Download and digestive

Digestion
Types of digestion systems
2/23
• in unicellular and primitive multicellular
organisms intracellular digestion
• in more developed multicellular organisms –
extracellular digestion
• diverse extracellular digestion systems exist
in animals – there are three basic types based
on the functioning of the „reactor”
– intermittent, stirred – saclike; one portion in,
digestion, undigested remnants out (e.g. hydra)
– continuous flow, stirred – continuous intake,
content mixed, continuous output (e.g. ruminant
forestomach)
– plug-flow, unstirred – continuous input, continuous
output, tubelike reactor, composition depends on
place, but not on time (e.g. small intestine in
vertebrates) 
3/23
Alimentary canal in vertebrates
• topologically external to the body
• entrance and exit are protected by
sphincters and other devices
• ingested material is subjected to various
mechanical, chemical and bacterial effects
• digestive juices break down the ingested
material chemically, nutrients are absorbed,
undigested, unabsorbed material is expelled
with the feces
• tubular organization allows for functional
specialization (i.e. acidic and alkaline
environment)
• parts of the alimentary canal: headgut,
foregut, midgut, hindgut 
Headgut
4/23
• food enters here – structures related to feeding
and swallowing: mouth-parts, buccal (oral)
cavity, pharynx, bills, teeth, tongue, salivary
glands, additional structures to direct the flow
of ingested materials and inspired water or air
• most multicellular organisms have salivary glands
to help swallowing (mucin - mucopolysaccharide)
• saliva may contain: enzymes, toxins,
anticoagulants (vampires, leeches, etc.)
• tongue from chordates on – mechanical digestion,
swallowing, grasping food (chameleon, anther),
chemoreception (taste buds)
• snakes take olfactory samples from the air and
wipe the samples in the Jacobson’s organ
(vomeronasal organ)
Foregut
5/23
• in most species it consists of the esophagus and
the stomach
• esophagus carries food from headgut to stomach
• in infrequently feeding animals it can contain a
saclike expanded section – crop (leeches) to
store food, birds might use it to feed nestlings
• digestion starts in the stomach
• in most vertebrates: pepsinogen and HCl
• monogastric stomach in omnivorous and
carnivorous vertebrates
• invaginations with gastric pits with gland cells 
• digastric stomach in ruminants: fermentative
(rumen + reticulum - cellulose) and digestive
(omasum + abomasum [enzymes only here]) parts
• camel, llama, alpaca, vicuñas: similar stomach
• fermentation occurs before the stomach also in
other species: kangaroo, chickenlike birds
• birds might have a muscular gizzard following the
stomach – chyme moves back and forth
Midgut I.
6/23
• in vertebrates it consists of the small intestine
(duodenum, jejunum, ileum), it is separated from
the stomach by the pylorus
• shorter in carnivores, longer in herbivores –
dynamic changes
• in tadpoles longer than in frogs relative to body
size
• duodenum: production of mucus and fluids +
receives secretions from liver and pancreas –
neutralization of stomach acid and digestion
• jejunum: secretion of fluids, digestion,
absorption
• ileum: mainly absorption, some secretion
• small intestine is characterized by a large
surface epithelium: gross cylindrical surface
would be 0.4 m2, but circular folds, intestinal
villi, brush border - 200-300 m2 
Midgut II.
7/23
• circular folds also slow down the progress of
food – more time for digestion
• each villus (approx. 1 mm long) sits in a circular
depression (crypt of Lieberkühn) – inside:
network of arterioles, capillaries and venules, in
the middle: central lacteal (lymph vessel) 
• longitudinal smooth muscle fibers – their
contraction empties the lymph vessels
• epithelium is made up of enterocytes (lifespan
3-6 days) proliferating at the bottom of the
crypts (chemotherapy!) and bearing brush border
(~1  long, 0.1  wide, 200,000/mm2); tight
junctions, desmosome 
• on the microvilli (brush border) glycocalyx:
hydrolases (glycoproteins) and luminal
transporters, inside actin filaments – in the
basolateral membrane Na-K-pumps and different
transporters
• among the enterocytes sporadic goblet cells
(mucus)
Hindgut
8/23
• stores remnants of digested food – absorption of
inorganic ions and water
• in vertebrates it consists of the final portion of
small intestine and of the large intestine (colon)
• the hindgut is the major site of fermentation in
many herbivores
– colon fermentation (plug-flow reactor) – large animals,
like horses, zebras, elephants, rhinos, sirenians (sea
cows), etc.
– cecal fermentation (continuous-flow, stirred reactor)
- smaller animals, like rabbits, many rodents, koalas,
opossums, etc. 
