Topic 8: chpt 21 Flashcards
(224 cards)
1
Q
What constitutes the beginning of the digestive system, and what are its components?
A
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.
2
Q
What is the gastrointestinal tract and its primary function?
A
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.
3
Q
What organs contribute secretions to the digestive process?
A
Digestive secretions are added to ingested food by secretory epithelial cells and accessory glandular organs, including the salivary glands, liver, gallbladder, and pancreas
4
Q
Describe the structural features of the GI tract.
A
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.
5
Q
How does food move and get absorbed in the GI tract?
A
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.
6
Q
How does waste exit the GI tract, and what is the role of bacteria?
A
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.
7
Q
Why is the GI tract considered part of the external environment?
A
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.
8
Q
What roles do the pancreas and liver play in digestion?
A
The pancreas and liver contribute crucial digestive enzymes and bile, respectively, that enter the duodenum to aid in the breakdown and absorption of nutrients.
9
Q
What is the function of the stomach in digestion?
A
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.
10
Q
What are the three sections of the stomach, and what are their functions?
A
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.
11
Q
Describe the structure and digestive function of the small intestine.
A
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.
12
Q
What is the role of the large intestine in digestion?
A
The large intestine, or colon, absorbs water and electrolytes from the remaining indigestible food matter, transforming it from watery chyme to semisolid feces.
13
Q
What triggers the defecation reflex?
A
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.
14
Q
Describe the anatomy and function of the anus in the digestive system.
A
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.
15
Q
How long is the digestive system, and what structural changes occur post-mortem?
A
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.
16
Q
What are the four layers of the gastrointestinal wall from the inner to the outer layer?
A
The four layers are:
1. Mucosa (inner lining)
2. Submucosa
3.Muscularis externa
4. Serosa (outer covering)
17
Q
What is the mucosa, and what are its three components?
A
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)
18
Q
Describe the role and structure of the mucosal epithelium in the gastrointestinal tract.
A
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.
19
Q
What is the significance of the cell junctions in the gastrointestinal epithelium?
A
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.
20
Q
What is the lamina propria, and what does it contain?
A
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).
21
Q
What is the function of the muscularis mucosae in the gastrointestinal tract?
A
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.
22
Q
What does the submucosa contain and what is its role?
A
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.
23
Q
What are the two layers of the muscularis externa, and what are their functions?
A
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.
24
Q
What is the serosa, and how is it connected to the rest of the abdominal cavity?
A
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.
25
What are the four basic processes of the digestive system?
The four basic processes are:
1. Digestion - Chemical and mechanical breakdown of food.
2. Absorption - Movement of substances from the GI lumen to the extracellular fluid.
3. Secretion - Movement of water and ions from the ECF to the GI tract lumen, and release of synthesized substances.
4. Motility - Movement of material in the GI tract due to muscle contraction.
26
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.
27
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.
28
What are the two meanings of secretion in the digestive system?
In the GI tract, secretion can refer to:
1. The movement of water and ions from the extracellular fluid to the digestive tract lumen.
2. The release of substances like enzymes and mucus synthesized by GI epithelial cells into the lumen or the ECF.
29
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.
30
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.
31
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.
32
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.
33
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.
34
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.
35
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.
36
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.
37
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.
38
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.
39
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
40
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.
41
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.
42
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.
43
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.
44
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.
45
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.
46
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.
47
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.
48
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
49
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.
50
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.
51
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.
52
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.
53
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.
54
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.
55
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.
56
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.
57
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).
58
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.
59
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.
60
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.
61
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.
62
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.
63
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.
64
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.
65
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.
66
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").
67
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.
68
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.
69
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.
70
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.
71
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.
72
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.
73
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.
74
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.
75
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.
76
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.
77
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.
78
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.
79
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.
80
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+.
81
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.
82
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.
83
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.
84
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.
85
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.
86
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.
87
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.
88
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.
89
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.
90
What is "dumping syndrome"?
Dumping syndrome is a side effect of surgery that removes parts of the stomach or small intestine, causing rapid gastric emptying into the small intestine, leading to diarrhea due to overwhelmed digestion and absorption.
91
How is gastric motility regulated during a meal?
Enhanced gastric motility is primarily under neural control, stimulated by stomach distension and regulated by short reflexes to mix food with gastric secretions effectively.
92
What stimulates gastrin secretion in the stomach?
Gastrin secretion is stimulated by amino acids and peptides in the stomach, stomach distension, coffee, and neural reflexes mediated by gastrin-releasing peptide (GRP) from parasympathetic neurons of the vagus nerve.
93
What are the main functions of gastric acid (HCl) in the stomach?
Gastric acid activates pepsin, denatures proteins, helps kill ingested microorganisms, and stops carbohydrate digestion by inactivating salivary amylase.
94
Describe the parietal cell mechanism for acid secretion
Parietal cells secrete HCl by pumping H+ into the stomach lumen in exchange for K+, with Cl- following the electrical gradient. This process is targeted by proton pump inhibitors (PPIs) to treat acid hypersecretion.
