UMD BSCI 202 Digestive System

digestive functions

ingestion, propulsion, absorption, defecation

propulsion

peristalsis and segmentation

peristalsis

waves of contractions that squeezes food along GI tract

segmentation

moves materials back and forth to aid with mixing. Churns and fragments a bolus

gastroenteritis

inflammation of GI tract

appendicitis

inflammation of appendix

old age digestive

metabolism decreases, fewer digestive juices, peristalsis slows, diverticulosis and cancer more common

alimentary canal

GI tract- mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus

accessory digestive organs

teeth, salivary glands, pancreas, liver, gallbladder

movement of digestive materials

visceral smooth muscle rhythmic cycles caused by **pacemaker cells**, peristalsis, segmentation

control of digestive activity

* stimulated by parasympathetic


* inhibited by sympathetic
* neural and hormonal mechanisms coordinate glands
* reflexes triggered by chemical and mechanical receptors in organ walls

digestive activity stimuli

* stretch of organ
* pH of contents
* presence of breakdown products

mouth

lips, cheeks, uvula (projection of SP), hard palate (anterior), soft palate (posterior), vestibule, oral cavity, tongue, tonsils (palatine and lingual)

tongue

attached ay hyoid bone and styloid processes of skull and lingual frenulum to mouth floor-

* mechanical processing
* chewing and swallowing
* sensory
* lubrication

mechanical breakdown by the mouth

mastication, mixing masticated food with saliva, sense of taste

chemical digestion by mouth

food mixed with saliva, salivary amylase breaks down starch into maltose

teeth function

mastication (chewing)

sets of teeth

* deciduous (baby teeth, 20 by 2)


* permanent (32) (wisdom are 3rd molars)

classifications of teeth

* incisors (cutting)


* canines (tearing),
* premolars and molars (grinding)

salivary glands

empty secretions into the mouth-

* parotid
* submandibular
* sublingual

saliva

**mucus** and **serous fluid**, helps to form food bolus, **salivary amylase** for starch digestion, dissolves chemicals to be tasted

* mucus is a viscous secretion containing mucin
* Serious fluids are a watery secretion containing amylase

pharynx

passageway for food liquid and air, stratified squamos epithelium, pharyngeal and palatal muscles assist in swallowing, two muscle layers outer longitudinal and inner circular propel food to esophagus by alternating contractions of muscle layers (peristalsis)

deglutination

1. **buccal** phase- **voluntary**, mouth,


1. food formed into bolus and forced into pharynx by tongue


2. **pharyngeal** esophageal phase- **involuntary**,


1. passageways except to stomach are **blocked**,
2. peristalsis moves bolus towards stomach,
3. **cardioesophageal** sphincter opens when food presses against it

esophagus

10 inches, pharynx to stomach through diaphragm, conducts food by peristalsis, passageway for food only, no digestive function

stomach functions

bulk storage of undigested food,

* mechanical breakdown,
* acids and enzymes break chemical bonds,
* intrinsic factor (without leads to **pernicious anemia**)
* protein chemical breakdown begins,
* delivers chyme to small intestine,
* site of some food breakdown

stomach anatomy

* left side of abdominal cavity
* food enters and **cardiesophageal** sphincter and empties at **pyloric** sphincter,
* cardiac region,
* fundus (expanded portion lateral to cardiac region),
* body,
* pylorus (funnel-shaped terminal end),
* lesser curvature and greater curvature,
* rugae

rugae

internal folds of the mucosa of the stomach

* create turbulence

stomach mucosa

simple columnar epithelium- mucus neck cells, gastric glands, chief cells, parietal cells, enteroendocrine cells

mucous neck cells

stomach mucosa- produce sticky **alkaline** mucous

* alkaline means it is rich in bicarbonate and can **neutralize hydrochloric acid**

gastric glands

stomach mucosa- in gatric pits and secrete gastric juice

* gastric juice has hydrochloric acid in it produced by parietal cells

chief cells

stomach mucosa- produce **pepsinogens** (pepsinogens are activated by hydrochloric acid in the lumen of the stomach)

parietal cells

stomach mucosa- produce hydrychloric acid

enteroendocrine cells

stomach mucosa- produce **gastrin** (from G cells) and **somatostatin** (from D cells)

peptic ulcers

gastric (lower stomach, more common) and duodenal-

* all ages,
* 1 in 10 Americans,
* result of damage to the cells that produce a layer of mucus and other functions that neutralize acid
* cells lining the stomach normally renewed
* problem is when cell function is disrupted by **helicobacter pylori** infection or **long term hunger**.

