structural & functional adaptation of the digestive system

Gastrointestinal tract
S4-Phase 1
2018-2019

Structural & functional adaptations of the digestive system


OBJECTIVES

1. Outline function & a general structural plan of the digestive
tract.
2. Introduce some regional variations in structure that relate to
functional.
3. describe the fluid balance of the gut
4. Describe properties of hormonal and enteric nervous
system.


Function of GIT system

?Digestion allows food to be converted to a sterile, neutral, and
isotonic structure containing small, sugars, amino acids, small
peptides, small particles of lipids and other small molecules. This is
now ready for absorption and excretion ,which is accomplished by
the action of HCl , bile and a variety of enzymes secreted by exocrine
glands – secretion

Motility the contractions of the smooth muscles in the walls of the tract,
mix and move the ingested food from mouth to anus.
Absorption products of digestion are then taken into the blood.

Initial physical disruption à Ingestion and transport
to storage à Initial chemic disruption and creation of
suspension (chyme) à Disinfection à Controlled
release of Chyme à Dilution and neutralisation à
Completion of chemical breakdown à Absorption of
nutrients and electrolytes à Final absorption of water
and electrolytes à Producing faeces for controlled
excretion.

Digestion

?The process of digestion can be divided into :
?The non-specific phase involves making the larger food particles
physically smaller, and
?the specific phase breaks down the food molecules chemically via
specific enzymes to smaller molecules.

?This process involves the production of chyme, which is a
semiliquid form of ingested food & it is sterile due to the action of the
stomach acid on any ingested microbes.


Digestion

Absorption

??Absorption is the specific (active or passive) uptake of nutrients molecules,
water, and electrolytes.


??Fluid passes very slowly through the small intestine and the small intestine
also has a very high surface area due to the villi and microvilli, allowing for
greater rate of absorption.


??In the large intestine, the remaining water and electrolytes are absorbed. The
slow transit through the large intestine allows water to be reabsorbed by the gut
and leaves dry faeces to accumulate in the sigmoid colon and anal canal.

Small intestine absorbs about (12.5L )of the fluid,
Large intestine absorbs about 1.35L.

Excretion


?Faeces are propelled periodically into the rectum, causing an urge to
defecate. When the sphincters relax around the anus, the faeces are
expelled.

?From the table , it is easy to understand why diarrhoea and vomiting
are major causes of dehydration as water added to the solution is lost
before being reabsorbed. There will also be GI disturbances such as
constipation as a result.

broad functions of the various regions of

Mouth and Oesophagus
??Mastication
?Saliva production
1. Protects mouth

the GI tract
· Wets / Bacteriostatic / Alkaline / High Ca2+
2. Lubricates food for mastication and swallowing
· Wet / Mucus
3. Starts digestion
· Sugars
??Swallowing
?Formation of bolus
?Rapid oesophageal transport


Stomach

Storage
Relaxes to accommodate food
Initial disruption
· Contracts rhythmically to mix and
disrupt
· Secretes acid and Proteolytic
enzymes to break down tissues &
disinfect as Chyme
Delivers
Chyme slowly into the Duodenum

Duodenum


Dilution and neutralisation of Chyme
o Water drawn in from ECF. Stomach
impermeable, Duodenum permeable.
o Alkali (bile) added from Liver and
Pancreas
o Enzymes added from pancreas

Balance of gut
Description Water in GI tract lumen
Eat 1kg a food per day 1L

Add 1.5l of saliva 2.5L

Add 2.5l of gastric secretion 5L

Addition of 9l of fluid for isotonic
and neutral 14L

Small intestine absorbs 12.5l of water 1.5L

Large intestine absorbs 1.35l 0.15 L

Remaining in faeces 0.15kg


Small Intestine

??Absorption of nutrients &
electrolytes
Fluid passes very slowly through
the small intestine
o Large surface area
o Epithelial cells absorb molecules,
some actively some passive
o Often coupled to Na+ ,absorption
pass into hepatic portal circulation
(first pass…)
??Absorbs the majority of water (1.5L vs.
0.15L large intestine)

Large Intestine

· Final absorption of water (0.15)
· Very slow transit

· Faeces form and accumulate in the
descending and sigmoid colon


Rectum


· Faeces propelled periodically
into rectum

· Urge to defecate

· Controlled relaxation of
sphincters and expulsion of
faeces


Enteric nervous system and its relationship to
the autonomic nervous system


The enteric nervous system is a subdivision of the autonomic nervous
system that directly controls motility the GI system.


The enteric nervous system is made up of two nerve plexuses in the wall
of the gut which may act independently of the CNS (short reflex pathway).

Parasympathetic control is the most significant. It coordinates both
secretion and motility using a range of neurotransmitters.














Hormonal Activiation


?A dozen or more peptide hormones are released by endocrine cells
in the walls of the gut. Some of these

1. - a paracrine action,
2. - hormone with true endocrine action.

hormones comprise 2 structurally related "groups" :
gastrin group and secretin group.

?These hormones are released from one part of the gut to affect the
secretions or the motility of other parts.