GIT 1 Flashcards
What is the GIT?
A complex system from the mouth to the anus
State the functions of the GIT
-digesting and absorb the substances we ingest and eliminate the waste
Enteric motility- activity of smooth muscle to move substances through GIT
Secretions- enzymes and fluid for digestion, mucus to transport/absorption
Digestion- chemical breakdown of ingested food into absorbable molecules
Absorption -movement of nutrient, water and electrolytes into the cell and then blood
What is the order of the main functions do occur in the GIT?
Ingestion—> digestion and absorption —>
Explain the overall function of the GIT
To absorb nutrients, electrolytes and water and eliminate waste products
Mechano- and chemo receptors respond to: Stretch, osmolarity and pH; presence of substrate and end products of digestion. They initiate reflexes that activate or inhibit certain digestive glands. Physiological processes to achieve these include motility, secretion of digestive juices, digestion of food, absorption of digestive products and fluids, the circulatory system to carry the absorbed substances away and the control of all these functions by the nervous and hormonal systems
Most food is taken in as solids and as macromolecules…
That cannot be readily transported across the intestinal epithelial cells. Digestion which involves physical and chemical alterations of the foo$ occurs first. This is facilitated by secretions from the G.I. tract and associated organs
What is mechanical disruption and when does it begin?
Mechanical disruption of ingested food begins in the mouth with chewing (mastication). Individuals without teeth usually require their solid food to be cut into smaller pieces before eating. The mechanical processes that alter food composition to facilitate composition absorption continue in the stomach both to initiate protein and lipid enzymatic digestion and to allow passage of gastric contents through the pylorous into the duodenum. This change in the size and consistency of gastric contents is necessary because solids that are greater than 2 mm in diameter do not pass through the pylorous
Why are a complex series of chemical reactions unique to each nutrient?
The chemical form in which different nutrients are ingested and absorbed varies according to the specific nutrient in question. Thus, complex series of chemical reactions are required to convert macromolecules into respective micronutrient
Explain digestion requiring enzymes secreted in the mouth, stomach, pancreas, and small intestine
Digestion involves the conversion of dietary food nutrients to a form that the small intestine can absorb. Digestion is completed in the small intestine by the action of enzymes. Digestion by these brush border enzymes is referred to as membrane digestion. The material presented to the small intestine includes both dietary intake and secretory products. The food material entering the small intestine differs considerably from that of the ingested material because of the mechanical and chemical changes just discussed. The load to the small intestine is also significantly greater than that of the ingested material. Dietary fluid intake is 1.5 to 2.5 L/day, whereas the fluid load presented to the small intestine is 8 to 9 L/day. The increased volume results from substantial quantities of salivary, gastric, biliary, pancreatic, and small intestinal secretions. These secretions contain large amounts of protein, primarily in the form of the digestive enzymes.
What happens after nutrients are metabolized?
Through a series of chemical processes- making it possible for cells to continue living. Waste substances from the ingested food are stored and excreted in the G.I. tract. Also excreted in the G.I. tract are products from the liver such as bile acids formed from cholesterol, bile pigments formed from hemoglobin degradation and drug metabolites
What is the function of the mouth?
Salivary enzymes begin carbohydrates digestion and breaking up of food particles
What is the function of salivary glands?
Moistens and lubricates food. The enzyme amylase digests carbohydrates
What is the function of the pharynx?
Directs food into the stomach to prevent choking
What is the function of the esophagus ?
Carries food down from the mouth to the stomach
What is the function of the liver?
The largest organ inside of the body. Makes bile (fluid that helps break down fats and gets rid of wastes in the body;); changes food into energy; and clears alcohol, some medicines, and poisons from the blood . Stores vitamins and iron. Destroys old blood cells
What is the function of the stomach?
Secretes hydrochloric acid and gastric juice and activates enzymes, continues to kill bacteria and pathogens. Stores and churns food. Enzyme pepsin digests protein
What is the f7nctio; 9f the gall bladder?
Stores the bile made in the liver, then empties it into the small intestine to help digest fats. Stones can form within.
What is the function of the pancreas?
A gland that makes enzymes for digestion and bicarbonate to neutralize stomach acid. Also makes the hormone insulin, which helps the body turns food into energy, and regulates blood sugar levels
What is the function of the small intestine ?
Digests protein, fats and carbs. Bacterial metabolism plus nutrient and excess water absorption. Surface is covered in villi for greater absorption. Site of fructose malabsorption and coeliac disease.
What is the function of the large intestine?
Also called the colon. It absorbs water and electrolytes from stool. It also contains beneficial bacteria which help produce certain vitamins. Forms and stores feaces
What is the function of the appendix?
A pouch attached to the first part of the large intestine. No one knows it’s function
What is the function of the rectum?