• hindgut terminates in the cloaca in many
vertebrates (cyclostomes, sharks, amphibians,
reptiles, birds and egg laying mammals), or in
the rectum
• defecation and urination are under behavioral
control 
• the alimentary canal in invertebrates have many
differences, but similarities as well 
Motility of alimentary canal I.
• motility is the ability of the alimentary canal to
contract
• its roles:
– propulsion of food from intake to excretion
– grinding and kneading the food to mix it with digestive
juices and to convert it to a soluble form
– stirring the gut contents to ensure the continuous
renewal of material in contact with the epithelium
• in arthropods and chordates it is achieved
exclusively by muscular motility, in other animal
groups ciliary motility might play a supplemental
or exclusively role
• in vertebrates at the entrance (buccal cavity,
pharynx, first third of the esophagus) and exit
(external anal sphincter) of the alimentary canal
striated muscles – providing an at least partial
voluntary control, in other places smooth muscles
and the enteric nervous system dominates
9/23
Motility of alimentary canal II.
• layers of the alimentary canal in vertebrates:
serosa, longitudinal and circular muscle,
submucosa, muscularis mucosa, lamina propria,
epithelium 
• there are two basic forms of motility: peristalsis
(longitudinal and circular muscles) and
segmentation (circular muscles) 
• sphincters: upper and lower esophageal, cardia
(functional), pylorus, ileocecal valve (between the
small and large intestine), internal and external
anal
• swallowing is a complex reflex: tongue presses
the food to the palate, soft palate closes the
nasal cavity, food is propelled into the pharynx,
mechanoreceptors induce the reflex, swallowing
is unstoppable
• upper esophageal sphincter relaxes, peristalsis
moves the food toward the stomach, lower
esophageal sphincter relaxes, cardia opens, food
enters the stomach
10/23
Motility of alimentary canal III.
• vomiting – complex reflex, helped by the
respiratory muscles – reverse peristalsis in the
small intestine, inspirational muscles contract –
negative pressure in the chest, abdominal
muscles contract – large pressure difference –
lower esophageal sphincter relaxes, chyme
enters the esophagus
• chyme returns to the stomach during retching
• during vomiting expiratory muscles contract,
upper esophageal sphincter relaxes
• centers in the medulla: central vomiting (without
retching), retching (without vomiting),
chemoreceptive trigger zone
• stimuli: direct (meningitis, disgust), chemical
(e.g. apomorphine), mechanical (back of the
throat), visceral (peritoneum, uterus, renal
pelvis, testis), organ of equilibrium
• reflux – cardia is leaking, acidic chyme reenters
the esophagus – can lead to inflammation, cancer
• regurgitation: in ruminants – chyme reenters the
buccal cavity without vomiting
11/23
Motility of alimentary canal IV.
• peristalsis in the stomach by partially closed
contraction ring - mixing, but it is not complete
– rat experiment with differently colored food
• small intestine – circumscribed expansion induces
peristalsis
• obstruction of passage – very dangerous
• causes: mechanical (e.g. tumor), physiological
(sympathetic hyperactivity – caused by peritoneal
excitation) - mechanism not completely clear
• large intestine absorption of water and ions,
excretion of feces
• following eating gastrocolic reflex distal
movement of the chyme – might involve mass
movement – frequently occurs in babies: eating
leads to defecation
• defecation is a complex process: posture,
contraction of abdominal wall, sphincters
• internal sphincter autonomic, external voluntary
regulation
12/23
Regulation of the intestines I.
• intrinsic control: contraction is myogenic in the
alimentary canal – smooth muscle is capable of
inducing electrical activity (rhythmic hypo-, and
repolarization – might lead to Ca-spike and
contraction; influenced by stretching and
chemical stimuli from the chyme 
• extrinsic control: enteric nervous system, central
nervous system, peptide hormones
• enteric nervous system
– myenteric (Auerbach's) and submucosal (Meissner’s)
neuronal networks
– local reflexes
– sensory neurons: transmit information of mechano-,
chemo-, and osmoreceptors - substance-P
– interneurons: n-Ach (excitatory), enkephalinergic,
somatostatin releasing (inhibitory)
– effector neurons: excitatory: colocalized ACh and
tachykinin (e.g. substance-P); inhibitory: VIP, NO,
ATP - morphine excites the latter neurons, longlasting contraction, constipation; on glands VIP can
also be excitatory
13/23
Regulation of the intestines II.
