Chapter 26 Digestive System Flashcards
Digestive System
Consists of the organs comprising the gastrointestinal tract (stomach, small intestines) and the accessory digestive structures (liver, pancreas), which function to obtain nutrients from our diet.
Digestive System: 2 Categories of Organs
- Gastrointestinal Tract: form a continuous tube that includes the oral cavity (mouth), pharynx (throat), esophagus, stomach, small intestine, and large intestine. It ends at the anus. It is 30 feet in length in an adult cadaver- due to smooth muscle tone, it is shorter in a living individual. The disassembly of molecules for absorption occurs within the lumen of the GI tract, and optimal digestion and absorption are dependent upon regulating and maintaining the environmental conditions within this space. **Materials within the lumen of the GI tract are not considered part of the body until they are absorbed.
- Accessory digestive organs are connected to the GI tract and develop as outgrowths from the tract. These organs assist in the breakdown of food. Accessory digestive glands produce secretions that empty into the lumen of the GI tract and include salivary glands, liver, and pancreas. Other accessory digestive organs are not glands. They include teeth and tongue, which participate in the chewing and swallowing of food, and the gallbladder, which concentrates and stores the secretions (bile) produced by the liver.
Digestive System: 6 main functions
- Ingestion - introduction of solid and liquid nutrients into the oral cavity. It is the first step in the process of digesting and absorbing nutrients.
- Motility - both involuntary muscular contractions (by skeletal muscle) and involuntary muscular contractions (by smooth muscle) for mixing and moving materials through the GI tract.
- Secretion - is the process of producing and releasing substances that facilitate both digestion and the movement of contents within the GI tract. Secretions are produced by both the accessory digestive glands (salivary glands, liver, pancreas) and the wall of the GI tract.
- Digestion is the breakdown of ingested food into smaller components that may be absorbed from the GI tract. Can be mechanical or chemical. Mechanical is the breaking of ingested material into smaller pieces without changing its chemical structure (no enzymes are involved). Example ice cube being crushed into ice chips. Chemical involves the activity of specific enzymes to break down complex molecules into smaller molecules so that they can be absorbed. (**also performed by bacteria within the large intestine.)
- Absorption - involves membrane transport of digested molecules, electrolytes, vitamins, and water across the epithelial lining of the GI tract into the blood or lymph. Occurs within the small intestine.
- Elimination - is the expulsion of indigestible components through the anal canal.
GI tract: 4 tunics (innermost - outermost)
Mucosa, submucosa, muscularis, adventitia (serosa)
- Mucosa - inner-lining mucous membrane. Consists of epithelium, an underlying layer called the lamina propria, and a thin layer of muscular mucosae. Muscularis mucosae facilitates the release of secretions from the mucosa into the lumen and increases contact of materials in the lumen with the epithelial layer of the mucosa for more efficient absorption.
- Submucosa - composed of areolar and dense irregular connective tissue. Blood vessels, lymph vessels, nerves, and glands are within submucosa. The autonomic motor neurons within this plexus innervate both the smooth muscle and glands of the mucosa and submucosa.
- Muscularis - composed of smooth muscle tissue. Arranged in an inner circular layer, which contains muscle cells oriented circumferentially within the GI tract wall, and an outer longitudinal layer, which is composed of muscle cells oriented lengthwise within the GI tract wall. Motility is the function. Contractions of the circular layer constrict the lumen of the tube, whereas contractions of the longitudinal layer shorten the tube. The collective contractions of these smooth muscle layers are associated with two primary types of motility mixing and propulsion.
Mixing is a “backward and forward” motion that blends secretions with ingested material within the GI tract, but does not result in directional movement of the lumen contents. It includes mixing waves (by the stomach) and segmentation (by the small intestine).
Propulsion is the directional movement of materials through the GI tract, and it occurs by the muscular of the GI tract by peristalsis. Peristalsis is the sequential contraction of the muscularis within the GI tract wall that moves like a wave within the different regions of the GI tract (esophagus, stomach, small intestine, and large intestine) . Peristalsis results in one-way movement of the lumen contents from the esophagus to the anus.
Sphincter is positioned between regions of the GI tract. These rings of smooth muscle relax (open) and contract (close) to a.) control the movement of materials into the next section of the GI tract b.) prevent its back flow. The pyloric sphincter, for example, regulates the movement of material from the stomach into the small intestine. - Adventitia or Serosa - outermost tunic is composed of areolar connective tissue with dispersed collagen and elastic fibers. A serosa is the adventitia plus an outer covering of a serous membrane called the visceral peritoneum. Only those digestive organs that are intraperitoneal have a serosa as their outermost tunic.
Enteric Nervous System
Has both sensory neurons and motor neurons, which extend from the esophagus to the anus. Forms both the submucosal nerve plexus and the myenteric nerve plexus within the GI tract wall. Innervates the smooth muscle and glands of the GI tract and mediates the complex coordinated reflexes for the mixing and propulsion of materials through GI tract.
Autonomic Nervous System
GI tract wall is also innervated by both parasympathetic and sympathetic divisions in the ANS. Parasympathetic and sympathetic axons synapse with smooth muscle and glands of the GI tract wall (to control these structures directly) and with neurons within the ENS (to regulate these structures indirectly). Parasympathetic innervation promotes GI tract activity: it stimulates GI motility and release of secretions, and relaxes GI tract sphincters. Sympathetic innervation opposes GI tract activity. It inhibits GI tract motility and release of secretions, contracts GI tract sphincters, and vasoconstrictor blood vessels within the GI tract wall. Any conditions that activated the sympathetic division (exercise, anger, stress) may slow or interfere with digestion.
Nerve Reflexes
Both ENS and ANS control the GI tract wall through nerve reflexes. In response to stimulation, either a short reflex or a long reflex is initiated.
Short reflex is a local reflex that only involves the ENS (and does not involve the central nervous system) . Sensory input detected by either baroreceptors or chemoreceptors is relayed to neurons within the ENS to alter smooth muscle contraction and gland secretion of the GI tract wall. These reflexes function in coordinating small segments of the GI tract to changes in stimuli.
Long reflex involves sensory input relayed to the central nervous system (CNS), which serves as the integration center. Autonomic motor output is then relayed to alter smooth muscle contraction and gland secretion of the GI tract wall. ** autonomic motor output is often relayed to other structures, including the accessory digestive organs (salivary glands, pancreas, liver). The results are coordinated smooth muscle contractions and secretory activity of potentially many different components of the digestive system.
Hormonal Control
Circulating hormones, which are released into blood (gastrin released from the stomach, which stimulates stomach motility and its release of digestive secretions; which inhibit stomach motility and its release of digestive secretions). In addition local hormones are released and influence adjacent cells (histamine released from endocrine cells within the stomach stimulates H+ release from adjacent parietal cells.).
Receptors
Receptors that initiate GI reflexes include baroreceptors, which detect stretch of the GI tract wall, and chemoreceptors, which monitor the chemical content of the material within the lumen, including the presence of protein and acid.
Serous Membranes of Abdominal Cavity
Peritoneum is the serous membrane associated with the abdominopelvic cavity. It consists of two serous membrane layers that are continuous with one another along the posterior abdominal wall. Parietal peritoneum is the serous membrane that lines the inner surface of the abdominal wall, whereas the visceral peritoneum is the serous membrane that covers the surface of internal organs within the abdominopelvic cavity. Between the parietal and visceral peritoneum is a potential space called the peritoneal cavity, which contains serous fluid. This fluid, which is produce by both the parietal peritoneum and visceral peritoneum, lubricates both the internal abdominal wall and the external organ surfaces. It allows the abdominal organs to move freely and reduces friction resulting from this movement.