95
What enzymes are secreted by the stomach, and what are their roles?
The stomach secretes pepsin for protein digestion and gastric lipase for fat digestion, though less than one-third of fat digestion occurs in the stomach.
96
What are the roles of paracrine secretions in the stomach?
Paracrine secretions include histamine, which stimulates acid secretion, somatostatin, which inhibits gastric secretion and motility, and intrinsic factor, which is crucial for vitamin B12 absorption.
97
How does the stomach protect itself from autodigestion?
The gastric mucosa is protected by a mucus-bicarbonate barrier, where mucus forms a physical barrier and bicarbonate acts as a chemical buffer to maintain a near-neutral pH at the cell surface despite highly acidic conditions.
98
What is Zollinger-Ellison syndrome?
Zollinger-Ellison syndrome involves excessive gastrin secretion, typically from tumors, leading to peptic ulcers due to overwhelming the stomach's protective mechanisms.
99
What are the most common causes of peptic ulcers?
The most common causes are NSAIDs, which inhibit prostaglandin production, and Helicobacter pylori infection, which induces inflammation of the gastric mucosa.
100
How have discoveries about acid secretion influenced treatments for hyperacidity?
Discoveries about parietal cell acid secretion led to the development of H2 receptor antagonists and proton pump inhibitors, which are now mainstays in treating conditions like gastroesophageal reflux disease and peptic ulcers.
101
What initiates the gastric phase of digestion even before food arrives in the stomach?
The gastric phase is initiated by the long vagal reflex of the cephalic phase, which begins with anticipatory stimuli related to food and activates neurons in the medulla to signal the stomach.
102
What mechanisms are activated in the stomach once food enters?
Once food enters, gastric lumen stimuli like distension and the presence of peptides/amino acids activate endocrine cells and enteric neurons, triggering hormones, neurotransmitters, and paracrine molecules that affect motility and secretion.
103
How does the stomach's structure facilitate its function as a food storage organ?
The upper half of the stomach acts as a storage area, where food is held until it can be processed, relying on receptive relaxation to expand and accommodate incoming food.
104
How do peristaltic waves function in the stomach?
Peristaltic waves in the lower stomach mix food with gastric secretions and push chyme toward the pylorus, allowing small amounts to pass into the duodenum, ensuring optimal digestion and absorption
105
What are the primary cell types in the gastric glands and their secretions?
Gastric glands contain multiple cell types that produce gastric acid (HCl), enzymes, hormones, and paracrine molecules which collectively facilitate digestion and regulate gastric environment.
106
How does the secretion of gastrin function and what stimulates it?
Gastrin secretion by G cells is stimulated by amino acids, peptides, and stomach distension, enhancing acid secretion directly and indirectly by increasing histamine release.
107
Describe the parietal cell mechanism for secreting gastric acid.
Parietal cells transport H+ ions into the stomach lumen in exchange for K+, with Cl- following to maintain electrical neutrality, resulting in HCl secretion.
108
What are the functions of gastric acid?
Gastric acid activates pepsin, denatures proteins, kills bacteria, and stops carbohydrate digestion, creating an environment conducive to digestive processes.
109
What roles do pepsin and gastric lipase play in the stomach?
Pepsin digests proteins, particularly effective on collagen, while gastric lipase, although less significant, aids in fat digestion.
110
What are the effects of histamine, somatostatin, and intrinsic factor secretions in the stomach?
Histamine stimulates acid secretion; somatostatin inhibits acid and enzyme secretion and gastric motility; intrinsic factor is crucial for vitamin B12 absorption in the intestine.
111
How does the stomach protect itself from the harsh conditions created by its own secretions?
A mucus-bicarbonate barrier on the gastric mucosa forms a physical and chemical protection against the acidic environment, preventing autodigestion.
112
Explain the pathological mechanism and consequences of Zollinger-Ellison syndrome.
This syndrome involves excessive gastrin secretion from tumors, leading to overwhelming acid production, breaking down protective gastric mechanisms, and often causing peptic ulcers.
113
What are the modern treatments for excess acid secretion and how do they work?
Modern treatments include H2 receptor antagonists and proton pump inhibitors, which reduce acid secretion by blocking key pathways in acid production.
114
What initiates the intestinal phase of digestion?
The intestinal phase of digestion begins once chyme passes into the small intestine. This phase is crucial for controlling the entry of chyme to avoid overwhelming the small intestine and ensuring efficient digestion and nutrient absorption.
115
How is motility in the small intestine controlled during the intestinal phase?
Intestinal motility is controlled through a combination of segmental and peristaltic contractions. These movements mix chyme with digestive enzymes and expose nutrients to the mucosal epithelium for absorption, ensuring that chyme moves slowly enough to allow complete digestion and absorption.
