Gastrointestinal Physiology Flashcards
(69 cards)
1
Q
Three regions of the small intestine (from proximal to distal)
A
- duodenum
- jejunum
- ileum
2
Q
Duodenum
A
The shortest and most fixed portion of the small intestine. The duodenum can be divided into four parts: superior, descending, inferior and ascending. Together, these parts form a ‘C’ shape, which is around 25cm long, and which wraps around the head of the pancreas.
3
Q
Jejunum
A
Middle portion of the small intestine
4
Q
Ileum
A
- Longest part of the small intestine
- The ileum joins the cecum, the first portion of the large intestine, at the ileocecal sphincter (or valve).
5
Q
Main artery supplying the small intestine
A
The superior mesenteric artery
6
Q
Small intestine
A
The primary digestive organ in the body. Not only is this where most digestion occurs, it is also where practically all absorption occurs.
7
Q
Large intestine
A
The terminal part of the alimentary canal. The primary function of this organ is to finish absorption of nutrients and water, synthesize certain vitamins, form feces, and eliminate feces from the body.
8
Q
Four main regions of the large intestine
A
- cecum
- colon
- rectum
- anus
9
Q
Cecum
A
The first part of the large intestine is the cecum, a sac-like structure that receives the contents of the ileum, and continues the absorption of water and salts.
10
Q
Colon
A
- Upon entering the colon, the food residue first travels up the ascending colon on the right side of the abdomen.
- At the inferior surface of the liver, the colon bends to form the right colic flexure (hepatic flexure) and becomes the transverse colon.
- The region defined as hindgut begins with the last third of the transverse colon and continues on.
- Food residue passing through the transverse colon travels across to the left side of the abdomen, where the colon angles sharply immediately inferior to the spleen.
- From there, food residue passes through the descending colon, which runs down the left side of the posterior abdominal wall.
- After entering the pelvis inferiorly, it becomes the s-shaped sigmoid colon, which extends medially to the midline
- The ascending and descending colon, and the rectum are located in the retroperitoneum.
- The transverse and sigmoid colon are tethered to the posterior abdominal wall by the mesocolon.
11
Q
System that prevents bacteria from crossing the mucosal barrier
A
- Upon entering the colon, the food residue first travels up the ascending colon on the right side of the abdomen.
- At the inferior surface of the liver, the colon bends to form the right colic flexure (hepatic flexure) and becomes the transverse colon.
- The region defined as hindgut begins with the last third of the transverse colon and continues on.
- Food residue passing through the transverse colon travels across to the left side of the abdomen, where the colon angles sharply immediately inferior to the spleen.
- From there, food residue passes through the descending colon, which runs down the left side of the posterior abdominal wall.
- After entering the pelvis inferiorly, it becomes the s-shaped sigmoid colon, which extends medially to the midline
- The ascending and descending colon, and the rectum are located in the retroperitoneum.
- The transverse and sigmoid colon are tethered to the posterior abdominal wall by the mesocolon.
12
Q
The four layers of the intestinal wall
A
- mucosa
- submucosa
- muscularis propria
- serosa
13
Q
Suspensory muscle of the duodenum
A
Located at the duodenojejunal junction is a slip of muscle called the suspensory muscle of the duodenum. Contraction of this muscle widens the angle of the flexure and aids movement of the intestinal contents into the jejunum.
14
Q
Ileocaecal valve
A
The ileocaecal valve represents the separation between the small and large intestine. Its main function is to prevent the reflux of enteric fluid from the colon into the small intestine. It is also used as a landmark during colonoscopy, indicating that the limit of the colon has been reached and that a complete colonoscopy has been performed.
15
Q
Features of the large intestine
A
- Attached to the surface of the large intestine are omental appendices - small pouches of peritoneum, filled with fat.
- Running longitudinally along the surface of the large bowel are three strips of muscle, known as the teniae coli. They are called the mesocolic, free and omental coli.
- The teniae coli contract to shorten the wall of the bowel, producing sacculations known as haustra.
- The large intestine has a much wider diameter compared to the small intestine.
16
Q
Mucosa
A
The mucosa consists of a single layer of epithelium, which is highly folded to increase its surface area for absorption. The mucosa contains invaginations, which form tubular exocrine glands that secrete mucus, electrolytes, water, and digestive enzymes. It also houses endocrine glands, which release Gi hormones such as cholecystokinin (CCK). The lamina propria, a connective tissue layer, supports the epithelium.
