Download Digestion

Unit 4—Maintenance of the
Human Body
Dr. Achilly
Part 1: The Digestive System
“Concepts” chapter 26
Make sure you know
the
position/orientation
of the organs of the
GI tract
The digestive system--overview
The food we eat contains lots of
nutrients, but most of them are too large
to be taken in by our cells.
 We must “digest” those nutrients, or
make them smaller, so that they can be
used.
 The digestive system performs 6 basic
functions.

The digestive system -overview
1.
2.
3.
Ingestion—taking food into mouth.
Secretion—cells that line digestive tract
secrete chemicals that aid in the
breakdown of nutrients.
Mix & propel—myo contraction helps to
mix and move the food thru the tract.
The digestive system -overview
4.
5.
6.
Digestion—a mechanical & chemical
process.
Absorption—when the products of
digestion are brought into the cells that
line the GI tract.
Defecation—anything that’s not
absorbed (cells & bacteria too!) leave
the body as feces via the anus.
The digestive system -overview

The digestive system is basically a tube
formed from 4 layers of tissue.
Mucosa is the inner lining. It contains cells
that secrete enzymes & chemicals for
digestion. It contains blood & lymphatic
vessels. There are also lymphatic nodules
that hold immune system cells.
 Submucosa is the next layer. It is mostly
connective tissue as well as blood & lymph
vessels.

The digestive system -overview
Muscularis layer contains muscle tissue that
is involuntary (not under your control).
These myo’s help to propel and break up
food.
 Serosa is the outermost layer.

The digestive system -overview
None of the digestive system is under
voluntary control.
 The myo’s and cells of the GI tract
respond to chemical & stretch receptors
located thru out the digestive system.
 Outside influences (fear, stress, anger)
can alter GI fxn.

The digestive system

A large serous
membrane called the
peritoneum covers
many of the
digestive organs.
 It has many folds
that bind the organs
to each other &
contain blood &
lymphatic vessels.
The peritoneum
is outlined in pink
The digestive system -overview
The largest fold of the peritoneum is
called the greater omentum. It drapes
over the small & large intestines like a
“fatty apron.”
 Contains many fat cells and lymph
nodes. These lymph nodes hold
immune cells which help to combat
infection.

The digestive system -overview
The digestive system--mouth

Digestive system
begins at the mouth
or oral cavity.
 Mouth is formed by
cheeks, & the hard &
soft palates.
The digestive system--mouth

Salivary glands




There are 3 pairs.
Parotid
Submandibular
Sublingual
The digestive system--mouth

Saliva




Fxns to begin chemical digestion & to lubricate
food.
Contains water, mucus, enzymes that destroy
bacteria, & some that start chemical digestion.
Salivary amylase is an enzyme that begins starch
breakdown.
Lingual lipase is released in mouth, but not
activated until it hits the acid in the stomach. It
begins fat breakdown.
The digestive system—mouth

Tongue




Forms the floor of the oral cavity. Has receptors for
taste.
It is considered an accessory digestive organ
because it is needed for digestion, but food does
not pass thru it. Composed of myo & mucous
membrane.
There are extrinsic myo’s of tongue which originate
outside of tongue, but connect to it. They move
tongue from side to side & in & out.
Intrinsic myo’s originate & insert w/in tongue. They
alter size & shape of tongue.
The digestive system—mouth

Teeth

Accessory digestive organs located in sockets
in the gingivae which covers the mandible &
maxilla.
The digestive system—mouth
As food enters mouth, mechanical
digestion results from chewing or
mastication.
 A soft, flexible mass results which is
called a bolus. It can be easily
swallowed.
 It passes from the mouth into the
pharynx.

The digestive system

Pharynx is made of
myo & lined by a
mucous membrane.
 Divided into 3 parts:




Nasopharynx
Oropharynx
Laryngopharynx
Contractions propel
food into esophagus.
The digestive system—swallowing

Voluntary stage—tongue pushes food
backward against palate.
 Involuntary stage—once food goes into
oropharynx, medulla oblongata is stimulated.



