Digestive System: Structure and Function

General Structure and Functions of the Digestive System

• Organs of the digestive system ingest food.
• Organs transport food.
• Organs digest food into smaller usable components.
• Organs absorb necessary nutrients into the bloodstream.
• Organs expel waste products from the body.
• The digestive system is composed of digestive and accessory digestive organs.
• Accessory digestive organs do not form the GI tube and can develop as outgrowths, connecting to the GI tract through ducts.
• Accessory digestive organs assist the GI tract with digestion.
• Accessory digestive organs include teeth, tongue, salivary glands, liver, gallbladder and pancreas.

GI tract organs

• These organs include the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.
• It is a continuous tube approximately 30 feet (9 to 10 meters) long, extending from the mouth to the anus.
• Smooth muscle in the wall pushes materials from one end to the other for motility.

Oral Cavity (Mouth)

• It serves as the entrance to the GI tract.
• The initial site of digestion, involving mechanical digestion via mastication and chemical digestion via enzymes in saliva.
• The two regions of the oral cavity are the vestibule and the oral cavity proper.
• The vestibule is the space between the cheeks or lips and the gums.
• Lips contain the orbicularis oris muscle.
• Lips possess keratinized stratified squamous ET.
• The labial frenulum is a band of fibroelastic tissue that originates in the lip and inserts in the attached gingiva to the maxilla (superior labial frenulum) or the mandible (inferior labial frenulum).
• Gingivae, or gums are made of dense regular CT and nonkeratinized ET.
• Bound anteriorly by the teeth and lips.
• Bound posteriorly by the oropharynx.
• Hard and soft palates form the superior boundary.
• The floor/inferior surface, contains the tongue and the mylohyoid muscle covered with mucosa.
• Cheeks, containing buccinator muscles, form the lateral walls.

Palate

• The hard palate makes up the anterior two-thirds of the palate and is hard and bony.
• The soft palate makes up the posterior one-third, is soft and muscular, primarily contains skeletal muscle, and has the uvula extending inferiorly.

Tongue

• It is an accessory digestive organ formed from skeletal muscle, covered with lightly keratinized stratified squamous epithelium.
• Forms the bolus, assists with swallowing.
• It attaches to the floor of the oral cavity via the lingual frenulum, on its inferior surface.

Salivary Glands

• Collectively, salivary glands produce and secrete saliva, which assists in the initial activities of digestion.
• The volume of saliva secreted daily ranges between 1.0 and 1.5 L.
• Three pairs of salivary glands exist: the parotid, submandibular, and sublingual glands.

Parotid Glands

• The parotid glands are the largest salivary glands and are located anterior and inferior to the ear, partially overlying the masseter muscle.
• They produce about 25–30% of saliva.
• Secretions are conducted through the parotid duct to the oral cavity.

Submandibular Glands

• The 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

• The sublingual glands are inferior to the tongue and open onto the inferior surface of the oral cavity.
• They contribute only about 3–5% of the total saliva.

Teeth

• Responsible for mastication, which is the first part of mechanical digestion.
• A tooth has an exposed crown, a constricted neck, and one or more roots.
• 20 deciduous teeth, also called "milk teeth," erupt between 6 months and 30 months after birth and are eventually lost and replaced by 32 permanent teeth.

Pharynx

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

Esophagus

• The esophagus is a muscular tube, measuring 25 cm in length, beginning from C6 to the cardiac opening of stomach at T10.
• The Esophagus passes in the neck, thorax, and abdomen.
• It lies in the median plane, front vertebral column, behind trachea, aortic arch,, bronchus and heart.

Esophagus Muscles

• The upper 1/3 of the esophagus is voluntary muscle.
• The lower 1/3 is involuntary muscle.
• The middle 1/3 is mix of the 2 muscle types.
• The superior esophageal sphincter consists of skeletal muscle, located where the pharynx and esophagus meet.
• The inferior esophageal sphincter is the orifice between the esophagus and stomach.

Organization of Abdominal GI Organs

• Peritoneum has Parietal peritoneum, Visceral peritoneum, and a Peritoneal cavity.
• Intraperitoneal organs include the stomach, liver, spleen, tail of pancreas, first part of the duodenum, jejunum, ileum, appendix, transverse and sigmoid colon.
• Retroperitoneal organs include the kidneys, adrenal glands, and ureters, pancreas (except for the tail), ascending and descending colon and the aorta.

