Topic 8: chpt 21 Flashcards
What constitutes the beginning of the digestive system, and what are its components?
The digestive system begins with the oral cavity (mouth and pharynx), which serves as a receptacle for food. Swallowed food then enters the gastrointestinal tract (GI tract), consisting of the esophagus, stomach, small intestine, and large intestine.
What is the gastrointestinal tract and its primary function?
The gastrointestinal tract (GI tract), also known as the gut from the stomach to the anus, is where digestion, the chemical and mechanical breakdown of food, takes place primarily in the lumen.
What organs contribute secretions to the digestive process?
Digestive secretions are added to ingested food by secretory epithelial cells and accessory glandular organs, including the salivary glands, liver, gallbladder, and pancreas
Describe the structural features of the GI tract.
The GI tract is a long tube with muscular walls lined by secretory and transporting epithelium. It features sphincters at intervals that function to separate the tube into segments with distinct functions.
How does food move and get absorbed in the GI tract?
Food is propelled through the GI tract by waves of muscle contraction. Digestion products are absorbed across the intestinal epithelium into the interstitial fluid and then into the blood or lymph.
How does waste exit the GI tract, and what is the role of bacteria?
Waste exits through the anus, and the GI tract houses a variety of bacteria, especially in the large intestine. This bacterial presence, part of the human microbiome, is beneficial and a topic of active research.
Why is the GI tract considered part of the external environment?
Since the GI tract opens to the outside world and its contents are not contained within the tissues of the body, its lumen and contents are technically part of the external environment, likened to a hole passing through a bead.
What roles do the pancreas and liver play in digestion?
The pancreas and liver contribute crucial digestive enzymes and bile, respectively, that enter the duodenum to aid in the breakdown and absorption of nutrients.
What is the function of the stomach in digestion?
The stomach mixes food with acid and enzymes, continuing the digestion started in the mouth and controls the rate at which chyme enters the duodenum through the pyloric valve, ensuring the intestine manages digestion and absorption effectively.
What are the three sections of the stomach, and what are their functions?
The stomach is divided into the upper fundus, the central body, and the lower antrum. These sections mix food with digestive juices to form chyme, which is then regulated into the small intestine via the pyloric valve.
Describe the structure and digestive function of the small intestine.
The small intestine consists of the duodenum, jejunum, and ileum. Most chemical digestion and nutrient absorption occur here, facilitated by digestive enzymes and secretions from the pancreas and liver.
What is the role of the large intestine in digestion?
The large intestine, or colon, absorbs water and electrolytes from the remaining indigestible food matter, transforming it from watery chyme to semisolid feces.
What triggers the defecation reflex?
The defecation reflex is triggered when feces enter the rectum, causing its walls to distend and initiating a reflex that results in the expulsion of feces through the anus.
Describe the anatomy and function of the anus in the digestive system.
The anus is the final section of the digestive tract, featuring an external anal sphincter of skeletal muscle under voluntary control, which allows for the expulsion of feces from the body.
How long is the digestive system, and what structural changes occur post-mortem?
In a living person, the digestive system from mouth to anus is about 450 cm long, with most of this being the intestines. Post-mortem measurements can be nearly double due to relaxation of the intestinal muscles.
What are the four layers of the gastrointestinal wall from the inner to the outer layer?
The four layers are:
1. Mucosa (inner lining)
2. Submucosa
3.Muscularis externa
4. Serosa (outer covering)
What is the mucosa, and what are its three components?
The mucosa is the inner lining of the gastrointestinal tract, consisting of:
- A single layer of mucosal epithelium facing the lumen
- Lamina propria (subepithelial connective tissue)
- Muscularis mucosae (a thin layer of smooth muscle)
Describe the role and structure of the mucosal epithelium in the gastrointestinal tract.
The mucosal epithelium is highly variable, changing from section to section. It includes transporting epithelial cells, endocrine and exocrine secretory cells, and stem cells. It functions in the secretion of ions, enzymes, mucus, and paracrine molecules into the lumen, and in absorption from the lumen.
