The Digestive System - Dr. Jerry Cronindrjerrycronin.weebly.com/uploads/5/9/7/4/5974564/chapter_24a.pdf · The Digestive System The GI tract, and accessory organs like the liver and

Chapter 24

The

Digestive System

The Digestive SystemGastroenterology is the study of the gastrointestinal system. The digestive system

functions are ingestion,secretion, digestion (chemical and mechanical), mixing and propulsion,absorption and defecation.

The Digestive SystemThe digestive system, also called the

gastrointestinal system, is composed of the

alimentary canal (GI tract), and the accessory

organs.

The alimentary canal extends from the mouth to the

anus through the ventral body cavity (approximately 9

m, or 30 ft.).

The accessory organs include the teeth, tongue, salivary

glands, liver, gallbladder, and pancreas.

The Digestive SystemThe GI tract, and accessory organs like the liver and pancreas, are responsible for facilitating the body’s metabolic processes. Catabolism: Larger molecules are broken into

smaller molecules (mouth, stomach, duodenum).• In the GI tract, this is called digestion and can occur by

either mechanical or chemical means.

Anabolism: Smaller molecules are used as building blocks for larger molecules (liver).

The Digestive SystemMechanical digestion includes all movements that facilitate catabolic processes: Mastication Swallowing Mixing

• Increase contact of food with digestive chemicals

Peristalsis• Movement of muscles within the GI tract that facilitates

movement of food

The Digestive SystemChemical digestion is mainly accomplished by

using water to break chemical bonds

(hydrolysis).

Fats are broken down into fatty acids and glycerol.

Carbohydrates are broken down from

polysaccharides into monosaccharides.

Proteins are broken down into polypeptides and

amino acids.

The Digestive System(Interactions Animation)

Enzyme mediated hydrolysis

You must be connected to the internet to run this animation

http://www.wiley.com/college/jenkins/0470227583/animations/index_23_02_04.html

Anatomy OverviewThe wall of the GI tract from the lower esophagus to the anal canal has the same basic, four-layered arrangement of tissues. The four layers of the tract, from deep to

superficial, are the mucosa, submucosa, muscularis, and serosa/adventitia.

The lumen is the inside ofthe tube.

Anatomy OverviewThe mucosa is a mucous membrane made of

various types of epithelium sitting on a loose

connective tissue called the lamina propria.

Nonkeratinized stratified squamous epithelium (for

protection) lines the pharynx, esophagus, and anus.

Simple columnar epithelium (for secretion/absorption)

lines the stomach and intestines.

• Located among the epithelial cells are various glandular cells

that secrete mucus and fluid into the lumen of the tract

(exocrine).

The lamina propria contains a prominent lymphoid tissue

(mucosa-associated lymphatic tissue or “MALT”) that

protect against disease. Underneath it is the muscularis

mucosae, a thin layer of smooth muscle that throws

the lining of the stomach

and small intestines into

tiny folds (increases

surface area to aid

digestion/absorption).

Anatomy Overview

Anatomy OverviewThe submucosa is composed of loose

connective tissue that binds the mucosa to the

muscularis.

It contains blood and lymphatic vessels

(to receive absorbed substances)

and an extensive network

of neurons known as

the submucosal

plexus.

The muscularis of the mouth, pharynx, superior and middle parts of the esophagus, and anal sphincter contains skeletal muscle that allows for voluntary swallowing and control of defecation. Throughout the rest of the tract, it is smooth muscle, arranged in inner circular and outer longitudinal sheets, with the myenteric nerve plexus continuing between them.

Anatomy Overview

Anatomy Overview

The serosa/adventitia is the outermost layer.

If attached to surrounding tissues (e.g. around the

esophagus), it is called adventitia - a fibrous connective

tissue arranged around the organ which it supports.

If contained in the peritoneal cavity, it is called serosa ,

which has a slippery mesothelium surface layer.

• Serosa covers the intra-abdominal organs as the visceral

peritoneum.

Anatomy Overview

Anatomy OverviewThe peritoneum is the body’s largest serous membrane, and it wraps around most abdominopelvic organs. The visceral peritoneum forms the “serosa” of the

alimentary canal and covers other intra-abdominal organs.

It then continues around the abdominal wall as the parietal peritoneum.

Anatomy OverviewUnlike the pericardium and pleurae, which smoothly cover the heart and lungs, the peritoneum contains large folds that bind the organs to one another and to the cavity walls. There are five major peritoneal folds:

• greater omentum• falciform ligament• lesser omentum• mesentery• mesocolon

Anatomy Overview

The greater omentum is the largest peritoneal fold.

It drapes over the transverse colon and the anterior coils

of the small intestine like a “fatty apron”.

• It contains many lymph nodes that help combat

and contain infections

of the GI tract.

• The large amount of adipose tissue can greatly

expand (as seen in people with “beer bellies”).

Anatomy OverviewThe falciform ligament attaches the liver to the anterior abdominal walland diaphragm.

