06-10-21 - GI System Flashcards
What is the primary function of the GI tract?
How does digestion aid absorption?
What are the 4 main processes that take place in the GIT?
- The primary function of the GIT is absorption of nutrients
- Digestion aids absorption by breaking down complex food molecules using physical methods (chewing) and chemical methods (enzymes)
- The 4 processes that take place in the GIT are
- Motility – movement of food
- Secretion – pancreas secretes enzymes
- Digestion
- Absorption – nutrients and water.
Label this diagram of the GI system
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What are the 3 major arteries that feed the GI tract?
What areas are each responsible for?
How is blood flow regulated in the GI tract?
- Celiac artery – liver, pancreas, stomach
- Superior mesenteric artery – smallest intestine and most of large intestine
- Inferior mesenteric – terminal portions of large intestine and rectum
- Blood flow is regulated by:
- The autonomous nervous system
- Sympathetic (vasoconstrictor)
- Parasympathetic (vasodilator)
- Enteric (vasodilator)
- Hormonal (endocrine) control
- Adrenaline (epinephrine) – vasoconstriction and vasodilation
- Gastrin (vasodilator)
- Local (paracrine) control
- Low Po2 (vasodilator)
What does the surface of the GIT have to be equipped to deal with?
Why are their different layers in the GIT?
Label the layers of the GIT.
- The GIT is always exposed to the external environment, so must be equipped to deal with this
- The GIT has many different layer, that each have a different function.
What are the layers from top to the bottom in the microanatomy of the GIT?
What does each layer consist of?
- The outer most layer is the mucosa, which consists of epithelium, lamina propria (connective tissue), and the muscularis mucosa
- The submucosa layer consists of blood vessels and nerve bundles that form the submucosal plexus (Meissner plexus), which is an integral part of the enteric nervous system.
- The muscularis externa consists of 2 smooth muscle layers – circular muscle and longitudinal muscle. Between these 2 layers lies the myenteric (Auerbach) plexus)
- The serosa is the bottom layer, and consists of connective tissue and squamous epithelial cells
What happens to the surface area of the mucosa layer as the GIT progresses?
How does this happen?
Why does this happen?
What is a specialist area of the GIT?
- As the GIT progresses, the surface area of the mucosa layer increases
- This is done by villi, which are also covered in epithelial cells, which have microvilli on their apical surfaces.
- This increase in surface area allows for more efficient absorption of nutrients in the small intestine.
- There are certain specialist areas in the GIT, such as on the duodenum (first part of the small intestine), where there is a bile and pancreatic duct that the pancreas can secret digestive enzymes into.
What are the 3 divisions of the autonomic nervous system (ANS) that regulate the GI system?
- Parasympathetic nervous system (PSNS)
- Sympathetic nervous system (SNS)
- Enteric nervous system (ENS)
Where do the nerves from the PSNS system that enter the GI system originate?
Where do they innervate in the GI system?
What do the sensory and motor components respond to?
What signals does the PSNS give to the GI tract?
- Cranial nerves 7 and 9 from the Pons (nerve fibres that connect medulla and cerebellum) innervates (supplies nerves to) salivary glands and gets the sensation from the tongue.
- Vagus nerve (cranial nerve 10 from medulla oblongata) innervates the GI tract from the oesophagus to most of the large intestine
- Pelvic splanchnic nerve (Sacral spinal nerves S2-S4) innervates the sigmoid area (last area) of the large intestine to the rectum.
- This innervation has sensory and motor components that respond to: stretch, pressure, temperature, osmolarity.
- The PSNS signals GI secretions and motility, which facilitates digestion and the absorption of nutrients (rest and digest)
Where do the nerves from the SNS system that enter the GI system originate?
Where do they innervate in the GI system?
What signals does the SNS give to the GI tract?
- Nerve that synapse in the superior cervical ganglion innervate the salivary glands and the top of the oesophagus
- Nerves that originate in the Thoracic (T5-T12) and Lumbar (L1-L3) regions innervate the rest of the nervous system.
- There are 3 ganglia (collection of neuronal bodies) that innervate the rest of the nervous system:
- Celiac – stomach and top part of oesophagus
- Superior mesenteric – Most of small intestine and the beginning of the large intestine
- Inferior mesenteric – bottom of the large intestine.