Intraperitoneal and Retroperitoneal Organs
Organs within the abdomen that are completely surrounded by visceral peritoneum are called Intraperitoneal Organs. The outermost layer of each of these organs is a serosa (not adventitia). They include the stomach, most of small intestine, parts of the large intestine (cecum, vermiform appendix, transverse and sigmoid colon) and most of the liver.
Retroperitoneal organs lie outside the parietal peritoneum directly against the posterior abdominal wall, so only their anterolateral portions are covered with parietal peritoneum. ** these organs are not completely enveloped by a visceral peritoneum. The outermost layer of retroperitoneal organs is an adventitia (not a serosa). Retroperitoneal digestive organs include the pancreas, esophagus (abdominal portion), most of the duodenum (first part of the small intestine), parts of the large intestines (ascending and descending colon) and the rectum.
Mesentery
Mesentery refers to the double layer of peritoneum that attaches to the posterior abdominal wall and supports, suspends, and stabilizes the intraperitoneal GI tract organs. Blood vessels, lymph vessels, and nerves that supply the GI tract are sandwiched between the two layers of a mesentery. A mesentery contains multiple tissues, some anatomists classify this structure as an organ.
1. Greater omentum extends inferiorly like an apron from the inferolateral surface of the stomach (greater curvature) and covers most of the abdominal organs. It often accumulates large amounts of adipose connective tissue, thus is referred to as the “fatty apron” and insulates the abdominal organs and stores fat.
2. Lesser omentum connects the superomedial surface of the stomach (lesser curvature) and the proximal end of the duodenum to the liver.
3. Falciform ligament is a flat, thin, crescent-shaped peritoneal fold that attaches the liver to the internal surface of the anterior abdominal wall.
4. Mesentery proper which is sometimes referred to as the mesentery, is a fan-shaped fold of peritoneum that suspends most of small intestine (the jejunum and ileum) from the internal surface of the posterior abdominal wall.
5. Mesocolon is a fold of the peritoneum that attaches the large intestine to the posterior abdominal wall. The mesocolon has several distinct sections, each named for the portion of the colonist suspends. For example, transverse mesocolon is associated with the transverse colon, whereas sigmoid mesocolon is associated with the sigmoid colon.
Upper Gastrointestinal Tract
Consists of the oral cavity (where salivary glands release their secretions), the pharynx, esophagus, stomach, and duodenum. It is where the initial mechanical and chemical processing of ingested material takes place
Oral cavity and Salivary Glands
Mechanical digestion (mastication) begins in the oral cavity. Saliva is secreted from the salivary glands in response to food being present within the oral cavity. It is mixed with the ingested materials to form a globular, wet mass called a bolus. One component of saliva is salivary amylase, an enzyme that initiates the chemical digestion of starch (amylose).
Pharynx
The bolus is moved into the pharynx during swallowing. Mucus secreted in saliva and in the superior part of the pharynx provides lubrication to facilitate swallowing.
Esophagus
The bolus is transported from the pharynx through the esophagus into the stomach. Mucus secretion by the esophagus lubricates the passage of the bolus.
Stomach
The bolus is mixed with gastric secretions as the muscularis in the stomach wall contracts. These secretions are released into the stomach lumen by epithelial cells of the stomach mucosa and include acid (hydrochloric acid HCL), digestive enzymes, and mucin. The mixing continues as an acidic puree called chyme is formed.
Duodenum (1st part of small intestines)
*is the first part of the small intestine. It is also included in the upper GI tract; it will be described with the rest of the small intestine.
Oral cavity (mouth)
the entrance to the GI tract. Food is digested into the oral cavity, where it undergoes the initial processes of mechanical and chemical digestion. 2 distinct spatial regions a.) vestibule (or buccal cavity), which is the space between the gums, lips, and cheeks b.) the oral cavity proper, which lies central to the teeth. The oral cavity is bounded laterally by the cheeks and anteriorly by the teeth and lips, and it leads posteriorly into the oropharynx.
Salivary glands
Produce saliva, are located both within the oral cavity (intrinsic salivary glands) and outside the oral cavity (extrinsic salivary glands)
Intrinsic salivary glands are unicellular exocrine glands that continuously release small amounts of secretions independent of the presence of food. Only the secretions from the intrinsic salivary glands contain lingual lipase, an enzyme that begins the digestion of triglycerides (after the bolus enters the stomach).
Extrinsic salivary glands are most saliva, that is produced from multicellular exocrine glands outside the oral cavity.
Parotid Gland
Parotid salivary glands are the largest salivary glands. Each parotid gland is located anterior and inferior to the ear, partially overlying the master muscle. The parotid salivary glands produce a portion (25-30%) of saliva, which is transported through the parotid duct to the oral cavity. The parotid duct extends from the gland, across the external surface of the master muscle, before penetrating the buccinator muscle and opening into the vestibule of the oral cavity near the second upper molar. Mumps is an infections of the parotid glands by a virus called myxovirus. Children are protected against mumps when immunized with MMR (measles, mumps, and rubella) vaccine. Produce serous secretions.
Submandibular salivary glands
both inferior to the floor of the oral cavity and medial to the body of the mandible. The submandibular salivary glands produce most of the saliva (about 60-70%). A submandibular duct opens from each gland through a papilla in the floor of the oral cavity on either side of the lingual frenulum. Produce both mucus and serous secretions.
sublingual salivary glands
Are inferior to the tongue, and medial and anterior to the submandibular salivary glands. Each sublingual salivary gland extends multiple tiny sublingual ducts that open onto the inferior surface of the oral cavity, posterior to the submandibular duct papilla. These small glands contribute only a limited amount (about 3-5%) of the total saliva. Produce mucus and serous secretions.
Saliva
Secreted daily ranges between 1 and1.5 liters. Most saliva is produced during a mealtime, but smaller amounts are produced continuously to ensure that the oral cavity mucous membrane remains moist. Composed of 99.5% water and mixture of solutes. Saliva is formed as water and electrolytes are filtered from plasma within blood capillaries, then through cells (acini) of a salivary gland. Other components are added by cells of the salivary glands, including salivary amylase, mucin, and lysozyme. The functions of saliva include:
1. Moistens ingested food as it is formed into a bolus, a globular, wet mass of partially digested material that is more easily swallowed.
2. Initiates the chemical breakdown of starch (a polymer of glucose molecules) in the oral cavity because of the salivary amylase it contains.
3. Acts as a watery medium into which food molecules are dissolved so taste receptors may be stimulated
4. Cleanses the oral cavity structures
5. Helps inhibit bacterial growth in the oral cavity because it contains antibacterial substances, including lysozyme and IgA antibodies (IgA is formed by plasma cells in the lamina propria and transported across the epithelial cells.
Regulation of Salivary Secretions
Salivary nuclei within the pons regulate salivation. A basal level of salivation in response to parasympathetic stimulation ensures that the oral cavity remains moist. Input to the salivary nuclei is received from chemoreceptors or baroreceptors in the upper GI tract. These receptors detect various types of stimuli, including the introduction of substances into the oral cavity, especially those that are acidic, such as a lemon; and arrival of foods into the stomach lumen, especially foods that are spicy or acidic. Input is also received by the salivary nuclei from the higher brain centers in response to the thought, smell, or sight of food. Stimulation of the salivary nuclei by either sensory receptors or higher brain centers results in increased nerve signals relayed along parasympathetic neurons within both the facial nerve (CN VII), which innervates the submandibular and sublingual salivary glands, and the glossopharyngeal nerve (CN IX), which innervates the parotid salivary glands, and additional saliva is released.
sympathetic stimulation, which occurs during exercise or when an individual is excited or anxious, results in a more viscous saliva by decreasing the water content of saliva. (This occurs because sympathetic stimulation constricts blood vessels of the salivary gland, which decreases the fluid added to saliva).
