Gastrointestinal Tract Flashcards
Structure and Functions of the GI Tract Organs:
Mouth: Chewing (mastication) begins mechanical digestion; saliva contains enzymes for chemical breakdown.
Pharynx and Esophagus: Transport food from the mouth to the stomach via peristalsis.
Stomach: Mechanically mixes food with gastric juices (HCl, pepsinogen) to form chyme; acid and enzymes begin protein digestion.
Small Intestine (Duodenum, Jejunum, Ileum): Primary site for digestion and nutrient absorption, with sections specialized for different enzymes and processes.
- Duodenum: Receives chyme from the stomach and digestive enzymes from the pancreas and bile from the liver, breaking down carbohydrates, proteins, and fats.
- Jejunum: Absorbs most nutrients, including vitamins, minerals, and amino acids.
- Ileum: Completes nutrient absorption, particularly vitamin B12 and bile salts, before moving content to the large intestine.
Pancreas: Secretes digestive enzymes (lipase, amylase, and proteases) into the duodenum to aid in the breakdown of fats, carbohydrates, and proteins. It also produces bicarbonate to neutralize stomach acid and regulates blood sugar through insulin and glucagon.
Liver: Produces bile, which is stored in the gallbladder and released into the duodenum to emulsify fats for easier digestion. The liver also processes absorbed nutrients, detoxifies harmful substances, and stores glycogen, vitamins, and minerals.
Gall Bladder: Stores and concentrates bile produced by the liver. It releases bile into the small intestine as needed to aid in fat digestion.
Large Intestine: Absorbs water, electrolytes; houses gut microbiota that contribute to fermentation of undigested carbs; forms and stores feces
Rectum: Stores feces until they are ready to be expelled.
Anus: The final part of the GIT, where waste exits the body through the process of defecation, controlled by internal and external anal sphincters.
Digestive Secretions and Enzymes:
Saliva: Secreted by salivary glands; contains amylase for carbohydrate digestion.
Gastric Juices:
- HCl: Secreted by parietal cells; activates pepsinogen, denatures proteins, and provides acidic environment.
- Pepsinogen: Secreted by chief cells; activated to pepsin by HCl to digest proteins.
- Mucus: Secreted by goblet cells; protects stomach lining from acid.
- Pancreatic Secretions: Amylase, lipase, proteases (trypsin, chymotrypsin), and bicarbonate ions (neutralize stomach acid)
Proteases and peptidases split proteins into small peptides and amino acids.
Lipases break triglycerides into fatty acids and glycerol or
monoglycerides (which is glycerol attached to a single fatty acid) diglycerides (which is glycerol attached to two fatty acids).
Amylases digest starch/glycogen into smaller molecules yielding maltose, which in turn is cleaved into two glucose molecules by maltase.
Nucleases split DNA/RNA into nucleotides.
Phases of Gastric Secretion:
Cephalic Phase:
Trigger: The sight, smell, taste, or thought of food before it even enters the stomach.
Mechanism: The brain (specifically, the hypothalamus and medulla) stimulates the vagus nerve (CN X), which sends signals to the stomach to begin the secretion of gastric juices and gastrin.
Effect: This phase prepares the stomach for digestion by stimulating the release of gastric juice production (e.g., hydrochloric acid (HCl), and pepsinogen), as well as increasing gastric motility.
Gastric Phase:
Trigger: The presence of food in the stomach, which causes stomach distension (stretching) and the presence of proteins and amino acids.
Mechanism: The distension activates stretch receptors, and partially digested proteins stimulate the release of gastrin.
pH > 4 gastrin release ↑
pH < 3 gastrin release ↓
Increased gastrin leads to an increase in gastric juice production (including HCl and pepsinogen) and enhances stomach contractions.
Effect: This phase is responsible for the majority (about 60-70%) of gastric secretion, ensuring the proper breakdown of ingested food.