stomach digestion

preliminary digestion of proteins, permits digestion of carbs

stomach absorption

very little nutrient absorption- aspirin and alcohol absorbed

food breakdown in stomach

gastric juice regulated by neural local and hormonal factors- presence of food or rising pH causes gastrin to be released

gastrin released causes...

stomach glands to secrete protein digesting enzymes (pepsinogen), mucus, hydrochloric acid

Acidic pH in the stomach causes...

pepsinogen to pepsin, hostile environment for microorganisms

protein digesting enzymes

pepsin (active), Renin (milk proteins in infants, not adults)

Gastric Secretion phases

cephalic, gastric, intestinal

Cephalic Phase

first phase of gastric secretion-

* prepare stomach for food,
* short (minutes),
* stimulated by vagus nerve and submucosal plexus,
* stimulates mucus, pepsinogen, HCl production which increases gastric juice-
* stimulated gastrin release by G cells

Gastric Phase

second phase of gastric secretion-

* enhance secretion,
* acidify chyme,
* digestion of proteins-long (hours) (so eat slowly),
* neural, hormonal, and local mechanisms, increase acid and pepsinogen production and mixing
* segmentation

gastric phase neural mechanisms

stretch receptors as stomach fills, chemoreceptors as pH increases

* Stretch and chemoreceptors stimulate submucosal and myenteric plexus

gastric phase hormonal mechanisms

gastrin release from G cells through parasympathetic activity and peptides and amino acids in chyme

gastric phase local mechanisms

stomach fills and mast cells secrete histamines

intestinal phase

third phase of gastric secretion-

* controls rate of chyme into duodenum,
* long (hours),
* enterogastric reflex from duodenum distention-
* CCK, GIP, secretin released from presence of lipids, carbs, and acid-
* undigested proteins cause gastrin - inhibition and reduction of gastrin and pepsinogen and mixing

stomach propulsion

* segmentation (mix chyme with digestive juices and propel food),


* gastroenteric reflexes,
* gastroilieal reflexes,
* peristalsis (lower stomach),
* pylorus measured chyme into small intestine (30 mL at a time),
* stomach empties in 4-6 hours

gastroenteric reflexes

initiated by stretch receptors in stomach

gastroilieal reflexes

triggers relaxation of **illeocecal** valve

* why you go to the bathroom about an hour after eating

ghrelin

hormone produced by the stomach that stimulates hunger

Small intestine

major digestive organ,

* site of nutrient absorption into the blood,
* extends from **pyloric** sphincter to **illeocecal** sphincter/valve,
* suspended by **mesentery** from posterior abdominal wall,
* majority of chemical digestion begins here

chemical digestion in small intestine

enzymes produces by intestinal cells and pancreas,

* pancreatic ducts carry enzymes to small intestine,
* bile (formed by liver) enters via bile duct

small intestine divisions

* **duodenum** (shortest, widest),
* **jéjunum** (larger, thicker walled, more vascular, more circular folds),
* **illeum** (more peyers patches filled with **leukocytes**)

illeocecal sphincter

transition between large and small intestine

crohns disease

immune system attacks gut bacteria and foods,

* inflammation usually found in **ileocecal** region,
* inflammatory bowel disease (causes inflammation),
* intestine blockage,
* treated by drugs and surgery

small intestine structural modifications that increase surface area

microvilli, villi, circular folds (plicae)

microvilli

tiny projections of the plasma membrane

* create brush border appearance (similar to mass rabsorber in the kidneys, the **proximal tubule**)
* need a microscope

villi

fingerlike structure formed by mucosa

* no need for microscope (macroscopic)

circular folds

plicae circulares - deep folds of mucosa and sub mucosa

lacteals

terminal lymphatics in villi- help absorb fat

intestinal glands

lined by enteroendocrine, goblet, and stem cells

small intestine digestion

* brush border enzymes - break polysaccharides into simple sugars
* some protein and fat digestion


* pancreatic enzymes -
* starches/carbs using **carbohydrases**
* proteins using **proteases**
* fats using **lipases**
* nuclear acids using **nucleases**


* alkaline content neutralizes chyme (rich in **bicarbonate**)
* where we get that extra bicarbonate for the 20/1 ratio of bicarbonate to CO2

lactose intolerance

less lactase as we age to break down lactose-

* ends up in the large intestine where digested by bacteria and causes gas
* excess lactose also **pulls water out** of neighboring tissue causing diarrhea
* Lactose persistence is the mutation that allows you to keep producing lactase

intestinal juices functions

moisten chyme, buffer acids, maintain digestive material in solution

duodenal glands

(brunners glands)- produce mucus, buffers, urogastrone (inhibits gastric secretion and motility)

peyers patches

most present in illeum- aggregated lymphoid nodules

small intestine absorption

* water (along entire length of small intestine),


* end products of digestion
* Most substances are absorbed by active transport through cell membrane,
* lipids by diffusion