17
Q
Submucosa
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The submucosa, a connective tissue layer, houses blood and lymphatic vessels that support the GI tract. The submucosal (Meissner) plexus is found in this layer.
18
Q
Goblet cells
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Arise from pluripotent stem cells with a function of secreting mucin to create a protective mucus layer. Goblet cells are also thought to be involved with immunoregulation.
19
Q
M cells
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Highly specialised cells which play a central role in the initiation of mucosal immune responses by transporting antigens and microorganisms to the underlying lymphoid tissue.
20
Q
Intestinal motor function
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The ability to assess the complex motor functions of the gastrointestinal tract accurately has been of tremendous value to understanding and treating digestive diseases.
21
Q
Intestinal permeability
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The non-mediated intestinal passage of medium-sized hydrophilic molecules occurs towards a concentration gradient without the assistance of a carrier system and can be measured by recording the passage of permeability markers over the epithelium via the paracellular or the transcellular route.
22
Q
Solutes can pass the intestinal epithelium via four routes
A
- Paracellular route (larger hydrophilic solutes)
- Transcellular route (small hydrophilic and lipophilic solutes).
- Transcellular route via aqueous pores (small hydrophilic solutes) or active carrier-mediated absorption (nutrients)
- Endocytosis, followed by transcytosis and exocytosis (larger particles, peptides and proteins).
23
Q
Techniques used to assess intestinal barrier function (in vivo)
A
- Orally Ingested Probes Assessed in Urine
- Biomarkers for Assessment of Intestinal Permeability
- Zonulin
- Fatty Acid Binding Proteins (FABP)
- Citrulline
- Glucagon-Like Peptide (GLP)-2
- LPS
- LPS-Binding Protein (LBP)
- Fecal Markers of Intestinal Permeability and Markers of Intestinal Inflammation (alpha (α) −1-antitrypsin (AAT), lipocalin 2 (LCN2), serum amyloid A) - Confocal laser endomicroscopy (CLE)
24
Q
Techniques used to measure tight junctions
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Tight junction alterations are readily indicated by the changes of TER
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Zonulin
Zonulin was shown to reversibly open tight-junctions in a protease-activated receptor 2 (PAR2) and epidermal growth factor receptor (EGFR) dependent manner resulting in dislocation of ZO-1 from the tight junction.
Increased zonulin concentrations have been reported in many conditions, including celiac disease, type 1 diabetes, IBD, obesity, schizophrenia, etc.
26
Bile acids
Although bile acids were initially discovered for their role as emulsifiers in fat digestion, they have also recently been found to regulate host immune response and signaling pathways, as well as play a critical role in host-microbe interaction. They exist in multiple forms, free carboxylic acids, hydroxyl group conjugates and amide conjugates (called bile amidates), and all are critical for maintaining host health
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Digestion
The human digestive tract can conveniently be considered as four distinct sub-processes run in series, with complex controls on the passage of digesta from one process to the next.
The four processes in terms of human anatomy correspond to the mouth (oral processing), the stomach (gastric processing), the small intestine (intestinal processing) and the large intestine or colon (fermentation).
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pH of the mouth
Neutral
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pH of the stomach
2
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pH of the small intestine
7-8
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pH of the colon
5-8
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Intestinal processing (duodenum)
The duodenum is the first approximately 30 cm of the small intestine and is functionally important as the region where the digesta move from an acidic environment to a neutral pH. In this region the pancreas secretes a range of hydrolytic enzymes, collectively referred to as pancreatin, that begin to react with the digesta to break down proteins, fats and carbohydrates. The pancreas also secretes bicarbonate, which raises the pH to neutrality and thus provides a favourable environment for these enzymes. Bile acids are also introduced from the duct of the gall bladder and act to emulsify fats, which facilitates breakdown by pancreatic lipase. Peristalsis in the small intestine is important both as a mixing function and as a transport function.
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Intestinal processing (ileum and jejunum)
The ileum and jejunum have the function of being a reactor to allow the digestive enzymes to do their work, and have a large surface area to allow absorption of the small molecules produced. In fact, the digestive tract is the organ with the largest area facing the ‘external’ environment, with an absorptive area of approximately 30–35 m2, of which about 30 m2 is the surface of the small intestine.8 This large area is due to the intensively convoluted surface of the intestines and the structures of villi and microvilli that line the surface, giving an effective 60- to 120-fold enhancement of surface area. Once hydrolysed, the digestible fragments (amino acids, small oligopeptides, mono- and disaccharides, etc.) must diffuse from the locus of hydrolysis to the absorptive surface, a process that is in competition with the rate of transit through the small intestine. Any hydrolysis products not absorbed by the time the digesta reach the colon are likely to be consumed by the colonic microbiota rather than directly absorbed.
