DIGESTIVE ORGAN
Divided into two groups:
Alimentary tract (GI tract):
• Mouth
• Pharynx
• Esophagus
• Stomach
• Small intestine
• Large intestine
• Anus
Accessory digestive organs
• Teeth
• Tongue
• Salivary glands
• Liver
• Gallbladder
• Pancreas
SIX BASIC PROCESSES:
Ingestion – this process involves taking foods and liquids into the mouth (eating).
Secretion – production and release from a cell or a gland of a physiologically active substance
Mixing and propulsion – alternating contraction and relaxation of smooth muscle in the walls of the GI tract mixing food and secretions and propel them towards the anus.
Digestion – mechanical and chemical process
• Mechanical digestion: teeth and smooth muscles
• Chemical digestion: use of digestive enzymes produced by the salivary glands
Absorption – the passage of digested foods from the gastrointestinal tract into blood or lymph.
Defecation – wastes, indigestible substances, bacteria, and digested materials that are not absorbed leave the body through the anus in the process of defecation.
Layers of the GI tract
MUCOSA
Inner lining of the GI tract which is a mucous membrane
Epithelium
• Squamous epithelium:
serves as protective function
Found in the mouth, pharynx, esophagus, and anal canal
• Squamous columnar:
Secretion and absorption
Lines the stomach and the intestines
Contains two types of cells:
Exocrine cells are present - secretes mucus and fluid into the lumen of the tract
Enteroendocrine cells - secretes hormone into the bloodstream
• Simple columnar epithelial cells
Are firmly sealed to each other by tight junctions that restricts leakage between cells
Lamina propria
– Is an areolar connective tissue containing many blood and lymphatic vessels, which are the routes by which nutrients absorbed into the GI tract reach the other tissues of the body.
– It also contains MALT (mucosa – associated lymphatic tissue). MALT are lymphatic nodules that contains immune system cells that protects against diseases.
MALT is present a along the GI tract, especially in the tonsils, small intestine, appendix, and large intestine
Muscularis mucosae
– The one that form the folds of the mucous membrane of the stomach and small intestine. This folds increases the surface area for digestion and absorption.
– It ensures that a absorptive cells are fully exposed to the contents of the GI tract
SUBMUCOSA
Consists of areolar connective tissue that binds the mucosa to the muscularis
It has many blood vessels and lymphatic vessels that receive absorbed food molecules
Submucosal plexus or Plexus of Meissner (a network of neurons) . These neurons are part of the enteric nervous system. It contains sensory and motor enteric neurons, plus parasympathetic and sympathetic postganglionic neurons that innervate the mucosa and submucosa.
Also contains glands and lymphatic tissue
MUSCULARIS
Consists of:
– Skeletal muscles - produces voluntary swallowing and it is found in the mouth, pharynx, superior and middle parts of the esophagus
– Smooth muscles – found in two sheets. The 1. Inner sheet of muscular fiber and 2. Outer sheet of longitudinal fibers. It is found all throughout the rest of the tract.
Involuntary contractions of the smooth muscle help break down food physically, mix it with digestive secretions, and propel it along the tract.
Myenteric plexus
Contains enteric neurons, parasympathetic ganglia, parasympathetic postganglionic neurons, and sympathetic postganglionic neurons that innervates the muscularis.
Controls GI tract motility (frequency and strength of the muscularis)
SEROSA
A superficial layer of those portion of the GI tract that are suspended in the abdominopelvic cavity
A serous membrane composed of areolar connective tissue and simple squamous epithelium.
PERITONEUM
It is the largest serous membrane of the body.
It consist of a layer of simple squamous epithelium with an underlying supporting layer of connective tissue.
Two divisions:
Parietal peritoneum – lines the wall of the abdominopelvic cavity
Visceral peritoneum – covers some of the organs in the cavity
Peritoneal cavity – the slim space found between the parietal and viscera cavity.
Contains large folds that weave between the viscera.
PERITONEAL FOLDS
GREATER OMENTUM
Omentum (fat skin)
The largest peritoneal fold.
Drapes over the transverse colon and coils of the small intestine
It is a double sheet that folds back upon itself
Contains considerable amount of adipose tissue
It has many lymph nodes (contributes macrophages and antibody – producing plasma cells that help combat and contain infections of the GI tract).