What is the significance of the cell junctions in the gastrointestinal epithelium?
In the stomach and colon, junctions form a tight barrier limiting passage between cells. In the small intestine, junctions are looser, allowing some substances to be absorbed between cells (paracellular pathway). These junctions have plasticity and their tightness can be regulated.
What is the lamina propria, and what does it contain?
The lamina propria is subepithelial connective tissue that holds the epithelium in place. It contains nerve fibers, small blood and lymph vessels, and immune cells. It also contains lymphoid tissue like Peyer’s patches, part of the gut-associated lymphoid tissue (GALT).
What is the function of the muscularis mucosae in the gastrointestinal tract?
The muscularis mucosae is a thin layer of smooth muscle that separates the lamina propria from the submucosa. It helps in moving the villi to increase the surface area for absorption, similar to the tentacles of a sea anemone.
What does the submucosa contain and what is its role?
The submucosa is a connective tissue layer containing larger blood and lymph vessels. It houses the submucosal plexus (Meissner’s plexus), which innervates the epithelial layer and the muscularis mucosae.
What are the two layers of the muscularis externa, and what are their functions?
The muscularis externa consists of:
- An inner circular layer that decreases the lumen’s diameter.
- An outer longitudinal layer that shortens the tube.
It is involved in the peristaltic movements that propel food through the GI tract.
What is the serosa, and how is it connected to the rest of the abdominal cavity?
The serosa is the outer covering of the digestive tract, a connective tissue membrane that continues with the peritoneal membrane lining the abdominal cavity. The peritoneum also forms mesenteries that support the intestines and prevent them from tangling.
What are the four basic processes of the digestive system?
The four basic processes are:
- Digestion - Chemical and mechanical breakdown of food.
- Absorption - Movement of substances from the GI lumen to the extracellular fluid.
- Secretion - Movement of water and ions from the ECF to the GI tract lumen, and release of synthesized substances.
- Motility - Movement of material in the GI tract due to muscle contraction.
What is digestion, and what does it involve?
Digestion is the chemical and mechanical breakdown of foods into smaller units that can be absorbed across the intestinal epithelium into the body. It involves the secretion of enzymes that break down complex macromolecules into smaller absorbable units.
Define absorption in the context of the digestive system.
Absorption is the process of moving digested nutrients, water, and electrolytes from the lumen of the GI tract across the intestinal epithelium into the extracellular fluid.
What are the two meanings of secretion in the digestive system?
In the GI tract, secretion can refer to:
- The movement of water and ions from the extracellular fluid to the digestive tract lumen.
- The release of substances like enzymes and mucus synthesized by GI epithelial cells into the lumen or the ECF.
What challenges does the digestive system face regarding autodigestion?
The digestive system must prevent autodigestion, where digestive enzymes digest the cells of the GI tract itself. If protective mechanisms fail, it can lead to the development of peptic ulcers.
What is the challenge of mass balance in the digestive system?
The digestive system must maintain mass balance by matching fluid input with output. About 9 liters of fluid (2 liters ingested and 7 liters secreted) enters the GI tract daily, and efficient reabsorption is crucial to prevent dehydration.
How does the digestive system manage the risk of dehydration from fluid loss?
Normally, intestinal reabsorption is efficient, losing only about 100 mL of fluid in the feces daily. However, conditions like vomiting and diarrhea can cause significant fluid loss, threatening blood pressure and ECF volume.
How does the digestive system coordinate its processes to overcome its challenges?
The body coordinates motility and secretion to maximize digestion and absorption while balancing the need to protect against pathogens and prevent autodigestion or excessive fluid loss.
How much fluid passes through the adult GI tract daily, and what are the sources?
Daily, 9 liters of fluid pass through the GI tract, of which only 2 liters are ingested. The remaining 7 liters come from body water secreted along with ions, enzymes, and mucus.
How does water move through the epithelial cells in the GI tract?
Water moves through the epithelial cells via channels or through leaky junctions between cells, known as the paracellular pathway, following the osmotic gradient created by the transfer of solutes.