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Digestion in the small intestine
The small intestine is essentially an enzyme bioreactor where food macromolecules are broken down by the hydrolytic reactions of the proteases and peptidases, carbohydrases and lipases and low molecular weight hydrolysis products diffuse out, to be transported into the circulation. In principle, these enzyme reactions are well known in terms of the specificity of the individual enzymes and their rates of reaction at pH around 8. In practice, because of the wide range of structures, both physical and chemical, of the macromolecules, the rates of hydrolysis and products of hydrolysis of real foods are only understood in very broad terms. It is possible to distinguish two types of effects that contribute to this complexity: physical effects and enzyme effects.
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What are the primary layers of the gastrointestinal (GI) wall, and what are their functions?
The GI wall consists of four main layers:
(1) the mucosa, responsible for secretion, absorption, and immune defence
(2) the submucosa, which contains blood vessels, lymphatics, and the submucosal (Meissner's) nerve plexus
(3) the muscularis externa, comprising circular and longitudinal smooth muscle layers that drive peristalsis and segmentation, coordinated by the myenteric (Auerbach's) plexus; and
(4) the serosa or adventitia, providing structural support and anchoring the gut to surrounding tissues.
36
How is gastric acid secreted and regulated?
Gastric acid is secreted by parietal cells in the gastric glands of the stomach. The secretion is stimulated by three primary agonists: acetylcholine (via vagal stimulation), histamine (via H2 receptors), and gastrin. These pathways converge to activate H+/K+ ATPase pumps on the apical membrane. Acid secretion is modulated by negative feedback from somatostatin, which inhibits G cells and parietal cells when pH becomes too low.
37
What is the role of the enteric nervous system in GI motility?
The enteric nervous system (ENS) is a semi-autonomous network of neurons located in the gut wall, primarily in the submucosal and myenteric plexuses. It coordinates local reflexes that regulate motility, secretion, and blood flow without central nervous system input. ENS modulates peristalsis, segmentation, and the migrating motor complex (MMC), adapting responses based on luminal stimuli and inter-neuronal communication.
38
Describe the phases of gastric motility
Gastric motility includes the following phases: (1) receptive relaxation of the fundus to accommodate food, mediated by vagovagal reflexes; (2) peristaltic contractions in the antrum to mix and grind food; and (3) gastric emptying, controlled by pyloric sphincter tone and regulated by the caloric and osmotic content of chyme. Gastric motility is coordinated by slow waves generated by interstitial cells of Cajal.
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How are carbohydrates digested and absorbed in the small intestine?
Carbohydrate digestion begins with salivary amylase and continues with pancreatic amylase in the small intestine, breaking polysaccharides into disaccharides. Brush border enzymes (e.g., lactase, sucrase, maltase) hydrolyze disaccharides into monosaccharides. Glucose and galactose are absorbed via SGLT1 (sodium-dependent), while fructose uses GLUT5. All three exit enterocytes via GLUT2 into the portal circulation.
40
What mechanisms regulate pancreatic enzyme secretion?
Pancreatic secretion is regulated by both neural and hormonal mechanisms. Acetylcholine and CCK stimulate acinar cells to release digestive enzymes (zymogens), while secretin primarily stimulates ductal cells to secrete bicarbonate. CCK release is triggered by fats and amino acids in the duodenum, and secretin is released in response to acid. These processes ensure that enzyme secretion matches the digestive load
41
What is the role of bile salts in lipid digestion?
Bile salts, synthesized from cholesterol in the liver and stored in the gallbladder, emulsify dietary fats into micelles, increasing their surface area for pancreatic lipase action. They form mixed micelles that facilitate the transport of fatty acids and monoglycerides to the brush border of enterocytes. After absorption, bile salts are reabsorbed in the ileum and recirculated via the enterohepatic circulation.
42
Describe the process of protein digestion and absorption.