- Generally, the SNS decreases GI secretions and motility (fight or flight)
What does the enteric nervous system (ENS) consist of?
What are its functions?
What is unique about the enteric nervous system?
• The enteric nervous system consists of 2 plexuses, the submucosal (Meissner) plexus and the myenteric (Auerbach) plexus
- The submucosal plexus:
- Regulates intestinal secretions and area specific absorption
- Interacts with blood vessels, circular and longitudinal muscle and muscularis mucosa
- The myenteric plexus:
- Regulates intestinal smooth muscle that helps generate tonic and rhythmic contractions
• The ENS still operates if the PSNS and SNS fibres are cut, so ENS can function autonomously via intrinsic neural and sensory reflexes.
What are the 3 phases of digestion?
- Cephalic phase
- Gastric phase
- Intestinal phase
Is the cephalic phase of digestion conscious or unconscious?
What are receptors stimulated by?
What nerve is responsible for this?
What does this cause to happen in the stomach?
- This is the only phase of digestion under conscious control
- Chemoreceptors and mechanoreceptors located in the oral and nasal cavities are stimulated by tasting, chewing, swallowing, smelling, and even thinking of food.
- This is done by the vagus nerve causing vagal reflexes (cranial nerve 10)
- This results in salivatory secretions and parasympathetic excitation in the stomach
- This causes pepsinogen digests (pepsin precursor - a zymogen) to be secreted by gastric chief cells in the stomach, and gastrin to be produced from G-cells in the stomach, which stimulates the secretion of HCl from parietal cells in the stomach, and the secretion of enzymes from pancreatic acinar cells
When does the gastric phase of digestion begin?
What reflexes does this phase involve?
What are these reflexes responding to in the stomach?
What is produced in this phase?
- This phase begins when the bolus (food) enters the stomach
- The gastric phase involves a lot of reflex responses, most of these are local reflexes in nature, which are called vagovagal responses (vagal cholinergic responses for pancreas and gall bladder)
- These local reflexes enact or cause a lot of the responses in the stomach, whether it is from distension of the stomach, acid, or another substance
- Gastrin and histamine secretion are stimulated at this time too, both of which will increase acid production.
How does the intestinal phase begin?
What happens in this phase?
What hormones are secreted?
What are they secreted by?
What do these hormones do?
What can these hormones work together to do?
- This phase begins with chyme (food) entering the duodenum of the small intestine
- There is further breakdown of material during this phase, but also absorption of nutrients.
- The release of CCK hormone from the duodenum is stimulated by amino acids and fatty acids
- The release of Secretin hormone from the small intestine is also stimulated by H+
- CCKs main function is to allow for the contraction of the gallbladder to increase bile secretion (which breaks down fat into fatty acids), and stimulation of the acinar cells in the pancreas to secrete digestive enzymes.
- Secretin aids in amplification of bile production by the liver, but also stimulates the pancreas to secret its exocrine components, like enzymes and bicarbonate, which can neutralise the acidic chyme, allowing for a better environment for the digestive enzymes to work in.
- CCK and secretin can also act together to inhibit peristaltic movements to allow for proper absorption.
What parts does the Upper and lower GI tract consist of?
- The Upper GI tract is considered to be the mouth, the oesophagus, the stomach and the duodenum of the small intestine.
- The lower GI tract consist of The Jejunum and ileum of the small intestine, the large intestine, and the anus
What are the 3 types of salivary glands?
Where are they located?
What do they secrete?
What are salivary glands functions?
- Parotid salivary gland – in the cheeks beside the ear – watery secretion
- Submandibular salivary glands – under jaw – watery secretion
- Sublingual salivary glands – floor of mouth – mucous secretion
- Functions of salivary glands:
- Lubrication
- Protection e.g lysozyme, calcium salts
- Digestion
- Taste
What are the 4 phases of swallowing?
What occurs at each phase?
- Voluntary phase – tongue pushes food (bolus) towards back of throat
- Pharyngeal phase – Pharynx contracts, and the glottis contracts to cover the airways
- (no name phase) – bolus is moved through to the top of the oesophagus
- Oesophageal phase – Oesophagus generates a wave of contraction that moves the bolus down the oesophagus into the stomach
What are the 3 main motility functions of the stomach?
