Final Exam

digestive system functions

digestive system functions

ingestion, secretion, digestion (chemical and mechanical), mixing and propulsion, absorption, defecation, responsible for facilitating metabolic processes

digestive system parts

alimentary canal and accessory organs (teeth, tongue, salivary glands, liver, gallbladder, and pancreas)

alimentary canal

a ‘tube’ through the ventral body canal, approximately 30 ft long

catabolism

larger molecules are broken into smaller molecules, happens in the mouth, stomach, and duodenum, in gi tract aka digestion, can be either mechanical or chemical

anabolism

smaller molecules are used as building blocks for larger molecules, happens in the liver

mechanical digestion

includes all movements that facilitate catabolic processes, mastication, swallowing, mixing, peristalsis

purpose of mixing

increase contact of food with digestive chemicals

peristalsis

movement of muscles within the gi tract that facilitates movement of food, a progression of coordinated contractions and relaxations of the circular and longitudinal layers of the muscularis which push the bolus onward

chemical digestion

mainly accomplished by hydrolysis, fats are broken down into fatty acids and glycerol, carbohydrates are broken down from polysaccharides into monosaccharides, proteins are broken down into polypeptides and amino acids

hydrolysis

using water to break down chemical bonds

layers of the gi tract

from deep (lumen is deep) to superficial: mucosa, submucosa, muscularis, serosa/adventitia, lumen is the inside space of the tube

mucosa

a mucous membrane made of various types of epithelium sitting on the lamina propria, has various glandular cells among the epithelial cells that secrete mucous and fluid into the lumen of the tract

lamina propria

loose ct under the epithelium in mucosa, contains MALT

where is nonkeratinized stratified squamous epithelium? (mucosa)

pharynx, esophagus, anus, there for protection

where is simple columnar epithelium? (mucosa)

the stomach and intestines, there for secretion/absorption

MALT

aka mucosa associated lymphatic tissue, a prominent lymphoid tissue in the lamina propria, protects against disease

muscularis mucosae

a thin layer of smooth muscle underneath the lamina propria (still in mucosa layer), makes the tiny folds of the lining of the stomach and small intestines, increases surface area to aid digestion and absorption

submucosa

the second layer (deep) of the gastrointestinal tract, composed of loose ct that binds the mucosa to the muscularis, contains blood and lymphatic vessels (to receive absorbed substances) and the submucosal plexus

submucosal plexus

an extensive network of neurons in the submucosa

muscularis

different in the mouth, pharynx, superior and middle parts of the esophagus, and anal sphincter vs rest of the gi tract

muscularis (mouth, pharynx, superior and middle parts of esophagus, and anal sphincter)

contains skeletal muscle that allows for voluntary swallowing and control of defacation

muscularis (everywhere else)

consists of smooth muscle arranged in 2 sheets, inner circular and outer longitudinal, has the myenteric nerve plexus continuing between them

where does the muscularis layer have skeletal muscle?

mouth, pharynx, superior and middle parts of esophagus, anal sphincter

serosa

outermost later of the gi tract when its contained in the peritoneal cavity, has a slippery mesothelium surface layer, covers the intraabdominal organs as a visceral peritoneum

adventitia

outermost layer of the gi tract when its attached to surrounding tissues (ex. around the esophagus), consists of a fibrous ct arranged around the organ which it supports

peritoneum

bodys largest serous membrane, wraps around most abdominopelvic organs, the visceral peritoneum forms the serosa of the alimentary canal and covers other intraabdominal organs, it then continues around the abdominal wall as the parietal peritoneum

how does the peritoneum differ from the pericardium and pleurae?

the pericardium and pleurae smoothly cover the heart and lungs while the peritoneum has large folds that bind the organs to one another and to the cavity walls, there are 5 major folds

5 major peritoneal folds

greater omentum, falciform ligament, lesser omentum, mesentery, mesocolon

greater omentum

the largest peritoneal fold, drapes over the transverse colon and the anterior coils of the small intestine like a ‘fatty apron’, contains many lymph nodes that help combat and contain infections of the gi tract, the large amount of adipose tissue can greatly expand (as seen in people with ‘beer bellies’)

falciform ligament

attaches the liver to the anterior abdominal wall and the diaphragm

lesser omentum

suspends the stomach and duodenum from the inferior edge of the liver, forms a pathway for blood vessels (hepatic portal vein and common hepatic artery) to enter the liver, contains the common bile duct

mesentery and mesocolon

mesentery- of the small intestine, mesocolon- of the large intestine, together attach the bowel to the posterior abdominal wall, hold the intestines loosely in place as muscular contractions mix and move the luminal contents along the gi tract

retroperitoneal organs

organs that lie in the retroperitoneal space, they are covered by the peritoneum only on their anterior surfaces, ‘retroperitoneal’ only refers to the part of the organ that is actually behind the peritoneum