Mechanical Digestion: Mastication
Mastication (chewing) requires the coordination of teeth, skeletal muscles in lips, tongue, cheeks, and jaws that are controlled by nuclei within the medulla oblongata and pons, called the mastication center. The primary function is to mechanically reduce its bulk into smaller particles to facilitate swallowing. Chemical digestion and absorption are affected very little by. Chewing that the surface area of food is increased, which facilitates exposure to and action by digestive enzymes. Mastication also promotes salivation to help soften and moisten the food to form a bolus.
teeth
Collectively known as the dentition. A tooth has an exposed crown, a constricted nest, and one or more roots that anchor it to the jaw. The roots of the teeth fit tightly into dental alveoli, which are sockets within the alveolar processes of both the maxillae and the mandible. The roots, the dental alveoli, and the periodontal membranes that the roots to alveolar processes form a gomphosis joint.
gingivae are the gums. They are composed of dense irregular connective tissue, with an overlying nonkeratinized stratified squamous epithelium that covers the alveolar processes of the upper and lower jaws and surrounds the neck of the teeth.
Pharynx
The pharynx is a funnel-shaped, muscular passageway with distensible (stretchable) lateral walls that serves as the passageway for both air and food. Three skeletal muscle pairs called the superior, middle, inferior pharyngeal constrictors form the wall of the pharynx. The oropharynx and laryngopharynx are line with nonkeratinized stratified squamous epithelium that provides protection against abrasion associated with swallowing ingested materials.
Esophagus
The esophagus is a normally collapsed tubular passageway. It is about 25 cm long in an adult and begins at approximately the level of the cricoid cartilage of the larynx, with most of its length within the thoracic cavity. This tube is directly anterior to the vertebral bodies and posterior to the trachea until it passes through an opening in the diaphragm called the esophageal hiatus. Only the last 1.5 cm of the esophagus is located within the abdominal cavity, where its inferior end connects to the stomach.
the superior esophageal sphincter is a contracted ring of circular skeletal muscle at the superior end of the esophagus. It is the area where the esophagus and the pharynx meet. This sphincter is closed during inhalation of air, so air does not enter the esophagus and instead enters the larynx and trachea.
The inferior esophageal sphincter is a contracted ring of circular smooth muscle at the inferior end of the esophagus. This sphincter is not strong enough alone to prevent materials from refluxing back into the esophagus; instead, the muscles of the diaphragm at the esophageal opening contract to help prevent materials from regurgitating from the stomach into the esophagus.
Motility: Swallowing Process
Swallowing, also called deglutition is the process of moving ingested materials from the oral cavity to the stomach. Swallowing has 3 phases: the voluntary phase, the pharyngeal phase, and the esophageal phase.
1. The voluntary phase occurs after ingestion. It is controlled by the cerebral cortex (primarily the temporal lobes and motor cortex of the frontal lobe). Ingested materials and saliva mix in the oral cavity. Chewing forms a bolus that is mixed and manipulated by the tongue and then pushed superiorly against the hard palate. Transverse palatine folds in the hard palate help direct the bolus posteriorly toward the oropharynx.
2. Pharyngeal phase is the arrival of the bolus at the entryway to the oropharynx (faucet) initiates the swallowing reflex. The pharyngeal phase is involuntary. Tactile sensory receptors around the faces are stimulated by the bolus and initiate nerve signals along sensory neurons to the swallowing center in the medulla oblongata. Nerve signals are then relayed along motor neurons to effectors to cause the following response:
a.) entry of the bolus into the oropharynx
b.) elevation of the soft palate and uvula to block the passageway between the oropharynx and nasopharynx
c.) elevation of the larynx by the extrinsic muscles move the larynx anteriorly and superiorly, resulting in the epiglottis covering the laryngeal inlet; this prevents ingested material from entering the trachea.
3. Esophageal phase is also involuntary. It is the time during which the bolus passes through the esophagus and into the stomach - about 5 to 8 seconds. The presence of the bolus within the lumen of the esophagus stimulates sequential waves of muscular contraction that assist in propelling the bolus toward the stomach.
Higher pressure occurs in the superior region of the esophagus relative to the inferior region.
The superior and inferior esophageal sphincters are normally closed at rest. When the bolus is swallowed, these sphincters relax to allow it to pass through the esophagus. The inferior esophageal sphincter contracts after passage of the bolus, helping to prevent reflux of materials and fluids from the stomach into the esophagus.
GERD: Gastroesophageal Reflux Disease
Frequent gastric reflux erodes the esophageal tissue in this condition, so over a period of time, scar tissue builds up in the esophagus, leading to narrowing of the esophageal lumen. In more advanced cases, the esophageal epithelium may change from stratified squamous to columnar epithelium, a condition known as Barrett esophagus. The secretions of columnar epithelium may provide protection from the erosive gastric secretions. Unfortunately, this metaplasia increases the risk of cancerous growths.
Stomach
Is a holding sac in the superior left quadrant of the abdomen immediately inferior to the diaphragm. Under normal conditions, between 3 and 4 liters of food, drink, and saliva enter the stomach daily and generally spend between 2 to 6 hours there, depending on the amount and composition of the ingested material. It mixes the ingested food with secretions released from the stomach wall and mechanically digests the contents into a semifluid mass called chyme. Chemical digestion of both protein and fat begins in the stomach, but absorption from it is limited to small, nonpolar molecules that are in contact with the mucosa of the stomach. Both alcohol and aspirin are examples of substances that are absorbed in the stomach. One significant function of the stomach is to serve as a “holding bag” for controlled release of partially digested materials into the small intestine, where most chemical digestion and absorption occur. One of the most vital functions performed by the stomach is the release of intrinsic factor (a substance required for the absorption of vitamin B12 which occurs within the small intestine.
Gross Anatomy of the Stomach
The stomach is a muscular, j-shaped organ. It has both a larger, convex inferolateral surface called the greater curvature and a smaller, concave superomedial surface called the lesser curvature. This organ is composed of 4 regions:
1. Cardia is a small, narrow, superior entryway into the stomach lumen from the esophagus. The internal opening where the cardia meets the esophagus is called the cardiac orifice, which is the location of the inferior esophageal sphincter (also known as the cardiac sphincter).
2. Fundus is the dome-shaped region lateral and superior to the esophageal connection with the stomach. Its superior surface contacts the inferior surface of the thoracic diaphragm. The funds has both weaker muscular contractions and a higher pH in its lumen area than other regions of the stomach.
3. Body is the largest region of the stomach; it is inferior to the cardiac orifice and the funds and extends to the pylorus.
4. Pylorus is the narrow, funnel-shaped terminal region of the stomach. Its opening into the duodenum of the small intestine is called the pyloric orifice. Surrounding this pyloric orifice is a thick ring of circular smooth muscle called the pyloric sphincter. The pyloric sphincter regulates the movement of material from the stomach into the small intestine.
Gastric Folds: Rugae
Internal stomach lining. These gastric folds are seen only when it is empty. They allow the stomach to expand greatly when it fills with food and drink and then return to its normal j-shape when it empties. In addition, the stomach is able to accommodate varying quantities of food due to the stress-relax response exhibited by the smooth muscle within the stomach wall. (Stress-relaxation response is a characteristic response of smooth muscle to a prolonged stretch. The smooth muscle initially contracts, but after a period of time it relaxes.)