116
What roles do parasympathetic innervation and gastrointestinal hormones play in intestinal motility?
Parasympathetic innervation and gastrointestinal hormones like gastrin and CCK (cholecystokinin) promote intestinal motility, facilitating the forward movement and mixing of chyme. Conversely, sympathetic innervation inhibits intestinal motility.
117
What is the total volume of substances entering the small intestine daily, and how much is absorbed?
Approximately 9 liters of food, fluid, and secretions enter the small intestine each day, which includes about 3.5 liters from hepatic, pancreatic, and intestinal secretions. About 7.5 liters of this is absorbed primarily in the duodenum and jejunum.
118
Describe the anatomy of the small intestine and its role in digestion.
The small intestine's anatomy maximizes its surface area for secretion, digestion, and absorption. It features villi and crypts macroscopically, and microvilli microscopically, which increase surface area and host enzymes and a glycocalyx coat on enterocytes, enhancing nutrient absorption.
119
What is the role of the hepatic portal system in nutrient absorption?
The hepatic portal system transports venous blood from the digestive tract to the liver, acting as a biological filter. It allows hepatic clearance of metabolites, drugs, and xenobiotics before they reach systemic circulation, illustrating the liver's critical filtering role.
120
How do intestinal secretions promote digestion?
Intestinal secretions include digestive enzymes, bile, bicarbonate, mucus, and isotonic NaCl solution. These components facilitate the chemical breakdown of nutrients, neutralize stomach acid, lubricate intestinal contents, and protect the intestinal epithelium.
121
What is the function of the isotonic NaCl secretion in the small intestine and colon?
Isotonic NaCl secretion involves the secretion of chloride into the lumen, drawing sodium and water along osmotic and electrical gradients, respectively. This process results in the secretion of isotonic saline, which lubricates gut contents and facilitates the movement and mixing of chyme.
122
What is the role of bile in the small intestine?
Bile, produced in the liver and stored in the gallbladder, is a nonenzymatic solution that emulsifies fats in the small intestine. This process increases the surface area of fats, making them more accessible to digestive enzymes and facilitating fat digestion.
123
Describe the bicarbonate secretion in the small intestine.
Bicarbonate is primarily secreted by the pancreas and to a lesser extent by the small intestine itself. It neutralizes the highly acidic chyme entering from the stomach, creating a more favorable pH for intestinal enzymes to function effectively.
124
What is the role of mucus in the small intestine?
Mucus secreted by intestinal goblet cells serves to protect the epithelium from mechanical and chemical damage and lubricates the intestinal contents, facilitating their smooth transit through the intestine.
125
How does the cystic fibrosis transmembrane conductance regulator (CFTR) channel function in isotonic NaCl secretion?
The CFTR channel facilitates the movement of chloride ions from intestinal crypt cells into the lumen. This ion movement creates an electrical gradient that draws sodium ions through the paracellular pathway, with water following osmotically, resulting in the secretion of isotonic saline.
126
What are the effects of parasympathetic and sympathetic innervation on the small intestine?
Parasympathetic innervation generally stimulates functions such as enzyme secretion and peristaltic movement, enhancing digestion and nutrient absorption. In contrast, sympathetic innervation tends to inhibit these processes, slowing down digestion and motility.
127
What are the two types of secretory epithelium in the pancreas?
The pancreas contains both endocrine and exocrine types of secretory epithelium. Endocrine secretions, such as insulin and glucagon, come from clusters of cells called islets. Exocrine secretions include digestive enzymes and a sodium bicarbonate (NaHCO3) solution.
128
What is the structure of the exocrine portion of the pancreas?
The exocrine portion of the pancreas consists of lobules called acini, similar to those of the salivary glands. Acinar cells secrete digestive enzymes, and duct cells secrete a solution of sodium bicarbonate. Ducts from the acini empty these secretions into the duodenum.
129
How are most pancreatic enzymes secreted and activated?
Most pancreatic enzymes are secreted as inactive zymogens. The activation begins in the intestine when brush border enteropeptidase converts trypsinogen to trypsin, which then activates other pancreatic zymogens into their active forms.
130
What triggers pancreatic enzyme release?
Pancreatic enzyme release is triggered by distension of the small intestine, the presence of food in the intestine, neural signals, and the GI hormone cholecystokinin (CCK).
131
How is bicarbonate secretion from the pancreas regulated, and what is its purpose?
Bicarbonate secretion into the duodenum neutralizes stomach acid entering from the stomach. It is produced from CO2 and water, requiring high levels of carbonic anhydrase. Bicarbonate is secreted by an apical Cl-/HCO3- exchanger, and H+ produced is expelled by basolateral Na+/H+ exchangers.
132
How does bicarbonate secretion interact with chloride and sodium ions?
Chloride for the bicarbonate exchange enters pancreatic cells on a basolateral NKCC cotransporter and leaves via an apical CFTR channel. Luminal Cl- then reenters the cell in exchange for HCO3- entering the lumen. Sodium follows negative ions from the ECF to the lumen through leaky junctions due to electrochemical gradients.
