Digestive Processes
food prepared for consumption by the cells through 5 basic activities

1. Ingestion--taking food into the body

2. Movement--passage of food along the gastrointestinal tract

3. Digestion--breakdown of food by both mechanical and chemical processes

4. Absorption--passage of food from the GI tract into the cardiovascular and lymphatic
systems for distribution to the cells who need nutrients

5. Defecation--elimination of indigestible substances from the GI tract

Chemical digestion
series of catabolic reactions that break down large molecules (carbohydrates, proteins, lipids) into smaller molecules that can be absorbed and used by the body

Mechanical digestion
-movements of the GI tract that aid in overall digestion
-teeth prepare food to be swallowed and smooth muscle of the stomach and small intestine churn the food so it mixes with enzymes that help in the chemical reaction

 

 

1. Gastrointestinal Tract (GI tract)
-also referred to as the alimentary canal
-continuous tube running through the ventral cavity from the mouth to the anus
-average length is 9 meters
-organs include: mouth, pharynx, esophagus, stomach, small intestine, large intestine

2. Accessory Structures
-teeth, tongue, salivary glands, liver, gall bladder, pancreas (all accessory structures lie outside the GI tract with the exception of the teeth and tongue)
-these structures produce or store secretions that aid in the chemical breakdown of food
-secretions reach the GI tract through ducts

4 basic layers throughout the GI tract from the esophagus to the anus

1. Mucosa--inner lining of the GI tract, mucous membrane

3 tissues of the mucosa
epithelium--
stratified in mouth and esophagus and simple throughout
the rest of the GI tract

lamina propria--
loose connective tissue that connects the epithelial layer
to the muscularis mucosae and provides space for blood
and lymphatic vessels

muscularis mucosa--
smooth muscle that puts folds into the intestine to increase
the surface area of the intestine

2. Submucosa
-dense connective tissue that binds the mucosa to the muscularis
-highly vascular and contains part of the autonomic nerve supply to the muscularis mucosa

3. Muscularis
-this layer in the mouth, pharynx and upper esophagus consist of some skeletal muscle that produces voluntary swallowing
-rest of the GI tract consist of smooth muscle found in 2 sheets inner sheet contains circular fibers outer sheet contains longitudinal fibers
-helps to fix food and move it through the GI tract

4. Serosa
-outermost layer of the GI tract
-also called peritoneum

parietal peritoneum lines the wall of the abdominal cavity
visceral peritoneum lines some organs
peritoneal cavity is the open space between the two peritoneums

one extension of the parietal peritoneum is called the

mesentery--
-tip of the mesentery is attached to the posterior abdominal wall
-binds the small intestine to the wall
-supply the small intestine with blood supply and nerves

 

mouth (oral cavity, buccal cavity)
formed by the checks, hard palate, soft palate and tongue

uvula-- tissue that protrudes past the end of the soft palate

tongue--
-accessory structure
-composed of skeletal muscle covered by mucous membranes

-medial septum--
-divides tongue into symmetrically lateral halves
-tongue attached inferiorly to the hyoid bone

-lingual frenulum--
-attaches tongue to the floor of the mouth
-limits the posterior movement of the tongue
-superior surface covered by papillae-- some of which are “taste buds”

-salivary glands
-accessory structures
-secrete fluid to keep mouth moist
-lubricate the food and start the chemical digestion of food

teeth
-accessory structures
-located in the alveolar processes of the mandible and maxillae

Deciduous teeth--
start appearing about 6 months ----20 total

Permanent dentition (teeth)
start appearing about 6 years ---- 32 total

your jaw continues to grow until you reach maturity
age 6 -- 1st set of molars
age 12 -- 2nd set of molars
age 18 -- 3rd set of molars “wisdom teeth”

 

Mastication (mechanical process)
-tongue moves food, teeth grind food, food mixes with saliva
-food reduced to a soft flexible bass called a Bolus

Salivary amylase
-starts the breakdown of starch (only chemical digestion that occurs in the mouth)

3 stages

1. voluntary
-bolus forced to the back of the mouth cavity and into the oropharynx by the tongue pressing up and back against the palates

2. pharyngeal
-bolus stimulates nerves in the oropharynx
-impulses cause soft palate and uvula to move up and seal off the
nasopharynx