LESSER OMENTUM
Arises as two folds in the serosa of the stomach and duodenum
It suspends the duodenum and the stomach from the liver.
MESENTERY
Is fan – shaped.
Binds the small intestine to the posterior abdominal wall.
It extends from the posterior abdominal wall to wrap around the small intestine and then return to its origin, forming a double layer.
MESOCOLON
Binds the large intestine to the posterior wall
It carries blood and lymphatic vessels to the intestines
Together the mesentery and mesocolon holds the intestine loosely in place, allowing for a great amount of movement as muscular contractions mix and moved the contents along the GI tract.
ALIMENTARY CANAL
- MOUTH
- PHARYNX
- ESOPHAGUS
- STOMACH
- SMALL INTESTINE
- LARGE INTESTINE
- ANUS
I.MOUTH/ ORAL/ BUCCAL CAVITY
• It is a cavity bounded externally by the lips and cheeks and leading into the pharynx.
• The roof is formed by the hard and soft palates
• And the anterior two thirds of the tongue fills the floor of the mouth.
• Lips
– are fleshy folds that surrounds the opening of the mouth.
– It is covered externally by skin and internally by a mucous membrane
– There is a transition zone where the 2 kinds of covering tissue meet. This portion of the lips is non-keratinized, and the color of the blood in the underlying blood vessels is visible through the transparent surface layer
• Hard Palate
– The anterior portion of the roof of the mouth
– Is formed by the maxillae and palatine bones
– It forms a bony partition between the ora and nasal cavity
• Soft Palate
– Posterior portion of the roof of the mouth
– An arch-shaped muscular partition between the oropharynx and nasopharynx that is lined by mucous membrane
• Uvula
– a conical muscular process that hangs from the border of the soft palate
– Uvula and soft palate closes off the nasopharynx and oropharynx during swallowing. This prevents swallowed food and liquids from entering the nasal cavity.
– Lateral to the base of the uvula are two muscular folds that run down the lateral sides of the soft palate:
• Palatoglossal arch – anteriorly, extends to the sides of the base of the tongue
• Palatopharyngeal arch – posteriorly, extends to the sides of the pharynx
• Palatine tonsils
– Located between the arches
• Soft Palate
w Posterior portion of the roof of the mouth
w An arch-shaped muscular partition between the oropharynx and nasopharynx that is lined by mucous membrane
• Uvula
w a conical muscular process that hangs from the border of the soft palate
w Uvula and soft palate closes off the nasopharynx and oropharynx during swallowing. This prevents swallowed food and liquids from entering the nasal cavity.
w Lateral to the base of the uvula are two muscular folds that run down the lateral sides of the soft palate:
s Palatoglossal arch – anteriorly, extends to the sides of the base of the tongue
s Palatopharyngeal arch – posteriorly, extends to the sides of the pharynx
• Palatine tonsils
w Located between the arches
II. PHARYNX (throat)
It is a funnel-shaped tube that extends from the internal nares to the esophagus posteriorly and to the larynx anteriorly.
It is composed of skeletal muscle and lined by mucous membrane.
Deglutition – or the movement of food from the mouth into the stomach is achieved by the act of swallowing. It is facilitated by saliva and involves the mouth, pharynx, and esophagus.
Swallowing occurs in three stages:
Voluntary stage: the bolus is forced to the back of the oral cavity and into the oropharynx by the movement of the tongue upward and backward against the palate
Pharyngeal stage:
when the bolus is going into the oropharynx, the involuntary pharyngeal stage begins The respiratory passage closes, and breathing is temporarily interrupted
The bolus stimulates receptors in the oropharynx, which sends impulses into the deglutition center in the brain stem particularly the lower pons and medulla oblongata
The returning impulses causes the soft palate and uvula to move upward to close off the nasopharynx, and the larynx is pulled forward and upward under the tongue. The movement of the larynx pulls the vocal chords together further sealing off the respiratory tract. The bolus passes through the laryngopharynx and esophagus in 1 – 2 seconds
Pharyngeal stage:
when the bolus is going into the oropharynx, the involuntary pharyngeal stage begins The respiratory passage closes, and breathing is temporarily interrupted
The bolus stimulates receptors in the oropharynx, which sends impulses into the deglutition center in the brain stem particularly the lower pons and medulla oblongata
The returning impulses causes the soft palate and uvula to move upward to close off the nasopharynx, and the larynx is pulled forward and upward under the tongue. The movement of the larynx pulls the vocal chords together further sealing off the respiratory tract. The bolus passes through the laryngopharynx and esophagus in 1 – 2 seconds
Esophageal stage
the passage of food from the laryngopharynx into the esophagus is regulated at the entrance to the esophagus by a sphincter called the UES (upper esophageal sphincter).
