Chapter 10

digestion

The gastrointestinal (GI) tract consists of a tube that extends from mouth

to anus

The contents of this tube are surprisingly not part of our bodies until they are

digested and absorbed into our blood stream to be used for cellular metabolism and growth

Mechanical digestion begins in the

mouth (buccal cavity) by mastication (chewing)

Mechanical digestion does what

This increases surface area to volume ratio and moistens food with secretions from salivary glands

Once swallowing is initiated, one direction movement is accomplished by

rhythmic, muscular contractions (peristalsis) of the muscularis externa

Chemical digestion is accomplished by

enzymes from the mouth, stomach, and most importantly, pancreatic secretions into the small intestines.

Our digestive tract breaks down ingested food to

macromolecules such as glucose, amino acids, fatty acids, and nucleic acids using enzymes during the process of digestion.

Absorption of water and nutrients occurs through the

intestinal lining

The small intestine absorbs most

nutrients, vitamins, and water

The large intestine hosts a plethora of

microbes, completes water reabsorption, and compacts feces

The entire length of the gi tract is made of four layers:

• the mucosa

• submucosa

• muscularis externa

• and serosa or adventitia

what is the serosa

outermost thin layer of the last portion of esophagus to the sigmoid colon

what is the adventitia

outermost layer of the pharynx and most of esophagus and rectum

Both the serosa and adventitia serve to anchor the

tube to surrounding tissue

Regional specializations occur in each major division of the

GI tract

Modifications of the mucosa and submucosa layers dictate the type of

secretion introduced into the lumen of the tube

the secretions from the entrire GI tract averages about

7–8 liters/day

• This large amount of body water must be reabsorbed, mainly by the intestines or dehydration can occur.

The digestive tract, or alimentary canal, consists of the

esophagus, stomach, small intestines, and large intestine

• Each region contains specialized glands and modifications.

The esophagus secretes

mucous to reduce friction as food is swallowed.

what is mucous

esophageal glands in the submucosa

The stomach produces

hydrochloric acid (HCl) and pepsinogen from mucosal gastric glands

The small intestine contains

villi and microvilli

what is modifications of the mucosa

microvilli

The large intestine possesses

crypts which aid in water reabsorption and mucous secretion

Peristalsis pushes food from mouth

to anus along the tube

what is Peristalsis

muscular wave of contraction of the muscularis externa

As food fills the stomach (now called chyme), it

stretches

• The gastric rugae (wrinkles) allow this to occur

The muscular wall contractions mix the ingested contents with

• secretions from mucous cells (mucous)

• parietal cells (HCl)

• chief cells

• enteroendocrine hormones that regulate digestion.

what is chief cells

pepsinogen, an inactive form of the protein digesting enzyme, pepsin

HCl (hydrochloric acid) is a strong acid that helps kill

ingested pathogens and activates pepsinogen into pepsin.

The duodenum is the first segment of the

small intestines and is about 25 centimeters or 10 inches in length

As chyme exits the pylorus of the stomach, in small amounts controlled by the pyloric sphincter into the duodenum, the acidic contents and stretching of the duodenal wall trigger the release of

enzymes, sodium bicarbonate, and zymogens from the pancreas

Bile salts from the gallbladder help emulsify

fats

what does emulsify mean

break large lipids into tiny, suspended droplets

Alkaline mucus secretions of duodenal glands and brush border enzyme activity of the duodenal epithelium help neutralize

stomach acid and activate pancreatic enzymes

The circular folds (plicae circulares) help mix the chyme with

enzymes, mucus, and bile salts.

All enzymes of digestion are produced by the

pancreas

Starch, fat, protein, DNA, and RNA are all broken down to the major

macromolecules

• glucose, fatty acids, amino acids, and nucleotides

The pancreas produces and secretes —— into the duodenum

enzymes, zymogens, and sodium bicarbonate

what us zymogens

inactive form of protein digesting enzymes

what does sodium bicarbonate do

buffers the acidic chyme from the stomach

The liver produces thousands of unique

proteins for various of functions

Bile formation has the only role in

digestion

Bile is composed of

bile salts, minerals, and pigments.

Pigments are produced by the breakdown of the heme portion of

hemoglobin into bilirubin

Only bile salts contribute to digestion by emulsifying and preparing

fats for absorption

Bile salts are derived by the addition of

glycine or taurine to cholesterol

• This creates ampipathic molecules that separate fat globules into lipid droplets (emulsify)

Bile salts then combine with fatty acids and monoglycerides and form

tiny micelles, about 4–7 nanometers in diameter

• can be absorbed into columnar epithelial cells of the small intestines.

The jejunum is where most

chemical digestion and absorption of nutrients takes place

facts about the jejunum

It’s about a meter and a half long and is highly vascula

Pancreatic enzymes continue to actively digest introduced

chyme

villi and microvilli greatly increase the surface area of the

jejunum’s epithelium

jejunum’s epithelium is where

where membrane transport pumps move nutrients into the circulatory system or lymphatic (fats) system

About the last half of the small intestines is the

ileum

• More digestion and absorption of nutrients takes place here

The ileum transforms in structure as it approaches the

cecum of the large intestine

Clusters of lymphatic tissue (Peyer’s patches) become more

prevalent, housing numerous lymphocytes to resist infection.

Villi and microvilli greatly increase the surface area of the small intestines about

200 times

• Without these structures, our intestines would need to be about 1,200 feet long to provide the same surface area

All nutrient absorption occurs through the

simple columnar epithelial cells of the mucosa

The large intestine is about

1.5 meters long and extends from the cecum to the rectum

The mucosa of the large intestine is

simple columnar epithelium that’s specialized for absorption

Motility is slower in the

large intestine

• allowing most of the water of chyme that enters to be reabsorbed

motility is important in preventing dehydration, which is a serious concern during extended bouts of

diarrhea

The colon is home to about 800 species of

bacteria that number in the billions

the bacteria in our colon do what

They further digest cellulose (we do not make enzymes that can break it down) and release glucose.

some bacteria produce

sulphur containing gas that gives rise to the foul odor of flatus

About 60% of the dry mass of feces is

bacteria

The epithelium makes a transition from simple columnar in the rectum to

stratified squamous in the anal canal.

Splenic flexure

the spleen can be seen through the colon wall at the sharp bend between the transverse and the descending colon

Cecum

beginning of the large intestine

Appendiceal orifice

opening into the appendix

Terminal ileum

last region of the small intestines

Ileo-cecal valve

prevents material in the colon from traveling back into the small intestine

Diverticulum in the ascending colon

diverticula are pouches that form in the mucosa and submucosa causing a bulge

Rectum

• about 12 cm long

• As material fills, the rectal walls expand and stimulate stretch receptors involved in defecation.

Parotid salivary gland

produces the majority of saliva, which helps moisten masticated food (bolus) through the esophagus

Wisdom teeth (3rd molars)

sometimes do not descend or have to be pulled to make room

a.

Anal canal

b

Mid-descending colon

c.

Splenic flexure

d

Mid-transverse colon

e

e. Cecum

f

f. Appendiceal orifice

g

g. Appendiceal orifice

h

h. Terminal ileum

i

i. Terminal ileum

j

j. Ileo-cecal valve

k.

Diverticulum in the ascending colon

L

L. Rectum

a

Ileum

b

Ileocecal valve

c

Cecum

d

Appendix

e

Ascending colon

f

transverse colon

g

Descending colon

h

Sigmoid colon

i

rectum

j

anus

a

Villus

b

Simple columnar epithelial cells