Histology: Gastric Pits & Gastric Glands
The stomach mucosa is composed of a simple columnar epithelium supported by lamina propria. The transition from stratified squamous in the esophagus to simple columnar epithelium in the stomach is abrupt. The simple columnar epithelial cells are replaced often (usually within a week) because of the harsh acidic environment of the stomach contents.
the lining is indented by numerous depressions called gastric pits.
Several gastric glands extend deep into the mucosa from the base of each gastric pit. The muscularis mucosae partially surrounds the gastric glands and helps expel gastric gland secretions when it contracts.
the muscular of the stomach varies from the general GI tract pattern in that it is composed of 3 smooth muscle layers instead of 2. An inner oblique layer, a middle circular layer, and an outer longitudinal layer. The presence of a third (oblique) layer of smooth muscle assists the continued churning and blending of the swallowed bolus to help mechanically digest the food. The muscularis becomes increasingly thicker and stronger as it progresses from the body to the pylorus.
The outermost layer of the stomach is a serosa because the stomach is intraperitoneal. It produces serous fluid that lubricates the external surface of the stomach to decrease friction associated with stomach motility.
Gastric Secretions: 5 types of secretory cells
Five types of secretory cells of the gastric epithelium are integral contributors to the process of digestion. 4 of these cell types produce the approximately 3 liters per day of gastric juice that are released into the stomach lumen. The fifth type of cell (G-cell) secretes a hormone into the blood.
Surface mucous cells
Line the stomach lumen and extend into the gastric pits. They continuously secrete an alkaline mucin onto the gastric surface. Mucin becomes hydrated, producing a 1-3 mm mucus layer that coats the epithelial lining. This mucus layer, along with a high rate of cell turnover in the mucosa, helps to prevent ulceration of the stomach lining upon exposure to both the high acidity of the gastric fluid and gastric enzymes.
Mucous neck cells
Are located immediately deep to the base of the gastric pit and are intersperse among the parietal cells. Mucous neck cells release a less alkaline mucin that differs structurally and functional from the alkaline mucin released by the surface mucous cells. The mucus produced by both types of mucous cells has lubricating properties to protect the stomach lining from abrasion or mechanical injury.
chief cells
Are the most numerous secretory cells within the gastric glands. These cells produce and secrete packets of zymogen granules, which primarily contain pepsinogen. Pepsinogen is the inactive precursor proteolytic enzyme pepsin. Pepsin must be produced in this inactive form to prevent the destruction of chief cell proteins.
pepsinogen is activated into pepsin following its release into the stomach. It is activated by both the low pH and active pepsin molecules already present within the stomach. Pepsin chemically digests denatured proteins in the stomach into smaller peptide fragments (oligopeptides)
Chief cells also produce the enzyme gastric lipase. Gastric lipase is one of the acidic lipases that has limited role in fat digestion.
Parietal cells
Are responsible for the addition of 2 substances into the lumen of the stomach.
1. Intrinsic factor is the production and release of intrinsic factor (a glycoprotein) is the only essential function performed by the stomach. Intrinsic factor is required for absorption of b12 in the ileum (the final portion of the small intestine). B12 is necessary for production of normal erythrocytes. A critical decrease or absence of B12 results in pernicious anemia.
2. Hydrochloric acid is not formed within the parietal cell; it would destroy the cell. Instead, the parietal cell forms H+ and releases both H+ and Cl- into the stomach lumen. HCL is responsible for the low pH of between 1.5-2.5 within the stomach.
Low pH: HCL
Facilitates the digestive processes of the stomach:
1. Food breakdown. The relatively tough plant cell walls and animal connective tissue are made easier to digest.
2. Protein denaturation. Proteins unfold and denature, facilitating chemical digestion by the proteolytic enzyme pepsin.
3. Pepsin activation. Pepsinogen (released from chief cells) is converted into pepsin (the active proteolytic enzyme).
4. Enhanced enzymatic activity. HCL creates the optimal pH environment for enzymatic activity of both pepsin and the acidic lipases (lingual lipase, released from intrinsic salivary glands and gastric lipase, released from the stomach).
G-Cells
Are a type of enteroendocrine cell that are hormone-producing cells in the gastric glands of the stomach. G-cells secrete the hormone gastrin into the blood. Gastrin stimulates stomach motility and stomach secretions. Other enteroendocrine cells produce different hormones, such as somatostatin, a peptide hormone that modulates the function of nearby enteroendocrine and exocrine cells.
Motility in the Stomach
Smooth muscle activity in the stomach wall has 2 primary functions: a) mixing the bolus with gastric juice to form chyme b)emptying chyme from the stomach into the small intestine.
- Contractions of smooth muscle in stomach wall mix bolus with gastric secretions to form chyme
- Peristaltic waves result in pressure gradients that move stomach contents toward the pyloric region.
- Pressure gradient increases force in pylorus against pyloric sphincter.
- Pyloric sphincter opens, and a small volume of chyme enters the duodenum.
- Pyloric sphincter closes, and retropulsion occurs.
Gastric mixing vs. Gastric emptying
Gastric mixing is a form of mechanical digestion that changes the semi digested bolus into chyme. Chyme has the consistency of a pastelike soup. Contractions of the stomach’s thick muscularis layer churn and mix the bolus with the gastric secretions, leading to a reduction in the size of swallowed particles.
Gastric emptying is the movement of acidic chyme from the stomach through the pyloric sphincter into the duodenum of the small intestine. This movement is facilitated by the progressive thickening of the muscularis layer in the pyloric region. As a wave of peristaltic muscular contraction moves through the pylorus toward the pyloric sphincter, a pressure gradient is established that drives the chyme toward the duodenum.
The peristaltic wave establishes a greater pressure on the contents in the pylorus than the pressure exerted by the pyloric sphincter to stay closed and prevent movement. A few millimeter (about 3 mm) of chyme are emptied into the duodenum. After the peristaltic wave has moved past the pyloric sphincter, the pressure of the sphincter is once again greater than the pressure on the contents , and the pyloric sphincter closes. As this sphincter closes, stomach contents are squeezed back toward the stomach body. This reverse flow event is retropulsion. Retropulsion not only results in the prevention of further chyme moving into the small intestine but also contributes to additional mixing of the stomach contents to further reduce the size of food particles.
Regulation of Digestive Processes: Stomach
Both the stomach’s motility and the release of its secretions are highly regulated so that ingested material is “pulverized” into chyme, which can then be effectively processed within the small intestine.
Pacemaker Cells
Are specialized cells located within the GI tract wall. In the stomach wall, these cells are located within the longitudinal layer of smooth muscle, and initiate the smooth muscle contraction. These cells spontaneously depolarize less than 4 times per minute and establish its basic muscular contraction rhythm. Electrical signals spread via gap junctions between the smooth muscle cells in the muscular layer of the stomach. These muscular contractions by the stomach wall are regulated by both nervous reflexes and hormones, which alter the force but not rate of contraction, which is constant. Secretory activity of gastric glands is also altered. How these occur is organized into 3 phases: cephalic phase, gastric phase, and intestinal phase. The cephalic and gastric phases involve the events before and during a meal, whereas the intestinal phase involves the events that occur after a meal, as the ingested materials are being digested.
The cephalic phase
initiated by thought, smell, sight, or taste of food (or even sounds of food preparation.)
