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.