Stomach

• A dilated part of the digestive system.
• A J-shape organ.
• Lies mostly in the upper left quadrant of the abdominal cavity (ULQ).
• It is inferior to the diaphragm and anterior to the spleen and pancreas.
• It has four region, the cardia, fundus, body and pylorus
• Rugae are longitudinal folds on its internal surface.
• Greater and Lesser curvature.
• Connects to the oesophagus through its upper opening and to the duodenum through its lower opening.
• The upper and lower openings remain closed at the time of gastric digestion.

Stomach Regions

• Fundus: uppermost portion, dome shaped, extends above the level of the cardiac sphincter, typically filled with air.
• Cardia: small region near the cardiac sphincter where the esophagus meets the stomach.
• Body: middle portion, located between the fundus and pyloric regions, the largest region with mucous folding
• Pylorus: the terminal portion that joins with duodenum at pyloric orifice, contains pyloric antrum, pyloric canal and pylorus,
• The Pyloric sphincter is a true anatomical sphincter that acts as a valve preventing back flow of food from the small intestine.

Small Intestine

• Finishes chemical digestion.
• Responsible for absorbing most nutrients.
• Has thin walls with a tube which is about 6 meters (20 feet) in length.
• Extends from the pylorus of the stomach to the cecum of the large intestine.

Adaptation of Small Intestine

• Its absorptive function is achieved by its 6 meter length.
• Its Plicae circularis (Folds of mucosa & submucosa) increases surface area 3 folds.
• Its Villi (outgrowth of intestinal mucosa = epithelium + CT corium ) increase surface area 10 folds.
• Its Microvilli (finger-like projections of the apical cell membrane) increase surface area 20 folds.

The Small Intestine Segments

• The Duodenum is the first segment of the small intestine roughly 25 centimeters (10 inches) long and is almost entirely retroperitoneal.
• Duodenum contains the superior, descending, horizontal and ascending parts.
• The jejunum is the middle region of the small intestine, approximately 2.5 meters (7.5 feet).
• The ileum is the last region, approximately 3.6 meters (10.8 feet) that forms three-fifths of the small intestine and terminates at the ileocecal valve.

Duodenum

• First segment of the small intestine approximately 25 cm (10 inches) long.
• Almost entirely retroperitoneal.
• Connects with the stomach via the pyloric sphincter.
• Located posterior to the abdominal wall and is C shaped around the head of the pancreas.
• Parts of duodenum: superior, descending, horizontal and ascending parts.
• Second part (descending part) of the duodenum has openings for the common bile duct, major duodenal papillae, minor duodenal papillae, and pancreatic duct.

Jejunum

• It is the middle region of the small intestine, approximately 2.5 meters (7.5 feet), making up two-fifths of its total length.
• The primary region for chemical digestion as well as nutrient absorption

Ileum

• Last region of the small intestine measuring about 3.6 meters (10.8 feet).
• It forms approximately three-fifths of the small intestine.
• Terminates at the ileocecal valve which is the sphincter that controls the entry of materials into the large intestine..

Peyer’s Patches

• Lymphatic collections are present in the small intestine.
• It is located in the mucous membrane and is more prominent in the ileum.

Jejunum & ileum

• Jejunum is in the Upper part of abdomen while the ileum is in the Lower part of abdomen.
• The length of Jejunum is Proximal 2/5 while that of the ileum is Distal 3/5
• The wall of the Jejunum is Thick while that of the ileum is Thin.
• Diameter is Wide for the Jejunum, but Narrow for the ileum.
• The Jejunum is Reddish while the ileum is Yellowish.
• 6-circular folds (Plicae circularis) are Numerous in the Jejunum, but Few in the ileum.
• The Jejunum has Many villi but the lleum has Few of them.
• The Jejunum is Absent of Peyer’s patches while those are Present in the ileum.

Large Intestine

• It is a muscular tube about 1.5 m.
• It has wider in diameter and shorter comparing with than the small intestine.
• Responsible for reabsorption of water and excretion of feces (defecation ).
• Part of the large intestine: Cecum, Appendix( about 9 cm average ), Ascending Colon, Transverse Colon, Descending Colon, Sigmoid Colon( S_ shape part about 35 cm), Rectum( consist of 3 pouches, about 12 cm), Anal Canal( terminal part, about 4 cm ).
• The transverse colon exhibits two flexures the Right colic flexure or hepatic and Left colic flexure or splenic.
• Characteristic features: taeniae coli, haustra, epiploic appendages.