ex of retroperitoneal organs

kidneys, ureters, most of the pancreas, adrenal glands, aorta, inferior vena cava

phases of the digestive activities of the gi tract

3 overlapping phases, cephalic phase, gastric phase, intestinal phase

cephalic phase

the smell, sight, thought, or initial taste of food activates neural centers in the cerebral cortex, hypothalamus, and brain stem to prepare for digestion, the brain stem activates the facial and glossopharyngeal nerves to stimulate secretion of saliva, while the vagus nerves stimulate secretion of gastric juice

gastric phase

begins when food reaches the stomach, neural and hormonal mechanisms promote secretion of gastric juice and increase gastric motility

intestinal phase

begins once food enters the small intestine, neural and hormonal responses promote the continued digestion of foods that have reached the small intestine

buccal cavity

aka oral cavity, formed by the cheeks, hard and soft palates, and the tongue, mechanical and chemical digestion occurs here

mechanical digestion in buccal cavity

happens through mastication, enables food to be mixed with saliva to form a soft flexible bolus that can be easily swallowed

chemical digestion in oral cavity

saliva starts the process of chemical digestion

saliva

99.5% water with tiny amounts of dissolved ions, IgA, lysozome (a bacteriolytic enzyme), and salivary amylase (a digestive enzyme that acts on starch)

what is the only thing chemically digested in the mouth?

starch

salivary glands

secrete saliva, 3 large salivary glands secrete most of the saliva (parotid, submandibular, sublingual), smaller glands are found on the lips (labial), cheeks (buccal), palate (palatal), and tongue (lingual)

average daily salivary secretions

1-1.5 liters

salivary regulation

under control of the ans, parasympathetic stimulation promotes secretion while sympathetic stimulation decreases it, salivary centers are in the brain stem, efferent nerve impulses are transmitted by the facial and glossopharyngeal nerves, additional salivary stimulators: touch, smell, taste, and psychological factors

tongue

composed of skeletal muscle (under voluntary somatic control), forces the moistened food bolus into position for deglutition and places the bolus into contact with the teeth for chewing, extrinsic muscles of the tongue attach to bones in the area and move the tongue from side to side, intrinsic muscles originate within the tongue and alter its shape and size for speech and swallowing

dentes

aka teeth, located in sockets of the alveolar processes of the mandible and maxillae, the sockets are lined by the periodontal ligament, a typical tooth has 3 major external regions (crown, root, neck), the neck of each tooth is covered by the gingivae which extends slightly into each socket

periodontal ligament

a dense fibrous ct that anchors the teeth to the socket walls and acts as a shock absorber during chewing

gingivae

aka gums

dentin

calcified ct that forms most of the tooth, the dentin of the crown is covered by enamel, it also encloses the pulp cavity (a space filled with pulp)

enamel

a harder than bone calcified material that covers teeth

pulp (teeth)

ct containing blood vessels, nerves, and lymphatic vessels

how many dentitions do humans have? (and what are dentitions?)

2 dentitions, sets of teeth

deciduous teeth

aka baby teeth, 20 teeth which begin to erupt around 6 months of age, all are lost btwn 6-12 yrs of age

permanent teeth

aka adult teeth, 32 teeth, numbered from right to left, top (1-16) and bottom (17-32), includes molars, premolars, cuspids (aka canines), and incisors

how many molars are there?

3 on each side, top and bottom (12 total)

how many premolars are there?

2 on each side, top and bottom (aka 8 total)

how many cuspids are there?

1 on each side, top and bottom (aka 4 total)

how many incisors are there?

4 top and bottom (the 4 front teeth), (aka 8 total)

wisdom teeth

the third molars (teeth 1, 16, 17, and 32)

deglutition

swallowing food, has 3 stages, involves many muscles subject to a number of voluntary and involuntary controls

why is deglutition so complicated?

the oropharynx and laryngopharynx have respiratory functions in addition to digestive functions and swallowed food has to transit them both on the way to the esophagus and it has to do that without going into the nasal cavity or airway

3 stages of deglutition

voluntary, pharyngeal, esophageal

voluntary part of deglutition

the tongue forces the bolus to the back of the oral cavity and into the oropharynx

pharyngeal part of deglutition

begins as the bolus passes into the oropharynx, receptors send impulses to the deglutition center in the medulla and pons, returning impulses cause the soft palate to move superiorly and posteriorly to close the nasopharynx, the epiglottis moves slightly inferiorly to close the glottis

esophageal part of deglutition

begins once the bolus enters the esophagus, peristalsis pushes the bolus onward

digestive function of the esophagus

only propulsion (moving food into the stomach

esophagus

a muscular tube that begins inferior to the laryngopharynx and is positioned posterior to the trachea, leaving the neck it traverses the thoracic cavity down the posterior mediastinum before piercing the diaphragm through the esophageal hiatus

what does the mucosa of the esophagus consist of?

