Digestive System

main functions of the digestive system

• take in food

• break it down into nutrient molecules

• absorb molecules into the bloodstream

• rid body of any indigestible remains

alimentary canal (gastrointestinal or GI tract or gut)

• continuous muscular tube that runs from the mouth to anus

• digests food: breaks down into smaller pieces

• absorbs pieces through lining into blood

• organs include mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus

accessory digestive organs

• teeth

• tongue

• gallbladder

• digestive glands: produce secretions that help break down food (salivary glands, liver, pancreas)

6 processes of food

1. ingestion

2. propulsion → peristalsis

3. mechanical breakdown

4. digestion

5. absorption

6. defecation

ingestion

eating

propulsion

movement of food through the alimentary canal, includes swallowing and peristalsis (wave contraction and relaxation)

mechanical breakdown

includes chewing, mixing of food with saliva, churning food in stomach, and segmentation

segmentation

local constriction of intestine that mixes food with digestive juices

digestion

series of catabolic steps that involves enzymes that break down complex food molecules into chemical building blocks

absorption

passage of digested fragments from lumen of GI tract into blood or lymph

defecation

elimination of indigestible substances via anus in form of feces

peritoneum

serous membranes of abdominal cavity that consists of

• visceral peritoneum

• parietal peritoneum

visceral peritoneum

membrane on external surface of most digestive organs

parietal peritoneum

membrane that lines body wall

peritoneal cavity

• fluid-filled space between two peritoneums

• fluid lubricates mobile organs

mesentery

double layer of peritoneum; layers are fused back-to-back

• extends from body wall to digestive organs, provides routes for vessels, lymphatics, nerves, and holds organs in place, and stores fat

intraperitoneal (peritoneal) organs

organs that are located within the peritoneum

retroperitoneal organs

located outside or posterior to the peritoneum; includes most of pancreas, duodenum, and parts of large intestine

peritonities

• inflammation of peritoneum, can be caused by piercing to abdominal wound, ulcer, or ruptured appendix

• peritoneal coverings sticking together helps; dangerous and legal if it spreads; treatment is debris removal and antibiotics

four layers or tunics of all digestive organs

• mucosa

• submucosa

• muscularis externa

• serosa

mucosa layer

• tunic layer that lines lumen

• functions: different layers perform one or all three

→ secretes mucus, digestive enzymes, and hormones

→ absorbs end products of digestion

→ protects against infectious disease

made up of three sublayers

1. epithelium

2. lamina propria

3. muscularis mucosae

epithelium layer of mucosa

• simple columnar epithelium and mucus secreting cells

• secretes mucus

• protects digestive organs from enzymes

• eases food passage

• may secrete enzymes and hormones

lamina propria layer of mucosa

• loose areolar connective tissue

• rich supply of capillaries (nourishment and absorption)

• contains lymphoid follicles that help defend against microorganisms

muscularis mucosae layer of mucosa

smooth muscle that produces local movements of mucosa

submucosa layer

• areolar connective tissue

• contains blood and lymphatic vessels, lymphoid follicles, and submucosal nerve plexus that supply GI tract tissue

• has abundant amount of elastic tissues which helps organs restore shape after large meal

muscular externa layer

• muscle layer responsible for segmentation and peristalsis

• contains inner circular muscle layer and out longitudinal layers

• circular layer forms sphincters

serosa layer

• outermost layer, made up of the visceral peritoneum

• formed from areolar connective tissue covered with mesothelium in most organs

splanchnic circulation

arteries that branch off abdominal aorta to serve digestive organs

hepatic portal circulation

• drains nutrient-rich blood from digestive organs

• delivers blood to liver for processing

enteric nervous system

• nervous system of the GI tract

• contains more neuran’s than spinal cord; known as gut brain

enteric neurons

• what the gut brain is made up of, they communicate extensively with each other and controls motility of GI tract

