Download Mechanical Digestion

Digestive
System
Gastrointestinal Tract
1. Mouth
2. Pharynx
3. Esophagus
4. Stomach
5. Small Intestine
6. Large Intestine
Accessory Structures
1. Teeth
2. Tongue
3. Salivary Glands
4. Liver
5. Gallbladder
6. Pancreas
The Processes of Digestion
1. Ingestion
taking food into the mouth
2. Secretion
GI tract and accessory cells secrete water, acid, buffers, and enzymes into lumen
3. Mixing and Propulsion
alternating contraction and relaxation of smooth muscles in the walls of the GI tract
4. Digestion
Breaking down of larger food particles into smaller molecules
Mechanical Digestion
Chemical Digestion
5. Absorption
small molecules pass from the lumen into the blood and lymph
6. Defecation
elimination of indigestible substances and bacteria from the GI tract
Mouth (Oral Cavity)
Mechanical Digestion
mastication (chewing)
tongue manipulates food
teeth grind food saliva
mixes so that food is
reduced to a soft flexible
mass called a bolus
that is easily swallowed
Salivary Glands
Chemical Digestion
salivary amylase
initiates the breakdown of
starch ingested disaccharides
and starches must be broken
down into monosaccharides
lingual lipase
initiates digestion of
triglycerides into fatty acids
and monoglycerides
Esophagus and Swallowing
Stomach
connects the esophagus to the duodenum
(first part of the small intestine)
Functions:
1. serves as a mixing area for saliva,
food and gastric juice
2. serves as a reservoir for holding food
before release into the SI
3. secretes gastric juice (HCL, pepsin,
intrinsic factor, gastric lipase)
HCL kills bacteria, denatures protein
pepsin begins digestion of proteins
intrinsic factor aids absorption of
vitamin B12
gastric lipase aids digestion of
triglycerides
4. secretes gastrin (digestive hormone)
into the blood
Physiology of Digestion and
Absorption in the Stomach
Mechanical Digestion
after food enters the stomach, gentle
mixing waves pass over the stomach
every 15-25 seconds
it macerates the food and mixes it
with gastric juice, forming chyme
the chyme moves back and forth
and keeps mixing until it is sent
through the pyloric sphincter into
the duodenum
Chemical Digestion
lingual lipase and salivary amylase
continue to break down fats and
carbohydrates respectively
pepsin and HCL break down proteins
gastric lipase splits triglycerides into
fatty acids and monoglycerides
Peristalsis
Segmentation
Regulation of Gastric Secretion and Motility
•
Cephalic Phase:
–
Sight, smell, taste, and thought initiate reflex
•
•
Cortex and hypothalamus send impulses to medulla
Parasympathetic (Vagus nerve) impulses from medulla promote:
–
–
•
Gastric Phase:
–
–
Stretch receptors detect distention of stomach
Chemoreceptors detect increased pH of gastric juice
•
–
Stimulates stomach churning and secreting
ACH from parasympathetic activity stimulates gastrin secretion
•
•
•
Peristalsis in stomach
Gastric glands to secrete:
» Pepsinogen
» HCL
» Mucus
» gastrin
Stimulates gastric juice secretion
Stimulates stomach motility
Intestinal Phase
–
–
Stretch receptors detect distention of duodenum
Chemoreceptors detect fatty acids and glucose in duodenum
•
•
Impulses from small intestine inhibit stomach secretions and motility
Stimulates release of:
–
–
–
Gastric inhibitory peptide (GIP)
» Inhibits gastric secretions and gastric motility in stomach
Secretin
» Decreases gastric secretions
Cholecystokinin (CCK)
» Inhibits stomach emptying
» Inhibits gastric secretions and gastric motility in stomach
Stomach Cell Layers
Stomach Cells
Small Intestines
source of the major events of digestion and absorption
its long length (over 10 feet) is to increase the surface area
it has many circular folds, villi and microvilli
Functions:
1. mixes chyme with digestive
juices and brings food in
contact with surface
mucosa for absorption
2. completes digestion of
carbohydrates, proteins,
& lipids
begins & completes
digestion of nucleic acids
3. is responsible for the
absorption of about 90%
of all nutrients
Small Intestines
Physiology
Digestion
chyme remains in SI for 3-4 hours
Mechanical Digestion
mixes chyme with digestive
enzymes
undergoes peristalsis
Chemical Digestion
pancreatic juice assists in
digestion
Absorption
food products pass through
mucosa absorptive cells
passage of these molecules from
the GI tract to the blood or lymph
is called absorption
Small Intestine Cell Layers
Small Intestine Cells
Liver and Gall Bladder
Brush Border Enzymes
• Carbohydrate digesting:
–
–
–
–
Alpha-dextrinase
Maltase
Sucrase
Lactase
• Protein digesting:
– Aminopeptidase
– Dipeptidase
• Nucleic Acid digesting
– Nucleosidase
– Phosphatase
Glucose
Absorption
Carbohydrates are
broken down into
monosaccharides like
glucose
Glucose passes through
SI Absorptive cells via
facilitative diffusion,
actually with the help
of a Na+ co-transporter
Glucose then passively
diffuses into the blood
Protein
Absorption
Proteins are broken down into
amino acids
Amino acids and the slightly
larger di-peptides passively
diffuse across the membranes
of the SI Absorptive cells
Amino acids then passively
diffuse into the blood
Pancreatic Juice Enzymes
• Carbohydrate digesting:
– Pancreatic amylase
• Protein digesting:
–
–
–
–
Trypsin
Chymotrypsin
Carboxypeptidase
Elastace
• Lipid digesting
– Pancreatic lipase
• Nucleic Acid digesting
– Ribonuclease
– Deoxyribonuclease
Fat
Absorption
Large fat droplets are emulsified
into monoglycerides and fatty
acids by bile salts from the liver
and are packaged into micelles
Micelles merge with SI Absorptive
cells and via a type of endocytosis
deposit the fat fragments in the cells
The fat fragments are then
reassembled into triglycerides in the
cell and packaged in chylomicrons
Chylomicrons then exit the cell and
enter the lymph on their way to the
liver for final digestion and storage
Large Intestine (Colon)
Functions:
1. completion of absorption
2. manufactures vitamins
(some B’s & K)
3. formation of the feces
4. expulsion of the feces
Structures:
cecum
ascending colon
transverse colon
descending colon
sigmoid colon
rectum
anal canal
anus
anal sphincters
Rectum
and
Anus
Internal Anal Sphincter
External Anal Sphincter
Larger Intestine Cell Layers
Large Intestine Cells