Vermiform Appendix

• Length: 7.5 – 12.5 cm
• Position: it is attached dorsomedially to the end of the cecum.
• Base: Mc Burney’s point
• Appendix lacks taeniae, haustra, semilunar folds and appendices epiploicae.
• Apex: variable where it maybe retrocecal, pelvic, Paracaecal or Preileal, Retroileal.
• The Vermiform Appendix is supplied blood by the appendicular artery as innervated by the nerve T10
• Appendicitis: Pain: vague, referred to umbilicus (T10)

Rectum

• Distal portion of large intestine, between sigmoid colon and anal canal.
• It storage of feces before defecation.
• The Rectum is situated in in the posterior part of lesser pelvis, front of sacrum and coccyx, behind urinary bladder in males and behind uterus in female
• Doesn't have taenia coli, Haustra, and Epiploic appendages.

Anal Canal

• It is the terminal part of large intestine.
• The canal is located in perineum, below the pelvic diaphragm.
• It is surrounded by the inner involuntary and outer voluntary sphincter
• There are no taenia coli, sacculation, and Epiploic appendages in the anal canal.

Difference Between Small and Large Intestine

The Liver

• Largest gland in the body, weighing about 1½ - 2.3Kg.
• Wedge-shaped, with the apex directed to the left and the base to the right.
• Located in the upper right part of the abdominal cavity.
• The liver is composed of four incompletely separated lobes, which includes the right, left, caudate, and quadrate lobe.

Liver Ligaments

• Falciform ligament: attaches the anterior surface of the liver to the anterior abdominal wall, a sickle-shaped ligament.
• The ligamentum teres, a piece of remnant from the umbilical vein that is located on its free edge.
• Coronary ligament: attaches the superior surface of the liver to the inferior surface of the diaphragm, which also defines the liver's exposed area.
• Triangle ligaments: triangle ligaments on the liver's left and right lobes, formed by the union of the anterior and posterior folds

Functions of The Liver

• Produce bile, a greenish fluid that breaks down fats into small droplets to assist in chemical digestion
• Detoxify drugs, metabolites, and poisons.
• Store excess nutrients and vitamins and release them when they are needed.
• Synthesize blood plasma proteins such as albumins, globulins, and proteins required for blood clotting.
• Phagocytize debris in the blood.
• Help break down and recycle components of aged erythrocytes, re-purposing damaged or worn-out formed elements.

Gallbladder

• Has The Pear shape.
• Gallbladder Parts: Fundus, body, neck, Cys, and duct.
• Relations:-anteriorly: liver + abdominal wall posteriorly: duodenum & Transvers colon.
• About 500 ml of bile is secreted by the liver daily, consisting of water, mineral salts, mucus, bile pigments mainly bilirubin, bile salts, and cholesterol.

Function of Gallbladder

• Concentrates and store bile produced by the liver until needed for digestion
• Typically can hold approximately 40 to 60 milliliters of concentrated bile

Biliary Tract

• Hepatic duct
• Cystic duct
• Common bile duct
• The structure connects with pancreatic duct, forming the hepatopancreatic duct which ends by hepato-pancreatic ampulla in the middle section of the duodenum

Biliary Apparatus.

• A network of thin ducts that carry bile from the liver and gallbladder to the duodenum.
• left and right lobes of the liver drain bile into the left and right hepatic ducts, respectively,
• the left and right hepatic ducts merge to form a single common hepatic duct, and the cystic duct attaches to the common hepatic duct and carries bile to and from the gallbladder

Pancreas

• Acts as a mixed gland because it exhibits both endocrine and exocrine functions.
• Located behind the stomach that extend across, intersecting the abdomen from the duodenum to the spleen: it is retroperitoneal.
• Four parts: Head, Neck, body and Tail.
• Endocrine functions are performed by the pancreatic islets.
• Exocrine activity results in the secretion of digestive enzymes, known as pancreatic juice, that is secreted into the duodenum.

Explore the digestive system's organs and their roles in food ingestion, transport, digestion, nutrient absorption, and waste expulsion. Learn about the GI tract organs and accessory digestive organs.