• The lesser omentum is a peritoneal fold that suspends the stomach and duodenum from the inferior edge of the liver.

– It forms a pathway for blood

vessels (hepatic portal

vein and common

hepatic artery)

to enter the liver,

and it contains the

common bile duct.

Anatomy Overview

Together, the mesentery (of the small intestine)

and mesocolon (of the large intestine) attach the

bowel to the posterior abdominal wall,

holding the intestines

loosely in place as

muscular contractions

mix and move the

luminal contents

along the GI tract.

Anatomy Overview

Anatomy OverviewSome abdominopelvic organs are covered by visceral peritoneum only on their anterior surfaces. The portion of the organ that lies behind the peritoneum is said to be “retroperitoneal”. Organs in theretroperitoneal space include: The kidneys and ureters Most of the pancreas The adrenal glands The aorta and inferior vena cava

Physiology OverviewDigestive activities of the gastrointestinal tract

occur in three overlapping phases:

1. The cephalic phase

2. The gastric phase

3. The intestinal phase

As we study the glands of the

mouth, stomach, and small

intestine – as well as the

secretions of the accessory organs the liver and pancreas –

we will learn about the specifics of all 3 phases.

Physiology OverviewDuring the cephalic phase of digestion, the smell, sight, thought, or initial taste of food activates neural centers in the cerebral cortex, hypothalamus, and brain stem to prepare for digestion. The brain stem activates the facial (CN VII) and

glossopharyngeal (CN IX) nerves to stimulate secretion of saliva, while the vagus nerves (CN X) stimulate secretion of gastric juice.

Physiology OverviewOnce food reaches the stomach, the gastric phase of digestion begins. Neural and hormonal mechanisms (the hormone

gastrin is a key player) promote secretion of gastric juice and increase gastric motility.

The intestinal phase of digestion begins once food enters the small intestine. Neural and hormonal responses promote the

continued digestion of foods that have reached the small intestine.

The MouthThe oral or buccal cavity, is formed by the

cheeks, hard and soft palates, and the tongue.

Mechanical digestion of food through mastication

(chewing) enables it to be mixed with saliva to form

a soft flexible bolus that can be easily swallowed.

Saliva starts the process of chemical digestion of

food

• Saliva is 99.5% water, with tiny amounts of dissolved ions,

IgA, lysozyme (a bacteriolytic enzyme), and salivary

amylase (a digestive enzyme that acts on starch).

Three large salivary glands secrete most of the

saliva: the parotid, submandibular, and sublingual

glands. The smaller glands are found on the lips

(labial), cheeks (buccal), palate

(palatal), and tongue

(lingual).

Daily salivary

secretions average

1–1½ liters.

The Mouth

The MouthSalivary regulation is under the control of the ANS Parasympathetic stimulation promotes secretion of a

moderate amount of saliva. Salivary centers are located in the brain stem and efferent nerve impulses are transmitted by the facial (VII) and glossopharyngeal (IX) nerves.• Touch (pressoreceptors), smell, taste (taste buds), and

psychological factors are also salivary stimulators.

Sympathetic stimulation decreases saliva secretions.

The Mouth(Interactions Animation)

Carbohydrate Digestion in the Mouth




Lipid Digestion in the Mouth



The MouthThe tongue is composed of skeletal muscle under

voluntary somatic motor control - it forces the

moistened food bolus into position for swallowing

(deglutition) and places the bolus into contact with

the teeth for chewing.

The extrinsic muscles of the tongue attach to bones in

the area and move the tongue from side to side.

Intrinsic muscles originate within the tongue and alter

its shape and size for speech and swallowing

The Mouth

The MouthThe teeth or dentes are located in sockets of the

alveolar processes of the mandible and maxillae.

The sockets are lined by the periodontal ligament -

a dense fibrous connective tissue that anchors the

teeth to the socket walls and acts as a shock

absorber during chewing.

A typical tooth has three major external regions: the

crown, root, and neck.

• The neck of each tooth is covered by the gingivae , or gums,

which extend slightly into each socket.

The MouthDentin is a calcified connective tissue that

forms most of the tooth.

The dentin of the crown is

covered by enamel, a

harder-than-bone

calcified material and

encloses the pulp cavity,

a space filled with pulp

(a connective tissue

containing blood vessels,

nerves, and lymphatic vessels).

The MouthHumans have two dentitions or sets of teeth. There are 20 deciduous or “baby teeth” which

begin to erupt around 6 months of age.• All are lost between 6–12 yrs. of ages

There are 32 permanent teeth numbered from right to left, top (1–16) to bottom (17–32).• The third molars (teeth 1,

16, 17, and 32) are the wisdom teeth.

Mastication




DeglutitionDeglutition is the act of swallowing food. The oropharynx and laryngopharynx have digestive as well as respiratory functions, and swallowed food must transit them both on the way to the esophagus – and it must do so while not going into the nasal cavity or the airway. This complex process involves many muscles subject

to a number of voluntary and involuntary controls.• Deglutition has 3 stages: voluntary, pharyngeal, and

esophageal.