* transported to liver by **hepatic portal vein** or lymph

large intestine

larger in diameter but shorter then small,

* frames internal abdomen,
* no villi, goblet cells, teniae coli,
* no digestive enzymes produced

large intestine parts

* cecum,


* colon (ascending, transverse, descending, sigmoid),
* rectum,
* anal canal,
* appendix

appendix

hangs from the cecum- accumulation of lymphatic tissue and stores good bacteria

large intestine functions

* reabsorption,
* compact material into feces (proportional to how long it stays in large intestine),
* mass movements,
* absorb vitamins from bacteria,
* store fecal matter
* no enzymes produced (resident enteric bacteria digest)

large intestine reabsorption

* water,
* vitamins K, biotin, and B5 produced by **enteric bacteria**
* do anerobic respiration so produce gases
* prevents invasion of **C. difficile colitis**


* organic wastes,
* **toxins** if fat soluble
* bile salts

defecation reflex

triggered by dissension of rectal walls, presence of feces in rectum

Meat consumption

Can lead to cancer and the more toxins the animal has the more toxins your large intestine will absorb

Large Intestine Movements

* sluggish peristalsis
* Mass Movements one to four times per day
* Length of time in GI tract is directly proportional to calorie content
* Longest to shortest - fats, proteins, carbs, fiber
* in there longer means more chance for toxins in there to be reabsorbed (biomagnification)

teniae coli

muscularis externa reduced to three bands of muscle- cause wall to pucker and form haustra (pocket like sacs)

feces

undigested food, water, mucus, bacteria

IBS

spastic colon-

* cramping and diarrhea
* most common disorder (20%),
* spastic motility,
* **serotonin** (a neurotransmitter) **decreased** (you become depressed)

anus

opening of large intestine -

* external (voluntary, skeletal muscle) anal sphincter
* internal (involuntary, smooth muscle) anal sphincter

rectum

* **last** **portion** of digestive tract,
* terminates at **anal canal**

hemorrhoids

swollen and inflamed veins around anus or lower rectum-

* caused by pregnancy, aging, or diarrhea


* increasing fiber and fluids in diet can prevent

pancreas

digestive enzyme production that are secreted into **duodenum**,

* **insulin** and **glucagon**


* posterior to parietal peritoneum, extends across abdomen from spleen to duodenum-

pancreatic duct

penetrates duodenal wall

pancreas endocrine functions

Secrete hormones into the **bloodstream**

* insulin and glucagon

pancreas exocrine functions

majority pancreatic secretions, **pancreatic juice** secreted into **small** **intestine**-

* carbohydrases,
* lipases,
* nucleases,
* proteolytic enzymes

insulin

decreases blood sugar levels

glucagon

increases blood sugar levels

goblet cells in large intestine function

produce **alkaline** **mucus** which **lubricates** feces passageway

insulin pathologies

type 1, type 2, **excess glucose** circulating in the blood

* leads to **glycosylation** of **proteins**
* cells forced to resort to other pathways to generate **ATP**

type 1 diabetes

**autoimmune** disease-

* insulin synthesizing cells in the pancreas are attacked

type 2 diabetes

**insulin resistance**-

* body does not properly respond to insulin-
* linked to BMI and abdominal fat

liver

picks up fats and fatty acids

* **largest** gland in the body,
* several roles in **digestion**,
* can **regenerate**,
* central role in **metabolism**

fats and fatty acids picked up by the liver

some are **oxidized** to provide energy **for liver cells**,

* rest broken down to **simpler compounds** and released into blood

liver digestion roles

* manufacture **bile**,
* **detox** drug and alch,
* degrades **hormones**,
* produces **cholesterol** and blood proteins (**albumins** and **clotting** proteins)

metabolic functions of the liver

* glycogenesis,


* glycogenolysis,
* gluconeogenesis

glycogenesis

**glycogen formation**-

* glucose molecules converted to glycogen and **stored in liver**

glycogenolysis

glycogen splitting-

* **glucose** **released** from liver after conversion form glycogen

gluconeogenesis

formation of **new sugar**-

* **glucose produced** from fats and proteins

bile

emulsify fat by physically breaking larbe fat globules into smaller ones

* produced by cells in liver,-
* composed of
* bile salts,
* bill pigments (**bilirubin** from **Hb breakdown**)
* cholesterol,
* phospholipids,
* electrolytes