• they make up two two main interconnecting intrinsic nerve plexuses (submucosal or myentric)

submucosal nerve plexus

regulates glands and smooth muscle in submucosa

myenteric nerve plexus

controls GI tract motility, muscularis externa

short reflexes

mediated by enteric nerve plexuses (gut brain); responds to stimuli in GI tract

long refelxes

respond to stimuli arising inside or outside of gut, such as from autonomic nervous system

• parasympathetic system enhances digestive process while sympathetic system inhibits digestion

three concepts regulate GI activity

1. digestive activity is provoked by a range of mechanical and chemical stimuli

2. effectors (an organ or cell that acts in response to a stimulus) of digestive activity are smooth muscle and glands

3. neurons (intrinsic and extrinsic) and hormones control digestive activity

mouth and associated organs

• mouth

• tongue

• salivary glands

• teeth

the pharynx

food passes from mouth into oropharynx and then into laryngopharynx; allows passage of food, fluids, and air

the esophagus

• pierces diaphragm at esophageal hiatus

• join stomach at cardial orifice

• gastroesophageal (cardiac) sphincter surrounds cardial orifice

→ keeps orifice closed when food is not being swallowed

→ mucus cells on both sides of sphincter help protect esophagus from acid reflux

stomach

• temporary storage tanks that starts chemical breakdown of protein digestion

• converts bolus of food to paste-like chyme

rugae

the many folds stomach mucosa forms when empty

regions of the stomach

• cardial part (cardia): surrounds cardial orfice

• fundus: dome-shaped region beneath diaphragm

• body: midportion

• pyloric part: lower part

pyloric part

wider and more superior portion of pyloric region, antrum, narrows into pyloric canal that terminates in pylorus

→ pylorus is continuous with duodenum through pyloric valves (sphincter controlling stomach emptying)

oblique (diagonal) layer

the muscularis externa layer in the stomach is modified to include this extra third layer

mucosa layer modification in stomach

dotted with gastric pits, which leads into gastric glands to produce gastric juices

types of gland cells

• mucous neck cells

• parietal cells

• chief cells

• enteroendocrine cells

mucous neck cells

secrete thin, acidic mucus of unknown function

parietal cells

secretions include:

• hydrochloric acid (HCI)

• intrinsic factor (a glycoprotein required for absorption)

chief cells

secretions include:

• pepsinogen (inactive enzyme)

• lipases (digests ~15% of lipids)

enteroedocrine cells

secrete chemical messengers into lamina propria

• act as paracrines → serotonin and histamine

• hormones → somatostatin and gastrin e.g. G cells

mucosal barrier

• harsh digestive conditions require stomach to be protected

• it protects stomach and is created byL thick layer bicarbonate-rich mucus, tight junctions between epithelial cells, damaged epithelial cells

three phases of gastric secretions

1. cephalic (reflex) phase

2. gastric phase

3. intestinal phase

cephalic (reflex) phase

conditioned reflex triggered by aroma, taste, sight, thought

gastric phase

lasts 3-4 hours and two-thirds of gastric juice is released

stimulation of gastric phase

• Food in the stomach causes the stomach to stretch and activates stretch receptors.

• Partially digested proteins, caffeine, and low acidity stimulate G cells to release gastrin.

• Gastrin stimulates parietal cells to release hydrochloric acid and increases enzyme secretion

inhibition of gastric phase

low pH (acidic) inhibits gastrin secretion

→ occurs between meals

→ occurs during digestion as negative feedback mechanism

intestinal phase

begins with a brief stimulatory component followed by inhibition

stimulation of intestinal phase

partially digested food enters small intestine, causing brief release of intestinal gastrin

inhibition of intestinal phase

four main factors:

• swelling of duodenum due to entry of chyme

• presence of acidic chyme

• presence of fatty chyme

• presence of hypertonic chyme

inhibition achieved in two ways: enterogastric reflex and enterogastrones

enterogastric reflex

duodenum inhibits acid secretion in stomach by:

