Biology 2402 Anatomy &Physiology II - Digestive system notes - Ch. 15
Digestive system processes the food used as fuel and nutrients for the body.
Composed of a tube through the body (digestive tract, gastrointestinal tract or alimentary
canal) and accessory organs (salivary glands, liver, gall bladder and pancreas).
Tube has specialized regions to perform specialized functions.
Fig 15.1 – 15.2
The tube is composed of four layers “from mouth to anus”, modifications in each region. Fig 15.3
lumen - the opening in the center of the tube where the food is located
mucosa - inner lining; mucous membrane made of epithelial tissue with
supporting connective tissue and smooth muscle, numerous mucus glands.
- type of epithelium and contour specialize by region
submucosa - loose connective tissue, holds mucosa in place, allows movement
- contains large blood vessels, lymphatic vessels, nerves, glands
muscularis - two layers of muscle, an inner circular layer and an outer
longitudinal layer and an oblique layer in the stomach
- composed of smooth muscle (skeletal muscle in the mouth, pharynx,
upper esophagus, anal canal)
- causes movement of food along tract and other mixing movements
adventitia / serosa - outer layer
- adventitia is fibrous connective tissue in head, neck and thoracic cavity
and rectum that attaches digestive tract to surrounding structures
- serosa is a smooth slick serous membrane in the abdominal cavity
also called the visceral peritoneum, surrounds the digestive
tract, forms a membrane (the mesentery) that suspends the digestive tract
in the abdominal cavity, then lines the abdominal cavity (parietal
peritoneum).
- the visceral and parietal peritoneum secrete peritoneal fluid as a lubricant
*List the 4 layers of the digestive tract and the tissue that forms each.
*List the major type tissue that forms each of these layers.
*How can the digestive tract move within the abdominal cavity and yet not be damaged by friction?
*Make a drawing showing the four layers of the alimentary tract and the lumen.
*What is the difference between the adventitia and the serosa?
Several processes occur in the digestive tract:
- ingestion - taking food into the mouth
- mechanical processing mixing movements (mastication, segmentation) produce mechanical digestion
propelling movements (swallowing, peristalsis) move food along tract
- chemical digestion - breakdown of food by breaking chemical bonds
- secretion - production and release of substances into lumen
- absorption - movement of small molecules into cells
- excretion (defecation) - removal of waste, indigestible material
Specialized regions of the digestive tract
mouth (oral cavity and vestibule)
15.5 – 15.10
- area where food is ingested; mechanical breakdown begins (mastication); food is
lubricated by mucus and saliva; some chemical digestion begins (carbohydrates);
analysis of food occurs (taste, texture, temperature).
- bounded by cheeks, lips, hard palate, soft palate with uvula, tongue
- prominent tooth bearing ridges covered with gingivae partially divide cavity
- tongue forms most of floor; used to manipulate food and sense details of food.
- lymphatic nodules beneath surface help remove bacteria, etc. that enter mouth:
lingual, palatine, pharyngeal tonsils
- salivary glands (accessory glands) 3 pairs - parotid, sublingual, submandibular
- 1.0-1.5 liter saliva /day - water dissolves food
- mucus lubricates food (mucous glands common around mouth)
- lysozyme and antibodies are antibacterial agents to kill bacteria
- salivary amylase is digestive enzyme to digest starches
- pH buffers reduce acidity
- teeth - hard structures used to masticate food
- root (in socket by periodontal ligament), neck and crown (above gingivae)
- enamel covers crown; dentin forms bulk of tooth; pulp cavity is central
area that contains blood vessels and nerves; cementum around root
- incisors (8) clipping or cutting, cuspids (4), bicuspids (8) shearing and
chopping , molars (12) crushing and grinding
- mucosa made of stratified squamous epithelium (resist abrasion)
- muscularis made of skeletal muscle
pharynx - connects mouth and esophagus
- oropharynx and laryngopharynx (nasopharynx is above soft palate and is not part
of digestive tract)
- mucosa and muscularis same as oral cavity (stratified squamous epi. and skeletal muscle).