1. Receptors: special sense receptors (nose, eyes)
2. Sensory input: increased nerve signals relayed from cerebral cortex and hypothalamus to medulla oblongata.
3. Medulla oblongata integrates input from higher brain centers
4. Motor output: increased nerve signals relayed along vagus nerve to stomach.
5. Effector: stomach stimulated to increase both its force of contraction and release of secretions.
**hormones secreted: none
The gastric phase
initiated by the presence of food in stomach.
1. Receptors: baroreceptors in stomach wall detect stretch; chemoreceptors detect protein or high pH in stomach contents.
2. Sensory input: increased nerve signals relayed to medulla oblongata
3. Medulla oblongata integrates sensory input
4. Motor output: increased nerve signals relayed along vagus nerve to stomach
5. Effector: stomach stimulated to increase both its force of contraction and release of secretions
**hormone secreted: gastrin (increases force of stomach contractions and release of secretions, contracts pyloric sphincter. )
The intestinal reflex
initiated by presence of acidic chyme in duodenum
1. Receptors: chemoreceptors in intestinal wall detect acidic chyme or low pH in duodenum contents.
2. Sensory input: decreased nerve signals relayed to medulla oblongata
3. Medulla oblongata: integrates sensory input
4. Motor output: decreased nerve signals relayed along vagus nerve to stomach.
5. Effector: stomach inhibited to decrease bot its force of contraction and release of secretions
**hormones secreted: cholecystokinin (CCK) (decreases the forces of contraction in the stomach); secretin (inhibits release of stomach secretions)
Lower Gastrointestinal tract
Composed of the small and large intestine. The lower GI tract continues the processes of digestion and functions in the absorption of nutrients. Material that cannot be digested and absorbed is then eliminated.
Small Intestine
The small intestine is divided into 3 continuous regions (duodenum, jejunum, and ileum). The small intestine receives acidic chyme from the stomach that is then mixed with accessory digestive organ secretions. Most chemical digestion of macromolecules and absorption of nutrients, water, and electrolytes occur within the small intestine.
Accessory digestive organs
Liver, gallbladder, and pancreas. Their collective secretions include bile and pancreatic juice. Bile is produced by the liver and then stored, concentrated, and released by the gallbladder. Pancreatic juice contains numerous digestive enzymes and is produced and released by the pancreas. Accessory digestive organ secretions - both bile and pancreatic juice contain HCO 3- (weak base), which neutralizes acidic chyme entering the duodenum.
Large intestine
Primarily absorbs water, electrolytes, and vitamins (including vitamins B and K produced by bacteria within the large intestine). The digestive process is completed as the semifluid mass of partly digested food is converted to feces and them eliminated through the anus.
Small Intestine
A long tube that extends between the stomach and the large intestine. About 9 to 10 liters of ingested food, water, and digestive system secretions enter the small intestine daily. Ingested nutrients typically spend at least 12 hours in the small intestine. The small intestine finishes chemical digestion and is responsible for absorbing almost all of the nutrients and a large percentage of water, electrolytes and vitamins.
Gross Anatomy: Small intestine
Duodenum
Duodenum forms the first segment of the small intestine. It is approximately 25 cm long and originates at the pyloric sphincter, which regulates movement of chyme from the stomach into the small intestine. The duodenum is arched into a c-shaped around the head of the pancreas and becomes continuous with jejunum at the duodenojejunal flexure. Most of the duodenum is retroperitoneal, although the very initial portion is intraperitoneal.
The most significant function of the duodenum is to serve as an “anatomic blender” that allows for efficient chemical digestion. The duodenum receives a.) acidic chyme from the stomach and b.) the secretions from the abdominal accessory digestive organs. These secretions include bile from the liver and gallbladder and pancreatic juice from the pancreas. It is within the lumen of the duodenum where all of these substances are mixed, and where digestive enzymes (with many released from the pancreas) have contact with ingested molecules and chemical digestion primarily occurs.
Gross Anatomy: Small intestine
Jejunum
Is the middle region of the small intestine. Extending approximately 2.5 meters, it makes up about 2/5 of the small intestine’s total length. The jejunum is the primary region within the small intestine for nutrient absorption (where the chemically digested contents that are received from the duodenum are moved from the small intestine lumen into either the blood or the lymph.
Gross Anatomy: Small intestine
Ileum
Is the last region of the small intestine. At about 3.6 meters in length, the ileum forms approximately 3/5 of the small intestine. Its distal end terminates at the ileocecal valve, a sphincter that controls the entry of materials from the small intestine into the large intestine. Absorption of digested materials continues in the ileum along with the absorption of bile salts and vitamin B12. Both the jejunum and the ileum are intraperitoneal organs and are suspended within the abdomen by the mesentery proper.
Small intestine structures:
Increase surface area
Circular folds
3 structures increase surface area: circular folds, villi, and microvilli.
Circular folds are macroscopic structures that are easily seen by the naked eye: they are projections formed by both the mucosal and submucosal tunics of the small intestine. Circular folds also act as “speed bumps” to slow down the movement of chyme and ensure that it remains within the small intestine for maximal nutrient absorption. Circular folds are more numerous the duodenum and jejunum and least numerous in the ileum.
Small intestine structures:
Increase surface area
Villus
A villus is a small, fingerlike projection of the simple columnar epithelium and lamina propria of the mucosa. They increase the surface area of the epithelial lining through which nutrients are absorbed. Villi are larger and most numerous in the jejunum, where much of the absorption takes place. The epithelium and lamina propria of each villus appears analogous to a glove (epithelium) covering a finger (lamina propria). Each villus contains an arteriole, a rich blood capillary network, and a venue. Most nutrients are absorbed into the blood capillaries. A lacteal is a type of lymphatic capillary also within the villus. A lacteal is responsible for absorbing lipids and lipid-soluble vitamins that are too large to be absorbed by the blood capillaries.
Small intestine structures:
Increase surface area
Microvilli
Brush border
Brush border enzymes
Are microscopic extensions of the plasma membrane of the simple columnar epithelial cells lining the small intestine. Microvilli further increases the surface area of the small intestine. Individual microvilli are not clearly visible in light micrographs of the small intestine; instead, they appear as a fuzzy edge of the simple columnar cells called the brush border. Embedded within this brush border are various enzymes that complete the chemical digestion of most nutrients immediately before absorption. Collectively, these are called brush border enzymes. Located in close proximity and also embedded within the plasma membrane are the required proteins for membrane transport of digested molecules. Between the intestinal villi are invaginations of the mucosa called intestinal glands, which secrete intestinal juice. These glands extend to the base of the mucosa and slightly resemble the anatomy of the gastric glands of the stomach.
Small intestine secretions
4 secretory cells
Goblet cells
Goblet cells within the simple columnar epithelium produce mucin that when hydrate form mucus, which lubricates and protects the intestinal lining. These cells increase in number from the duodenum to the ileum, because more lubrication is needed as digested materials (and water) are absorbed and undigested materials (and less water) remain in the lumen.
Smal intestine secretions
4 types of secretory cells
Enteroendocrine cells
Release hormones such as CCK and secretin into the blood.
Small intestine secretions
4 types of secretory cells
Paneth cells
Are located only in the base of the intestinal crypts. These cells assist with the functioning of the innate immune system by secreting lysozyme, as well as some other antimicrobial agents, to help protect against potentially harmful substances (virus) that are within the small intestine.
Brunner gland
Also called duodenal submucosal gland. This gland produces a viscous, alkaline mucus secretion that protects the duodenum from the acidic chyme entering the duodenum from the stomach.