Protein digestion begins in the stomach with pepsin, activated by acidic pH. In the small intestine, pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) further break down peptides. Brush border peptidases complete digestion into amino acids, di- and tripeptides. These are absorbed via specific transporters: amino acids via Na+-dependent symporters, and small peptides via H+-dependent PepT1 transporters.
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How is water absorbed in the gastrointestinal tract?
Water absorption in the GI tract is primarily passive and follows osmotic gradients established by solute (mainly sodium) absorption. In the small intestine, sodium is absorbed via cotransport with nutrients and via Na+/H+ exchangers. Water moves paracellularly or transcellularly to maintain isotonicity. The colon fine-tunes water reabsorption, concentrating stool before defecation.
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What is the migrating motor complex and what function does it serve?
The migrating motor complex (MMC) is a cyclic, recurring motility pattern in the fasting state, consisting of phases of quiescence, irregular contractions, and strong peristaltic waves. It occurs approximately every 90–120 minutes and serves to clear residual food, secretions, and bacteria from the stomach and small intestine. It is regulated by motilin and suppressed by feeding.
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What are the major gastrointestinal hormones and their functions?
The major GI hormones include gastrin (stimulates acid secretion and mucosal growth), cholecystokinin (CCK, stimulates gallbladder contraction and pancreatic enzyme secretion), secretin (stimulates bicarbonate secretion from pancreas and bile ducts), GIP (glucose-dependent insulinotropic peptide, stimulates insulin secretion), and motilin (stimulates the MMC). These hormones integrate digestive function with nutrient presence and motility patterns.
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How is the small intestine structured to optimize absorption?
The small intestine has several anatomical adaptations to maximize surface area: circular folds (plicae circulares), villi, and microvilli on the apical membrane of enterocytes. These features provide a large absorptive surface. Enterocytes contain transporters for nutrients, and a rich capillary and lymphatic network in the villi allows for efficient nutrient absorption and distribution.
47
Describe the role of the colon in fluid and electrolyte balance.
The colon reabsorbs water and electrolytes (mainly Na+ and Cl−) and compacts feces. Sodium absorption occurs via epithelial sodium channels (ENaC), stimulated by aldosterone. Chloride follows passively or via Cl−/HCO3− exchange, and water follows osmotically. The colon also secretes potassium and bicarbonate under certain conditions.
48
What is the function of goblet cells in the intestinal tract?
Goblet cells secrete mucins, which form mucus upon hydration. Mucus protects the epithelium from mechanical damage, pathogens, and autodigestion. It also facilitates the movement of luminal contents and plays a role in maintaining microbial balance and barrier function.
49
How do Peyer’s patches and M cells contribute to gut immunity?
Peyer’s patches are lymphoid aggregates in the ileum that monitor intestinal bacteria and generate immune responses. M cells overlie Peyer’s patches and transport antigens from the lumen to immune cells below. This helps the gut immune system maintain tolerance to commensals and mount defenses against pathogens.
50
What is the role of the liver in digestion?
The liver produces bile, essential for lipid emulsification and absorption. It processes nutrients absorbed via the portal vein, including carbohydrate storage (glycogen), amino acid metabolism, and lipid handling. The liver also detoxifies substances, synthesizes plasma proteins, and contributes to immune surveillance via Kupffer cells.
51
Describe the cephalic phase of digestion.
The cephalic phase is initiated by sight, smell, thought, or taste of food and prepares the GI tract for digestion. It involves parasympathetic (vagal) stimulation leading to increased salivation, gastric acid secretion, pancreatic enzyme release, and motility. This anticipatory response enhances digestive efficiency.
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How does gastric emptying influence nutrient absorption?
Gastric emptying regulates the rate at which chyme enters the duodenum. Slower emptying enhances nutrient digestion and absorption by matching digestive enzyme activity and transport capacity. Nutrient composition affects emptying: fats and hyperosmolar solutions delay it, regulated by feedback from intestinal hormones like CCK and secretin.
53
What is the function of Brunner’s glands?
Brunner’s glands are located in the submucosa of the duodenum. They secrete an alkaline mucus rich in bicarbonate, which neutralizes gastric acid and protects the duodenal epithelium. This also optimizes pH for pancreatic enzyme function.
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How is iron absorbed in the gastrointestinal tract?
