Digestive System

What are the two main components of the GI tract or digestive system?

alimentary canal and accessory organs

alimentary canal (+ length)

extends from mouth to anus through ventral body cavity

~ 30 ft in cadavers, slightly shorter in living body

components of the alimentary canal

mouth, pharynx, esophagus, stomach, intestines, anus

accessory organs

teeth, tongue, salivary glands, liver, gallbladder, pancreas

catabolism (+ structures that do that)

large molecules broken down into smaller molecules

• mouth

• stomach

• duodenum

anabolism (+ structures that do that)

small molecules used to build larger molecules

• liver

functions of the digestive system

1. ingestion

2. secretion

3. mixing/propulsion

4. digestion

5. absorption

6. defecation

secretion of digestive system

cells lining the GI tract secrete enzymes, water, and buffer to break down and lubricate food

mixing/propulsion of the digestive system

smooth muscle of the muscularis moves food through GI tract and mixes the food

mechanical digestion (+ types)

movements that facilitate catabolism

• mastication

• swallowing

• mixing

• peristalsis

• segmentation

segmentation

local contractions in the small intestine

how is chemical digestion mainly done?

by hydrolysis or enzymes

How are the different macromolecules broken down during chemical digestion?

fats: fatty acids + glycerol

carbs: monosaccharides

proteins: polypeptides + amino acids

nucleic acid: nucleotides

absorption of the digestive system

broken down food/liquid is taken into cells lining the GI tract and into blood and lymph

Where is 90% of absorption done?

small intestine

defecation

unabsorbed waste is eliminated as feces out anus

layers of the GI tract from deep to superficial

1. mucosa

2. submucosa

3. muscularis

mucosa (+ sublayers)

a mucous membrane

1. epithelium

2. lamina propria

3. muscularis mucosae

What are the two types of epithelium in the mucosa, where are they located, and what are their functions?

non-keratinized stratified squamous: pharynx, esophagus, anal canal (protection)

simple columnar: stomach, intestines (secretion and absorption)

lamina propria (+ function)

a loose connective tissue connecting the epithelium to muscularis mucosae

• scattered lymphoid follicles, part of MALT

• defends against pathogens

MALT

mucosa-associated lymphoid tissue

muscularis mucosae

thin layer of smooth muscle that produces local movements of mucosa

submucosa (+ function)

loose CT that connects mucosa to muscularis

• contains blood and lymphatic vessels and submucosal plexus

• allows organs to stretch as food enters then regain their shape as it passes

muscularis

contains inner circular and outer longitudinal muscles responsible for peristalsis and segmentation

serosa

visceral pericardium in abdominopelvic cavity

What replaces the serosa layer in organs outside of the abdominopelvic?

adventitia (dense CT)

peritoneum (+ visceral and parietal)

wraps around most of abdominopelvic organs

visceral: covers organs

parietal: continues around abdominal wall

enteric nervous system (+ what it consists of)

part of the nervous system for the digestive system

• consists of enteric neurons that communicate to regulate digestive activities and form submucosal and myenteric nerve plexuses

submucosal nerve plexus (location and function)

• in submucosa

• regulates secretions

myenteric nerve plexus (location and function)

• between layers of muscularis

• regulate peristalsis and segmentation

peritonial folds

1. greater omentum

2. falciform ligament

3. lesser omentum

4. mesentary

5. mesocolon

peritoneal cavity

space between two membranes containing serous fluid

mesentary (+ location, function)

double layer of peritoneum (layers fused back-to-back)

• extends from body wall to digestive organ (most are dorsal and attach to posterior wall)

• stores fat, provides routes for blood vessels, lymphatics, and nerves to reach organs

intraperitoneal organs

surrounded by peritoneum and anchored to body wall by mesentary

retroperitoneal and the organs in retroperitoneal space

portions of organs that lie behind peritoneum

• kidneys and ureters

• most of pancreas

• aorta and inferior vena cava

peritonitis (+ cause, treatment)

inflammation of peritoneum

causes: ruptured organ (usually appendix) where bacteria leaks out, piercing abdominal wound, perforating ulcer, and poor surgical sterile technique

treatment: infectious debris removed and megadoses of antibiotics

What are the three overlapping phases of digestion?