133
What is the role of sodium bicarbonate secretion in cystic fibrosis?
In cystic fibrosis, defects in the CFTR channel disrupt the secretion of Cl- and fluid, leading to thick mucus that blocks pancreatic ducts and prevents enzyme secretion. This is due to a mutation causing the CFTR protein to be defective or absent, also affecting the mucociliary escalator in the respiratory system.
134
Describe the fluid movement in pancreatic secretion.
In the pancreas, the secretion of a watery sodium bicarbonate solution is a passive process driven by electrochemical and osmotic gradients. As Na+ and HCO3- are transferred from ECF to the lumen, water follows osmotically, completing the secretion process.
135
What is bile and where is it secreted from?
Bile is a nonenzymatic solution secreted from hepatocytes, or liver cells. It plays a crucial role in the digestion and absorption of fats.
136
What are the key components of bile?
The key components of bile include:
1. Bile salts - Facilitate enzymatic fat digestion.
2. Bile pigments (e.g., bilirubin) - Waste products of hemoglobin degradation.
3. Cholesterol - Excreted in the feces.
137
What role do bile salts play in digestion?
Bile salts act as detergents to make fats soluble during digestion, enhancing the enzymatic breakdown of fats.
138
How and where is bile stored and concentrated?
Bile secreted by hepatocytes travels in hepatic ducts to the gallbladder, which stores and concentrates the bile solution.
139
What triggers the release of bile from the gallbladder?
During a meal that includes fats, contraction of the gallbladder sends bile into the duodenum through the common bile duct.
140
What is the fate of bile salts after fat digestion?
Bile salts are not altered during fat digestion and are reabsorbed in the terminal section of the small intestine (ileum). They are then recycled back to the liver through the hepatic portal vein.
141
How are bile salts recycled in the body?
Reabsorbed bile salts are sent back into circulation, return to the liver, and are resecreted by hepatocytes. This recirculation is essential for fat digestion, as the body’s pool of bile salts must cycle from two to five times for each meal.
142
What happens to bilirubin and other waste products in bile?
Bilirubin and other wastes secreted in bile cannot be reabsorbed and are excreted in the feces. This process helps in the elimination of waste products from the body.
143
How does the transition from stomach to small intestine affect protein digestion?
Protein digestion initiated in the stomach stops when chyme enters the small intestine because the enzyme pepsin, which functions at a low pH, is inactivated by the higher pH of the small intestine.
144
What completes the digestion of proteins, carbohydrates, and fats in the small intestine?
Pancreatic and brush border enzymes in the small intestine finish the digestion of proteins, carbohydrates, and fats. These enzymes break down the macromolecules into smaller molecules that can be easily absorbed.
145
What is the impact of the small intestine's pH on enzymatic activity?
The higher pH of the small intestine deactivates pepsin (from the stomach) but activates pancreatic and intestinal enzymes that are crucial for the final stages of chemical digestion.
146
What are the primary types of lipids found in the Western diet, and what percentage of fat calories come from triglycerides?
The Western diet primarily includes triglycerides, cholesterol, phospholipids, long-chain fatty acids, and fat-soluble vitamins. Nearly 90% of fat calories come from triglycerides, the main form of lipid in both plants and animals.
147
Why is fat digestion complicated and what role do bile salts play in this process?
Fat digestion is complicated due to the poor water solubility of most lipids, resulting in a coarse emulsion of large fat droplets in the chyme. Bile salts secreted by the liver into the small intestine help break down this coarse emulsion into smaller, more stable particles, increasing the surface area available for enzymatic digestion.
148
What is the structure and function of bile salts in fat digestion?
Bile salts are amphipathic, meaning they have both hydrophobic regions and hydrophilic regions. The hydrophobic regions associate with the surfaces of lipid droplets, while the polar side chains interact with water, creating a stable emulsion of small, water-soluble fat droplets.
149
How are fats enzymatically digested in the small intestine?
Enzymatic fat digestion involves lipases that remove two fatty acids from each triglyceride molecule, resulting in one monoglyceride and two free fatty acids. Colipase, a protein cofactor from the pancreas, assists by displacing some bile salts and allowing lipase access to the fats.
150
What are the roles of micelles in fat digestion and absorption?
As digestion proceeds, fatty acids, bile salts, mono- and diglycerides, phospholipids, and cholesterol form small disk-shaped micelles. These micelles transport lipids to the enterocytes, where lipophilic substances like fatty acids and monoglycerides are absorbed primarily by simple diffusion.
151
How is cholesterol absorbed in the small intestine?
Initially thought to diffuse passively, cholesterol absorption involves specific, energy-dependent transport proteins on the brush border membrane, including NPC1L1, which is targeted by the cholesterol absorption inhibitor ezetimibe.
152
Describe the process of reassembling and transporting fats within enterocytes.