Iron is primarily absorbed in the duodenum. Dietary non-heme iron (Fe³⁺) is reduced to Fe²⁺ by duodenal cytochrome B and transported into enterocytes via divalent metal transporter 1 (DMT1). Heme iron is absorbed via a separate heme carrier protein. Inside enterocytes, iron is stored as ferritin or exported into circulation by ferroportin, where it binds transferrin for systemic transport. Hepcidin regulates absorption by degrading ferroportin.
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What are the major roles of the gut microbiota in physiology?
Gut microbiota ferment indigestible carbohydrates to produce short-chain fatty acids (SCFAs), synthesize certain vitamins (e.g., vitamin K, biotin), modulate immune function, prevent colonization by pathogens, and influence gut motility and barrier integrity. Dysbiosis is linked to metabolic and inflammatory diseases.
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What are the key features of intestinal epithelial cell turnover?
Enterocytes are renewed every 3–5 days from stem cells in the crypts of Lieberkühn. New cells migrate up the villus, differentiating into absorptive or secretory cells. At the villus tip, senescent cells undergo apoptosis and are shed into the lumen.
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How is gastric acid neutralized in the duodenum?
Acidic chyme entering the duodenum stimulates secretin release, which promotes bicarbonate secretion from the pancreas and biliary tract. Brunner’s glands also secrete bicarbonate-rich mucus. This neutralizes gastric acid, protecting the mucosa and optimizing pH for enzyme activity.
58
What role does the vagus nerve play in GI physiology?
The vagus nerve provides parasympathetic innervation to most of the GI tract. It modulates motility, secretion, and reflexes like the receptive relaxation of the stomach and pancreatic secretion. It also facilitates the cephalic phase of digestion and communicates with the ENS.
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How are lipids transported after absorption?
Monoglycerides and fatty acids are re-esterified into triglycerides in enterocytes, packaged into chylomicrons along with cholesterol and fat-soluble vitamins. Chylomicrons enter lacteals (lymphatic capillaries) and are transported via the thoracic duct into systemic circulation.
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How is glucose absorbed in the small intestine?
Glucose is absorbed via sodium-dependent glucose transporter 1 (SGLT1) on the apical membrane of enterocytes. It is co-transported with sodium, driven by the Na+/K+ ATPase gradient. Glucose exits basolaterally via GLUT2 into the bloodstream.
61
How are fat-soluble vitamins absorbed?
Vitamins A, D, E, and K are absorbed with dietary lipids. They are solubilized in micelles formed by bile salts, then diffuse into enterocytes, where they are incorporated into chylomicrons and enter the lymphatic circulation.
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What is the enterogastric reflex?
The enterogastric reflex is triggered by duodenal distension, low pH, or nutrient content. It inhibits gastric motility and secretion via sympathetic stimulation and enteric inhibitory signals, slowing gastric emptying to allow intestinal processing.
63
What are short-chain fatty acids and their roles in gut physiology?
Short-chain fatty acids (SCFAs), such as acetate, propionate, and butyrate, are produced by bacterial fermentation of fibre. They serve as energy sources for colonocytes, regulate water and sodium absorption, and modulate inflammation and gut barrier integrity.
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How does the GI tract detect luminal contents?
The gut uses chemoreceptors, mechanoreceptors, and enteroendocrine cells to sense pH, osmolarity, nutrient composition, and distension. These signals modulate ENS activity and hormone secretion to regulate digestion and motility.
65
How does the colon handle undigested carbohydrates?
Undigested carbohydrates are fermented by colonic microbiota into SCFAs and gases (e.g., CO₂, H₂, CH₄). SCFAs are absorbed and provide energy to colonocytes, while gases are expelled as flatus.
66
How does the ileocecal valve function in GI physiology?
The ileocecal valve prevents backflow from the colon to the small intestine. It opens in response to ileal distension and closes with colonic distension or pressure. This ensures unidirectional flow and limits microbial overgrowth in the small intestine.
67
How is cholesterol absorbed in the intestine?
Cholesterol is solubilized in bile salt micelles and absorbed by enterocytes via Niemann-Pick C1-like 1 (NPC1L1) transporter. Some is esterified by ACAT and packaged into chylomicrons. Absorption is limited by ABCG5/ABCG8 transporters that efflux excess cholesterol back into the lumen.
68
What role does the lymphatic system play in GI absorption?
The lymphatic system absorbs chylomicrons containing lipids and fat-soluble vitamins from enterocytes via lacteals in the villi. These lymph vessels drain into the thoracic duct, bypassing the portal circulation, and deliver nutrients directly to the systemic circulation.
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