DIGESTION

moving contents (motility, peristalsis)

Gut muscle isotonic: maintains same contraction over a wide range of lengths

secreting enzymes, lubricants

chemical breakdown

absorbing products of breakdown

elimination of solid and non-polar waste

glands

exocrine

endocrine

blood flow regulated by local metabolic factors

post-prandial dilation of blood vessels

 

Anatomy (Vander p. 560)

 

salivary glands                                                                            pancreas & liver

              ß                                                                                      ß

          Mouth  Þ pharynx  Þ esophagus  Þ  stomach  Þ  small intestine (duodenum, jejunum, ileum) Þ  large intestine Þ   rectum

 

layers

serosa

longitudinal muscle

myenteric plexus (Auerbach)

circular muscle

submucous plexus (Meissner)

          submucosa (connective tissue, larger blood vessels, lymph vessels)

mucosa (exocrine & endocrine secretory cells, epithelial cells absorb)

 

 

 

Control of digestion

1.     Pacemaker activity of smooth muscle itself (slow wave potentials (may be due to Na+ pump)

gap junctions between the cells)

2.     short [intrinsic] and long [extrinsic] reflexes

3.     myenteric and submucous plexi

sensory

          osmoreceptors, mechanoreceptors, chemoreceptors

motor innervation

4.     sympathetics and parasympathetics, motor

e.g. vagus, differential control of different processes

5.     hormones

 

THE ENTERIC NERVOUS SYSTEM (b & l)

 

Controls motor and secretory activities

as many neurons as the spinal cord (~100,000,000)

ganglia interconnected by fine unmyelinated fibers

sensory (mechano-, particular chemical, temperature, and pain)

motor (smooth muscle, exocrine, endocrine glands, dilates bl. vessels)

interneurons.

parasympathetics Þ enteric nervous system

sympathetics Þblood vessels, some smooth muscle

also Þ enteric nervous system

MYENTERIC NEURONS

          Þ smooth muscle, (excitatory, inhibitory)

SUBMUCOSAL NEURONS

          Þ secretory cells (endocrine, exocrine) and are vasodilators

ENTERIC REFLEXES

          stimulates motility orally, inhibits motility anally

 

 

 

 

 

 

 

 

 

3 phases

cephalic, gastric and intestinal

 

Mouth: Chewing; No absorption, minimal digestion

Salivary secretions, amylase, lysozyme, water, mucus, HCO3-

under control of autonomics

basal secretion by tonic parasympathetic input

reflexes, chemoreceptors, mechanoreceptors Þ parasymp. stimulation

cortex Þ parasympathetic stimulation

Pharynx (throat) and esophagus:

swallowing (deglutition)

mucous

Stomach

Gastric motility

receptive relaxation (inhibition of vagus)

[separate from vagus Þ acid secreting cells]

distention of the stomach Þ muscle to contract (directly, long and short reflexes)

CHYME

enterogastric reflex

fat, acid , hypertonic fluids, distention of duodenum Þ sensory neurons

inhibits gastric emptying (long and short reflexes)

duodenal distention Þ enterogastrones inhibit gastric motility

acids Þ secretin

a.a., f.a. Þ cholecystokinin

fat, glucose Þ GIP =glucose-dependent insulinotropic peptide

 

 

 

 

Gastric secretion

Body and fundus

secretes thick mucus, protects stomach from its own digestive juices

 OXYINTIC MUCOSA

gastric pits: neck cells (watery mucous)

parietal cells (HCl, intrinsic factor [B12 absorbed in ileum])

 

INTERSTITIAL                                                                                   LUMEN

FLUID                                                                    H+               H+

 

CO2              CO2 + H2O  Þ H2CO3  Þ H+ + HCO3-

 

HCO3-                                       HCO3-

Cl-                                                          Cl-                     Cl-

 

Pyloric gland area (PGA)

chief cells (zymogen granules, exocytosis (pepsinogen Þ pepsin)

other cells secrete gastrin into blood

Gastrin Þ acid from parietal cells, pepsinogen from chief cells

Gastrin release

3 mechanisms (2 excitatory and 1 inhibitory)

cephalic phase is excitatory

gastric phase, excitatory

a.a. & peptides, distention, drugs (alcohol and caffeine)

Þ short and long reflexes

intestinal phase; inhibitory, enterogastric reflex,  enterogastrones

 

 

 

 

 

SMALL INTESTINE

Enzymes from pancreas

PANCREAS

islets, endocrine

acini, exocrine

proteolytic - trypsinogen (Þ trypsin by enterokinase in the brush border) chymotrypsinogen and procarboxypeptidase (both activated by trypsin)

carbolytic - amylase (polysaccharides into disaccharides)

lipolytic - lipase (triglycerides into monoglycerides and free fatty acids)

other – ribonuclease, elastase

watery HCO3-

regulation of pancreatic secretion

cephalic phase - vagus; small

gastric phase - gastrin, small

intestinal phase - esp. acid in duodenum Þ

acid Þ secretin (Þ HCO3-)

a.a Þ cholecystokinin (Þ enzymes)

 

 

 

 

LIVER

Bile

bile salts (cholic acid salts), HCO3-, cholesterol, lecithin, and bilirubin

emulsifies fats  Þ water soluble (lipase is water soluble)

Bile salts, polar and non-polar parts, recycled from intestine

bile secretion Þ gall bladder (storage, concentration) Þ small intestine

secretin Þ HCO3- secretion

parasympathetic Þ bile secretion beginning in cephalic phase

Blood supply

          Hepatic artery Þ

                                      Sinusoids lined with specialized cells (Kupffer cells)

Portal vein Þ                  surrounded by system of hepatic ducts

         

Liver Functions

  1. metabolic processing of absorbed nutrients
  2. degradation of molecules

drugs, hormones, waste products, cholesterol, bilirubin, destroy old RBC, bacteria

oxidation, methylation, renders lipid soluble Þ water soluble for kidney

can activate some drugs

  1. synthesis of plasma proteins

transporter proteins, albumin, lipoproteins, globulins, fibrinogen & clotting factors

  1. storage of glycogen, fats, vitamins (A, D, B12), Fe, Hb
  2. activates vitamins K, D
  3. makes bile Þ gall bladder (storage, concentration) Þ small intestine

 

 

ABSORPTION OF NUTRIENTS

(large intestine absorbs water and salt, secretes mucus, absorbs vitamin K)

water soluble vitamins absorbed by bulk flow

fat soluble vitamins along with fats (below)

B12 via intrinsic factor in terminal ileum