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

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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- Motility2- Secretion3- Digestion & absorption

• The digestive system also must get ride of the undigested waste materials this is achieved through the process of defecation.

= Movements (contractions)

= Exocrine + Endocrine

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.

Submucosal plexus

Myenteric plexus

Innervation of the G. I. T.

1- Autonomic nerve system (ANS) supply.a) Sympatheticb) 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.

Decrease GI activityIncrease GI activity

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

During swallowing bothnasopharynx and epiglottis close to prevent food moving into the nose and trachea, respectively.

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 contractLongitudinal relax Circular relax

Longitudinal contract

Direction ofpropulsion

Presence of bolus causes local distention small intestine, and this activates myenteric plexus between circular and longitudinal muscle layers of intestine.

Aboral propulsion

Mouth Anus

Structure and Function of the Stomach

Structure: divided into:1- Fundus2- Body3- Antrum

Fuction:

1- Storage of food2- Mixing of food3- Emptying of food into small intestine

Storage

Mixing &Emptying

Retropulsion

Fundus

Body

Antrum

The process of mixing food in the stomach is called “retropulsion”

Control of Intragastric Pressure

RR

Empty Stomach DistensionFood Entering Stomach

Laplace law:

Pressure = 2 x Surface Tension

Radius

- To keep the intragastric pressure constant, after food enters the stomach the peristalsis is inhibited for about 1 hour.

Both surface tension and radiusincrease proportionally thereforethe pressure stays constant

Structure and Function of the StomachGastric glands have several types of cells that secrete different materials which collectively are called gastric juice:

1-Mucus = lubrication & protection

3- Pepsiongen = Digestion

4- Gastrin = Stimulate HCl secretion

6- Histamine = Stimulate HCl secretion

5- Somatostatin = Inhibit gastrin secretion

1- Gobblet cells- secret mucus.2- Parietal (oxyntic) cells – sceret HCl 3- Chief cells – sceret enzyme (pepsinogen) 4- G cells – secret hormone gastrin5- D Cells – secret somatostatin6- Enterochromaffin-like (ECL) cells –secret histamine.

2- HCl = Help digestion

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

Inactive enzymeActive enzyme

Small Intestine

Both Pilace circularis and villi increase surface area

A Villus

Intestine folds pilacae circularisand on these folds there are villi which have a layer of epithelialcells which their membrane isalso has folds called microvilli.These arrangement increase theSurface area of the small intestine.

Microvilli

Intestinal Motility

Types of contractions in small intestine:

1- Segmentation: (mixing movement)

Intestinal Motility

2- Peristalsis: (propulsive movement)

Bolus

Circular contractLongitudinal relax Circular relax

Longitudinal contract

Direction ofpropulsion

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.

Structure and Motility of the Colon

LIVER- Is the largest internal organ in the body.- Has many functions:

1- Detoxification of blood2- Regulating metabolism3- Protein synthesis4- Storage site for vitamins and iron5- Secretion of bile.

* Bile secretion is the principal digestive function of the liver.

- Bile contains:- Bile salts- Urobilinogen- Cholestrol- Lecithin- Bilirubin- Electrolytes- Bicarbonate

Small intestine

LIVER

5-Secretion of Bile

LIVER

Enterohepatic CirculationEntero = intestine Hepatic = liver

Gallbladder

The bile which is secreted by the liver isstored and concentrated in gallbladder.

When bile is needed for digestion, the walls of gallbladder contract and thisejects the bile through the bile ductinto duodenum.

PancreasWeighs about 100g, and produces about 1L of juice/day. Secretions are both endocrine and exocrine.

Islets of LangerhansSecret insulin and glucagon

Acini: secret pancreatic juice

Metabolicfunction

Digestive function

Pacreatic Juice

• Pancreatic juice contains:

– Water and electrolytes (Na+, K+, Cl+, Ca++)– Bicarbonate (HCO3-)– Digestive enzymes:

• Amylase• Trypsin• Lipase• Ribonuclease• Deoxyribonuclease• Etc.

Sodium BicarbonateFor neutralization of acid

Enzymes for digestionof 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 stomachFrom small intestine

Inhibit activity of GI tractStimulate activity of GI tract

Control contractions

Control Secretions & Blood flow

Regulation of Gastric Secretion

• Contractions and secretions of the stomach to some extent are automatic. For example: the presence of food causes distension in the stomach wall and this will lead to contractions. Presence of some food materials in the stomach stimulate its glands to secret gastric juice.

• However, when ANS or hormones are active their effects dominate the function of the stomach.

• There are 3 phases in the control of gastric secretion:

1- Cephalic phase: sight, smell, taste, even thoughts of food can initiate this phase.

2- Gastric phase: when food enters the stomach this phase becomes active.

3- Intestinal phase: when food enters small intestine this phase becomes active.

Cephalic PhaseSightSmellSound Thoughts

Cortex

TasteTouchTemp.

Etc.

MouthAppetite

centerHypothalamus

- Cephalic phase is abolished by vagotomy.

Vagal Nuclei

Parietal cells HClChief cells EnzymeMucus cells MucusG cells Gastrin

Gastric Gland

PancreasSmall intestine

ACh

Vagus

Gastric Phase

Food entering stomach

Gastric Gland

PancreasSmall intestine

ACh Distension, protein digestion products

Enteric Plexus

ACh

- Gastric secretion is abolished after administration of atropine

Vagal Nuclei

Parietal cells HClChief cells EnzymeMucus cells MucusG cells Gastrin

Vago-Vagal

Local

Vagus

Blocker of ACh

Intestinal Phase

Secretin is release in responseto acid and CCK is released in response Fat.

1- Inhibition: At low duodenal pH and presence of food

Decrease acid Secretion

Blood

Gastric gland

CCK

S cell

Food entering small intestine

Secretin

Somatostatin

2- Stimulation: Occurs when duodenal pH is relatively high.

Increased acid Secretion

Food entering small intestineG cell

Blood

Gastrin

Gastric gland

WEAK

STRONG