Download The Digestive System Chapter 16

The Digestive System
Chapter 16
Function of the Digestive System
 To break down food into a “usable”
(absorbable) form
 To supply our cells with the nutrients
(carbohydrates, proteins, fats, vitamins,
minerals, water) they need for energy,
growth & repair
Organs of the Digestive System
 Gastrointestinal tract (GIT) –
continuous passageway which
contains the food from the time it
enters the body, until it leaves; organs
include:
 mouth (oral cavity), pharynx,
esophagus, stomach, small intestine,
large intestine, rectum, anus
 Accessory organs - participate in
digestive processes; organs include:
 teeth,
tongue, salivary glands, liver, gall
bladder, pancreas
Processes of Digestion
1. Ingestion
2. Movement along GIT
 Voluntary – e.g. swallowing
 Involuntary – e.g. peristalsis
3. Secretion – release of water, enzymes, acids,
buffers, mucous, etc. into GIT for physical
(mechanical) & chemical digestive processes
Processes of Digestion
4. Digestion
 Mechanical processing – physical breakdown
of food; e.g. mastication, emulsification, mixing
waves, segmentation
 Chemical digestion – chemical breakdown of
food; disassembling of organic molecules into
their component parts; requires enzymes
 carbohydrates (polysaccharides)
disaccharides  monosaccharides
 proteins  amino acids
 lipids (triglycerides) fatty acids
Processes of Digestion
5. Absorption – movement of nutrients from GIT into
blood capillaries (monosaccharides, amino acids,
H2O, vitamins, minerals) or lymphatic capillaries
(fatty acids)
6. Excretion (Defecation) – removal of waste
products from GIT
Histology of the GIT
4 layers of tissue
surround the lumen
of the GIT
 epithelium – stratified squamous or simple
Mucosa columnar
 lamina propria – loose CT
 muscularis mucosa – smooth muscle
Submucosa
CT with BV’s,
nerves &
lymphatics
Muscularis externa
Skeletal muscle at beginning & end of
GIT, smooth muscle (inner circular;
outer longitudinal layer) from lower
esophagus to rectum
Serosa (a.k.a.
visceral peritoneum)
Peritoneum & Mesenteries
 The abdominal cavity is lined with parietal
peritoneum & many of the organs within are
covered with visceral peritoneum
 Folds of peritoneum called “mesenteries” attach
some organs to others
 greater omentum
 lesser omentum
 mesentery proper
Mouth (oral cavity)
 Regions include the vestibule & oral cavity proper
 Roof comprised of hard & soft palate; floor primarily
comprised of tongue
 Mucosa of stratified squamous epithelium (nonkeratinized)
Tongue –
 intrinsic & extrinsic muscles
 papillae
 filiform
 fungiform
 circumvallate
Mouth (oral cavity)
Salivary glands –
secrete saliva – made
of H2O, salts &
“salivary amylase”
Parotid
gland
Sublingual
gland
Submandibular
gland
Teeth –
involved in “mastication”
Pharynx
Common passageway for air & food
 oropharynx & laryngopharynx lined with
stratified squamous epithelium (nasopharynx
lined with PSCC)
 uvula & epiglottis protect airway when
swallowing (“deglutition”)
nasopharynx
uvula
oropharynx
epiglottis
laryngopharynx
Esophagus
 muscular tube extending from pharynx, posterior to trachea, down thoracic
cavity, through “esophageal hiatus” of diaphragm, to lower esophageal (a.k.a.
cardiac) sphincter at junction of stomach
 functions in “deglutition” through action of peristalsis
 mucosa is stratified squamous epithelium
 variations in muscularis externa – begins as skeletal muscle at upper 1/3,
mixed skeletal & smooth muscle in middle, smooth muscle by lower 1/3
Stomach - Gross Anatomy
Lower esophageal (cardiac) sphincter
Pyloric sphincter
Stomach - Histology
Rugae – folds of mucosa & submucosa to allow
for expansion of stomach
Mucosa of simple
columnar epithelium with
mucous cells
Gastric pit leading to
gastric glands
Stomach – Histology (cont)
- Secrete mucus to protect
epithelial cells from enzymes
& acid
- Secrete HCl (for protein
digestion)
(Peptic)
- Secrete pepsinogen which
gets converted to “pepsin”
when mixed with HCl; for
protein digestion
Stomach
 Modifications in stomach include 3 layers of smooth
muscle in muscularis externa – outer longitudinal, middle
circular, innermost oblique layer
Functions of stomach include:
 temporary storage of food
 mechanical breakdown of food to “chyme” through
powerful mixing waves
 start of chemical digestion of proteins –
pepsin
proteins
HCl
polypeptides
Small Intestine - Anatomy
- connects stomach to large intestine; 15-20’ long; 1” diameter; held
together in abdominal cavity by “mesentery proper”
- site for completion of chemical digestion & absorption of nutrients
- comprised of three regions:
Duodenum – 10” in length;
receives chyme from
stomach, secretions from
liver, gallbladder & pancreas
