Bio 132 - Digestive System Part 6: Lactose Intolerance, Large intestine, Absorption and Motility, Defecation, Disorders of L. Intestine, Endocrine and ANS Control of Digestion

Lactose Intolerance

lactose passes undigested into large intestine

-increases osmolarity of intestinal contents

-causes water retention in the colon and diarrhea

-gas production by bacterial fermentation of the lactose

lactose intolerance occurs in many parts of the population

15% American whites,

90% of American blacks,

70% of Mediterraneans;

and nearly all of Asian descent

large intestine receives

500 mL indigestible residue/day

-reduces it to about 150 mL of feces by absorbing water and salts

-eliminates feces by defecation

large intestine begins at

the cecum inferior to ileocecal valve

where is the appendix attached

to the lower end of the cecum

•densely populated with lymphocytes; it's a source of immune cells and "good" bacteria

What part of the large intestine frame the small intestine

ascending colon,

right colic (hepatic) flexure,

transverse colon,

left colic (splenic) flexure,

and descending colon

sigmoid colon

S-shaped portion leading into pelvis

rectum

portion ending at anal canal

•has small curves and infoldings

anal canal

final 3 cm of the large intestine

•terminates at the anus

•anal columns and sinuses - exude mucus into anal canal during defecation

•hemorrhoids

hemorrhoids

permanently distended veins that protrude into the anal canal or form bulges external to anus

muscular externa of colon

haustra,

taenia coli,

internal anal sphincter,

external anal sphincter

haustra

pouches in the colon caused by the muscle tone of the taeniae coli

taenia coli

longitudinal bands of muscle that maintain the haustra

internal anal sphincter

smooth muscle, involuntary

external anal sphincter

skeletal muscle, voluntary

large intestine mucosa

simple columnar epithelium through entire large intestine

-anal canal has nonkeratinized stratified squamous epithelium in its lower half

large intestine has no

circular folds or villi

large intestine lamina propria and submucosal layers have

large amount of lymphatic tissue

-provide protection from the bacteria that densely populate the LI

bacterial flora populate large intestine

-about 800+ species of bacteria

-ferment cellulose & other undigested carbs

•we absorb resulting sugars

-help in synthesis of vitamins B and K

flatus

intestinal gas

-average person produces 500 mL per day from 7 to 10 L of gas present but reabsorbed

-most is swallowed air, but hydrogen sulfide, indole and skatole produce odor

•hydrogen gas may explode during electrical cauterization used in surgery

Absorption and Motility

•LI takes ~ 12-24 hours to reduce residue to feces

-reabsorbs water & electrolytes, but doesn't digest

•feces consist of 75% water and 25% solids - bacteria, fiber, fat, mucus and sloughed epithelial cells

•haustral contractions occur every 30 minutes

-a form of segmentation

-distension of a haustrum stimulates it to contract

•mass movements occur 1 to 3 times a day

-triggered by gastrocolic and duodenocolic reflexes

•filling of the stomach and duodenum stimulates

defecation

stretching of the rectum stimulates defecation reflex

-parasympathetic defecation reflex involves spinal cord

•stretching of rectum sends sensory signals to spinal cord

•pelvic nerves return signals, intensifying peristalsis & relaxing internal anal sphincter

•defecation occurs only if external anal sphincter is voluntarily relaxed

Crohn's disease

inflammation of portions of the small or large intestine caused by inappropriate immune response; runs in families

inflammatory bowel disorders (IBD's)

IBD (inflammatory bowel disease) cause

digestive upset, weight loss, fever

Diverticulitis

small pouches form in the lining of the intestine; food may get trapped in these pouches, causing inflammation and pain

• correlated with a low-fiber diet

colorectal cancer

• common and deadly form of cancer

• starts as precancerous polyps

• runs in families

• correlated with a high-animal-fat diet

• curable if caught early using digital rectal exams or colonoscopy

3 overlapping phases of digestion

1.Cephalic phase

2. Gastric phase

3. Intestinal phase

cephalic phase

stimuli from food (smell, sight, taste) reaches cerebral cortex, hypothalamus, and brain stem

- Brain stem signals cranial nerves VII and IX to signal salivary glands, and cranial nerve X stimulates gastric glands

Gastric phase

• stretch receptors and chemoreceptors respond to food in stomach, and signal parasympathetic and enteric neurons to trigger peristalsis

• at the same time, gastrin is released by G cells in the stomach (because of presence of food, acid, and acetylcholine from ANS)

• gastrin enters blood, comes back to stomach, and signals release of gastric juice, plus increases gastric motility

intestinal phase

•slows chyme entering sm. intestine

• stretch receptors cause enterogastric reflex - send impulses to medulla to inhibit parasympathetic stimulation, so gastric motility is reduced

• cholecystokinin (CCK) and secretin secreted by intestinal glands (each enhances the other)

-CCK causes secretion of enzyme-rich pancreatic juice, release of bile, and causes feeling of satiety

-secretin causes flow of bicarbonate-rich pancreatic juice, inhibits gastric juice