What is the significance of the polarization of gastrointestinal epithelial cells?
GI epithelial cells are polarized, with distinct apical and basolateral membranes featuring specific membrane proteins that dictate the direction of solute and water movement across the epithelium.
How are digestive enzymes secreted and activated in the GI tract?
Digestive enzymes are synthesized on the rough ER, packaged by the Golgi into vesicles, and stored until needed. They are released by exocytosis and often remain bound to the apical membranes of intestinal cells. Some are secreted as inactive zymogens and must be activated in the GI lumen.
What are zymogens and how are they activated?
Zymogens are inactive proenzyme forms of digestive enzymes, allowing stockpiling without damaging cells. They are activated in the GI lumen; common zymogens include pepsinogen, which becomes pepsin.
What are the primary functions of mucus in the GI tract?
Mucus primarily serves to protect the GI mucosa and lubricate the gut contents. It is composed of glycoproteins called mucins.
Where and how is mucus produced in the digestive system?
Mucus is produced by mucous cells in the stomach and salivary glands, and by goblet cells in the intestine. Goblet cells constitute 10% to 24% of intestinal cells
What stimulates mucus release in the GI tract?
Mucus release is stimulated by parasympathetic innervation, neuropeptides in the enteric nervous system, cytokines from immunocytes, and is increased by parasitic infections and inflammatory processes to strengthen the protective barrier.
How does the GI system use mechanical processes in digestion?
The GI system uses chewing and churning to break food into smaller pieces, increasing the surface area exposed to digestive enzymes for more effective chemical breakdown.
At what pH do stomach and small intestine enzymes function best, and why?
Stomach enzymes function best at acidic pH due to the stomach’s acidic environment, while enzymes in the small intestine operate optimally at alkaline pH, reflecting their respective locations and functions.
Where does most nutrient absorption occur in the GI tract?
Most nutrient absorption occurs in the small intestine, with additional absorption of water and ions in the large intestine.
How is motility facilitated in the GI tract?
Motility is facilitated by the spontaneous contraction of GI smooth muscle, which helps move food from the mouth to the anus and mixes the food to increase exposure to digestive enzymes.
What are the two types of contractions in the GI smooth muscle, and where do they occur?
Tonic contractions, sustained for minutes or hours, occur in some sphincters and the anterior stomach. Phasic contractions, lasting a few seconds per cycle, occur in the posterior stomach and small intestine.
What generates slow wave potentials in the GI tract?
Slow wave potentials are generated by the interstitial cells of Cajal (ICCs), which function as pacemakers and are located between the smooth muscle layers and intrinsic nerve plexuses.
How do slow wave potentials influence smooth muscle contraction?
Slow waves that reach the threshold cause voltage-gated Ca²⁺ channels to open, leading to muscle contraction. The strength of contraction is graded by the duration of the slow wave and the amount of Ca²⁺ that enters.
What is the role of ICCs in relation to GI motility disorders?
Researchers are investigating the link between ICCs and functional bowel disorders like irritable bowel syndrome and chronic constipation, as ICCs coordinate GI motility
What is the relationship between slow waves and action potentials in GI smooth muscle?
Slow waves must reach a threshold to trigger action potentials. If they do not reach threshold, no muscle contraction occurs. When they do, the resulting Ca²⁺ influx initiates contraction.
How does the enteric nervous system affect GI motility?
The enteric nervous system primarily influences whether slow waves reach the threshold to fire action potentials, thereby controlling the rhythmic contractions of GI smooth muscle.
What is the migrating motor complex and its function in the GI tract?
The migrating motor complex is a series of contractions that sweeps food remnants and bacteria from the stomach through the GI tract to the large intestine, functioning as a “housekeeping” tool to clean the upper GI tract between meals.
Describe peristalsis in the gastrointestinal tract.
Peristalsis involves progressive waves of contraction that move a food bolus forward through the GI tract. Circular muscles contract behind the bolus, pushing it into a relaxed receiving segment that subsequently contracts to continue the movement forward.