3. esophageal
-bolus pushed through the esophagus by involuntary muscular movements called peristalsis

peristalsis--
-function of the muscularis that is controlled by the medulla
-circular fibers contract and force food down
-longitudinal fibers below bolus contract and shorten the portion of the esophagus below the bolus which increases the opening above (similar to a Chinese finger toy)
-passage of solid food from the mouth to the stomach is about 4 to 8 seconds. liquids about 1 second

esophagus
-muscular collapsible tube that lies behind the trachea
-22-26 cm long
-passes through the mediastinum just anterior to the vertebral column
-pierces the diaphragm through the opening called the
esophageal hiatus

stomach
-J-shaped, inferior to to the diaphragm
-sphincter between esophagus is called the cardiac sphincter

composed of 4 parts
1. cardia-- closest to the esophagus
2. fundus-- rounded portion above the cardia
3. body-- central portion
4. pylorus-- narrow, inferior region prior to the small intestine

*stomach can only absorb some water, electrolytes, certain drugs (aspirin) and alcohol

histology
made of the same four layers as the rest of the GI tract (these layers are modified)

rugae--large folds of an empty stomach

chief cells--secrete pepsin, which is as enzyme

parietal cells--secrete HCL and intrinsic factor-- helps absorption of vitamin B₁₂

mucous cells

enteroendocrine cells
-produce gastrin--

-hormone that stimulates HCL and pepsin production
-closes cardiac sphincter and relaxes pyloric sphincter

**collection of above juices is called gastric juice

-mixing waves (peristalic movements) pass over the stomach every 15 to 25 seconds
-gastric juices reduce bolus to a thin liquid called chyme
-excess food stored in the fundus
-mixing waves force chyme toward the pyloric sphincter
-sphincter only allows a portion of the chyme to pass through

Chemical digestion
-starts the breakdown of protein
-pepsin breaks the peptide bonds that hold the amino acids together
-pepsin only works in an acidic environment (stomach pH of 2)
-alkaline mucous lines the stomach which should prevent the breakdown of the stomach walls

3 phases trigger gastric secretions

1. cephalic (reflex) phase--
-before food enters the stomach
-sight, smell, taste or thought of food stimulates production of gastric juices

2. gastric phase--
-food enters the stomach
-distention of the stomach triggers this phase
-partially digested proteins and caffeine stimulate secretion of

stomach gastrin--increases production of gastric juice, increase motility of GI tract, relaxes pyloric and ileocecal sphincters

3. intestinal phase--
-chyme enters the intestine 2-6 hours after ingestion
-overall effect is the inhibition of gastric secretions
-three hormonal secretions cause the inhibition of gastric juices

1. secretin-- stimulates pancreatic secretions
2. cholecystokinin-- induces the contraction of the gallbladder which releases bile
3. gastric inhibiting peptide (GIP)-- stimulates the release of insulin which maintains proper blood sugar levels

pancreas
-accessory structure of the GI tract
-oblong gland 12.5 cm long and 2.5 cm thick
-lies posterior to the greater curvature of the stomach
-connected by 2 ducts to the upper part of the small intestine

pancreatic duct--
joins with the common bile duct from the gallbladder to form the hepatopancreatic ampulla which empties into the small intestine about 10 cm below the pylorus

-2.5 cm above the hepatopancreatic ampulla is the other duct

histology
-groups of glandular epithelial cells
-about 1% of these cells are called the islets of Langerhan-- endocrine part of the pancreas that secrete hormones (insulin)

-other 99% are called acini-- exocrine glands that secrete digestive enzymes called
pancreatic juice--
-slightly basic
-enzymes for all foods especially lipids
-secretion triggered by acid chyme entering the small intestine

liver (LD)
-about 1.4 Kg in an average adult
-under diaphragm in the right hypochondrium and part of the epigastrium of the abdomen
-2 principal lobes

histology
lobules--
-functional unit of the liver
-each lobule surrounds a central vein
-cells of the lobule secrete:

bile---yellow, brown, olive green liquid with
a pH of 7.6-8.6

-mostly H₂O with bile salts

bile salts--emulsifiers that break down large fat globules
into digestible sizes

-lining of spaces between cells are phagocytic

Kupffer’s cells--
-destroy worn out RBC and white blood cells, bacteria and toxic substances
-filters blood (all things absorbed from the GI tract go to the liver first)

physiology
1. carbohydrate metabolism--maintains normal blood glucose levels

2. fat metabolism---synthesizes cholesterol and digest cholesterol and stores fat

3. protein metabolism--
-loss of this function results in death with in a few days
-removal of nitrates
-conversion of NH₃ into urea
-synthesis of plasma proteins
-synthesis of fibrenogen (blood coagulant)
-synthesis of anticoagulants
-conversion of one amino acid to another

4. removal of drugs and hormones
-detoxify penicillin, ampicillin
-excrete or alter into bile, estrogen, or aldosterene