nonkeratinized stratified squamous epithelium

type of muscle in the muscularis in the esophagus

varies by region, superior 1/3 is skeletal muscle, inferior 1/3 is smooth muscle, intermediate 1/3 is both

upper esophageal sphincter

a sphincter at the upper end of the esophagus

lower esophageal sphincter

aka cardiac sphincter, regulates the movement of food from the esophagus into the stomach, incompetence of the les results in gerd (gastroesophageal reflux disease) which manifests as ‘heart burn’

stomach

a j shaped enlargement of the gi tract, situated between the esophagus and the first part of the small intestine, also serves as a mixing chamber and holding reservoir, the gastric muscularis has a 3rd layer of muscle (oblique muscle) to facilitate the mixing action of mechanical digestion

rugae

large folds in the mucosa of the empty stomach which enable gastric distention depending on the amount of stomach contents

size and position of the stomach

position and size changes continually as the diaphragm moves it with each breath, empty its about the size of a large sausage but its the most distensible part of the gi tract and can expand to accomodate a large quantity of food

why does the stomach also serve as a mixing chamber and holding reservoir?

because a meal can be eaten much more quickly than the intestines can digest and absorb it so the stomach helps slow the process down for the intestines

4 main regions of the stomach

cardia, fundus, body, pylorus

what type of mucosa does the stomach have?

simple columnar epithelial cells which secrete a protective mucous, columns of secretory cells extend down into the lamina propria forming gastric glands, several gastric glands open into the Botton of narrow channels called gastric pits

types of exocrine and endocrine cells in the gastric glands and pits

exocrine: mucous neck cells (produce mucous), parietal cells (produce intrinsic factor and HCl), chief cells (secrete the protease pepsinogen and gastric lipase), endocrine: g cells (located mainly in the pyloric antrum, secrete the hormone gastrin into the bloodstream)

gastric juice

formed by the secretion of the mucous, parietal, and chief cells, totals 2-3 l/day

how is the stomach protected from gastric juice?

1-3mm thick layer of alkaline mucous secreted by surface mucous cells and mucous neck cells

what does the strongly acidic nature of gastric juice do?

kills many microbes, partially denatures proteins in food, converts pepsinogen into pepsin

pepsin

the only proteolytic enzyme in the stomach

gastric lipase

splits triglycerides

intrinsic factor

aka if, needed for absorption of vitamin b12 in the terminal ileum (vitamin b12 is needed for rbc production)

gastric erosion

erosion of the stomachs epithelial lining, caused by a disturbance btwn hydrochloric acid production, pepsin secretion, and mucosal defenses, can be caused by too much alcohol consumption or use of an nsaid drug (probably too much use)

mixing waves

gentle, rippling, peristaltic movements that pass over the stomach every 15-20 sec, macerate food, mix it with secretions of the gastric glands and reduce it to chyme

chyme

a soupy liquid that is food mixed with gastric juices

does absorption occur in the stomach?

technically yes but very little bc the epithelial cells in the stomach are impermeable to most materials

timing of digestion

within 2-4 hrs after eating a meal the stomach has emptied its contents into the duodenum, foods rich in carbs spend the least time, high protein foods remain somewhat longer, emptying is slowest after a fat laden meal containing large amount of triglycerides

how does the intestinal phase begin?

at appropriate intervals, the stomach allows a small amount of chyme to pass through the pyloric sphincter and enter the duodenum to begin the intestinal phase of digestion

completion of digestion

a collective effort of pancreatic juice, bile, and intestinal juice in the small intestine

what does digestion and absorption in the small intestine depend heavily on?

secretions from the pancreas and gallbladder (liver)

pancreas

an oblong gland located posterior to the stomach in the retroperitoneal space, connected to the duodenum by the hepatopancreatic ampulla and accessory ducts, secretes enzymes (which digest food in the small intestine) and sodium bicarbonate (which buffers the acidity of chyme)

pancreatic acini

glandular clusters, about 99% of the them participate in exocrine secretion, only 1% of the clusters (pancreatic islets) form the endocrine portion of the gland

what do pancreatic islets secrete?

the hormones glucagon, insulin, somatostatin, and pancreatic polypeptide

pancreatic juice

alkaline fluid secreted into the duodenum (about 1-1.5 l/day), creates the proper ph for certain digestive enzymes in the small intestine

which digestive enzymes does pancreatic juice help and what to they do?

pancreatic amylase (digests starch), trypsin, chymotrypsin, carboxypeptidase, elastase (cleave polypeptides into dipeptides and single amino acids), pancreatic lipase (the major triglyceride digesting enzyme in adults)

liver

the bodys largest gland, second largest organ, has 2 main lobes (left and right, divided by the falciform ligament), covered by visceral peritoneum, made up of liver lobules