Motility: Small intestine
Smooth muscle activity of the muscular within the small intestine wall has 3 primary functions: a) mixing chyme with accessory gland secretions b) moving the chyme continually against the brush border, and c) propelling the contents through the small intestine toward the large intestine.
All these functions facilitate chemical digestion and absorption, employing the processes of segmentation and peristalsis. When chyme first enters the small intestine, segmentation is more prevalent than peristalsis. Segmentation mixes chyme with secretions from the accessory digestive organs through a “backward and forward” motion.
Peristalsis then propels material within the GI lumen by alternating contraction of the circular and longitudinal muscle layers in small regions. The rhythm of muscular contractions is more frequent in the duodenum than in the ileum; the net movement of intestinal contents is toward the large intestine.
Accessory Digestive Organ Ducts
Biliary apparatus
A series of ducts deliver secretions from the accessory digestive organs to the duodenum of the small intestine. These ducts include the biliary apparatus from the liver and gallbladder, and the pancreatic ducts from the pancreas.
Biliary apparatus is a network of thin ducts that include the right and left hepatic ducts, which drain the right and left lobes of the liver. The right and left hepatic ducts merge to form a single common hepatic duct. The union of the cystic duct from the gallbladder and the common hepatic duct forms the common bile duct, which extends to the hepatopancreatic ampulla.
Accessory Digestive Organ Ducts
Pancreatic ducts
Include both the main pancreatic duct and the accessor pancreatic duct. The main pancreatic duct transports the majority of the pancreatic juice. It joins with the common bile duct to form the hepatopancreatic ampulla. The accessory pancreatic duct is a smaller duct whereby limited amounts of pancreatic juice may also enter the duodenum. This duct penetrates the duodenal wall, forming the minor duodenal papilla.
Accessory Digestive Organ Ducts
Hepatopancreatic ampulla
Is a swelling either adjacent to or within the posterior duodenal wall, which penetrates through the duodenal wall forming a small projection called the major duodenal papilla. The hepatopancreatic ampulla receives bile from the common bile duct (coming from the liver and gallbladder) and pancreatic juice from the main pancreatic juice from the main pancreatic duct. The release of these accessory gland secretions (bile and pancreatic juice) is regulated by the hepatopancreatic sphincter that is located within the ampulla. This sphincter is normally closed, preventing accessory digestive gland secretions from entering the duodenum. Relaxation and opening of this sphincter permits the flow of these secretions into the duodenum (a process stimulated by cholecystokinin (CCK) hormone.)
Liver
An accessory digestive organ located in the right upper quadrant of the abdomen, immediately inferior to the diaphragm. it has numerous functions but its main function in digestion is the production of bile.
Gross Anatomy: Liver
The liver is the largest internal organ, weighing 1-2 kg, and it constitutes approximately 2% of an adult’s body weight. The liver is covered by a connective tissue capsule except at the port hepatitis. Covering the connective tissue capsule is a visceral peritoneum, except for a small region on its diaphragmatic surface called the bare area.
Gross Anatomy: Liver
Lobes
The liver is composed of 4 partially separated lobes and is supported by 2 ligaments. The major lobes are the right lobe and left lobe. The right lobe is separated from the smaller left lobe by the falciform ligament, a peritoneal fold that secures the liver to the internal surface of the anterior abdominal wall. In the inferior free edge of the falciform ligament lies the round ligament of the liver, which represents the remnant of the fetal umbilical vein. Within the right lobe are the caudate lobe and the quadrate lobe. The caudate lobe is adjacent to the inferior vena cava, and quadrate lobe is adjacent to the gallbladder.
along the inferior surface of the liver are several structures that collectively resemble the letter H; the inferior vena cava and the ligament venous form the vertical inferior parts. (Recall that the ligamentum venous is a remnant of the ductus venosus in the embryo. This vessel, which allows blood to bypass the liver, shunted blood from the umbilical vein to the inferior vena cava.) the porta, hepatic, the horizontal crossbar of the H, is the site at which blood and lymph vessels, bile ducts, and nerves extend from the liver. In particular, the hepatic portal vein and branches of the hepatic artery proper enter at the porta hepatic.
Histology: Liver
The liver’s connective tissue capsule branches throughout the organ and forms septa (walls) that partition the liver into thousands of microscopic polyhedral hepatic lobules, which are the structural and functional units of the liver. Within hepatic lobules are liver cells called hepatocytes. At the periphery of each lobule are several portal triads, composed of a bile ductule, and microscopic branches of both the hepatic portal vein and the hepatic artery. At the center of each lobule is a central vein that drains the blood flow from the lobule. Central veins collect the blood and merge throughout the liver to form left and right hepatic veins that eventually empty into the inferior vena cava. In a cross section, a hepatic lobule looks like a side view of a bicycle wheel. The central vein is like the hub of the wheel. At the circumference of the wheel (where the tire would be) are the portal triads that are usually equidistant apart. Cords of hepatocytes make up the numerous spokes of the wheel, and they are bordered by hepatic sinusoids, which transport blood.
Blood Flow: Liver Lobules
The cells of the liver receive blood from 2 sources; one is oxygenated and the other deoxygenated. The hepatic artery is a branch of the celiac trunk that extends off of the descending abdominal aorta and transports oxygenated blood to the liver. The hepatic portal vein is part of the hepatic portal system and transports deoxygenated and nutrient rich blood from the capillary beds of the GI tract, spleen, and pancreas. The hepatic portal vein delivers approximately 75% of the blood volume to the liver (the hepatic artery brings the other 25%). The hepatic artery and hepatic portal vein branch extensively into smaller vessels until microscopic branches form components of the portal triad. The oxygenated blood from the hepatic artery branch (within the portal triad) and the deoxygenated blood of the hepatic portal vein branch (within the portal triad) both enter a sinusoid where the blood is “ processed”. (Recall that sinusoids are thin-walled capillaries with large gaps between these cells, which make the sinusoids significantly more permeable than other capillaries.)Blood then drains into the central vein of the lobule. Central veins collect the blood from each lobule and merge throughout the liver to ultimately form left and right hepatic veins that empty into the inferior vena cava.
Hepatic sinusoids: Liver
Several significant events occur as blood is transported through hepatic sinusoids.
1. Nutrients are absorbed from the sinusoids and enter the hepatocytes.
2. Oxygen is delivered to hepatocytes for aerobic cellular respiration.
3. Stellate (or Kupffer cells), which are macrophages that line the liver sinusoid, engage in phagocytosis of potentially harmful substances (microbes). (Stellate cells are fixed macrophages of the immune system;)
Bile: Formation and Flow
Bile is a yellowish-green, alkaline fluid containing mostly water, bicarbonate ions (HCO 3-), bile salts, bile pigments (bilirubin), cholesterol, lecithin (a phospholipid), and mucin. Hepatocytes of the liver product bile at a rate of 0.5 to 1 liter per day.
Bile is released from hepatocytes into bile canaliculi. These small channels transport bile to bile ductules of portal triads. (Observe that bile flow, which is away from the central vein to the portal triad, is in the opposite direction of blood flow, which moves from the portal triad to the central vein.) Bile within the bile ductules flows into progressively larger bile ducts until reaching either the right or left hepatic duct. The ducts of the biliary apparatus transport the bile to the duodenum.
Bile has several functions:
1. Neutralizing acidic chyme within the small intestine through bicarbonate ions (HCO 3-)
2. Emulsification of lipids by bile salts and lecithin (which is a type of mechanical digestion)
3. Elimination of bilirubin, a waste product of erythrocyte destruction
**bile does not contain digestive enzymes for the chemical breakdown of nutrients within the GI tract. Instead, components of bile (bile salts) are dividing the larger aggregates lipid into smaller aggregates of lipid by mechanical digestion (like ice cubes broken into ice chips). This allows for more effective chemical digestion by pancreatic lipase.