esophagus - tube that carries food from pharynx to stomach, through mediastinun and
diaphragm into abdominal cavity
- mucosa is made of stratified squamous epithelium
- muscularis is skeletal muscle in upper portion and smooth muscle in lower
- adventitia - connective tissue that anchor esophagus to surrounding
- circular muscle bands form sphincter at each end - upper and lower esophageal
(cardiac) sphincter - prevent food from moving backwards
- deglutition (swallowing) moves food into esophagus from oral cavity
- food moved through esophagus by peristalsis
*What accessory glands occur in the mouth? List four secretions of these glands and tell what each
does.
*What type of epithelium occurs in the mouth, pharynx and esophagus?
*Describe three ways that the digestion of food begins in the mouth.
Swallowing (deglutition) - moving food from mouth to esophagus
- food is mixed with saliva and pressed into bolus by tongue
- the bolus is pushed up and back by the tongue
- this voluntary action pushes the bolus into the oropharynx, triggering the
involuntary reflex described below.
- the soft palate is pushed up to close the nasopharynx
- the larynx is pulled up and the epiglottis folds over the glottis - this opens the
esophagus and closes the larynx. Pulmonary ventilation is stopped.
- the bolus is moved into and along the esophagus by peristalsis.
Peristalsis - moving food along tube
Fig. 15.4c
- bands of circular muscle contract as waves behind bolus, decreasing diameter of
tube and pushing bolus forward
- longitudinal muscles in front of bolus contract, making tube shorter and wider so
that bolus moves easily forward
- sphincters relax ahead of bolus
*Describe two types of movement that occur in the mouth, pharynx and esophagus.
Stomach - located in the upper left abdominal cavity, just inferior to the diaphragm.
- temporary storage of ingested food, gradual release (3-4 hour processing)
- some mechanical breakdown of food by churning, some chemical digestion
(food at this point is a thick liquid called chyme)
- production of intrinsic factor necessary for some vitamin absorption
- several regions can be distinguished:
Fig 15.11
- cardiac , an area around the connection with the esophagus (this forms the
cardiac sphincter, which is the same as the lower esophageal sphincter)
- fundic, an area that extends above the cardiac region for temporary storage
- body, large main part of the stomach
- pylorus, narrow lower portion that connects to small intestine
- pyloric sphincter, valve that closes to regulate chyme passing into intestine
- the mucosa is made of simple columnar epithelium (cells specialized for
secretion and absorption - in the stomach little absorption occurs)
- large accordion - like fold, the rugae, allow the stomach to expand (1 -1.5 liters)
- mucus glands produce thick alkaline mucus which protects epithelium from
acids and digestive enzymes
- folds in epithelium form gastric pits and gastric glands; secrete gastric juice (2-3
liters/day)
- parietal cells secrete hydrochloric acid (HCl) and intrinsic factor (helps
absorb vitamin B12)
- chief cells secrete pepsinogen, which is activated by acid to pepsin
to begin the chemical digestion of proteins
- other cells secrete a hormone called gastrin (more on this later)
- the muscularis is composed of circular, longitudinal and oblique layers of smooth muscle
- this extra muscle layer improves mixing.
- the serosa (visceral peritoneum) forms two mesenteries:
- the lesser omentum suspends the stomach from the liver
- the greater omentum hangs down over the abdominal viscera
* Make a drawing of the stomach and label the four regions and two sphincters.
*Describe how the mucosa and the muscularis are different in the stomach compared to other regions
of the digestive tract.
*List three secretions of the gastric glands and tell what each does.
*What is the primary function of the thick alkaline mucus in the stomach?