Soft palate & uvula move up to prevent food
entering nasopharynx
Epiglottis closes, so that food doesn’t enter trachea.
Esophageal sphincter relaxes. Food enters
esophagus.
Swallowing
The digestive system--esophagus

Esophagus is a muscular tube.
Lies posterior to trachea. Travels
thru the mediastinum, anterior to
vertebral column. Pierces
diaphragm & ends at the superior
portion of the stomach.
The digestive system--esophagus
A wave-like motion of myo contractions
called peristalsis, forces food down
esophagus.
 When lower esophageal sphincter
relaxes, food enters stomach.
 Takes 4-8 seconds from mouth to
stomach.

The digestive system--stomach

Stomach performs many fxns:
Mixing
 Storage
 Chemical digestion
 Secretes the hormone gastrin.

The digestive system—stomach

1.
2.
3.
4.


Stomach has 4 main regions:
Cardia
Fundus
Body
Pylorus
It also has many ridges called rugae.
Pyloric sphincter open up to the
duodenum.
Stomach
The digestive system--stomach
The stomach contains the 4 basic cell
layers discussed before.
 It has specialized secretory cells called
gastric glands which line narrow
channels called gastric pits.

The digestive system—stomach
All of these secretions form gastric juice
(we make 2-3 qt/day).
 Gastric juice continues the chemical
break down.
 Proteins are broken into smaller
polypeptide chains, triglycerides into fatty
acids and monoglycerides, etc.
 Food + gastric juices = chyme.

The digestive system—stomach
While chemical digestion is occurring,
mechanical digestion continues.
 Stomach has rhythmic contractions.
 Small amounts of chyme are passed into
duodenum at regular intervals.
 Stomach takes 2-4 hours to empty.
 Very little absorption here—only water,
small ions, certain drugs & alcohol.

The digestive system
Food enters the small intestines next.
 The rest of the chemical digestion occurs
here & is dependent upon enzymes
secreted by many accessory organs:

Pancreas
 Liver
 Gall bladder

The digestive system—pancreas

Has an exocrine portion:



Secretions are put directly into a duct (pancreatic
juices enter the pancreatic duct)
This duct empties the secretions into the
duodenum
Endocrine portion:

These cells secrete hormones into the blood
stream which maintain homeostasis.



Insulin
Glucagon
others
The digestive system—pancreas

Pancreatic juice:
Sodium bicarbonate—neutralizes chyme
coming in from stomach.
 Pancreatic amylase—digests starch
 Trypsin, chymotrypsin, carboxypeptidase,
elastase— digest proteins
 Pancreatic lipase—digest triglycerides
 Ribonuclease, deoxyribonuclease—digest
DNA & RNA

The digestive system—pancreas
Many enzymes are secreted in inactive
forms.
 They are activated by other enzymes in
the small intestines once they arrive
there.

The digestive system—liver &
gallbladder

Next to the skin, liver
is heaviest organ in
body.
 Located inferior to
diaphragm.
 Gallbladder is a sac
located on the
posterior surface of
liver.
The digestive system—liver &
gallbladder

Liver has a large right lobe & smaller left lobe.
 Liver is suspended by the falciform ligament,
ligamentum teres (round ligament) & coronary
ligaments.
The digestive system—liver &
gallbladder
Fxn’l unit of liver is a
lobule.
 Shaped like hexagon.
Made of cells called
hepatocytes.
 They are arranged
around a central vein
and there are many
permeable capillaries
called sinusoids.

The digestive system—liver &
gallbladder
Bile is secreted by hepatocytes.
 Some enters small intestines right away,
some is stored in gallbladder.
 Brownish liquid = water, bile salts,
cholesterol, lecithin, bile pigments.
 Bile is part excretory product & part
digestive secretion.

The digestive system—liver &
gallbladder

Excretory
The pigment is bilirubin which comes from
the hemoglobin of broken down red blood
cells.
 Bilirubin is further broken down in intestines
to stercobilin which gives feces their brown
color.

The digestive system—liver &
gallbladder

Digestive secretion

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Bile salts help to emulsify
fats.
Emulsification breaks
down large lipid globules
into smaller ones, so that
pancreatic lipase can act
on them.
Salts also help with lipid
absorption.
The digestive system—liver &
gallbladder

Blood supply of liver:

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Receives oxygenated blood from
hepatic artery.
Gets deoxygenated, but nutrient
rich blood from hepatic portal
vein.
Blood goes to sinusoids.
Oxygen, nutrients & toxins enter
hepatocytes.
Products of hepatocytes &
unneeded nutrients secreted
back into blood via central vein to
hepatic vein.
The digestive system—liver &
gallbladder
Portal triad
The digestive system—liver &
gallbladder

The liver does a lot!!

