Digestive System: Functions & Organs

• The digestive system is a critical body system for processing food and nutrients
• Lecture # 5-6 covers the digestive system

Functions of the Digestive System

• It involves ingesting food, transporting it, and breaking the food down into smaller, usable parts
• It also includes absorbing the necessary nutrients into the bloodstream
• Finally, it expels waste products from the body

Categories of Digestive Organs

• Digestive organs
• Accessory digestive organs

GI Tract Organs

• Oral cavity
• Pharynx
• Esophagus
• Stomach
• Small intestine
• Large intestine
• A continuous tube, about 30 feet (9-10 meters) long, running from the mouth to the anus.
• Smooth muscle in the wall of the GI tract is responsible for motility
• It helps push materials from one end to the other.

Accessory Digestive Organs

• These organs do not form the GI tube itself
• They develop as outgrowths connected to the GI tract, some by ducts
• They assist the GI tract for digestion of food.
• Include teeth, tongue, salivary glands, liver, gallbladder and pancreas

Oral Cavity

• This is the entrance to the GI tract
• It is the initial site of digestion, both mechanical via mastication, and chemical via enzymes in saliva.
• Two regions comprise of it:
• • The vestibule, which is the space between the cheeks or lips and the gums
• • The oral cavity proper.
• Lips consist of the orbicularis oris muscle, keratinized stratified squamous ET
• The labial frenulum is a band of fibroelastic tissue that originates in the lip
• It inserts in the attached gingiva to the maxilla (Superior labial frenulum) or the mandible (Inferior labial frenulum).
• Gingivae, or gums, consist of dense regular CT and nonkeratinized ET
• The oral cavity is bounded anteriorly by the teeth and lips, and posteriorly by the oropharynx
• The superior boundary is formed by the hard and soft palates.
• The floor, or inferior surface, of the oral cavity contains the tongue and the mylohyoid muscle covered with mucosa
• The lateral walls are formed by the cheeks, which contain buccinator muscles

Palate

• The hard palate makes up the anterior two-thirds of the palate. It is hard and bony
• Posterior one-third of the palate is the soft palate
• The soft palate is soft, muscular, and primarily composed of skeletal muscle
• The uvula extends inferiorly from the posterior part of the soft palate.

Tongue

• This is an accessory digestive organ
• It is formed from skeletal muscle covered with lightly keratinized stratified squamous epithelium
• It forms the bolus and performs important functions in swallowing
• The inferior surface attaches to the floor of the oral cavity via the lingual frenulum

Salivary Glands

• They collectively produce and secrete saliva, a fluid that assists in the initial activities of digestion
• Volume of saliva secreted ranges between 1.0 and 1.5 L daily
• Three pairs of salivary glands: 1- parotid glands, 2- submandibular glands, and 3- sublingual glands
• Parotid Glands are the largest salivary glands
• They are located anterior and inferior to the ear and partially overlie the masseter muscle
• They produce about 25–30% of saliva, which flows via the parotid duct to the oral cavity
• Submandibular Glands: Are inferior to the body of the mandible
• They produce most of the saliva (about 60–70%)
• Their ducts open through a papilla in the floor of the mouth
• Sublingual Glands: Are located inferior to the tongue
• They open onto the inferior surface of the oral cavity and contribute only about 3–5% of the total saliva

Teeth

• They're responsible for mastication, which is the first part of mechanical digestion
• A tooth has an exposed crown, constricted neck, and one or more roots
• There are two sets:
• • 20 deciduous teeth (also called “milk teeth”), which erupt between 6 months and 30 months after birth
• • They are eventually lost and replaced by 32 permanent teeth

Pharynx

• The Fibromuscular tube extends from the skull base to the sixth cervical (C6) vertebra
• Lies behind the nasal cavity, mouth, and larynx and is wider superiorly
• It is a mix of respiratory and digestive functions
• It allows passage of air and food

Esophagus

• It is a muscular tube that is 25 cm in length.
• It begins from the sixth cervical vertebra (C6) to the cardiac opening of the stomach at the tenth thoracic vertebra (T10)
• It passes in the neck, thorax, and abdomen
• It lies in the median plane in front of the vertebral column, and behind the trachea, aortic arch, and bronchus

Esophagus Muscles

• Voluntary upper 1/3
• Involuntary lower 1/3
• Mixed in the middle
• The superior esophageal sphincter consists of skeletal muscle where the pharynx and esophagus meet
• The Inferior esophageal sphincter is the orifice between esophagus and stomach

Stomach

• It is a dilated part of digestive system with j-shape organ.
• It mostly lies in the upper left quadrant of the abdominal cavity
• Located just inferior to the diaphragm, and anterior to the spleen and pancreas.