Once inside the enterocytes, monoglycerides and fatty acids move to the smooth endoplasmic reticulum to recombine into triglycerides. These triglycerides then join with cholesterol and proteins to form chylomicrons, which leave the cell via exocytosis and enter lacteals due to their size.
153
What is the fate of chylomicrons and shorter fatty acids after their formation in enterocytes?
Chylomicrons, due to their large size, enter lacteals and pass through the lymphatic system to the venous blood near the heart. Shorter fatty acids, which are not assembled into chylomicrons, can cross capillary basement membranes directly into the blood.
154
What are the primary types of dietary carbohydrates, and what enzymes are involved in their digestion?
The primary dietary carbohydrates include starch, sucrose, glycogen, cellulose, lactose, maltose, glucose, and fructose. Amylase breaks down long glucose polymers into smaller chains and maltose. Salivary amylase starts starch digestion in the mouth and is denatured in the stomach; pancreatic amylase then continues in the small intestine. Disaccharides are further broken down by intestinal brush-border enzymes (disaccharidases) such as maltase, sucrase, and lactase.
155
What are the absorbable end products of carbohydrate digestion, and what restrictions exist on carbohydrate absorption?
The absorbable end products of carbohydrate digestion are monosaccharides: glucose, galactose, and fructose. Intestinal carbohydrate absorption is restricted to these monosaccharides, meaning all larger carbohydrates like starch and glycogen must be digested into monosaccharides to be absorbed. Cellulose remains undigested due to the absence of necessary enzymes and constitutes dietary fiber.
156
What is lactose intolerance, and what causes it?
Lactose intolerance occurs when individuals have insufficient lactase, the enzyme needed to digest lactose (milk sugar), leading to symptoms like diarrhea, bloating, and flatulence when consuming dairy products. This condition is common in adults except for some of European descent who inherit a gene allowing continued lactase production.
157
How are glucose and galactose absorbed in the small intestine?
Glucose and galactose are absorbed via the apical Na+ glucose SGLT symporter and the basolateral GLUT2 transporter, similar to the transport mechanism in the renal proximal tubule. These transporters also move galactose.
158
How is fructose absorbed in the small intestine, and how does it differ from glucose absorption?
Fructose is absorbed via facilitated diffusion, which is not dependent on sodium. It crosses the apical membrane of enterocytes through the GLUT5 transporter and the basolateral membrane via GLUT2.
159
How do enterocytes manage to keep intracellular glucose concentrations high enough to facilitate diffusion into the extracellular space?
Unlike most other cells, enterocytes (and proximal tubule cells) do not use glucose as their primary energy source; they use the amino acid glutamine instead. This allows absorbed glucose to pass unchanged into the bloodstream without being metabolized immediately.
160
What are the sources of proteins in the intestinal lumen, and what is their digestibility?
Most ingested proteins are polypeptides or larger, with plant proteins being the least digestible and egg protein being the most digestible (85–90%). Surprisingly, 30% to 60% of the protein in the intestinal lumen is not from food but from sloughed off dead cells and secreted proteins like enzymes and mucus.
161
What are the two broad groups of enzymes for protein digestion, and how do they function?
Protein digestion enzymes are classified into endopeptidases (proteases) and exopeptidases. Endopeptidases break internal peptide bonds, creating smaller peptide fragments. Examples include pepsin in the stomach and trypsin and chymotrypsin from the pancreas. Exopeptidases release amino acids from peptides one at a time; aminopeptidases act on the amino-terminal end, and carboxypeptidases act on the carboxy-terminal end.
162
How are proteases activated in the GI tract?
Proteases are secreted as inactive proenzymes (zymogens) by epithelial cells in the stomach, intestine, and pancreas. They become activated once they reach the GI tract lumen, ensuring that they do not damage the cells in which they were produced.
163
How are peptides and amino acids absorbed in the intestine?
Peptides are absorbed as free amino acids, dipeptides, and tripeptides via multiple transport systems. Free amino acids mostly use Na+-dependent cotransport proteins. Dipeptides and tripeptides use the H+-dependent cotransporter PepT1. Inside the cells, oligopeptides can be digested to amino acids by cytoplasmic peptidases or transported intact across the basolateral membrane.
164
What roles do oligopeptide transporters play in drug absorption?
The oligopeptide transporter PepT1 is not only essential for absorbing dietary peptides but also facilitates the intestinal uptake of various drugs, including certain antibiotics, angiotensin-converting enzyme inhibitors, and thrombin inhibitors.
165
What is the role of exopeptidases in protein digestion and how do they function?
Exopeptidases, such as aminopeptidases and carboxypeptidases, function by releasing single amino acids from the ends of peptide chains. Aminopeptidases act on the amino-terminal end, while carboxypeptidases target the carboxy-terminal end. The pancreatic carboxypeptidases, particularly, are crucial in the final steps of protein digestion, further breaking down peptides into absorbable units.