What speeds do peristaltic contractions achieve, and where are they most significant?
Peristaltic contractions can propel food at speeds between 2 and 25 cm/sec, being crucial in the esophagus to move materials from the pharynx to the stomach, and aiding in food mixing within the stomach.
What are segmental contractions and their role in digestion?
Segmental contractions involve short sections of the intestine alternately contracting and relaxing to churn and mix intestinal contents, ensuring thorough contact with the absorptive epithelium for effective digestion and nutrient absorption.
How do motility disorders affect the GI tract?
Motility disorders can range from esophageal spasms and delayed gastric emptying to constipation and diarrhea, impacting the normal movement of content through the GI tract and causing symptoms like abdominal pain and altered bowel habits.
What is guanylate cyclase-C (GC-C) and its role in GI health?
GC-C is a receptor-enzyme on the luminal side of intestinal epithelial cells, regulating fluid secretion in the intestine. Overactivation by pathogenic bacterial toxins can cause excessive fluid secretion, leading to diarrhea.
What are the four basic GI processes essential for moving substances from the external environment into the body’s internal environment?
The four basic processes are digestion (breakdown of food into smaller units), absorption (movement of substances into the ECF), secretion (movement of water and ions into the GI lumen and release of substances by GI cells), and motility (movement of material through the GI tract).
What challenges does the digestive system face in processing food?
The challenges include avoiding autodigestion, maintaining mass balance of fluids, and defending against pathogens due to the large surface area exposed to the external environment.
How does the digestive system prevent autodigestion?
The system prevents autodigestion by secreting powerful enzymes to digest food without harming GI tract cells themselves. Failures in this protective mechanism can lead to peptic ulcers.
Describe the significance of mass balance in the digestive system.
Mass balance involves matching fluid intake with output to prevent dehydration. Approximately 9 liters of fluid pass through the GI tract daily, with most being reabsorbed to maintain fluid homeostasis.
How does the digestive system protect the body from pathogens?
The GI tract uses physiological defense mechanisms like mucus, enzymes, acid, and gut-associated lymphoid tissue (GALT) to prevent pathogens in food from entering the body’s internal environment.
What role does the enteric nervous system (ENS) play independently of the central nervous system?
The ENS can function independently to regulate GI motility, secretion, and local reflexes, similar to the nerve networks of Cnidaria, such as jellyfish and sea anemones.
What are the similarities between the enteric and central nervous systems?
Both systems have intrinsic neurons, use similar neurotransmitters and neuromodulators, are supported by glial cells, and possess diffusion barriers to protect against uncontrolled substance flow.
What is the function of short reflexes in the ENS?
Short reflexes are confined to the GI tract where they start and end, controlling local conditions without input from the CNS. They manage processes like secretion and motility through the submucosal and myenteric plexuses.
How do long reflexes in the GI tract operate?
Long reflexes involve sensory inputs transmitted to and processed by the CNS, impacting GI function. These reflexes can originate from both within the GI tract and external stimuli like the sight and smell of food.
What are the roles of parasympathetic and sympathetic nerves in GI function?
Parasympathetic nerves generally stimulate GI functions, enhancing digestion and absorption (“rest and digest”), while sympathetic nerves inhibit these processes, often reducing secretion and motility during stress (“fight or flight”).
What roles do GI peptides play in the digestive system?
GI peptides excite or inhibit motility and secretion, and some act as hormones or paracrine signals. They may also influence the brain, such as cholecystokinin enhancing satiety or ghrelin increasing food intake.
What are the two forms of secretion for GI peptides?
GI peptides can be secreted into the lumen, affecting epithelial receptors, or into the extracellular fluid to influence neighboring cells.
How do GI peptides affect areas outside the gastrointestinal tract?
Some GI peptides have actions that involve the brain, influencing feelings like satiety or hunger.
Describe the discovery and significance of the first GI hormone.
Secretin was the first GI hormone discovered by Bayliss and Starling in 1902, identified when acidic chyme triggered pancreatic juice release, demonstrating hormonal communication via the bloodstream.
