Transcript Digestive System

Digestive System
• Functions of the digestive system.
• Structure and innervation of the digestive system.
• Swallowing, peristalsis, Esophagus and LES.
• Structure and secretions of the stomach.
• Mechanism of HCl secretion, GI protection, & Ulcers.
• Intestine, liver, gallbladder, and pancreas.
• Regulation of digestive system.
• Digestion and absorption of major food groups.
CHAPTER 18
Gastrointestinal
Tract and
its Accessory
Organs
Functions of the Digestive System
• The major functions of digestive system are to digest food
and absorb the final products into blood.
• Question) How the digestive system achieves the above goal?
• Answer) by the following activities:
1- Motility
2- Secretion
3- Digestion & absorption
• The digestive system also must get ride of the undigested waste materials
this is achieved through the process of defecation.
Layers of the G. I. T.
1- Mucosa.
- Direct contact with food,
- directly involved in secretion and absorption,
- contains lymph nodes, goblet cells, and other
secretory cells,
- muscularis mucosa, thin muscular layer
changes the surface area of mucosa.
2- Submucosa.
- Tissue below mucosa, which is supportive to
- mucosa, contains blood vessels, glands, nerve
plexus.
3- Mascularis.
- Responsible for the motility of the GIT,
- has inner circular and outer longitudinal
smooth muscle,
- also contains nerve plexuses.
4- Serosa.
- Outermost layer serves supportive and
protective functions.
Innervation of the G. I. T.
1- Autonomic nerve system (ANS) supply.
a) Sympathetic
b) Parasympathetic
2- Enteric nervous system (ENS). “intrinsic nervous
system of the gut”
a) Myenteric plexus.
Its stimulation causes increase in motor activity (motility)
of the gut.
b) Submucosal plexus.
Mainly controls secretion and blood flow of the GIT,
also serves many sensory functions.
Interaction of ANS with ENS
Swallowing
- Swallowing reflex is initiated when pressure receptors in walls of pharynx are
stimulated by food or drink.
- Swallowing is co-ordinated by the swallowing center in brain stem (medulla oblongata).
Upper Esophageal
Sphincter
Swallowing
The food passes down the esophagus
by peristalsis and enters into the
stomach via the Lower Esophageal
Sphincter (L.E.S.)
LES
Peristalsis
- Aboral propulsion of contractions at a velocity of 0.5 to 2 cm per sec. (much faster
in the proximal compared to distal part of intestine).
-Peristalsis waves are weak and die out after about 10 cm therefore giving a slow
rate of movement of chyme (1 cm/min).
Bolus
Circular contract
Longitudinal relax
Direction of
propulsion
Circular relax
Longitudinal contract
Presence of bolus causes local distention small intestine, and this activates
myenteric plexus between circular and longitudinal muscle layers of intestine.
Structure and Function of the Stomach
Structure:
divided into:
1- Fundus
2- Body
3- Antrum
Storage
Mixing &
Emptying
Fuction:
1- Storage of food
Retropulsion
2- Mixing of food
3- Emptying of food into small intestine
Control of Intragastric Pressure
Empty Stomach
R
Food Entering Stomach
Distension
R
Laplace law:
2 x Surface Tension
Pressure =
Radius
- To keep the intragastric pressure constant, after food enters the
stomach the peristalsis is inhibited for about 1 hour.
Structure and Function of the Stomach
Gastric glands have several types of cells that secrete different materials which
collectively are called gastric juice:
EXOCRINE
1- Gobblet cells- secret mucus.
3- Chief cells – sceret enzyme (pepsinogen)
2- Parietal (oxyntic) cells – sceret HCl
4- G cells – secret hormone gastrin
5- Enterochromaffin-like (ECL) cells –secret histamine.ENDOCRINE
6- D Cells – secret somatostatin
PARACRINE
How the Mucosa of GI is
Protected against Digestion
Acid (HCl) and enzyme (pepsin) help in digestion of food substances. By this action
the food is broken into smaller molecules which can be absorbed into blood.