Intestinal Phase:
Trigger: The entry of partially digested chyme from the stomach into the small intestine (duodenum).
Mechanism: Hormones such as secretin, CCK, GIP and GLP1 are released, inhibiting further gastric secretion and
slowing gastric emptying. Secretin stimulates HCO3- (bicarbonate) secretion and stimulates the flow of bile from the liver to the gallbladder CCK triggers the gallbladder to contract and release bile, and it stimulates the pancreas to secrete enzymes into the duodenum, where they aid in digestion GIP and GLP1 stimulate insulin secretion
Effect: These hormones slow gastric emptying, allowing the small intestine to process chyme at an optimal rate and prevent over-acidification, while the digestive process shifts from the stomach to the intestines.
Understand the location and function of the enteric nervous system
Location:
The ENS is embedded in the walls of the entire gastrointestinal tract, from the esophagus to the anus.
It consists of two main networks (or plexuses) of neurons:
- Myenteric (Auerbach’s) Plexus: Located between the circular and longitudinal muscle layers, primarily responsible for controlling gastrointestinal motility.
- Submucosal (Meissner’s) Plexus: Found in the submucosa, mainly regulating enzyme secretion, blood flow, and local absorption within the gut.
Function:
Regulation of Motility: The ENS coordinates smooth muscle contractions for peristalsis and segmentation, promoting the forward movement of food and the mixing of digestive contents.
Control of Secretions: It regulates the release of digestive enzymes, mucus, and other secretions needed for digestion.
Blood Flow Regulation: The ENS helps control local blood flow within the gut, ensuring that the intestines receive the necessary blood supply to support digestion and absorption.
Reflex Activities: The ENS conducts short reflexes (entirely within the gut wall) that respond to stimuli such as stretch, pH, and nutrient content, adjusting gut function in real-time.
Define motility in the GIT and explain it in the context of peristalsis, segmentation, and sphincters.
Peristalsis:
Alternating waves of contraction and relaxation move GI contents forward along the digestive tract (propulsion).
Occurs in the esophagus, stomach (mixing and propulsion), small intestine, and large intestine.
Peristalsis ensures that food progresses through the digestive system, allowing it to be digested and absorbed in different stages as it moves along the GIT.
Segmentation:
Localised contractions mix chyme with digestive juices but do not propel chyme forward.
Enhances nutrient absorption by increasing contact between chyme and intestinal walls.
Predominantly occurs in the small intestine (with a slower form in the large intestine)
This mixing action brings food into contact with digestive enzymes and absorptive surfaces, facilitating digestion and nutrient absorption.
Sphincters:
Sphincters (smooth and skeletal muscles) regulate passage and storage of luminal contents into different sections of the GI-tract (and prevent the backflow of luminal contents).
Blood Supply to the Liver
Dual Blood Supply: The liver receives blood from two sources:
- Hepatic Artery: Supplies oxygen-rich blood from the systemic circulation.
- Hepatic Portal Vein: Delivers nutrient-rich but oxygen-poor blood from the digestive tract and spleen, allowing the liver to process nutrients absorbed from the intestines.
Blood Flow: Blood from the hepatic artery and portal vein mixes within the liver’s sinusoids and eventually drains into the central vein of each lobule, which flows into the hepatic veins and exits the liver to return to systemic circulation.
Structure of Liver Lobules
Lobules: The liver is composed of small, hexagonal structural units called liver lobules.
- Each lobule has a central vein at its center, surrounded by hepatocytes (liver cells) arranged in rows.
- At each corner of the lobule is a portal triad, which consists of a branch of the hepatic artery, a branch of the portal vein, and a bile duct.
Sinusoids: The mixed blood flows through spaces called sinusoids (capillary-like channels) between rows of hepatocytes, allowing exchange of substances between blood and hepatocytes.
Kupffer Cells: Specialized macrophages called Kupffer cells are located within the sinusoids and play a role in removing pathogens and old red blood cells from the blood.