166
What is transcytosis and its significance in peptide absorption?
Transcytosis is a process where peptides larger than three amino acids are absorbed by binding to membrane receptors on the luminal surface of the intestine and then transported across the cell. This process is crucial for the absorption of certain peptides that may act as antigens, potentially leading to allergic reactions.
167
How does peptide absorption contribute to the development of food allergies and intolerances?
Peptides absorbed through transcytosis can act as antigens, stimulating antibody formation. This immune response can result in food allergies and intolerances. Understanding peptide absorption is significant for addressing and managing allergic reactions to certain foods.
168
How does peptide absorption change from birth in infants, and what are the implications?
In newborns, peptide absorption primarily occurs in intestinal crypt cells, where the intestinal villi are initially very small. As the infant grows and the villi develop, reducing the crypts' exposure to chyme, peptide absorption rates decline. Delaying exposure to allergy-inducing peptides can allow the gut to mature, potentially decreasing the likelihood of developing allergies.
169
What impact has delaying the introduction of gluten to infants had on childhood gluten allergies?
The incidence of childhood gluten allergies has decreased since parents began delaying the introduction of gluten-based cereals until several months after birth. This delay allows the intestinal system to mature, reducing the chance of gluten acting as an allergen that triggers immune responses.
170
What is DDAVP and how does it utilize the principles of peptide absorption?
DDAVP (1-deamino-8-D-arginine vasopressin) is a synthetic analog of the hormone vasopressin that is designed to be absorbed without being digested, thanks to its slightly altered structure. This makes DDAVP effective when administered orally, unlike natural vasopressin, which would be digested if ingested.
171
What are the steps involved in the digestion of nucleic acids?
Nucleic acids such as DNA and RNA are digested first into nucleotides by pancreatic and intestinal enzymes, and then further broken down into nitrogenous bases and monosaccharides. These components are absorbed into the body through active transport (for bases) and facilitated diffusion and secondary active transport (for monosaccharides).
172
How are fat-soluble vitamins absorbed and what concerns exist with certain fat substitutes?
Fat-soluble vitamins (A, D, E, and K) are absorbed along with dietary fats in the small intestine. Concerns arise with non-absorbable fat substitutes like Olestra and the lipase inhibitor orlistat, as they can prevent the absorption of these vitamins. Users are often advised to take multivitamins to avoid deficiencies.
173
What is unique about the absorption of vitamin B12?
Vitamin B12 (cobalamin) is absorbed in the ileum but only when complexed with intrinsic factor, a protein secreted by gastric parietal cells. This absorption can be hindered by long-term use of proton pump inhibitors, which reduce gastric acid secretion, potentially leading to vitamin B12 deficiency and pernicious anemia.
174
What is pernicious anemia and how is it treated?
Pernicious anemia is a severe vitamin B12 deficiency that occurs when there is an absence of intrinsic factor, leading to significantly reduced red blood cell synthesis. It is treated with vitamin B12 injections since oral supplements would not be absorbed due to the lack of intrinsic factor.
175
How is iron absorbed in the digestive tract and regulated?
Dietary iron is absorbed in two forms: heme iron from meat via a specific apical transporter, and ionized iron (Fe2+) through cotransport with H+ on the divalent metal transporter 1 (DMT1). Iron absorption is regulated by the hormone hepcidin, which inhibits iron uptake by binding to and causing the degradation of the ferroportin transporter when iron stores are high.
176
Describe the absorption process of calcium in the small intestine.
Calcium absorption involves both passive movement through paracellular pathways and hormonally regulated active transport. Active calcium transport occurs via apical calcium channels and is pumped out across the basolateral membrane by a calcium ATPase or a sodium-calcium antiporter, particularly in the duodenum. Regulation of calcium absorption is influenced by vitamin D
177
What impact do dietary choices and conditions have on the absorption of minerals like calcium and iron?
The absorption of minerals such as calcium and iron can be influenced by dietary sources (e.g., plant vs. animal sources), presence of other dietary components that may enhance or inhibit absorption, and the body's physiological needs and conditions, such as hormonal levels and existing mineral stores.
178
Where does most water absorption occur in the digestive tract?
Most water absorption occurs in the small intestine, with an additional 0.5 liter per day absorbed in the colon.
179
How does nutrient absorption influence water movement in the intestine?
The absorption of nutrients and ions creates osmotic gradients that drive water movement from the intestinal lumen into the extracellular fluid (ECF).
180
What are the three main membrane proteins involved in sodium absorption in enterocytes and colonocytes?
1. Sodium channels such as ENaC.
2. Na+Cl- symporter.
3. Na+-H+ exchanger (NHE).
181
How is sodium absorbed in the small intestine aside from ion-specific transporters?
In the small intestine, significant sodium absorption also occurs through organic solute uptake via transporters like the SGLT and amino acid-dependent sodium transporters.
182
What is the primary transporter for sodium on the basolateral side of enterocytes and colonocytes?