Stomach Regions

• Cardia
• Fundus
• Body
• Pylorus (Antrum – Canal – Sphincter)
• The stomach has curvature
• • Greater curvature
• • Lesser curvature
• The Rugae are longitudinal folds on internal surface
• Its upper opening is connected to the esophagus
• The lower opening is connected to the duodenum
• Both openings remain closed during gastric digestion

Stomach Regions Detail

• Fundus or fundic region - This is the uppermost portion of the stomach. It is dome shaped
• It extends above the level of the cardiac sphincter, and it’s normally filled with air
• Cardia or cardiac region - This is a small region very near the cardiac sphincter where the esophagus meets the stomach
• Middle portion of the stomach is the Body
• Located between the fundus and pyloric region, this is the largest region
• Empty stomach shows mucous folding
• Pyloric part - This is the terminal portion of the stomach
• It joins with duodenum at pyloric orifice and has three parts:
• • Pyloric antrum
• • Pyloric canal
• • Pylorus
• The Pyloric sphincter is a true anatomical sphincter acting as a valve preventing back flow of food from the small intestine

Small Intestine

• It finishes chemical digestion
• Responsible for absorbing most of the nutrients.
• A thin-walled tube about 6 meters (20 feet) in the length and is coiled
• Extends from the pylorus of the stomach to the cecum of the large intestine

Small Intestine - Absorptive Function

• 6 m long
• Plicae circularis (Folds of mucosa & submucosa), increases surface area by 3 folds
• Villi: (outgrowth of intestinal mucosa which is epithelium + CT corium ) increase surface area 10 folds
• Microvilli: (finger-like projections of the apical cellmembrane) increase surface area 20 folds

Small Intestine Regions

• Duodenum: It is the first segment of the small intestine
• Jejunum: It is the middle region of the small intestine and primary region for chemical digestion and nutrient absorption
• Ileum: It is the last region of the small intestine and terminates at the ileocecal valve --Sphincter controls the entry of materials into the large intestine
• The Duodenum
• • First segment, approximately 25 centimeters (10 inches) long
• • Almost entirely retroperitoneal and connects to the stomach via the pyloric sphincter.
• • Site is posterior abdominal wall, C shape around the head of pancreas
• The Four Parts of duodenum are: 1st Superior part, 2nd Descending part, 3rd Horizontal part, and 4th Ascending part
• Second (Descending part) has opening for common bile duct, major/minor duodenal papillae, and pancreatic duct.
• The Jejunum
• • It is middle region of the small intestine, approximately 2.5 meters (7.5 feet)
• • It makes up approximately two-fifths of the small intestine’s total length and primary region for chemical digestion and nutrient absorption
• The lleum
• • Its the last region of the small intestine that is about 3.6 meters (10.8 feet) in length that forms approximately three-fifths of the small intestine.
• • Terminates at the ileocecal valve, which is a sphincter that controls the entry of materials into the large intestine
• Peyer’s Patches: lymphatic collections present in the small intestine
• The Ileum is prominent in: mucous membrane
• Jejunum & Ileum have the following characteristics:
• • Jejunum: Upper part of abdomen, Proximal 2/5Length, Thick wall, wide diameter, reddish colour, Numerous 6-circular folds (Plicae circularis), many Villi, and absent Peyer’s patches
• • Ileum: Lower part of abdomen, Distal 3/5 Length, Thin wall, narrow diameter, Yellowish color, Few 6-circular folds (Plicae circularis), Few Villi, and Present Peyer’s patches