The stomach protects itself from digestion by:
1- The chief cells of the gastric mucosa are extremely impermeable to HCl.
2- A layer of alkaline (HCO3-) mucus covers the mucosa.
3- The cells of mucosa are tightly fused to each other so that the HCl does not
leak to the submucosa.
4- Rapid rate of cell division in the mucosa (entire epithelium is replaced
every 3 days).
5- The enzyme pepsin is secreted in inactive form (pepsinogen).
Activation of Pepsin
Small Intestine
A Villus
Intestine folds pilacae circularis
and on these folds there are villi
which have a layer of epithelial
cells which their membrane is
also has folds called microvilli.
These arrangement increase the
Surface area of the small intestine.
Intestinal Motility
Types of contractions in small intestine:
1- Segmentation: (mixing movement)
Intestinal Motility
2- Peristalsis: (propulsive movement)
Bolus
Circular contract
Longitudinal relax
Direction of
propulsion
Circular relax
Longitudinal contract
Structure and Motility of the Colon
1- Mixing movements:
Haustrations = occur in cecum, entire colon, and rectum.
Are characterized by contractions of
inner circular muscles.
2- Propulsive movement:
Mass peristalsis = strong peristalsis
which directed aborally, occur
several times a day in cecum and
entire colon.
Reverse peristalsis = orally directed
which gives more time to the fecal
matter to be expose of water
absorption.
Regulation of Intestinal Function
The GI has its own nervous system (enteric nervous system), it can sense and
form reflex actions due to the presence of food.
In the intestine the enteric plexus is very extensive and this system helped by the autonomic
nervous system are involved in several reflexes:
1- Gastroilial reflex =
increase gastric activity causes increased motility of ileum
and movement of chyme (food mixed with digestive juice)
through the ileocecal valve.
2- Ileogastric reflex =
when ileum is distended this decreases the motility and emptying
of the stomach.
3- Intestino-intestinal reflex =
over-distension of one part of the intestine causes reflex relaxation
of the rest of the intestine.
LIVER
- Is the largest internal organ in the body.
- Has many functions:
1- Detoxification of blood
2- Regulating metabolism
3- Protein synthesis
4- Storage site for vitamins and iron
5- Secretion of bile.
5-Secretion of Bile
* Bile secretion is the principal digestive function of the liver.
- Bile contains:
- Bile salts
- Urobilinogen
- Cholestrol
- Lecithin
- Bilirubin
- Electrolytes
- Bicarbonate
LIVER
Small intestine
LIVER
Enterohepatic Circulation
Entero = intestine
Hepatic = liver
Gallbladder
The bile which is secreted by the liver is
stored and concentrated in gallbladder.
When bile is needed for digestion, the
walls of gallbladder contract and this
ejects the bile through the bile duct
into duodenum.
Pancreas
Weighs about 100g, and produces about 1L of juice/day.
Secretions are both endocrine and exocrine.
Metabolic
function
Islets of Langerhans
Secret insulin and glucagon
Digestive
function
Acini: secret pancreatic juice
Pacreatic Juice
• Pancreatic juice contains:
– Water and electrolytes (Na+, K+, Cl+, Ca++)
– Bicarbonate (HCO3-)
Sodium Bicarbonate
– Digestive enzymes:
For neutralization of acid
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Amylase
Trypsin
Lipase
Ribonuclease
Deoxyribonuclease
Etc.
Enzymes for digestion
of food
Regulation of the Digestive System
• The activities of the G.I. Tract is regulated by:
– Neural mechanisms:
• Automomic nervous system
– Sympathetic
– Parasympathetic
• Enteric nervous system
– myenteric
– Submucosal
– Action of hormones (endocrine):
• Hormones secreted from the glands located in the G.I. Tract act on effector
cells of the G.I. Itself. Examples of such hormones are: secretin, gastrin, CCK
(cholecystokinin)
From stomach
From small intestine