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Regulates blood glucose levels
Lipid metabolism
Protein metabolism
Processes drugs & hormones
Excrete bilirubin
Make bile salts
Storage
Phagocytosis of old cells & bacteria
Activates vitamin D
The digestive system—small
intestines

Small intestines has 3
sections:




Duodenum
Jejunum
Ileum
It joins the large
intestine at the
ileocecal spinchter.
The digestive system—small
intestines

The epithelium of sm. intestine contains
many cells which make up the intestinal
glands:
Absorptive—take in nutrients from chyme
 Goblet—secrete mucus
 Paneth—secrete bacteriocidal enzymes
 Hormone secreting cells (S, CCK & GIP)

The digestive system—small
intestines
The walls of the intestine are highly
folded.
 In addition, the folds have folds or fingerlike projections called villi. Lots of
surface area for absorption.
 Each villi contains an arteriole, venule &
lacteal (lymph vessel).

The digestive system—small
intestines
The digestive system—small
intestines

Each villi is covered with microvilli so
that a fuzzy brush border is formed lining
the intestines.
The digestive system—small
intestines
Some physical digestion occurs as myo’s
contract and move food along—called
segmentation. This also forces nutrients
against the wall of the intestine so that they
are more easily absorbed.
 Chemical digestion takes place in the lumen of
the small intestines & along the brush border.
 Digestion & absorption are aided by watery
mix of intestinal & pancreatic juice, & chyme.
The pH is slightly alkaline.

The digestive system—small
intestines
Chyme entering small intestines has
partially digested carbs, proteins & lipids.
 Digestion finishes in small intestines.
 Carbohydrates (are broken down to) 
monosaccharides
 Proteins  amino acids
 Lipids glycerol & fatty acid
 Nucleic acids nucleotides

The digestive system—small
intestines
Once all molecules are broken down,
they must be absorbed.
 After they enter the cells of the villi they
can be put into the circulatory system &
transported to other parts of the body
(liver first).

The digestive system—small
intestines
Nutrient
Monosaccharides
Method of absorption into
cells
Amino acids
Active or facilitated
diffusion
Active transport
Electrolytes
Active
Vitamins
Simple diffusion
Water (8.3 of the 9.3L)
osmosis
The digestive system—small
intestines

Absorption of lipids is more complicated
Bile salts surround fatty acids &
monoglycerides into spheres called
micelles.
 Micelle moves to brush border.
 Fatty acids & monoglycerides diffuse into
border cells.
 Inside cell, they reform into triglycerides
which clump together & get coated in
proteins. Now they’re called chylomicrons.

The digestive system—small
intestines
Chylomicrons enter lacteal.
 These lymphatic vessels eventually dump
their contents into the general circulation
(left subclavian vein).
 They eventually make their way to the liver
& adipose tissue.

The digestive system—large
intestines

Large intestine is the
end of the GI tract.
 Fxns:



Complete
absorption
Vitamin production
Form & expel feces
The digestive system—large
intestines
Ileocecal sphincter regulates passage of
chyme from small intestine into large.
 Gastroileal reflex strengthens
contractions of ileum & forces chyme into
cecum.
 Hormone gastrin also relaxes sphincter.
 Strong muscular waves in large intestine
propel chyme to rectum.

The digestive system—large
intestines

No enzymes are produced, but bacteria
help.
Ferment remaining carbs (release hydrogen
& methane gas & CO2)
 Break down remaining proteins & amino
acids
 Produce vit. K & some B vitamins

The digestive system—large
intestines
Chyme stays in large intestine 3-10
hours.
 Rest of water is absorbed.
 Feces formed:

Water
 Salts
 Dead cells
 Bacteria
 Indigestible parts of food

The digestive system—large
intestines
Peristalsis pushes feces into rectum.
 As rectum stretches, defecation reflex is
stimulated.
 Internal anal sphincter opens
involuntarily.
 External anal sphincter is under
voluntary control.

The End