Large Intestine

• It is a muscular tube about 1.5 m long with a wider diameter than the small intestine
• It provides reabsorption of water and excretion of feces (defecation)
• Consists of Cecum, Appendix (about 9 cm in average), Ascending Colon, Transverse Colon, Descending Colon, Sigmoid Colon(S_shape part about 35 cm), Rectum(consist of 3 pouches, about 12 cm), and Anal Canal( terminal part about 4 cm )
• Transverse colon has two flexures:
• • Right colic flexure or hepatic
• • Left colic flexure or splenic
• Defecation: Execration of undigested food as feces “.
• Characteristic features:
• • Taeniae coli: three bands of smooth muscle along the whole wall
• • Haustra: series of dilatation present in the wall of the large intestine caused by contraction of taeniae coli.
• • Epiploic appendages are small Fat-filled pouches of visceral peritoneum
• • In contrast to the cecum and the rest of the colon the appendix is lacks taeniae, haustra, semilunar folds and appendices epiploicae.
• --The Base of Base is Mc Burney’s point and Vertex is variables
• -The rectum is distal portion of large intestine between sigmoid colon and anal canal
• The rectum does not have taenia coli, Haustra, and Epiploic appendages, but stores feces before defecation
• Situated in the posterior of the lesser pelvis where infront of sacrum and coccyx.
• Its behind urinary bladder in males and behind uterus in female.
• The Anal canal: It is the terminal part of large intestine located in perineum and below the pelvic diaphragm.
• This surrounded by: Inner involuntary sphincter and Outer voluntary sphincter
• It is not having taenia coli, sacculation, and Epiploic appendages

Difference between small and large intestine

• Length: Small-6 meteres, Large-1.5 meters
• Lumen: Small- narrower, Large - wider
• Mobility: Small- more, Large- less
• Small intestine has villi but large intestine don't
• Small intestine absent Peyer’s patches but large intestine has
• Small intestine absent Taenia coli but large intestine has
• Small intestine absent appendice epiploicae but large intestine has
• Small intestine absent Sacculation but large intestine has

Accessory Digestive Organ Liver

• It is the largest gland in the body (1½ - 2.3Kg)
• This is a wedge with apex is directed to the left and the base is directed to the right side.
• Situated in upper right part of abdominal cavity and composed of four incompletely separated lobes:Right/Left/Caudate/Quadrate lobe
• Liver Ligaments include:
• • Falciform ligament: attach the anterior surface of the liver to the anterior abdominal wall; a sickle-shaped ligament
• • Ligamentum teres: a piece of the umbilical vein located on its free edge
• • Coronary ligament: attaches superior surface of the liver to the inferior surface of the diaphragm and that defines the liver's exposed area. This ligament consists of the anterior and posterior folds.
• • Triangle ligaments:triangle ligaments on the liver's that both Left and right lobes that are formed by the anterior and posterior folds which combined the anterior and posterior layers of the coronary ligament to produce: left triangular ligament

Liver Functions

• Produce bile.
• This greenish fluid that breaks down fats into small droplets that assist in their chemical digestion
• Detoxify drugs, metabolites, and poisons.
• Store excess nutrients & vitamins & release them when they are needed.
• Synthesize blood plasma proteins such as albumins, globulins & proteins required for blood clotting.
• Phagocytize debris in the blood and help break down and recycle components of aged erythrocytes & damaged or worn-out formed elements.
• The Portal triad (area): area that is isolated from liver parenchyma by limiting plate of modified hepatocytes
• • Portal triad includes branches of portal vein (blood from sup.& inf.mesenteric, splenic veins) but empties into sinusoids
• • Branch of hepatic artery:oxygenated blood from coeliac trunk of aorta that empties into sinusoids
• • The gallbladder concentrates 500 ml daily.
• Composition consists of water, mineral salts, mucus, bile pigments mainly bilirubin, bile salts, cholesterol

Accessory Digestive Organ Gallbladder

• This is pear shaped whose parts; Fundus, body, neck, Cys duct.
• Anterior relations: It’s found in liver + abdominal wall while posterior relations: duodenum & Transvers colon.
• The BILIARY TRACT:-Hepatic/Cystic/Common bile duct
• All the above connects with pancreatic duct forming hepato-pancreatic duct and ends by hepato-pancreatic ampulla in the middle part of duodenum Biliary apparatus: network of thin ducts to carry bile from liver, and gallbladder to the duodenum.

Biliary Apparatus Components

• Left/right lobes of the drain bile into the left/right hepatic ducts, respectively.
• the left and right hepatic ducts will merge to form single common hepatic duct
• The cystic duct (attaches to the common hepatic duct) and carries bile to and from the gallbladder Pancreas
• It is a mixed gland (exhibits both endocrine and exocrine functions)
• Located behind the stomach extends cross the abdomen from duodenum to spleen
• Retroperitoneal
• It has four parts: Head/Neck/body/Tail
• Endocrine functions are performed by the pancreatic islets
• Exocrine activity results in the secretion of digestive enzymes, called pancreatic juice, into the duodenum

Learn about the digestive system, a vital body system responsible for processing food and nutrients. This lesson covers the functions, digestive & accessory digestive organs. Includes the GI Tract organs, such as the oral cavity, pharynx, esophagus, stomach, and intestines.