Digestion & Absorption • Digestion is the process of breaking down food into molecules that are small enough to enter the body cells. • Absorption is the passage of these smaller molecules through the plasma membrane of cells lining the stomach and intestines into the blood and lymph.
Digestive System • The organs that perform the functions of digestion and absorption are collectively referred to as the digestive system. • Gastroenterology deals with the structure, function, diagnosis and treatment of diseases of the stomach and intestines. • Proctology deals with the diagnosis and treatment of disorders of the rectum and anus.
Components Of The Digestive System • The gastrointestinal (GI) tract or alimentary canal is a continuous tube that extends from the mouth to the anus through the ventral body cavity. • Organs of the GI tract include the mouth, most of the pharynx, esophagus, stomach, small intestine, and large intestine. • The length of the GI tract in a cadaver is about 9m (30 ft). It is shorter in a living person due to clonus.
Components Of The Digestive System • Accessory digestive organs include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas. • The teeth aid in the physical breakdown of food and the tongue assists in chewing and swallowing. • The other accessory digestive organs never come into direct contact with the food. They produce or store secretions that flow into the GI tract and aid in the chemical breakdown of food.
Functions Of The Digestive System • Ingestion • Secretion • Mixing and propulsion • Digestion • Absorption • Defecation
Ingestion • This process involves taking foods and liquids into the mouth (eating).
Secretion • Cells within the walls of the GI tract secrete about 7 liters of water, acid, buffers, and enzymes into the lumen of the GI tract daily.
Mixing & Propulsion • Alternating contraction and relaxation of smooth muscle in the walls of the GI tract mix food and secretions and propel them toward the anus. • This is referred to as motility.
Digestion • Mechanical digestion. • The teeth cut and grind food. • The smooth muscles of the stomach and small intestine churn the food to help it dissolve and mix with enzymes.
Digestion • Chemical digestion. • The large carbohydrate, lipid, protein, and nucleic acid molecules in food are split into smaller molecules by hydrolysis. • Digestive enzymes produced by the salivary glands, tongue, stomach, pancreas, and small intestines catalyze these catabolic reactions. • Amino acids, cholesterol, glucose, vitamins, minerals, and water can be absorbed without chemical digestion.
Absorption • Absorption is the entrance of ingested and secreted fluids, ions, and small molecules that are products of digestion into the epithelial cells lining the lumen of the GI tract. • The absorbed substances pass into the blood or lymph and circulate to all cells of the body.
Defecation • Substances that were not absorbed leave the body through the anus in a process called defecation. • These substances include wastes, indigestible substances, bacteria, cells sloughed from the GI tract, and digested materials that were not absorbed. • The eliminated material is called feces.
Layers Of The GI Tract • Mucosa • Submucosa • Muscularis • Serosa
Mucosa • The mucosa (inner lining) is a mucous membrane. • It is composed of a layer of epithelium in direct contact with the contents of the GI tract, areolar connective tissue, and a thin layer of smooth muscle (muscularis mucosae).
Mucosa • Epithelium. • Epithelium in the mouth, phaynx, esophagus, and anal canal is nonkeratinized stratified squamous epithelium and serves a protective function. • Epithelium in the stomach and intestines is simple columnar epithelium and functions in secretion and absorption.
Mucosa • Lamina propria. • Areolar connective tissue containing many blood and lymphatic vessels, which are routes through which nutrients are absorbed. • Mucosa-associated lymphatic tissue is also present to protect against microbes. • Muscularis mucosa. • A thin layer of smooth muscle fibers which creates folds in the stomach and small intestine to increase surface area.
Submucosa • The submucosa consists of areolar connective tissue that binds the mucosa to the muscularis.
Submucosa • It contains blood and lymphatic vessels that receive absorbed food molecules.
Submucosa • It also contains the submucosal plexus (plexus of Meissner) which is an extensive network of neurons. • These neurons are part of the enteric nervous system or “brain of the gut”. • They regulate movements of the mucosa and vasoconstriction of the blood vessels. • The nerves innervate secretory cells of the mucosal and submucosal glands.
Muscularis • The muscularis of the mouth, pharynx, and superior and middle parts of the esophagus contains skeletal muscle that produces voluntary swallowing. • Skeletal muscle also forms the external anal sphincter, which permits voluntary control of defecation.
Muscularis • The rest of the GI tract muscularis contains smooth muscle. • The myenteric plexus (plexus of Auerbach) is within the layers of smooth muscle. It is also part of the enteric nervous system and controls GI motility (i.E. GI frequency and strength of contraction).
