Download B. Human digestion

DIGESTIVE SYSTEM
I.
GENERAL
A. Definitions
1. Complete / incomplete digestive system
a) Complete digestive system
(1) Has two openings, a mouth and a anus, connected by an alimentary
canal
(2) This type of digestive system is allows specialization along different
regions of the tract
(a) Separate areas of ingestion, storage, digestion, and defecation
(3) Complete digestive systems are found in most pseudocoelomates and
coelomates
b) Incomplete digestive tract
(1) A single opening serves both as a mouth and anus
(2) The digestive system is called a gastrovascular cavity
(3) Found in radiates, turbellarians and trematodes
2. Intracellular / extracellular digestion
a) Intracellular digestion
(1) Digestion occurs within a cell
(2) Food is endocytized (e.g., phagocytosis), and then endosome fuses with
lysosome for final digestion
(a) Lysosomes contain many digestive enzymes
b) Extracellular digestion
(1) Digestion occurs outside cells
(a) Either outside the organism, or within a gastrovascular cavity or
alimentary canal
(2) Digestive enzymes are secreted, nutrients are digested extracellularly,
then soluble nutrients are transported across cell membranes
B. Human digestion
1. Digestion occurs within the alimentary tract (mouth to anus) by both mechanical
and chemical processes
2. Digestion of food is an extracellular process due to enzymes that are secreted
into the gut by glands
3. Parts of the digestive tract
a) Alimentary canal
(1) Long tube from mouth to anus
(2) Mouth, pharynx, esophagus, stomach, small intestines, and large
intestines
b) Accessory organs
(1) Needed for digestion, but food does not pass through
(2) Salivary glands, liver, gall bladder, and pancreas
4. Functions of the digestive system
a) Ingest the food
b) Digest it into small molecules that can cross the cell membrane
c) Absorb the nutrient molecules
d) Eliminate non-digestible wastes
II. MOUTH
A. Overall structure
1. Roof of the mouth separates the nasal chamber from the mouth cavity and
consists of the anterior hard palate and the posterior soft palate, which ends in
the uvula
B. Functions
1. Ingest (receive) food
2. Mechanically break it down into smaller pieces with the aid of teeth
3. Combine the food with saliva
4. Forms bolus for swallowing
5. Enzymatically begin the digestion of starches by salivary amylases
6. Tastes food
C. Saliva
1. Salivary amylase
a) Enzymatically breaks down polysaccharides into simpler sugars
2. Mucous
a) Coats the food to form a bolus and to make swallowing easier
3. Bicarbonate
a) Neutralizes acids
D. Teeth
1. Total number of adult teeth equals 32
a) Types of adult teeth per one-half jaw include
(1) Two incisors for biting
(2) One canine for tearing food
(3) Two flat premolars for grinding
(4) Three molars used for crushing
2. Structure of a tooth
a) Consists of a crown and root
b) Layers
(1) Hard enamel is outermost
(2) Middle layer of dentin (bone-like)
(3) Inner pulp layer that contains nerves and blood vessels
III. PHARYNX
A. A four was intersection
1. Connected to the mouth, esophagus, nasal cavity, and larynx
2. The nasopharyngeal opening closes when the soft palate moves back during
swallowing
3. The opening to the trachea (glottis) closes when the epiglottis folds over it
4. Heimlich maneuver or tracheometry may be needed to remove food stuck in the
glottis
IV. ESOPHAGUS
A. Function
1. Transports the bolus through the thoracic cavity to the stomach
a) Heartburn occurs when stomach acid backs up into esophagus through the
cardiac sphincter
B. Peristalsis
1. Rhythmical contractions pushes food along
V. STOMACH
A. Connects the esophagus to the duodenum of the small intestine
1. Chyme leaves the stomach via the pyloric sphincter into the duodenum
B. Functions
1. Stores food
2. Mixes food with gastric juice
a) Termed chyme when it leaves the stomach
3. Hydrochloric acid
a) Kills bacteria
b) Breaks down connective tissue in meat
c) Ulcers form when HCl penetrates the mucus barrier and disintegrates the
tissue
4. Pepsin begins the enzymatic breakdown of proteins into amino acids
VI. SMALL INTESTINE
A. Functions
1. Receives secretions from the liver and pancreas as well as chyme from the
stomach
2. Digestion of food
3. Absorbs nutrients
4. Transports undigested material to the large intestine
B. The structural unit of the small intestine is the villus
1. Microvilli
a) Increases surface area for final digestion and absorption of nutrients
2. Lacteal
a) A small lymph vessel that returns fluids and lipoprotein droplets back to the
bloodstream
3. Blood vessels
a) Absorb the sugars and amino acids of the acid chyme that were taken up
into the cells by active transport
VII. LARGE INTESTINE
A. Functions
1. Absorbs water
2. Prepares and stores non-digestible material until defecation
3. Absorbs vitamins produced from bacteria
B. Large intestine
1. Cecum
a) Lies below entrance of the small intestine and contains the appendix, which
may have a role in immunity
b) When inflamed, it causes appendicitis
2. Colon
a) Functions
(1) Fluid and vitamin absorption
b) Consists of the ascending colon, transverse colon, and the descending colon
c) Over 99% of colon bacteria are obligate anaerobes
3. Rectum
a) The last 20 cm of the large intestine
4. Anal canal
a) Last portion of rectum for defecation of feces
(1) Feces contains undigested residues, bile pigments, and the bacteria
VIII.
ACCESSORY ORGANS
A. Pancreas
1. Functions
a) Produces digestive enzymes and fluid that enters the duodenum by way of
the pancreatic duct
b) Enzymes
(1) Trypsinogen and chymotrypsinogen breakdown proteins
(2) Amylase breaks down starches
c) Fluids
(1) Contains bicarbonate
(a) Helps neutralize HCl of chyme to prevent duodenal ulcers
B. Liver
1. Functions
a) Produce bile
(1) Old red blood cells are destroyed, and hemoglobin is converted into bile
pigments (bilirubin and biliverdin) and bile salts (used to emulsify fat into
smaller droplets for lipase digestion)
(2) Bile is stored in the gallbladder and enters the duodenum via bile duct
(a) Choleocystokinin in response to fat will stimulate the gallbladder to
release bile
b) Detoxifies the blood by removing poisonous substances coming via the
hepatic portal vein from the intestine
c) Maintains a constant blood glucose concentration
(1) Converts absorbed glucose to glycogen if blood glucose levels exceed
0.1%
(2) If blood glucose is below 0.1%, glycogen broken down to glucose
IX. DIGESTIVE ENZYMES
A. Introduction
1. Digestive enzymes are hydrolytic enzymes that catalyze degradation of a
molecule by the addition of water at specific bond
2. These proteins are substrate specific and have a preferred pH
B. Amylase
1. Salivary amylase
a) Breaks down polysaccharides into simpler sugars
2. Pancreatic amylase
a) Breaks down polysaccharides into simpler sugars
C. Pepsin
1. Secreted by gastric glands
a) Pepsinogen
(1) Converted to pepsin by HCl
b) Pepsin is a protease that converts proteins to amino acids
D. Trypsin
1. A protease that digests proteins into amino acids
2. Trypsinogen made in the pancreas and converted to trypsin in the small
intestines
D. Chymotrypsin
1. A protease that digests proteins into amino acids
2. Chymotrypsinogen made in the pancreas and converted to chymotrypsin in the
small intestines
E. Lipase
1. Converts emulsified fat droplets to glycerol plus fatty acids that rejoin inside the
cells of the villi
2. Later they enter the lacteals