Gallbladder
Sac-like organ that stores, concentrates, and releases bile that the liver produces. The gallbladder has 3 tunics; an inner mucosa, a middle muscular, and an external serosa. The mucosa is thrown into folds that permit distension of the wall as the gallbladder fills with bile. The gallbladder drains bile into the cystic duct, which connects to the common bile duct.
At the neck of the gallbladder, a sphincter valve controls the flow of bile into and out of the gallbladder. Bile enters the gallbladder when the hepatopancreatic sphincter associated with the hepatopancreatic ampulla is closed. It backs up through both the common bile duct and the cystic duct into the gallbladder. The gallbladder can hold approximately 40 to 60 mL of concentrated bile. Concentrated bile is transported from the gallbladder through the cystic duct and then the common bile duct through the hepatopancreatic ampulla into the duodenum.
Pancreas
Has both endocrine and exocrine functions. Endocrine cells produce and secrete hormones such as insulin and glucagon. Exocrine cells produce pancreatic juice to assist with chemical digestion. Disorders that affect either a) the pancreatic ducts that lead from the pancreas into the duodenum (cystic fibrosis) or b) the pancreas (pancreatic cancer) have serious and potentially fatal effects on the ability to digest and absorb nutrients.
Gross Anatomy: Pancreas
The pancreas is approximately 5 to 6 inches in length and about 1 inch thick. It is a retroperitoneal organ that extends horizontally from the duodenum toward the left side of the abdominal cavity, where it has contact with the spleen. The pancreas exhibits a wide head adjacent to the curvature of the duodenum; a central, elongated body projecting toward the left lateral abdominal wall; and a tail that tapers as it approaches the spleen.
Histology: Pancreas
The pancreas contains modified simple cuboidal epithelial cells called acing cells that are arranged in saclike acini. These cells are organized into large clusters termed lobules. Acing cells produce and release digestive enzymes. Small ducts lead from each acinus into larger ducts that empty into either the main pancreatic duct or the accessory pancreatic duct, which lead to the duodenum. The simple cuboidal epithelial cells lining the pancreatic ducts have the important function of secreting alkaline fluid containing bicarbonate ion ( HCO 3-). This weak base functions to neutralize acidic chyme entering the small intestine.
Pancreatic Secretions
Together, secretions of acing cells and cells that line the pancreatic ducts form pancreatic juice. Pancreatic juice (approximately 1 to 1.5 liters per day) is an alkaline fluid containing mostly water, HCO 3-, and a versatile mixture of digestive enzymes. These enzymes include:
1. Pancreatic amylase
2. Pancreatic lipase for the digestion
3. Inactive proteases (trypsinogen, chymotrypsinogen, and procarboxypeptidase) that, when activated, digest protein.
4. Nucleases for the digestion of nucleic acids (DNA and RNA)
Regulation: Accessory Digestive Structures
The increase in vagal stimulation in the cephalic phase and gastric phase, in addition to stimulating stomach motility and secretion, also activates the pancreas to release pancreatic juice. Recall that in the intestinal phase, both cholecystokinin (CCK) and secretin are released. Cholecystokinin is a hormone released from the small intestine primarily in response to free fatty acids in chyme. The functions of CCK include:
1. Stimulating smooth muscle within the gallbladder wall to strongly contract, causing the release of concentrated bile (this primary function of stimulation the gallbladder, also called the cholecystokinin)
2. Stimulating the pancreas to release enzyme-rich pancreatic juice
3. Relaxing the smooth muscle within the hepatopancreatic ampulla, allowing entry of bile and pancreatic juice into the small intestine.
Secretin is released from the small intestine primarily in response to an increase in chyme acidity. Secretin primarily causes the release of an alkaline solution that contains HCO 3- from both the liver and ducts of the pancreas. Upon entering the small intestine, this alkaline fluid helps neutralize the acidic chyme. (Recall that CCK and secretin also inhibit stomach motility and release of gastric secretions.
Large Intestine
The large intestine, is a relatively wide tube that is significantly shorter than the small intestine. It is called the “large” intestine because its diameter is greater than that of the small intestine. Approximately 2 liters of digested material passes from the small intestine to the large intestine daily. Most nutrients (including water, electrolytes, and vitamins) have been absorbed within the small intestine. The large intestine absorbs water (about 1.8 liters per day, for an adult) . The large intestine also absorbs some electrolytes (primarily sodium [Na+] and chloride [Cl-] ions) from the remaining digested material that enters it from the small intestine (as well as vitamins B and K, which are synthesized by bacteria within the large intestine). It is estimated that only 200 milliliters of the water entering the colon daily is lost in feces. The watery chyme that first enters the large intestine, including all of the undigested materials as well as the waste products secreted by the accessory digestive organs (bilirubin by the liver), solidifies and is compacted into feces, or fecal material. The large intestine then stores this fecal material until it is eliminated through defecation.
Gross Anatomy: Large instestine
Cecum
3 major regions constitute the large intestine; the cecum, the colon, and the rectum.
The cecum is a blind sac. It is the first portion the large intestine and located in the right lower abdominal quadrant. This pouch extends inferiorly from the ileocecal valve. Chyme enters the cecum from the ileum. Projecting inferiorly from the posteromedial region of the cecum is the vermiform appendix, a thin, fingerlike sac lined by lymphocyte-filled lymphoid nodules. Both the cecum and the vermiform appendix are intraperitoneal organs.
Gross Anatomy: Large instestine
Colon
At the level of the ileocecal valve, the second region of the large intestine, the colon, begins and forms an inverted u-shaped arch. The colon is partitioned into 4 segments; the ascending colon, transverse colon, descending colon, and sigmoid colon.
The ascending colon originates at the ileocecal valve and extends superiorly from the superior edge of the cecum along the right lateral border of the abdominal cavity. The ascending colon is retroperitoneal, since its posterior wall directly adheres to the posterior abdominal wall, and only its anterior surface is covered with peritoneum. As it approaches the inferior surface of the liver, the ascending colon makes a 90-degree turn toward the left side and anterior region of the abdominal cavity. This bend in the colon is called the right colic flexure, or the hepatic flexure.
Gross Anatomy: Large instestine
Transverse Colon
Originate at the right colic flexure and curves slightly anteriorly as it projects horizontally to the left across the anterior region of the abdominal cavity. The transverse colon is intraperitoneal. As the transverse colon approaches the spleen in the left upper quadrant of the abdomen, it makes a 90-degree turn inferiorly and posteriorly. The resulting bend in the colon is called the left colic flexure, or the splenic flexure.
Gross Anatomy: Large instestine
Descending Colon
Is retroperitoneal and located along the left side of the abdominal and slightly posterior. It originates at the left colic flexure and descends vertically to the sigmoid colon.
Gross Anatomy: Large instestine
Sigmoid Colon
originates at the sigmoid flexure and turns inferomedially into the pelvic cavity. The sigmoid colon, like the transverse colon, is intraperitoneal. The sigmoid colon terminates at the rectum. Recall that type of mesentery, called the mesocolon, attaches each section of the colon to the posterior abdominal wall, with the mesocolon of each region specifically named (ascending mesocolon, transverse mesocolon).
Gross Anatomy: Large instestine
Rectum
Is the third major region of the large intestine. It is a retroperitoneal structure that extends from the sigmoid colon. The rectum is a muscular tube that readily expands to store accumulated fecal material prior to defecation. Three thick transverse folds of the rectum, called rectal valves, ensure that fecal material is retained during the passing of gas.