Serosa • The serosa is the superficial layer of the portions of the GI tract that are suspended in the abdominopelvic cavity. • Inferior to the diaphragm it is called the visceral peritoneum.
Peritoneum • The peritoneum is divided into parietal peritoneum which lines the wall of the abdominopelvic cavity and visceral peritoneum which lines some of the organs in the cavity. • The space between the parietal and visceral peritoneum is called the peritoneal cavity. • In some diseases, the peritoneal cavity becomes distended by the accumulation of fluid in a condition called ascites.
Retroperitoneal • Some organs lie on the posterior abdominal wall and are only covered by peritoneum on their anterior surface. • These organs are said to be retroperitoneal and include the kidneys and pancreas.
Peritoneum Functions • The peritoneum contains large folds that weave between the viscera. • These folds bind the organs to each other and to the walls of the abdominal cavity. • They also contain blood vessels, lymphatic vessels, and nerves that supply the abdominal organs.
Peritoneal Folds • Greater omentum. • Falciform ligament. • Lesser omentum. • Mesentery. • Mesocolon.
Greater Omentum • The largest peritoneal fold. • It drapes over the transverse colon and coils of the small intestine like a “fatty apron”. • It contains a considerable amount of fatty tissue. • It can greatly expand with weight gain, giving rise to the characteristic “beer belly”. • There are many lymph nodes in the greater momentum.
Falciform Ligament • The falciform ligament attaches the liver to the anterior abdominal wall and diaphragm. • The liver is the only digestive organ that is attached to the anterior abdominal wall.
Lesser Omentum • The lesser omentum suspends the stomach and duodenum from the liver. • It contains some lymph nodes.
Mesentery • The mesentery is fan-shaped and binds the small intestine to the posterior abdominal wall. • Blood vessels, lymphatic vessels, and lymph nodes lie between the two layers of mesentery.
Mesocolon • The mesocolon binds the large intestine to the posterior abdominal wall. • It carries blood vessels and lymphatic vessels. • The mesentary and mesocolon work together to loosely hold the intestines in place. This allows for great movement to allow them to mix food and propel food along the GI tract.
Peritonitis • Peritonitis is an acute inflammation of the peritoneum. • Contamination of the peritoneum by infectious microbes causes it.
Peritonitis • This is the result of accidental or surgical wounds in the abdominal wall. • Perforation or rupture of abdominal organs also causes this. • When inflamed peritoneal surfaces rub together, peritonitis can result.
Mouth • The mouth is also referred to as the oral or buccal cavity. • It is formed by the cheeks, hard and soft palates, and tongue.
Mouth • The lips (labia) are fleshy folds surrounding the opening of the mouth. • The labial frenulum is a midline fold of mucous membrane that attaches the inner surface of each lip to its corresponding gum. • The orbicularis oris and buccinator muscles keep food between the upper and lower teeth to assist in chewing.
Mouth • The vestibule of the oral cavity is the space bounded by the cheeks and lips externally and the teeth and gums internally. • The oral cavity proper is the space that extends between the teeth and gums to the fauces (opening between the oral cavity and throat).
Mouth • The hard palate is the anterior portion of the roof of the mouth and is formed by the maxillae and palatine bones.
Mouth • The soft palate is the posterior portion of the roof of the mouth. It is an arch-shaped muscular partition that is lined by mucous membrane. • The uvula is a conical muscular process hanging from the free border of the soft palate. During swallowing, the uvula and soft palate are drawn superiorly and closing off the nasopharynx to prevent foods from entering the nasal cavity.
Salivary Glands • A salivary gland is any cell or organ that releases saliva into the oral cavity. • Saliva cleanses the mouth and teeth. • When food enters the mouth, secretion of saliva increases. • Saliva lubricates, dissolves, and begins the chemical breakdown of food.
Minor Salivary Glands • Labial glands in the lips. • Buccal glands in the cheeks. • Palatal glands in the palate. • Lingual glands in the tongue.
Major Salivary Glands • These glands lie beyond the oral mucosa and empty their secretions into ducts that lead to the oral cavity. • Parotid glands and parotid duct. • Submandibular glands and submandibular ducts. • Sublingual glands and lesser sublingual ducts.
Composition & Functions Of Saliva • Saliva is 95/5% water and 0.5% solutes. • Lysozyme – a bacteriolytic enzyme. • Salivary amylase – a digestive enzyme that acts on starch.
Salivation • Salivation is the secretion of saliva. • It is controlled by the autonomic nervous system. • The feel and taste of food are potent stimulators of salivary gland secretions.