What are the three families of GI hormones?
The gastrin family (gastrin, CCK), the secretin family (secretin, VIP, GIP, GLP-1), and a third family with peptides like motilin.
How do gastrin and cholecystokinin (CCK) interact with receptors?
Due to their structural similarity, gastrin and CCK can bind to and activate the same CCKB receptor.
What was the original and revised function of the hormone GIP?
Originally known as gastric inhibitory peptide for blocking gastric acid secretion, it is now also called glucose-dependent insulinotropic peptide, reflecting its role in stimulating insulin release in response to glucose.
What is the function of GLP-1?
Glucagon-like peptide-1 (GLP-1), along with GIP, acts as a feedforward signal for insulin release, particularly in response to glucose in the intestinal lumen.
What role does motilin play in the GI tract?
Motilin secretion increases are associated with the migrating motor complex, helping to regulate the timing of gut motility between meals.
What triggers the cephalic phase of digestion?
The cephalic phase is triggered by anticipatory stimuli such as smelling, seeing, or thinking about food, which activate neurons in the medulla oblongata to prepare the digestive system by increasing secretion and motility.
How does the medulla oblongata contribute to the cephalic phase of digestion?
The medulla oblongata sends efferent signals through autonomic neurons to the salivary glands and through the vagus nerve to the enteric nervous system, enhancing gastrointestinal preparation for incoming food.
What are the four main functions of saliva?
Saliva functions to 1) soften and moisten food, 2) initiate starch digestion with salivary amylase, 3) enhance taste by dissolving food particles, and 4) provide defense with antibacterial enzymes and immunoglobulins.
What is the primary anatomical structure of salivary glands?
Salivary glands consist of acinar cells arranged in grapelike clusters, with ducts that converge into larger ducts leading to the mouth. These glands produce a hyposmotic saliva that varies in enzyme and mucus content.
Describe the composition and production process of saliva.
Saliva is initially secreted as an isotonic NaCl solution, which becomes hyposmotic as NaCl is reabsorbed and K+ and bicarbonate are secreted by duct cells. This process creates saliva that is high in K+ and low in Na+.
How is salivation controlled?
Salivation is primarily controlled by parasympathetic innervation, with some sympathetic influence. It can be triggered by sensory stimuli such as sight, smell, and thought of food.
How does swallowing (deglutition) function?
Swallowing is a reflex that moves food from the mouth to the stomach, beginning with tongue pressure that activates sensory neurons. This triggers muscle contractions that close the airway, open the esophagus, and push the bolus towards the stomach.
What are the phases of swallowing?
Swallowing involves elevating the soft palate, lifting and forwarding the larynx, folding the epiglottis, relaxing the upper esophageal sphincter, and peristaltic contractions moving the bolus through the esophagus.
What role does the lower esophageal sphincter play in digestion?
The lower esophageal sphincter maintains high muscle tension to separate the esophagus from the stomach, relaxing during swallowing to allow food passage and preventing gastric acid reflux when contracted.
How does the body respond to the risk of gastroesophageal reflux?
During inspiration, when intrapleural pressure drops, the esophageal walls expand, creating subatmospheric pressure that could draw stomach contents upward if the lower esophageal sphincter is relaxed.
What is gastroesophageal reflux disorder (GERD)?
ERD is a common disorder where improper contraction of the lower esophageal sphincter allows stomach acid to irritate the esophagus, often leading to heartburn.
What are the three main functions of the stomach?
The stomach functions to 1) store food and regulate its passage to the small intestine, 2) chemically and mechanically digest food into chyme, and 3) protect the body by destroying pathogens and protecting itself from its own secretions.
What triggers digestive activity in the stomach before food arrives?
Digestive activity is initiated by the long vagal reflex from the cephalic phase, even before food enters the stomach.
Describe the concept of “receptive relaxation” in the stomach.
Receptive relaxation is a neurally mediated reflex where the stomach relaxes and expands to accommodate incoming food from the esophagus, enhancing its storage capacity.