The primary transporter for sodium on the basolateral side is the Na+K+ATPase.
183
How is chloride absorbed in the intestinal cells?
Chloride is absorbed using an apical Cl-HCO3- exchanger and moves across the cells via a basolateral chloride channel.
184
What pathways do potassium and water primarily use for absorption in the intestine?
Potassium and water are primarily absorbed via the paracellular pathway in the intestine.
185
What role do sensory receptors in the intestine play in the regulation of the intestinal phase?
Intestinal sensors trigger neural and endocrine reflexes that regulate the rate of chyme delivery from the stomach and enhance digestion, motility, and nutrient utilization by sensing the composition of the meal.
186
What novel type of receptors do intestinal epithelial cells express, similar to taste buds?
Intestinal epithelial cells express G protein-coupled receptors and the taste-linked G protein gustducin, similar to taste buds, which help them detect the contents of the meal.
187
What are the primary control signals from the gut to the stomach and pancreas during the intestinal phase?
1. Enteric nervous system activation decreases gastric motility and secretion.
2. Secretin release in response to acidic chyme decreases acid production and gastric motility.
3. CCK release in response to fats slows gastric motility.
4. Incretin hormones (GIP and GLP1) released in response to carbohydrates promote insulin release and decrease gastric motility and acid secretion.
5. Osmolarity sensors in the intestine regulate gastric emptying based on the hyperosmotic nature of chyme.
188
How much unabsorbed chyme remains by the end of the ileum, and how much water is normally lost in feces?
By the end of the ileum, only about 1.5 liters of unabsorbed chyme remain. The colon absorbs most of this volume so that normally only about 0.1 liter of water is lost daily in feces.
189
Describe the ileocecal valve and its function.
The ileocecal valve is a tonically contracted region of the muscularis that narrows the opening between the ileum and the cecum. It relaxes each time a peristaltic wave reaches it and also relaxes when food leaves the stomach as part of the gastroileal reflex.
190
List the seven regions of the large intestine.
The large intestine has seven regions: the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus.
191
What is the cecum and what is located at its ventral end?
The cecum is a dead-end pouch with the appendix, a small finger-like projection, at its ventral end.
192
Describe the pathway of material through the large intestine.
Material moves from the cecum upward through the ascending colon, horizontally across the body through the transverse colon, then down through the descending colon and sigmoid colon, and finally into the rectum and anus.
193
What are the structural differences between the muscularis of the large intestine and the small intestine?
The muscularis of the large intestine has an inner circular layer and a discontinuous longitudinal muscle layer concentrated into three bands called the tenia coli, which pull the wall into bulging pockets called haustra.
194
Describe the mucosa of the colon.
The mucosa of the colon has two regions: the luminal surface, which lacks villi and appears smooth, composed of colonocytes and mucus-secreting goblet cells, and the crypts, which contain stem cells, goblet cells, endocrine cells, and maturing colonocytes.
195
How is chyme mixed in the colon, and what is mass movement?
Chyme in the colon is mixed by segmental contractions. Forward movement is minimal during mixing contractions and depends primarily on mass movement, a unique colonic contraction that sends a substantial bolus of material forward, occurring 3-4 times a day and associated with eating and stomach distension.
196
What triggers the defecation reflex and what processes are involved in defecation?
The defecation reflex is triggered by the distension of the rectum. It involves relaxation of the internal anal sphincter and peristaltic contractions in the rectum that push material toward the anus, while the external anal sphincter is consciously relaxed.
197
How can emotional stress affect defecation?
Emotional stress may increase intestinal motility, causing psychosomatic diarrhea, or decrease motility, causing constipation.
198
What causes constipation and how is it treated?
Constipation can be caused by consciously ignoring a defecation reflex or decreased motility, leading to continued water absorption and hard, dry feces. It can be treated with glycerin suppositories, which attract water and help soften the feces.
199
How do colonic bacteria contribute to digestion and absorption in the large intestine?
Colonic bacteria break down undigested complex carbohydrates and proteins through fermentation, producing lactate, short-chain fatty acids, and significant amounts of absorbable vitamins, especially vitamin K.
200
What are some end products of bacterial fermentation in the colon?
End products include lactate, short-chain fatty acids like butyric acid, and intestinal gases such as hydrogen sulfide. Fatty acids can be absorbed by simple diffusion and used by colonocytes for energy.
201
Why are some starchy foods, such as beans, notorious for producing intestinal gas?
Some starchy foods, like beans, are notorious for producing intestinal gas (flatus) because colonic bacteria break down these foods, producing gases as a byproduct.
202
What similarities exist between defecation and urination?
Both defecation and urination are spinal reflexes triggered by the distension of the organ wall. Defecation involves the relaxation of the internal anal sphincter and peristaltic contractions, similar to how urination involves relaxation of urinary sphincters and bladder contractions.
203
What is diarrhea and what causes it?