1. cephalic

2. gastric

3. intestinal

cephalic phase of digestion

• smell, sight, thought, or initial taste of food activates neural centers in cerebral cortex, hypothalamus, and brain stem to prepare for digestion

• the brainstem then activates facial nerve and glossopharyngeal nerve for saliva secretion and vagus nerve for secretion of gastric juices

gastric phase of digestion

once food reaches stomach

• neural and hormonal mechanisms promote secretion of gastric juices and increase gastric motility

What is an important hormone for the gastric phase of digestion?

gastrin promotes secretion of gastric juices

intestinal phase of digestion

once food enters small intestine

• neural and hormonal mechanisms promote the continued digestion of food

What is food called once it reaches the small intestine?

chyme

components of the oral cavity

cheeks, hard and soft palate, and tongue

composition of saliva

99.5% water and tiny amounts of dissolved ions, IgA, lysozome, and salivary amylase

salivary amylase

digestive enzyme that acts on starch

tongue (+ function)

skeletal muscle under voluntary somatic motor control

function: force bolus into position for swallowing and place bolus into contact with teeth for chewing

extrinsic tongue muscles

attach to bones in area and move tongue side to side

intrinsic tongue muscles

originate within tongue and alter its shape and size for speech and swallowing

3 stages of degultition

1. voluntary stage

2. pharyngeal stage

3. esophageal stage

voluntary stage of deglutition

tongue forces bolus to back of oral cavity and into oropharynx

pharyngeal stage of deglutition

bolus passes into oropharynx and receptors send impulses to deglutition center in medulla and pons

esophageal stage of deglutition

once bolus enters esophagus, peristalsis occurs

peristalsis

coordinated contractions/relaxations of muscularis to push bolus onward

upper/lower esophageal sphincters (UES and LES) (+ function of LES)

sphincters situated at the ends of the esophagus

• LES regulates movement of food from esophagus into stomach

gastroesophageal reflux disease (GERD)

incompetence of LES that manifests as “heart burn”

rugae

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

What is different about the muscularis of the stomach compared to other organs?

the stomach has a 3rd inner oblique layer of muscularis (only in body of stomach) to facilitate mixing for mechanical digestion

regions of the stomach

1. cardia: attaches to esophagus

2. fundus: the top round part

3. body

4. pylorus: attaches to duodenum

function of simple columnar cells in stomach

secrete protective mucus

gastric glands

formed by columns of secretory cells extending down into lamina propria

gastric pits

narrow channels where several gastric glands open into

exocrine cells in gastric glands/pits

• mucous neck cells

• parietal cells

• chief cells

mucous neck cells

produce mucous and protective mucous to protect the stomach from gastric juice

parietal cells in stomach

produce intrinsic factor and HCl

chief cells in stomach

secrete the protease pepsinogen and gastric lipase

What forms gastric juice and how much is produced a day?

secretions from mucous neck cells, parietal cells, and chief cells

2-3 L/day

endocrine cell in gastric glands/pits + their function, location

enteroendocrine G cells

• secrete hormone gastrin into blood which increases the production of HCl by parietal cells

• located mainly in pyloric antrum

How is the stomach protected by its own gastric juices?

a thick layer of alkaline mucous from surface mucous cells and mucous neck cells

What is the pH in the stomach?

2

What order do the macromolecules get liquified in the stomach?

carbs, proteins, lipids

pepsin

only proteolytic enzyme in stomach

gastric lipase

splits triglycerides

intrinsic factor

needed for absorption of vitamin B12 in terminal ileum

mixing waves

gentle, rippling peristaltic movements that pass over stomach every 15-25 seconds and mix food with gastric juice to make chyme

duodenum

the section of the small intestine immediately following stomach where most chemical digestion occurs

What sphincter does chyme pass through to move from stomach to small intestine?

pyloric sphincter

pancreas (+ location)

oblong gland located posterior to the stomach in the retroperitoneal space

pancreatic duct

connects pancreas to duodenum of small intestine

how many lobes does the liver have and how is it divided

2 lobes (right and left) divided by the falciform ligament

subunit of liver

liver lobules (repeating functional units)

hepatocyte (+ functions)

functional cells of liver

digestive functions:

• make, transform, and store proteins, carbs, and lipids

• detoxify and excrete a variety of exogenous and endogenous substances

non-digestive functions:

• phagocytosis of old/worn out cells

• make heparin and plasma proteins