digestion.
 Pancreas secretes Na+ bicarbonate into the duodenum to neutralize the acidic chyme.
 Pancreas produces and releases the digestive enzymes amylase, trypsin, & lipase to complete the chemical
breakdown of carbohydrates, proteins, & lipids.
C. Disorders of the Digestive System
1. Diarrhea – contents in the large intestine pass through too quickly and very little water is absorbed. Due to stress,
infection; can result in dehydration.
2. Constipation – contents in large intestine pass through too slowly; too much water is absorbed.
3. Ulcers – deterioration of stomach or intestinal wall caused by over-secretion of HCl. Painful; may result in
intestinal bleed.