DeglutitionThe first stage is the voluntary act of the tongue forcing the

bolus to the back of the oral cavity and into the oropharynx.

The involuntary pharyngeal stage begins as the bolus passes into

the oropharynx - receptors send impulses to the deglutition

center in the medulla and pons.

Returning impulses cause the soft palate to move superiorly

and posteriorly to close the nasopharynx.

The epiglottis moves slightly inferiorly to close the glottis.

DeglutitionThe esophageal stage of swallowing begins

once the bolus enters the esophagus.

Peristalsis, a progression of coordinated

contractions and relaxations of the circular and

longitudinal layers of the muscularis, push the bolus

onward.

Deglutition(Interactions Animation)

Deglutition



The EsophagusThe only digestive system function that occurs in the esophagus is propulsion (moving food into the stomach).The esophagus is a muscular tube thatbegins inferior to the laryngopharynx, and positioned posterior to the trachea. Leaving the neck, it traverses the

thoracic cavity down the posterior mediastinum before piercing the diaphragm through the esophageal hiatus.

The mucosa of the esophagus is nonkeratinzed

stratified squamous epithelium

The type of muscle in the

muscularis of the esophagus

varies by region

• the superior 1/3 is

skeletal muscle

• the intermediate 1/3 is

skeletal and smooth muscle

• the inferior 1/3 is

smooth muscle

The Esophagus

The EsophagusUpper and lower esophageal sphincters (UES

and LES) are situated at each end of the tube.

The LES regulates the movement of food from the

esophagus into the stomach.

• Incompetence of the LES

results in Gastroesophageal

Reflux Disease (GERD),

which manifests as

“heart burn”.

The stomach is a J-shaped enlargement of the GI tract situated between the esophagus and the first part of the small intestine (the duodenum).The position and size of the stomach varies continually - the diaphragm pushes it inferiorly with each inhalation and pulls it superiorly with each exhalation.

Rugae are large folds in the

mucosa of the empty stomach

which enable gastric

distension, depending

on the amount of stomach

contents.

The Stomach

The StomachEmpty, the stomach is about the size of a large

sausage, but it is the most distensible part of

the GI tract and can expand to accommodate a

large quantity of food.

Because a meal can be eaten much more quickly

than the intestines can digest and absorb it, one of

the functions of the stomach is to serve as a mixing

chamber and holding reservoir.

As a functional adaptation, the gastric muscularis

contains an additional 3rd inner oblique layer of

Note the additional oblique layer of smooth

muscle in the gastric muscularis, which is

limited primarily to the body of the stomach

The Stomach

The stomach has four main regions:

the cardia

the fundus

the body

the pylorus

The StomachSimple columnar epithelial cells (surface

mucous cells) line the mucosal surface and

secrete a protective mucous.

Columns of secretory

cells extend down into

the lamina propria

forming gastric glands.

Several gastric glands open

into the bottom of narrow

channels called

The StomachA variety of specialized exocrine and endocrine

cell types populate the gastric glands and pits.

Exocrine gland cells include:

• mucous neck cells which produce mucus

• parietal cells which produce intrinsic factor and HCl

• chief cells which secrete the protease pepsinogen and

gastric lipase

Enteroendocrine G cells, located mainly in the

pyloric antrum, secrete the hormone gastrin into the

bloodstream.

The StomachThe secretions of the mucous, parietal, and chief cells form gastric juice, which totals 2–3 l/d.The stomach is protected from its own gastricjuice by a 1–3 mm thick layer of alkaline mucus secreted by surface mucous cells and mucous neck cells.

The StomachThe strongly acidic nature of gastric juice kills many microbes, partially denatures proteins in food, and converts pepsinogen into pepsin. Pepsin is the only proteolytic enzyme in the

stomach. Gastric lipase splits triglycerides. Intrinsic factor (IF) is needed for absorption of

vitamin B12 in the terminal ileum. • Vitamin B12 is needed for RBC production.

The StomachDisturbing the balance between hydrochloric acid production, pepsin secretion, and mucosal defenses can lead to erosion of the stomach's epithelial lining. This graphic shows an

endoscopic view of a gastric erosion, possibly caused by consumption of too much alcohol or use of an NSAID drug such as aspirin or ibuprofen.

The Stomach• Gentle, rippling, peristaltic movements called

mixing waves pass over the stomach every 15 to 25 seconds.– These waves macerate food, mix it with secretions

of the gastric glands, and reduce it to a soupy liquid called chyme.

The Stomach(Interactions Animation)

Stomach Peristalsis



Chemical Digestion - Gastric Acid







Protein Digestion in the Stomach




The Digestive System - Dr. Jerry Cronindrjerrycronin.weebly.com/uploads/5/9/7/4/5974564/chapter_24a.pdf · The Digestive System The GI tract, and accessory organs like the liver and

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