• enteric nervous system short reflexes

• sympathetic nervous system

entrogastrones

duodenal enteroendocrine cells release two important hormones that inhibit gastric secretion → secretin and cholecystokinin

what is increase of HCO₃^- in blood leaving stomach referred as?

alkaline tide

liver

digestive function is production of bile

bile

fat emulsifier (stabilizer)

gallbladder

chief function is storage of bile

pancreas

supplied most of enzymes needed to digest chyme, as well as bicarbonate to neutralize stomach acid

composition of bile

yellow-green alkaline solution containing bile salts and bilirubin

bile salts

cholesterol derivatives that function in fat emulsification and absorption

bilirubin

pigment formed from heme; bacteria break down in intestine that gives poop its brown color

exocrine function of the pancreas

produce pancreatic juice

→ acini

→ ducts

acini

clusters of secretory cells that produce zymogen granule containing proenzymes

ducts

secrete to duodenum via main pancreatic duct; smaller duct cells produce water and bicarbonate

composition of pancreatic juice

• 1200-1500 ml/day is produced

• watery, alkaline solution (pH g) to neutralize acidic chyme

• electrolytes, primarily HCO_3^-

→ proteases (for proteins)

→ amylase (for carbs)

→ lipases (for lipids)

→ nucleases (for nucleic acids)

small intestine

• major organ of digestion and absorption

• from pyloric sphincter to ileocecal valve, which it joins large intestine

divisions of small intestine

• duodenum

• jejunum

• ileum

modifications of small intestine

• circular folds

• villi

• microvilli

circular folds in small intestine

permanent folds that force chyme to slowly spiral through lumen, allowing more time for nutrient absoprtion

villi in small intestine

fingerlike projections of mucosa with a core that contains dense capillary beds and lymphatic capillary called a lacteal for absorption

microvilli in small intestine

extensions of mucosal cell that give fuzzy appearance called the brush border that contains membrane-bound enzymes brush border enzymes, used for final digestion

intestinal crypts

tubular glands scattered between villi

peyer’s patches

found in great numbers in distal part of small intestine, where bacterial numbers increase

duodenal glands

glands of duodenum that secrete alkaline mucus to neutralize acidic chyme

sources of enzymes for digestine

• bile, bicarbonate, digestive enzymes are imported from liver and pancreas

• brush border enzymes bound to plasma membrane perform final digestion of chyme

regulating chyme entry

• delivery has to be slow to prevent loss of water from blood

• low pH of chyme has to be adjusted

• chyme has to be mixed with bile and pancreatic juice

• enterogastric reflex and enterogastrones control movement of food into duodenum to prevent it from being overwhelmed

motility of small intestine after a meal

segmentation is most common motion of small intestine

• mixes/moves contents toward ileocecal valve

• intensity is altered by long and short reflexes and hormones

large intestine three unique features

• teniae coli

• haustra

• epiploic appendages

teniae coli

three bands of longitudinal smooth muscle in muscularis

haustra

pocketlike sacs caused by tone of teniae coli

epiploic appendages

fat-filled pouches of visceral peritoneum

subdivisions of large intestine

• cecum

• appendix

• colon

• rectum

cecum

first part of large intestine

appendix

• masses of lymphoid tissue

• part of MALT immune system

• bacterial storehouse

• twisted shape makes capable to blockages

ascending colon

travels up right side of abdominal cavity to level of right kidney (ends in right-angle turn called right colic flexure)

transverse colon

travels across abdominal cavity (ends in right angle turn called left colic flexure)

descending colon

travels down left side of abdominal cavity

sigmoid colon

S-shaped portion that travels through pelvis

rectum

three rectal valves stop feces from being passed with gas

bacterial flora

consists of 1000+ different types of bacteria, enters small intestine or anus to colonize colon

fermentation function of bacterial flora

• ferment indigestible carbs and mucus

• release irritating acids and gases

vitamin synthesis function of bacterial flora

synthesize B complex and some vitamin K needed by liver to produce clotting factors