The elevation of the larynx during the pharyngeal stage of swallowing causes the sphincter to relax, making the bolus enter the esophagus.
During this stage, because of peristalsis (a progression of coordinated contractions and relaxations of the circular muscles and longitudinal layers of the muscularis, wavelike motion) the bolus is push onward and towards the stomach. The passage of solid and semi solid from the mouth to the stomach takes about 4 – 8 seconds; while very soft foods and liquids pass through in about 1 second.
The LES (lower esophageal sphincter), relaxes during swallowing and thus allowing the bolus to pass from the esophagus to the stomach.
III. Esophagus
It is a collapsible tube that lies posterior to the trachea
It is about 25 cm (10 inches) long
It begins at the level of the sixth cervical vertebra and descends through the mediastinum in front of the vertebral column and behind the trachea.
It passes through the diaphragm at the level of the 10th thoracic vertebrae and ends at the cardiac orifice of the stomach at the level of the eleventh thoracic vertebrae
The esophagus has four layers:
Fibrous outer layer: consists of areolar tissue containing many elastic fibers
Muscular layer: comprised of two layers, the outer with fibers running longitudinally and the inner layer consisting of circular fibers
Areolar or muscular layer:connects the mucous and muscular coats and contains the larger blood vessels and nerves, as well as the mucous glands.
mucous membrane
IV. STOMACH
Location:
Inferior to the diaphragm in the epigastric, umbilical, and left hypochondriac region.
J - shaped
Anatomy:
It has four main regions:
• Cardia: superior opening of the stomach
• Fundus: rounded portion
• Body: large central portion
• Pylorus: the region of the stomach that connects to the duodenum
Four layers of the stomach wall:
Outer serous layer
Muscular layer – this consists of three layers of smooth muscle fibers, the outer being longitudinal, the middle circular and the inner oblique
Submucous layer – this is composed of loose areolar tissue
The lining of the mucous membrane – it has numerous folds(rugae), which run longitudinally and flattens out once stomach is full
Functions of the stomach:
Mixes saliva, food, and gastric juice to chyme.
Reservoir for holding food before release into small intestine.
Secretes gastric juice (2000 – 300ml per day), which contains:
• HCl: kills bacteria and denatures proteins in food and stimulates secretion of hormones that promote the flow of bile and pancreatic juice
• Pepsin: protein-digesting enzyme
• Intrinsic factor: aids in absorption of Vitamin B12
• Gastric lipase: aids digestion of triglycerides
Mixes gastrin (a type of hormone that is secreted by the G cell which is located in the pyloric antrum) in the blood
Regulation of Secretion and Motility
Cephalic Phase:
The cephalic phase of gastric digestion consists of reflexes initiated by sensory receptors in the head.
Sight, smell, thought of food
the cerebral cortex and the feeding center in the hypothalamus send nerve impulses to the medulla oblongata
Stimulate the gastric glands to secrete pepsinogen, hydrochloric acid, and mucus into stomach chyme and gastrin into the blood; and increase gastric motility
Gastric Phase
Once food reaches the stomach
The stomach walls distends and a peristaltic movement called mixing waves pass over the stomach every 15 – 25 seconds.
Macerates, mixes the food with gastric juice and reduce the food into a soupy liquid (chyme).
chyme (10 –15 ml) will got to the duodenum passing the pyloric sphincter making the stomach wall less distended
The negative feedback cycle suppresses secretion of gastric juice
Intestinal Phase
This phase is an inhibitory phase wherein it prevents chyme from totally leaving the stomach and overloading the duodenum.