What 3 things does the stomach do between meals?
What does the stomach produce?
• The stomach has 3 primary motility functions:
Accommodation of food via receptive relaxation
• Mixing of food via slow-wave initiated contractions and retropulsion
• Gastric empty
- Between meals:
- Migrating motor complexes (MMCs)
- Prevent bacterial overgrowth, and keep stomach and intestine clean
- Stimulated by motilin (hormone released during fasted state)
• The stomach produces chyme, which is a semi-liquid material that consists of gastric juices and partly digested food.
What are 4 examples of gastric secretions?
How are H+ secretions controlled?
Where do the gastric secretions go?
- Gastric secretions include:
- Ions and water
- Mucous from mucous neck cells
- Pepsinogen (precursor to pepsin) from peptic (chief) cells
- Intrinsic factor (protein) and H+ from parietal (oxyntic) cells that line stomach wall
- Regulation of H+ secretion occurs at the level of the H+ K+ ATPase
- All secretions produced are moved to the body of the stomach.
What are the 3 parts of the small intestine?
What happens when chyme passes into the duodenum?
Why does this happen?
What occurs in the small intestine?
How is this facilitated?
- The parts of the small intestine are the duodenum, jejunum and ileum
- When chyme hits the duodenum, this stimulates further enzyme and bicarbonate secretion from the pancreas, which is dumped into gut.
- The bicarbonate neutralises the acid from the chyme and makes the environment more suitable for digestive pancreatic enzymes to function (around pH 6)
- Most macronutrient, vitamin, and mineral absorption occurs in the small intestine
- This is facilitated by the massive increase in surface area created by the villi (10x) and microvilli (20x) on the mucosal layer of the small intestine.
Where does carbohydrate digestion start in the GI tract?
Where does main carbohydrate digestion occur?
What enzymes are used?
How does absorption occur?
- Carbohydrate digestion starts in the mouth via salivary amylase
- Main digestion starts in the small intestine – pancreatic amylase breaks down membrane bound disaccharides from the starch in the mouth
- Absorption is via sodium-dependent co-transport
Where does protein digestion start in the GI tract?
Where does main protein digestion occur?
What enzymes are used?
How does absorption occur?
- Protein digestion starts in the stomach via pepsin
- Main digestion of protein occurs in the small intestine – enzymes used are pancreatic trypsin, chymotrypsin, and elastase
- Absorption is via sodium-dependent and H+-dependent co-transport
Where does fat digestion start?
Where does most fat digestion occur?
What fats can diffuse across the intestinal epithelium without modification?
What enzymes are used?
What is done to aid absorption of lipids?
- Fat digestion starts in the mouth (lingual lipase) and the stomach (gastric lipase)
- A majority of fat is broken down in the small intestine (pancreatic lipase)
- SCFAs and MCFAs can diffuse across the intestinal epithelium
- Bile salts from the liver/gall bladder combine with lipids to form mixed micelles, which help to emulsify lipids and make them soluble, which aids in absorption, as LCFA can’t be absorbed by themself
How does chyme pass from the ileum to the large intestine?
What are the different parts of the large intestine?
What are the 2 main functions of the large intestine?
• Chyme comes into the large intestine via the ileocecal sphincter, which regulates the flow through the ileum to the large intestine
- The parts of the large intestine are the:
- Caecum
- Ascending, transverse, descending, sigmoid colon
- Rectum
- Anus
- The main function of the small intestine is to absorb water and electrolyte ions, which helps to solidify the waste.
- The Large intestine is also responsible for motility of contents, mixing and propulsion:
- Segmentation
- Peristalsis
- Mass movement contraction – wave of contraction that starts behind the chyme and goes all the way around the large intestine
- This occurs after breaking a fast and creates room for the next lot of food.
How does the anal canal function normally?
What does gas or faeces in the anal canal stimulate?
Is this process reflex or conscious?
- The anal canal is tightly closed via contraction of anal sphincters
- Gas or faeces in the anal canal stimulates stretch receptors in the wall, which initiates rectosphinteric reflex
- This is the reflex relaxation of the smooth muscle of the internal anal sphincter and contraction of the external anal sphincter
- At this point, faeces stored in sigmoid colon is moved forward into the rectum
- Defecation is a reflex activity, but is subject to conscious control