Diarrhea is a pathological state where intestinal secretion of fluid is not balanced by absorption, resulting in watery stools. It occurs if normal intestinal water absorption mechanisms are disrupted or if there are unabsorbed osmotically active solutes holding water in the lumen.
204
What are some substances that cause osmotic diarrhea?
Substances that cause osmotic diarrhea include undigested lactose, sorbitol (a sugar alcohol used as an artificial sweetener in some chewing gums and foods for people with diabetes), and Olestra (a fake fat made from vegetable oil and sugar).
205
How is polyethylene glycol used in clinical settings?
Patients needing their bowels cleaned out before surgery or other procedures are often given 4 liters of an isotonic solution of polyethylene glycol and electrolytes to drink. Because polyethylene glycol cannot be absorbed, it passes into the colon, triggering copious diarrhea that removes all solid waste from the GI tract.
206
What causes secretory diarrhea and how does it work?
Secretory diarrhea occurs when bacterial toxins, such as cholera toxin from Vibrio cholerae and Escherichia coli enterotoxin, enhance colonic Cl- and fluid secretion. When excessive fluid secretion is coupled with increased motility, diarrhea results.
207
How can secretory diarrhea be viewed as adaptive and what are its risks?
Secretory diarrhea in response to intestinal infection can help flush pathogens out of the lumen, but it also has the potential to cause dehydration if fluid loss is excessive.
208
How can oral replacement fluids help in the treatment of diarrhea?
Oral replacement fluids can prevent morbidity and mortality associated with diarrhea by replacing salt and water loss. These solutions usually contain glucose or sucrose, Na+, and K+. The inclusion of sugar enhances Na+ and Cl- absorption.
209
What is the GI tract's role as an immune organ?
The GI tract is the largest immune organ, exposed to disease-causing organisms, and uses the immune cells of the GALT to prevent pathogens from entering the body through absorptive tissues.
210
What are the first lines of defense against pathogens in the GI tract?
The first lines of defense are enzymes and immunoglobulins in saliva and the highly acidic environment of the stomach.
211
How does the GALT respond if pathogens reach the small intestine?
Sensory receptors and immune cells in the GALT respond with actions such as diarrhea and vomiting.
212
What are M cells and what do they do?
M cells (microfold cells) overlie Peyer’s patches and provide information about lumen contents to the immune cells of the GALT. They use endocytosis and transcytosis to transport antigens to the basolateral membrane where macrophages and lymphocytes await.
213
Describe the structure of M cells.
M cells have fewer and more widely spaced microvilli than typical intestinal cells. Their apical surface contains clathrin-coated pits with embedded membrane receptors for antigens.
214
How do M cells transport antigens?
When antigens bind to receptors on the M cells, they are transported via endocytosis and transcytosis to the basolateral membrane, releasing them into the interstitial fluid where macrophages and lymphocytes are waiting.
215
What happens when immune cells detect threatening antigens?
They secrete cytokines to attract more immune cells, trigger an inflammatory response, and increase Cl-, fluid, and mucus secretion to flush out invaders.
216
What causes inflammatory bowel diseases, and what is a potential therapy?
Inflammatory bowel diseases like ulcerative colitis and Crohn's disease are caused by inappropriate immune responses to normal gut contents. Blocking cytokine action from GALT is a potential therapy.
217
How do certain bacteria exploit M cells to cross the intestinal barrier?
Bacteria like Salmonella and Shigella bind to M cell receptors, allowing the M cells to transport them across the epithelial barrier into the body, causing an immune response.
218
What are the effects of Salmonella and Shigella on the body?
Both bacteria cause diarrhea. Salmonella also causes fever and vomiting.
219
What is vomiting, and what is its purpose?
Vomiting, or emesis, is a protective reflex that expels gastric and duodenal contents from the mouth to remove toxic materials before they can be absorbed.
220
What are the potential consequences of excessive vomiting?
Excessive or prolonged vomiting can cause metabolic alkalosis due to the loss of gastric acid.
221
How is the vomiting reflex coordinated?
The vomiting reflex is coordinated through the vomiting center in the medulla, starting with sensory receptor stimulation and often accompanied by nausea.
222
What stimuli can trigger vomiting?
Vomiting can be triggered by chemicals in the blood (cytokines and drugs), pain, disturbed equilibrium (motion sickness), emotional stress, and physical stimuli like tickling the back of the pharynx.
223
What is the process of vomiting?
Efferent signals from the vomiting center initiate reverse peristalsis starting in the small intestine, aided by abdominal contractions increasing intra-abdominal pressure. The stomach relaxes, pushing contents back into the esophagus and out of the mouth. Respiration is inhibited, and the epiglottis and soft palate close off the trachea and nasopharynx to prevent inhalation of vomitus.
224
What are the risks associated with vomiting?
Inhalation (aspiration) of vomitus can damage the respiratory system and cause aspiration pneumonia if acid or food particles enter the airways.