Two hormones are release into the blood that affects the stomach:
• Secretin – decreases gastric secretion
• Cholecystokinin – inhibits stomach emptying
Small Intestine
It is where the major event of digestion and absorption occurs
Length: 3 meters (10 feet)
Divided into three regions:
• Duodenum – the shortest region
Only about 10 inches
Starts at the pyloric sphincter and merges with the jejunum
• Jejunum – about 1 m (3 feet)
• Ileum – measures about 2 m (6 feet)
Muscles of the Small Intestine
Mucosa and submucosa facilitates the process of digestion and absorption
Mucosa – forms a series of fingerlike villi
- each villi has a connective tissue which contains a venule, arteriole and a blood capillary network and a lymphatic capillary
- microvilli
- lined with epithelium that formed intestinal glands which secretes intestinal juice;
- epithelial cells in the mucosa are also goblet cells which secretes mucus
Submucusa – contains duodenal glands that secretes an alkaline mucus that helps neutralize gastric acid in the chyme.
Mechanical digestion in the Small Intestine
Two types of movement in the small intestine
A. Segmentation
B. Are localized, mixing contractions that occur in portions of intestine distended by a large portion of chyme. Chyme is mix with the digestive juices and bring the particles of food into contact with the mucosa for absorption.
C. Segmentation is usually faster in the duodenum at is slower at the ileum.
• Migrating motility complex (MMC)
• Begins when all the food is absorbed and segmentations stops and peristalsis begins.
• Starts at the lower portion of the stomach and pushes chyme forward a long stretch of small intestine before it dries up.
• It takes about 90 – 120 minutes to reach the ileum, so chyme stays in the smal intestine for about 3 – 5hours
Chemical digestion in the Small intestine
The chyme that enters the small intestine contains partially digested carbohydrates, proteins, and lipids
Digestion of carbohydrates:
Starches that are not broken by the salivary amylase re acted on by pancreatic amylase (an enzyme in pancreatic juice) except for cellulose.
Digestion of Proteins:
Protein starts its digestion in the stomach. The following enzymes that act on protein are the following: trypsin, cymotrypsin, carboxypeptidase, and elastase
Digestion of Lipids:
Triglycerides are emulsified by the the bile salts
Absorption
Carbohydrates:
It is absorbed as monosaccharides via a facilitated diffusion.
Proteins:
Is absorbed as amino acids via active transport
Lipids
Absorbed via simple diffusion
Electrolytes
Via an active transport
Vitamins
ADEK (fat soluble): absorbed via simple diffusion
Most B vitamins and Vitamin C: simple diffusion
Vitamin B12: combines with intrinsic factor
Water
Absorb through osmosis
Large Intestine
- It is the terminal portion of the GI tract
Length: 1.5 m (5 feet)
Diameter: 6.5 cm (2.5 in.)
From the anus to the ileum
Parts:
Ileocecal sphincter (valve)
Cecum – a blind pouch
Appendix – a coiled tube attached to the cecum
Colon : ascending, transverse, and descending
Rectum
Anal canal
Mechanical Digestion
The passage of chyme from the ileum into the cecum is regulated by the action of the Ileocecal sphincter.
Presence of food
Gastroileal reflex
Intensifies ileal peristalsis
Pushes chyme in the ileum into the cecum making the cecum distended and contractions t the ileocecal sphincter intensifies
Absorption and Feces formationAbsorption and Feces formation
Absorption and Feces formation
Chyme becomes solid or semi solid because of water absorption and it will becomes as feces.
Approximately 100 – 200 ml of water is reabsorb back.
The large intestine also absorbs ions, sodium and potassium, and some vitamins.
Defecation
Distention of the recital wall stimulates stretch receptors which in return will initiate the defecation reflex.
Send impulses to the sacral spinal chord which will initiate contraction of the rectal muscles thereby increasing the pressure within.
Internal anal sphincter is open.