• central nervous system
– parasympathetic innervation (preganglionic):
• acting mostly on interneurons of the enteric nervous system –
excitatory effect 
• to a smaller extent on efferent neurons – stomach functions,
sphincter relaxation (e.g. esophagus)
– sympathetic innervation (postganglionic):
• vasoconstriction
• pre-, and postsynaptic inhibition through 2-receptors
• direct excitatory effect on sphincters through 1 receptors
• local peptide hormones
– proved hormones: secretin, gastrin, CCK, GIP
(glucose-dependent insulinotropic peptide (formerly:
gastric inhibitory peptide) – many more candidates
– hormonal role is difficult to prove: measurement of
levels, administration (physiological vs. pharmacological
dose), antagonists
– gastrin family - five C-terminal amino acids of gastrin
and CCK are the same, both are active at different
lengths
– secretin family – secretin, GIP, glucagon, VIP
– produced by unicellular glands detecting the
composition and pH of the chyme directly – neuronal
regulation in some of them 
14/23
15/23
Gastrointestinal hormones
cell
G
CC
K
S
hormone
stimulus
stomach
gastrin
peptides,
amino acids in
the stomach
HCl
production
, motility
up
cholecystokinin
lipids, proteins
in the small
intestine
motility,
emptying
inhibited
secretin
acid in the
small intestine
emptying
inhibited
glucose-dependent
GIP
insulinotropic
peptide
HCl
carbohydrates production
in the small
,
intestine
emptying
inhibited
bile
pancreas
emptying
the gall
bladder
increased
enzyme
production
increased
HCO3secretion
glucose
dependent
insulin
secretion
Gastrointestinal secretions
16/23
• three types of secretion exist:
– secretion-reabsorption type – proteins, water,
electrolytes secreted in the acinus, reabsorption in
the secretory duct, e.g. salivary glands
– sequential secretion type – protein secretion in the
acinus, water and electrolytes in the secretory duct,
e.g. pancreas, liver
– parallel secretion type – e.g. stomach; chief cells:
pepsinogen, parietal cells: HCl, intrinsic factor, goblet
cells: mucin and HCO3–
• in one day about 5-6 l digestive secretions 
• production of saliva
– three pairs of large salivary glands: parotid,
submandibular, sublingual + many small ones in the
buccal cavity
– function: lubrication (dry mouth - thirst), mucin,
lysozyme, IgA, rinsing (dog-breath), amylase 
– serous and mucous acinus cells
– saliva is hyposmotic because of the reabsorption of
NaCl 
– mostly parasympathetic innervation, sympathetic
activation results in thick, viscous saliva
– unconditional and conditional reflexes – trumpet player
and licking of lemon
Secretion in the stomach
17/23
• secretion is parallel in the stomach; in addition,
G-cells produce gastrin
• fluid is acidic and isosmotic
• functions of the low pH: optimal environment for
pepsin, chemical degradation of the food
(denaturation), killing of bacteria
• large invaginations (canaliculi) in the membrane
of the parietal cells with H-K-ATPase molecules
- 106 concentration gradient – “world” record
(exit of Cl– and K+ through channels)
• source of H+: CO2 and water (carbonic
anhydrase, HCO3–/Cl– exchange) 
• facilitation: vagus nerve (m-ACh), gastrin,
histamine
• cephalic, gastric, intestinal phase
• inhibition: HCl level, fatty acids longer than 10
C in the small intestine
• secretion of chief cells increased by n-ACh, HCl

Secretion of the pancreas
18/23
• indispensable for digestion
• sequential secretion
– acinus cells:
• active enzymes (-amylase, lipase, DNAase, RNAase)
• proenzymes (trypsinogen, chymotrypsinogen,
procarboxipeptidases, prophospholipases, etc.)
– secretory duct:
• large amount of fluid with high HCO3– content (alkaline)
• CO2 - HCO3– and H+ (carbonic anhydrase), Na+/H+ antiporter,
Na+-pump, apical HCO3– exit
• secretory duct enters the duodenum along with
the bile duct at the ampulla of Vater
• enteropeptidase (enterokinase) secreted by the
duodenum activates trypsin, which in turn
activates all the other (there is also a trypsin
inhibitor in the pancreas) – during inflammation
early activation, necrosis and death can occur
• activation of acinus cells: CCK, m-ACh, VIP
• activation of HCO3– secretion - secretin
Functions of the liver
19/23
• secretion (bile acids) and excretion (bilirubin,
cholesterol, poisons, medicines, hormones, etc.)
• bile is produced by the parenchyma cells (75%)
and by the epithelium (25%) lining the bile ducts
the latter secretes electrolytes
• sinusoids with large-pored endothelium, between
them one-cell-thick parenchyma sheets –
between neighboring hepatocytes bile canaliculi,
surrounded by tight junctions – if damaged bile
enters circulation
• bile is concentrated in the gallbladder, emptied
three times a day (20-30 ml)
• 95% of bile acids are reabsorbed from the gut
• bilirubin is transformed by bacteria to
stercobilin giving the brown color of the stool
Digestion and absorption I.