Small intestine - major region of chemical digestion and absorption, divided into three regions:
- duodenum (20-30 cm), separated from the stomach by the pyloric sphincter
- ducts from pancreas and liver deliver secretions from these glands
- glands secrete thick alkaline mucus to neutralize chyme
- jejunum (2.5 meters), most chemical digestion and absorption occurs here
- ileum (3.6 meters), an area of some absorption
- the ileocecal valve is a sphincter that closes the distal end of the small intestine
- the mucosa is made of simple columnar epithelium
Fig. 15.3, 15.12
- has a series of permanent folds (plicae circulares), finger-like projections (villi)
and each cell has folds (microvilli) - increase surface area 600 times
- each villi contains a capillary bed and a large lymph capillary, a lacteal.
- the blood capillaries merge into hepatic portal system to the liver
- the lacteals pick up materials, especially fats, and transport them into blood.
- intestinal glands form at bottom of folds between villi -secrete intestinal juices
which contain mucus and water (1.8 liters/day).
- duodenal glands secrete thick alkaline mucus
- duodenal endocrine cells secrete several hormones important in regulation of the
digestive process.
- cells in mucosa secrete some digestive enzymes
- the muscularis is composed of longitudinal and circular smooth muscle
- peristalsis moves food slowly along small intestine
- segmentation is a type movement that mixes chyme
Fig.15.4b
*Describe the mucosa of the small intestine and explain how the surface area is greatly
increased. What type epithelium forms the mucosa and how are these cells shaped?
*What is the value of the small intestine having a large surface area (in contact with chyme)?
*Describe the process of segmentation and tell what it accomplishes.
*What are the primary functions of the small intestine? What accessory glands enter?
Large Intestine - an area of recovery where water (8-9 liters/day into digestive tract,
Fig. 15.18
liters/day out with feces), minerals, and some vitamins are recovered from the
mostly indigestible material and feces is formed and expelled.
- symbiotic bacteria are maintained and feed on what we cannot digest - the waste
contains some important substances like vitamin K and biotin.
- divided into several major regions - about 1.5 m total - cecum, colon and rectum
and anal canal.
- the colon is supported by mesocolon.
- cecum connects to the ileum separated by the ileocecal valve; a short section that
receives chyme from the small intestine.
- the vermiform appendix is a blind pouch that connects to the cecum.
- the colon is divided into the ascending, transverse, descending and sigmoid
colon
- the rectum is the last 20 cm and fills just prior to defecation
- anal canal is the last portion of the rectum that ends at the anus.
- the mucosa is made of simple columnar epithelium
- mucous glands are common but no digestive glands and no villi
- beginning in the anal canal the mucosa is made of stratified squamous
epithelium (for abrasion resistance).
- muscularis is of circular and longitudinal smooth muscle
- three thin bands of longitudinal muscle run along either side of the cecum and
colon, called the taenia coli.
- muscle tone in the taenia coli pull the colon into pouches called haustra
- mass peristalsis (mass movement) moves food from haustra to haustra
- the last portion of the circular muscle of the anal canal forms the internal anal
sphincter - this is smooth muscle under unconscious control.
- skeletal muscle of the abdominal floor forms the external anal sphincter - this is
muscle that is under conscious control.
Fig. 15.29
*Describe the anal sphincter. What type muscle forms it and why this is important.
*List the epithelial tissues in the digestive tract and the regions where each occurs.
*List the sphincters of the digestive tract and tell where each is located.
Accessory digestive glands: (salivary glands, pancreas, and liver) Fig. 15.10, 15.14-15.18
pancreas - a gland that is part endocrine (1%) and part exocrine (99%)
- the exocrine portion (glands with ducts that deliver its secretions) produces an
alkaline pH buffer (sodium bicarbonate) and several very effective digestive
enzymes for all types of molecules. (pancreatic juices 1-1.5 liters/day)
- the enzymes are secreted in inactive form and are activated in the intestine
- exocrine secretions enter small ducts that merge to larger ducts - pancreatic duct
joins with common bile duct just before entering the duodenum.