External anal sphincter is relaxed
Defecation occurs
Major Salivary glands
Where most saliva is secreted
Lies beyond the oral mucosa but has ducts that lead to the oral cavity
3 pairs of major salivary glands:
1. Parotid glands
• Are located inferior and anterior to the ears (between the skin and the masseter muscle)
• Secretes saliva into the oral cavity via a parotid duct
• Contains cells that secrete a watery (serous) liquid containing salivary amylase
2. Submandibular glands
• Found beneath the base of the tongue in the posterior aspect of the floor of the mouth
• Submandibular ducts – run under the mucosa on either side of the floor of the mouth
• Contains cells similar to those found in the parotid glands, plus some mucous cells
Sublingual glands
1. Are superior to the submandibular glands
2. The lesser sublingual ducts, open into the floor of the mouth in the oral cavity
3. It contains mostly mucous cells, so they secrete a much thicker fluid that contributes only a small amylase to the saliva
TONGUE
An accessory digestive organ that is composed of skeletal muscle covered with mucous membrane
Extrinsic muscles:
located on the side of the mouth
moves the tongue from side to side, in and out to maneuver food for for chewing, shape the food into a rounded mass, and force the food to the back of the mouth for swallowing.
Holds the tongue in position
Intrinsic muscles:
alter the shape of the tongue for speech and swallowing
Lingual frenulum:
A fold in the midline of the undersurface of the tongue and is attached to the floor
Limits the movement of the tongue posteriorly
TONGUE
3 MAJOR REGIONS:
CROWN – VISIBLE PORTION ABOVE THE LEVEL OF THE GUM
ROOT - SOCKET
NECK – CONSTRICTIVE JUNCTION OF THE CROWN AND ROOT NEAR THE GUM LINE
COMPOSITION:
Dentin – a calcified connective tissue that gives the teeth its basic shape and rigidity
Pulp cavity – lies within the crown and is filled with pulp
Enamel – the one that covers the dentin
Function:
Cut, tear, and pulverize food to reduce solids to smaller particles for swallowing
LiveR
The heaviest gland in the body
Weighs about 3 bs
And it is the 2nd largest organ of the body
Divisions:
Right lobe
Left lobe – the smaller of the two
Falciform ligament – the one that divides the two lobes
STRUCTURE:
FUNCTIONAL UNIT: LOBULES
Lobules: made up of epithelial cells called hepatocytes
BLOOD SUPPLY:
It receives blood from two sources:
Hepatic artery – receives the oxygenated blood
Hepatic portal vein – receives the deoxygenated blood containing newly absorbed nutrients, drugs, and possibly microbes and toxins from the GIT.
FUNCTION
- Metabolic function:
Desaturation – wherein is breaks down stored fats
Breaks down excess amino acids and converts to urea
Converts excess carbohydrates into fat for storage
Detoxifies drugs and poisons
Synthesizes Vit. A, plasma protein, and prothrombin and fibrinogen
It is the main heat producing organ in the body
Manufactures antibodies, antitoxins, and heparin
- Storage function
Vitamins A and D
Anti anemic factor
Iron from the diet and worn out blood cells
- Secretory function:
Secretes bile
GALBLADDER
It is a pear – shaped organ situated on the undersurface of the right lobe of the liver.
It has a cystic duct that passes backwards and downwards to form the common hepatic duct and together they form the bile duct.
It stores bile
When fatty foods enters the duodenum the sphincter at the entrance to the bile duct relaxes and bile that is stored goes to the intestine
PANCREAS
Location: lies posterior to the greater curvature of the stomach
Parts:
Head - attaches to the duodenum
Body
Tail
Ducts: 2 major ducts found
Pancreatic duct (duct of Wirsung) – larger of the two
• It joins the common bile duct and forms the hepatopancreatic ampulla (ampulla of Vater)
Accessory duct (duct of Santorini)
• Empties directly to the duodenum
Pancreas is made up of small clusters of glandular epithelial cells.
Acini – 99 percent of the pancreas
- exocrine portion of the organ
- Secretes pancreatic juice (a mixture of fluid and digestive enzymes)
- approx 1200-1500/day.
- clear, colorless liquid made up mostly of water, some salts, sodium bicarbonate and some enzymes
enzymes:
s pancreatic amylase – digests CHO
s trypsin, chymotrypsin, carboxypeptidase, and elastase – digests CHON
s Pancreatic lipase – digests triglycerides
Islet of Langerhans – 1% of the pancreas
s endocrine portion of the of the pancreas
s Secretes the following hormones: glucagon, insulin, somatostatin, and pancreatic polypeptide
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