20/23
• carbohydrates completely, while lipids and
proteins up to more than 90% are digested and
absorbed from the gut
• digestion of carbohydrates and proteins is a
two-step process: luminal digestion is completed
by enzymes (oligosaccharidases, exopeptidases)
on the surface (glycocalyx) of enterocytes
• absorption is mostly energized by the Na+
gradient that in turn is rebuild by the K+-Na+pump in the basolateral membrane
• carbohydrates:
– -amylase can break the 1-4 bond, but not the 1-6
– it is unable to break the -glycosidic bond in lactose,
this bond gets broken by the lactase (-galactosidase)
– the lack of this enzyme leads to lactose intolerance
– uptake of glucose and galactose is accomplished by a
Na+ cotransporter, fructose enters the cell using
GLUT-5 – it is slower, as it is a passive process
– all sugars are transported through the basolateral
membrane by GLUT-2 
– part of the plant fibers (cellulose) are fermented by
bacteria – not much usable energy is released, but
huge amount of CH4, CO2 is produced
Digestion and absorption II.
21/23
• proteins:
– luminal endopeptidases (pepsin, trypsin, chymotrypsin,
elastase) and exopeptidases (carboxypeptidases) digest
proteins to amino acids and smaller peptides
– enterocyte glycocalyx: different membrane peptidases
– membrane transport as amino acids (70-75%) or di-,
and tripeptides (25-30%), mainly by group-specific
Na+ cotransporters (active transport), partly through
facilitated diffusion
– at the basolateral membrane: facilitated diffusion
• vitamin B12:
– absorbed in protein-associated form
– demand: 1-2 microgram/day – reserves in liver are
sufficient for several years
– B12 binds to R-protein in the stomach, R is digested in
the duodenum, B12 binds to intrinsic factor
– in the ileum, receptor induced endocytosis, in blood
transported by transcobalamin II
– B12 is needed for erythropoiesis – anemia is most
frequently caused by the lack of the intrinsic factor
Digestion and absorption III.
22/23
• lipids:
– hydrophobic character, digestion is only possible at
the lipid-water border – micelles formed with the
help of bile acids
– most important enzyme: pancreatic lipase; in general
it cuts off 1,3 fatty acids
– fatty acids, 2-monoglycerids enter the enterocytes
from the micelles
– micelles also contain lipid-soluble vitamins (DEKA) –
lack of bile acids leads to low vitamin K levels and
disturbances in hemostasis
– lipids are reformed in the endoplasmic reticulum of
enterocytes and form lipoproteins containing
triglycerides, phospholipids, cholesterol and its
esters as well as apolipoproteins
– lipoproteins are classified according to their density:
VLDL, LDL, HDL – the largest are the chylomicrons
– lipoproteins are transported from the Golgi to the
lymphatic vessels through exocytosis 
– lipoproteins are also produced in the liver
Digestion and absorption IV.
23/23
• calcium:
– reabsorbed partly by paracellular diffusion, but mostly
by active transport
– regulation: parathormone and calcitriol (1,25dihidroxi-D3-vitamin)
– entrance by unknown mechanism – calcium binding
protein – active transport through the basolateral
membrane
• iron:
– stored in the enterocytes in the form of ferritin,
transported in the blood bonded to transferrin – if
the enterocyte is saturated absorption stops
– demand: in men 1 mg/day, in women (because of blood
loss during menses) 2-3 mg/day – iron is needed
mainly because of the renewal of enterocytes
• water and NaCl:
– Na+ channels in the apical membrane (their number is
regulated by aldosterone) - Na+-pump in the
basolateral membrane
– Cl– and water follows passively 
End of text
Reactor types
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-13.
Parts of the digestive tract
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-15.
Monogastric stomach
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-18.
Anatomy of the small intestine
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-20.
Structure of a villus
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-21a,b.
Brush border
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-21c,d.
Colon and cecal fermenters
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-22.
Behavioral control
The Far Side Gallery 3, G.Larson, Andrews and McMeel, Kansas City. 1994, p..21.
Digestive systems in vertebrates
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-17.
Cross-section of the intestine
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-22.
Motility of the intestine
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-24.
Basic membrane potential
rhythm
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-25.
Autonomic innervation
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-26.
Gastrointestinal hormones
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-34.
Digestive secretions
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-29.
Rinsing function of saliva
The Far Side Gallery 3, G.Larson, Andrews and McMeel, Kansas City. 1994, p..24.
Production of saliva
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-28.
HCl secretion in the stomach
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-32.
Pepsin secretion in the stomach
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-33.
Sugar transport in the intestine
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-35.
Lipid transport in the intestine
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-36.
Digestive fluid movements
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-37.