5. excretion of bile

6. synthesis of bile salts

7. storage--glycogen, vitamins A, B₁₂, D, E, K, iron, copper

8. phagocytosis--Kupffer’s cells

9. activation of vitamin D

gallbladder
-pear shaped sac about 7-10 cm long
-accessory structure
-located in the fossa of the visceral surface of the liver

physiology
-hormonal stimulation causes smooth muscle contraction and squeezes contents into the cystic duct and common bile duct

-stores and concentrates bile until small intestine needs

-empty small intestine = closed hepatopancreatic ampulla sphincter
(bile flows back to gallbladder)

-chyme in small intestine opens the sphincter

small intestine
-2.5 cm in diameter, 6.35 m long
-absorption and most digestion occur here

3 parts
1. duodenum (means 12)
-1st 12 fingers width from the pyloric sphincter (about 25cm)
2. jejunum (means empty) at death this portion is found empty--2.5m long
3. ileum
-longest part at 3.6m long
-joins the large intestine at the ileocecal sphincter (valve)

histology
-same 4 layers as the rest of the GI tract
-mucosa and submucosa modified for digestion and absorption

-intestinal glands--secrete juices
-duodenal glands--secrete alkaline mucous

large surface area = better absorption

4 things help the small intestine absorb better

1. length
2. microvilli
3. villi--
-projections about 1 mm high
-10 to 40 every square mm
-gives lining of the small intestine a velvety appearance
-each contain: arteriole, venule, capillary network, and lymphatic vessels

4. circular folds (plicae circulares)
-permanent ridges about 10 mm high
-start near the proximal portion of duodenum and stop at the midpoint of ileum
-causes chyme to “spiral” which enhances absorption by increasing the contact with villi

mechanical

1. segmentation--
-major movement of small intestine
-localized contraction to mix the chyme
-DOES NOT move chyme downward

2. peristalsis--
-moves chyme down
-weak compared to in the esophagus
-chyme moves at 1 cm per minute (food stays in the small intestine about 3 to 5 hours

chemical

-pancreatic juice, bile and intestinal juice complete the breakdown of carbohydrates, proteins and lipids

-food pieces need to be small enough to pass through the epithelial tissue of the villi and depends on diffusion, facilitated diffusion, osmosis, and active transport

carbohydrates --- monosacharides
proteins --- amino acids
fats, lipids --- fatty acids, glycerol, and glycerides

Monosacharides and Amino acids go into the blood -- liver -- then to general circulation.

Fatty acids and Glycerides-- -bile salts dissolve and form bubbles called

micelles--
-container for fatty acids and glycerides
-when they come in contact with epithelial cells they allow contents to diffuse out of the “bubble”
-the micelle stays in the small intestine

Lipids hitch rides with protein transporters to form lipoproteins

HDL’s (high density lipoproteins)
-remove cholesterol from arteriole walls to take to the liver
-good

LDL’s (low density lipoproteins)
-transport cholesterol to tissues for hormone and membrane production
-cause cholesterol to build up in your arteries
-bad

Water
-absorbed by osmosis
-on average, 9 liters of water enters the small intestine a day
-all but 1 liter is absorbed

Large Intestine
-completes absorption (less than 10%)
-forms and expels feces

Anatomy
1.5 meters in length and 6.5 centimeters in diameter

Cecum
-portion below the ileocecal valve
-the appendix is attached to the pouch of the cecum

Colon--divided into 4 parts

Ascending -- up
Transverse -- across
Descending -- down
Sigmoid -- angles toward the midline of the body

Rectum--last 20 centimeters of the gastrointestinal tract

Anal Canal--last 2 to 4 centimeters of the rectum

Anus
-opening of the anal canal
-internal anal sphincter -- smooth “involuntary muscle”
-external anal sphincter -- skeletal muscle

Histology
-same four layers as the rest of the GI tract
-no villi
-no circular folds

simple columnar epithelial cells absorb water
goblet cells secrete mucous

Digestion
Chemical
-no enzymes (digestion through bacteria action)
-bacteria ferment excess carbohydrates and this releases hydrogen, methane and carbon dioxide
-the bacteria breakdown of amino acids produce odor

Feces formation
-chyme stays in the large intestine for 3 to 10 hours, during which it is a semi-solid due to the absorption of water
-all but about 100 milliliter of water is absorbed

Feces
-consist of water, inorganic salts, sloughed off epithelial cells, products of bacteria decomposition and undigested parts of food

Defecation
-emptying of rectum
-triggered by distention of the rectum