The anal canal makes up the terminal few centimeters of the large intestine. The anal canal is lined by a stratified squamous epithelium, and it passes through an opening in the elevator ani muscles of the pelvic floor and terminates at the anus. The internal lining of the anal canal contains relatively thin longitudinal ridges, called anal columns, between which are small depressions termed anal sinuses. As fecal material passes through the anal canal during defecation. Pressure exerted on the anal sinuses causes their cells to release mucin to form mucus. The extra mucus lubricates the anal canal during defecation. At the base of the anal canal are the involuntary smooth muscle internal anal sphincter and voluntary skeletal muscle external anal sphincter, which close off the opening to the anal canal. The muscles composing these sphincters relax and allow the sphincter to open during defecation.
Diverticulosis vs. Diverticulitis
Diverticulosis is the presence of diverticula (small “bulges”) in the intestinal lining. These are formed typically when the colon tightens and narrows in response to low amounts of fiber or bulk in the colon.
Diverticulitis is inflammation of diverticula. Diverticulitis may be life-threatening if the diverticula rupture and fecal matter leaks into the abdominal cavity.
6 Essential Nutrients
Carbohydrates, proteins, lipids, minerals, vitamins, and water.
Carbohydrate Digestion
Carbohydrates are organized based upon the number of repeating units of simple sugars. Carbohydrates may be classified as monosaccharides (glucose, fructose, galactose), disaccharide (sucrose, maltose, lactose), and polysaccharides (starch, cellulose). Chemical digestion of carbohydrates consists of a) the breakdown of starch into individual glucose molecules and b) the breakdown of disaccharides into the individual monosaccharides that compose them. The oral cavity and small intestine are the main sites of carbohydrate digestion.
Carbohydrate Digestion
Breakdown: Oral Cavity
Digestion of starch begins in the oral cavity. It is catalyzed by salivary amylase that is synthesized and released from the salivary glands. Salivary amylase breaks the chemical bonds between glucose molecules, within the starch molecule, to partially digest the starch molecule. The extent of starch digestion is dependent upon the length of time the salivary amylase is allowed to act on the starch.
Salivary amylase is inactivated by the low pH of the stomach when the bolus is swallowed. This inactivation typically occurs within 15 to 20 minutes after the bolus enters the stomach. The larger the meal, the longer salivary amylase remains active. This extended activity occurs because it takes longer for the swallowed bolus to be mixed with the low pH of the gastric juices that inactivate the salivary amylase. This is more likely when the bolus is within the funds of the stomach, where smooth muscle contractions are the weakest and the pH is the highest. No new enzymes for carbohydrate digestion are introduced in the stomach.
Carbohydrate Digestion
Breakdown: Small Intestine
Starch digestion continues within the small intestine. Pancreatic amylase, which is produced and secreted from the pancreas into the small intestine (step 1), continues the digestion of starch into shorter strands of glucose (5 to 25 glucose molecules, oligosaccharides), maltose (disaccharide of two glucose molecules), and individual glucose molecules (step 2).
The completion of starch breakdown is accomplished by brush border enzymes embedded within the epithelial lining of the small intestine (step 3). These enzymes include dextrinase and glucoamylase, which break the bonds between glucose subunits of oligosaccharides, and maltase that breaks the bond between two glucose molecules that compose maltose.
Protein Digestion
Proteins are polymers composed of Aino acid subunits linked by peptide bonds. Digestion of protein releases individual amino acids so that the amino acids may be absorbed into the blood and transported to cells for the synthesis of new proteins.
Proteins are broken down into amino acids by enzymes that target peptide bonds between either specific adjacent amino acids within the protein or any amino acid from the end of a protein. All enzymes that digest protein, collectively called proteolytic enzymes or proteases, are released from both the stomach and pancreas as inactive enzymes. These enzymes must be activated (pepsinogen is activated to pepsin within the low pH of the stomach). This is because the proteolytic enzymes would destroy the proteins within the cells that produce them and, in the case of protein-digesting enzymes produced in the pancreas, would destroy the cells lining the main and accessory pancreatic ducts as they passed through those ducts.
Protein Digestion
Breakdown: Stomach
Protein digestion begins within the stomach lumen. Hydrochloric acid that is formed from parietal cells causes a low pH within the stomach that both denatures proteins to facilitate their chemical breakdown and activates the formation of pepsin from pepsinogen. Pepsin is a proteolytic enzyme that chemically digests proteins into shorter strands of amino acids (oligopeptides). Chyme is moved from the stomach into the small intestine before proteins are completely digested to amino acids.
Protein Digestion
Breakdown: Small Intestine
The high pH of the small intestine inhibits further action by pepsin on protein shortly following the entry of chyme into the small intestine. Three of the enzymes that continue the digestion of protein are synthesized and released from the pancreas into the small intestine in inactive forms-trypsinogen, chymotrypsinogen, and procarboxypeptidase (step 1). Once these inactive forms of the enzymes reach the small intestine, trypsinogen is activated by the enzyme enteropeptidase, an enzyme previously synthesized by the small intestine and released into the lumen of the small intestine (step 2). Enteropeptidase activates trypsinogen to trypsin, as well as chymotrypsinogen to chymotrypsin and procarboxypeptidase to carboxypeptidase.
Lipid Digestion
Lipids are highly variable structures that contain different arrangements of their building blocks. Lipids have one unifying property, which is that they are not water-soluble (they are hydrophobic). Two major ingested lipids are triglycerides (or neutral fats) and cholesterol. Tryglycerides are composed of three fatty acids bonded to a glycerol molecule, and enzymes are required to break the bonds between glycerol and fatty acids. Chemical digestion of cholesterol is not required its absorption.
Lipid Digestion
Breakdown: Stomach
Lingual lipase (produced by intrinsic salivary glands in the mouth; is a component of saliva in the oral cavity. The optimal pH of this enzyme, however, means it is not activated until it reaches the stomach. Once in the stomach, triglycerides undergo limited digestion by both lingual lipase and gastric lipase (an enzyme produced by chief cells of the stomach. These “acidic lipases” digest approximately 30% of the triglycerides to diglyceride and a fatty acid. Neither of these lipase enzymes requires the participation of bile salts.
Lipid Digestion
Breakdown: Small Intestine
Triglyceride digestion continues within the small intestine. The processing of lipids that occurs in the small intestine is facilitated by mechanical digestion that occurs through bile. Lipids are hydrophobic molecules and do not dissolve in the luminal fluids of the digestive system, but rather form relatively large lips masses. For example, when butter is added to water, the butter does not mix with the water but remains separate. The large lipid droplets must first be mechanically separated into smaller droplets before chemical digestion by pancreatic lipase can effectively occur. This process of mechanical digestion is called emulsification. (This is similar to breaking an ice cube into ice chips.) Emulsification occurs by the action of bile salts, which are part of bile. Bile is produced by the liver and stored, concentrated, and released from the gallbladder. Bile salts are amphipathic molecules composed of a polar head and non polar tail. The non polar tails position themselves around the lipid droplets with the polar heads next to the aqueous fluid in the lumen (like an inverted spiked ball with the lipid droplets in the center; step 1). This structure is called a micelle. The function of bile salts is to emulsify lipids so tat pancreatic lipase, which is produced and secreted from the pancreas into the small intestine, has greater “access” to the triglyceride molecules and may more effectively chemically digest the triglyceride molecules. (Note that the process of emulsification is facilitated by lecithin, which is a type of phospholipid molecule within bile.