Liver - large organ involved in many metabolic functions.
- composed of cells called hepatocytes arranged into functional units, lobules
- each lobule contains a sinusoid, a large permeable capillary, that empties into a
central vein. Hepatocytes are arranged as plates along the sinusoid.
- each lobule receives blood from two sources, the hepatic portal vein carrying
absorbed food from the intestine and the hepatic artery carrying oxygenated
blood.
- each lobule also contains bile canaliculi that collect the bile produced by
hepatocytes - the canaliculi merge to larger ducts to common bile duct to
duodenum or cystic duct to gall bladder.
- bile is a break down product of hemoglobin from the destruction of old red
blood cells. It contains bile salts is used to emulsify fats in the intestines.
- the gall bladder stores excess bile until needed
Chemical digestion
- large organic molecules (polymers) must be broken into small subunits
(monomers) in order to pass through cell membrane of mucosa (absorption)
- digestive enzymes break chemical bonds to form smaller molecules
- bile emulsifies lipids (fats)
- know the following enzymes, where they are produced and what they digest:
salivary glands - salivary amylase
pancreas - amylase (carbohydrates)
(carbohydrates)
- nuclease (nucleic acids)
gastric glands - pepsin (proteins)
- lipase (fats)
-trypsin, chemotrypsin,
liver - bile (emulsifies fats)
carboxypeptidase (proteins)
Regulation of digestion - movements and secretions within digestive system.
cephalic phase: taste, smell, sight, thought of food stimulates (via parasympathetic nerves) salivary
glands to secrete saliva, gastric glands to secrete gastric juices and the muscularis of stomach to begin
churning.
gastric phase: when food enters the stomach the wall is distended, triggering stretch receptors. This
reinforces the cephalic phase - gastric glands are stimulated, stomach churning is stimulated and the
hormone gastrin (produced in gastric glands) stimulates the lower esophageal sphincter (aka cardiac
sphincter) to contract. Gastrin also stimulates the gastric glands and muscularis of stomach and it
inhibits (so they relax and open) the pyloric sphincter and the ileocecal sphincter (this allows food to
be moved along the digestive tract).
intestinal phase: when acid chyme enters duodenum the acid stimulates sympathetic nerves that block
stimulation of parasympathetic nerve stimulation of stomach, inhibiting gastric glands and the
stomach muscularis churning.
Distention of the duodenum stimulates parasympathetic nerves that stimulate segmentation and
peristalsis.
The acid chyme stimulates the production of the hormone secretin which stimulates the pancreas to
produce pH buffer.
The proteins and fats in the chyme stimulate the production of the hormone cholecystokinin (aka
CCK) which stimulates the pancreas to secrete digestive enzymes, the gall bladder to contract forcing
bile into the duodenum, the pyloric sphincter to contract. CCK also inhibits the gastric glands and
stomach churning.
Why do things have to be so complicated!!
*Describe the muscularis of the colon. Tell how it is different from the muscularis in other regions
and how this affects movement in the colon.
*List three accessory glands in the digestive tract and tell what each secretes.
*Describe the vessels that connect to each lobule in the liver (4) and tell what each is
transporting.
*What is the function of the gall bladder?
*The pancreas is called both an exocrine and an endocrine gland. What does this mean?
*If a train leaves Chicago at 8:00 AM heading south at a constant velocity of 55 mph and a 200
pound man on that train smells a freshly baked cinnamon croissant, what phase of gastric
function will be stimulated?
*Describe the cephalic phase - tell what causes this phase and what is stimulated and what is
inhibited.
*Describe the gastric phase - tell what causes this phase and what is stimulated and what is inhibited.
*Describe the intestinal phase - tell what causes this phase and what is stimulated and what is
inhibited.
*List three hormones that are involved in regulating the digestive system and tell what triggers the
production of each, where each is produced and what each does.
*How is the duodenum protected from the acidic chyme that enters from the stomach?