Jejunum – 8’ long; most
digestion & absorption
occurs here
Ileum – 12’ long; connects to
cecum of large intestine at
iliocecal valve (sphincter)
Small Intestine
Modifications in mucosa & submucosa of intestinal wall designed to
increase functional surface area:
 Plicae circulares (circular folds) –
large transverse ridges; most abundant in
jejunum
 Villi – small finger-like projections of
mucosal folds across surface of intestine
Plicae
circulares
Small Intestine
Villi
 Villi lined with “absorptive cells” - mucosal epithelium of simple columnar
epithelium with microvilli “brush border” . These cells also produce
enzymes (disaccharidases & peptidases) for final digestion of carbs and
proteins
 Submucosa of each villus contains a capillary network & a “lacteal”
(lymphatic capillary) for absorption of nutrients
Pancreas
Stomach
Tail
Body
Head
Duodenum
Pancreatic
duct
 Retroperitoneal
elongated organ, deep to
stomach, from duodenum
to spleen
 Both endocrine
(pancreatic islets of
Langerhans – secretes
insulin & glucagon) &
exocrine gland
(pancreatic acini –
secrete pancreatic juice
aka pancreatin)
Pancreas
Pancreatic juice – mixture of enzymes & buffers (sodium
bicarbonate) secreted by acinar cells into pancreatic duct &
released into duodenum
 pancreatic amylase
Starch (polysaccharide)
 lipase
Lipids (triglycerides)
maltose (disaccharide)
fatty acids + monoglycerol
 proteases (trypsin, chymotrypsin, carboxypeptidase)
Proteins & polypeptides
 nucleases – digest RNA & DNA
small tripeptides
& dipeptides
 sodium bicarbonate – neutralizes acidic chyme because
enzymes in small intestine need an alkaline pH
Liver - Anatomy
 Largest organ within the body
 Comprised of 4 lobes:
 Large right & left lobes divided by falciform
ligament; small caudate & quadrate lobes
 Lobes of liver functionally divided into microscopic
lobules
Liver - Histology
 Lobules comprised of rows of Hepatocytes arranged radially around a
central vein
 Hepatocytes surround blood sinusoids (capillary structures) which are
partially lined with phagocytic Kupffer (aka stellate reticuloendothelial) cells
hepatocytes
central vein
sinusoids
Liver
 Hepatocytes produce bile, which gets secreted into bile
canaliculi of lobule
 Bile canaliculi merge to form bile ducts which eventually
merge to create the right & left hepatic ducts
Liver & gall bladder
 Right & left hepatic ducts unite to form common hepatic duct
which merges with cystic duct of gall bladder to form common
bile duct which enters the duodenum
Right hepatic duct
 Gall bladder – hollow
muscular sac under right lobe
of liver; stores &
concentrates bile
 Bile released into duodenum
functions in emulsification of lipids,
absorption of fats (due to presence
of bile salts), & excretion of bilirubin
Left hepatic duct
Liver - Functions
The liver has over 200 functions including:
 Bile production & excretion
 Metabolic regulation –
 storage of glycogen, fatty acids, fat-soluble vitamins &
minerals
 interconversion of nutrients (“gluconeogenesis”)
 detoxification & removal of drugs, toxins & hormones
 hematological regulation –
 phagocytosis of worn-out RBCs, bacteria & other
pathogens
 synthesis of plasma proteins
Blood Supply to Liver
In order for the liver to perform all of its functions, it receives
blood through 2 vessels:
 Hepatic artery - delivers oxygenated blood into sinusoids of
liver
 Hepatic Portal vein – delivers de-oxygenated, nutrient-rich
blood from digestive organs to sinusoids of liver
Liver uses O2 & nutrients
within blood of sinusoids
& then blood drains into
central veins of lobule &
eventually into IVC
Large Intestine
 Begins at the ilium & ends at the
anus; 5’ long; 3” in diameter
Transverse
colon
 3 regions:
cecum - blind pouch; has appendix
attached
colon – ascending, (hepatic flexure),
transverse, (splenic flexure),
descending, sigmoid
rectum – last 1” known as “anal canal”
Hepatic (rt.
Colic) flexure
Splenic (lt. colic)
flexure
Ascending
colon
Descending
colon
ileum
Ileocecal sphincter
Rectum
Anal canal
Cecum
Sigmoid colon
Vermiform appendix
Rectum
Anal canal
Internal anal
sphincter
Rectum
External anal
sphincter
Anus
Large Intestine
 main functions – H2O re-absorption; absorption of some vitamins &
minerals; formation & temporary storage of fecal material
no chemical (enzymatic) digestion but some bacterial
Simple columnar epithelium in mucosa,
except at anal canal (strat. Squam.)
No plicae circularis or villi
Modifications in muscularis externa &
serosa :
haustra
taeniae coli
 longitudinal muscle layer forms bands
called “taeniae coli” which create puckers
known as “haustra”
 serosa forms “epiploic appendages”
epiploic appendages
Overview of digestion & absorption