Download Digestive System

Digestive System
Gastrointestinal (GI) Tract
• GI or alimentary canal – continuous, coiled,
hollow tube that winds through ventral body
cavity from mouth to anus
– Entire GI is ~30 feet in cadaver, but shorter
in living person due to muscle contractions
– Includes mouth, pharynx, esophagus,
stomach, small intestine, large intestine, and
ends at the anus
– Other accessory organs include salivary
glands, pancreas, liver, gall bladder
Anatomy of Digestive System
Functions of Digestive System
1. Ingestion – putting food into mouth
2. Propulsion – moving food from one organ to the
next
3. Mechanical digestion – breaks down food
physically into smaller pieces
4. Chemical digestion – large molecules are broken
down into their building blocks by enzymes
5. Absorption – transport of digested end products
from the GI tract to the blood or lymph
6. Defecation – elimination of indigestible food
that leaves through the anus in the form of
feces
Activities of Mouth, Pharynx, and Esophagus
• Mouth
– Chewing begins mechanical breakdown of
food
• Pharynx
– Propels food into esophagus by peristalsis
(involuntary smooth muscle contractions)
• Epiglottis covers windpipe (trachea) to
prevent food from moving down into lungs
• Esophagus
– Propels food into stomach via peristalsis
Activities of Stomach
• Stomach – a storage tank
– C-shaped sac with sphincters at each end
• Cardiac or esophageal sphincter
prevents food from going back into
esophagus
• Pyloric sphincter opens to allow food to
move into the small intestine
– Physical digestion occurs as stomach grinds
and churns food
– Chyme - partially digested food that leaves
the stomach
– Chemical digestion occurs with enzymes
• Gastric glands – in stomach lining;
release gastric juice
–Gastric juice – includes digestive
enzymes, intrinsic factor,
hydrochloric acid (HCl), and mucus
»Intrinsic factor helps absorb
vitamin B12
• Pepsin – enzyme that breaks down
protein (activated by HCl)
Activities of the Small Intestine
• A tube that is 8 – 18 feet long and
twisted into coils
• The major digestive organ
• Three subdivisions
– Duodenum (5% of total length)
– Jejunum (40% of total length)
– Ileum (almost 60% of total length)
• Nearly all food absorption occurs in the
small intestine!
• Duodenum
– Pancreatic ducts dump pancreatic
juice into duodenum
– Bile duct (joins with pancreatic
ducts) dumps bile from liver into
duodenum
• Jejunum and ileum - primary
function is absorption
• Intestinal villi tiny projections of inner
wall of small intestine; greatly increase
surface area for absorption
– Each villus contains blood capillaries
and a lacteal into which foodstuffs
are absorbed
• Ileocecal sphincter – opens to let
chyme pass into large intestine
Activities of the Large Intestine
• Larger in diameter than the small
intestine, but not nearly as long (~5
feet)
• Major function is to dry out indigestible
food residues by absorbing water and
then eliminate them as feces
• Subdivisions: cecum, appendix, colon,
rectum, and anal canal
• Cecum – first part of large intestine;
sac-like with worm-like appendage called
appendix
• Colon – parts in order are: ascending
(travels up), transverse (travels across),
descending (travels down), and then
sigmoid (S-shaped)
• Rectum – stores feces and leads to anal
canal
• Anus – ending of GI; voluntary sphincter
opens to release feces
Activities of Accessory Organs
• Teeth – breakdown, grind, and tear food into
smaller pieces
• Salivary glands release saliva; contain mucus
to soften food and salivary amylase, which
begins carbohydrate digestion
– Parotid gland – largest, lie anterior and
somewhat inferior to each ear
– Sublingual gland – smallest; lie on floor of
mouth inferior to tongue
– Submanibular gland – lie in floor of mouth
on inside surface of lower jaw
• Pancreas – soft, pink, triangular
gland; secretes pancreatic juice
–Pancreatic duct – tube that
carries pancreatic juice to
duodenum
– Pancreatic juice contains the following:
• bicarbonate - neutralizes acidic chyme
coming from stomach
• amylase - enzyme that breaks down
carbohydrates
• lipase - enzyme that breaks down lipids
• trypsin, chymotrypsin, and
carboxypeptidase - enzymes that
breakdown proteins
• nuclease – enzyme that breaks down
nucleic acids
• Liver – largest gland in the body; digestive
function is to produce bile
– Bile – leaves liver through the hepatic duct
and enters duodenum through bile duct
• Yellowish-green liquid containing bile salts,
bile pigments (bilirubin), cholesterol,
electrolytes, and lipids (no enzymes)
• Only the bile salts and lipids help with
digestion
• Bile salts emulsify fats by physically
breaking large fat globules into smaller
ones
• Gall bladder – small, green sac under liver that
stores bile
Homeostatic Imbalances
• Gall stones – if bile is stored in gall bladder
too long or too much water is removed, the
cholesterol in it crystallizes forming gall
stones
– Can cause blockage of hepatic or bile ducts
and bile begins is released into bloodstream
instead
• Bile in blood and eventually tissues
causes jaundice
• Hepatitis – inflammation of liver; often cause
by virus (also called hepatitis)
• Cirrhosis – chronic inflammation of liver; can
be from drinking too much alcohol
• Heartburn – when cardiac sphincter
does not close tightly and allows acidic
stomach juices to enter esophagus
– Hiatal hernia – superior part of stomach
protrudes slightly above the diaphragm;
which weakens cardiac sphincter
• Peptic ulcers – erosion of stomach lining
(open sore in membrane), which exposes
the lining to acid; often caused by some
bacteria (Helicobacteri pylori)
• Pancreatitis – inflammation of pancreas
caused by pancreatic enzymes breaking
down the pancreatic tissue itself
• Diverticulitis – diverticula are formed
by the inner layer of the intestine
protruding through the large intestine
wall; causes inflammation and pain
– Caused by not eating enough bulk (fiber);
colon narrows and puts more pressure on
walls
• Diarrhea – large intestine does not absorb
enough water from feces; loss of
electrolytes and fluids can cause severe
dehydration
• Constipation – too much water is removed
from feces; can be caused by low fiber
diets
• Impacted teeth – teeth that remain
embedded in jawbone; can cause pressure
and pain; often occurs with wisdom teeth
• Appendicitis – infected appendix; bacteria
can accumulate because its twisted
• Hemorrhoids – enlarged and inflamed
branches of the rectal vein in the anal
canal that cause itching, pain, and
sometimes bright red bleeding
• Vomiting – complex reflex that empties
stomach through esophagus, pharynx,
and mouth (vomiting center in medulla
oblongata)