Concept of Liver Zonation
Blood flow along the lobule’s radial axis creates
gradients of oxygen, nutrients, and hormones.
This spatial variability results in the non-uniform
expression of key liver functions à liver zonation
Examples of zonated liver functions:
* Pericentral (Zone III):
* Drug detoxification
* Bile acid production
- Zone 2 (Intermediate Zone): Positioned between zones 1 and 3, this zone has intermediate functions that overlap with the other two zones.
- Periportal (Zone I):
- Cholesterol biosynthesis
- Protein secretion
Describe the key functions of the liver, including iron metabolism, lipid transport, and the three phases of detoxification
Metabolic Functions:
- Carbohydrate Metabolism: The liver regulates blood glucose by storing glucose as glycogen (glycogenesis) and releasing it during fasting (glycogenolysis and gluconeogenesis).
- Protein Metabolism: It synthesizes plasma proteins, like albumin and clotting factors, and converts ammonia from protein breakdown into urea for safe excretion.
- Iron Metabolism: The liver stores iron, which is bound to ferritin, and releases it as needed for red blood cell production. It also synthesizes transferrin, a protein that transports iron in the blood to the bone marrow for erythropoiesis (red blood cell formation).
Lipid Metabolism and Transport:
- Lipid Processing: The liver synthesizes, stores, and breaks down lipids and cholesterol. It produces bile, essential for emulsifying fats in the intestine and facilitating fat digestion and absorption.
- Lipoprotein Synthesis: The liver produces lipoproteins (such as very-low-density lipoproteins or VLDL) to transport triglycerides and cholesterol throughout the body. It also takes up and processes low-density lipoproteins (LDL), regulating cholesterol levels in the blood.
Detoxification Functions:
- Phase I (Modification): The liver uses enzymes, primarily cytochrome P450 enzymes, to modify toxic substances, making them more polar (water-soluble) through oxidation, reduction, or hydrolysis reactions. This step often produces intermediate metabolites, which may still be reactive and potentially harmful.
- Phase II (Conjugation): In this phase, the liver adds a polar group to the modified substances, further increasing their water solubility. This process involves conjugation reactions, such as glucuronidation, sulfation, or acetylation, which prepare the substances for excretion.
- Phase III (Secretion/Excretion): The now water-soluble conjugates are transported out of the liver cells and excreted from the body. They are either secreted into bile for elimination in the feces or released into the bloodstream for renal excretion via the kidneys.
Understand the concept of the circadian clock as an endogenous oscillator that regulates a wide range of physiological processes, including the functions of the GI
tract.
Key Features of the Circadian Clock:
- Endogenous Oscillator: The circadian clock is driven by an internal timing mechanism present in nearly every cell, with a master clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN receives light signals from the eyes, synchronizing the clock with the external environment.
- Molecular Mechanism: Within cells, the circadian rhythm is regulated by a set of “clock genes” that interact in feedback loops to produce cyclical patterns of gene expression, which in turn drive daily rhythms in physiology and behavior.
Circadian Regulation of Gastrointestinal (GI) Functions:
- Gastric Acid Secretion: Acid production in the stomach follows a circadian rhythm, with peak secretion often occurring in the late evening. This rhythm aids in food digestion at specific times, aligning with feeding patterns.
- Motility: GI motility, including the peristaltic waves that move food through the tract, is regulated by circadian rhythms. For instance, gastric emptying and bowel movements typically slow down at night, promoting digestive efficiency and allowing the body to rest.
- Nutrient Absorption and Metabolism: The absorption and processing of nutrients, such as glucose and lipids, also follow circadian rhythms. Enzymes involved in metabolism are expressed at higher or lower levels depending on the time of day, helping the body manage energy intake and storage effectively.
- Microbial Interactions: The composition and function of the gut microbiome fluctuate with circadian rhythms, as microbial populations shift in response to feeding patterns and host metabolism, impacting digestion and immune function.