The lower end of the large intestine, leading to the anus. Stores and expels faeces
What is the function of the anus
The. Opening at the end of the digestive tract where the bowel movements leave the body
Explain the secretions and motility patterns of the oral cavity and esophagus
Motility patterns: swallowing, chewing
Secretions: saliva(salivary glands)
Digestion: carbohydrates, fats(minimal)
No absorption
Explain the motility pattern, secretion and digestion of the stomach
Motility pattern: peristaltic mixing and propulsion
Secretions: HCl (parietal cells); pepsinogen and gastric lipase (chief cells ) ; mucus cells and HCO3- (surface mucous cells); gastrin (G cells) ; histamine( ECL cells)
Digestion: proteins, fats
Absorption: lipid-soluble substances such( as alcohol and aspirin )
Explain the motility patterns, absorption, digestion of the small intestine
Motility pattern: mixing and propulsion primarily by segmentation
Secretion: enzymes; HCO3- and enzymes (pancreas) ; bile (liver) ; mucus(goblet cells ); hormones : CCK, secretin, GIP, and other hormones
Digestion: carbohydrates, fats, polypeptides, nucleic acids
Absorption: peptides by active transport; amino acids, glucose, and fructose by secondary active transport ; fats by simple diffusion; water by osmosis; ions, minerals, and vitamins by act8ve transport
What is the motility pattern, absorption and digestion of the large intestines ?
Motility pattern: segmental mixing; mass movement for propulsion
Secretions: mucus(goblet cells)
Digestion: none except by bacteria
Absorption: ions, water, minerals, vitamins, and small organic molecules produced by bacteria
Briefly describe the gastrointestinal layers
The intestinal tract is composed of multiple layers, each possessing a distinct function. Depending on structure-function relations, a layer’s prominence changes along the length of the tube. Moving from the lumen to the outside of the tube, the layers include epithelium, lamina propria, muscularis mucousa, submucosa, sunmucosal plexus, circular muscle, myenteric plexus longitudinal muscle and serosa
Describe the mucosa layer of the gastrointestinal layers
Epithelium lamina propria and muscularis mucosa. The apical side of the epithelium faces the GI lumen and the bassolateral side faces the interstitium and vasculature. Apical surfaces may have villi and crypts
Describe the submucosa of the gastrointestinal system
Thicker layer with similar composition to the lamina propria. It contains the blood vessels and bundle of nerve fibers called the sub mucosal plexus (Meissner’s).
Describe the muscularis externa if the gastrointestinal system
Comprised of the circular and longitudinal muscle. The myenteric plexus (Auberch) is nestled between the two layers. The circular muscle punches the tube when it contracts; whilst the longitudinal muscle arranged parallelly shortens the tube when it contracts
Describe the serosa of the gastrointestinal system
Outermost layer of connective tissue and a layer of squamous epithelial cells. Some parts of the GI tract do not have a serosal layer but connect directly to the adventitia
What are the epithelial cells?
Are specialized for various secretions and absorptions of nutrients
What is the muscularis mucosa?
Contractions change the surface areas for secretion of nutrients
What are the purposes of circular and longitudinal muscles?
Circular muscle- contractions decrease diameter of the lumen
Longitudinal muscle- contraction shortens the segment of the GIT
What are the Submucosal plexus( Meissner’s) and Myenteric plexus (Auerbach)?
Integrate the motility, secretory and endocrine functions of the GIT
What regulates the external environment of the lumen of the GI?
- Stimulus at the gastrointestinal wall affects chemoreceptors, osmoreceptors, or mechanoreceptors stimulate both afferent impulses and local(enteric) plexus)
- afferent impulses taken to central nervous system , long reflexes stimulate efferent impulses which also local (enteric) nerve plexus
- Enteric nerve plexus stimulates eff3ctors: smooth muscle or gland. Response is change in contractile or secretory activity
- Short reflex is when stimulus stimulates chemoreceptors, osmoreceptors or mechanoreceptors which stimulates enteric/local plexus which then stimulates smooth muscle or gland- short reflex
Briefly describe neural regulation of the GIT
Neural regulation is by the three divisions of the autonomic nervous system
Neural extrinsic control by the CNS I.e. parasympathetic and sympathetic nervous system
Neural intrinsic control by local centers I.e. enteric nervous system
What parts of the autonomic system are part of the enteric nervous system?
Sympathetic and parasympathetic
What is the enteric nervous system?
A “Mini-brain”, with independent information processing capability and also a “program library” that initiates patterns of GI activity (e.g., inter-digestive, digestive, emetic activity etc)
Explain the anatomical arrangement of the ENS
The ENS is arranged anatomically into nerve networks in the myenteric and submucosal plexuses. The system contains around 100 million neurons (sensory, inter- and motor). Ganglion cells in the ENS resemble those in the CNS to form sophisticated integrated neural networks. Given its size, the ENS is often viewed as a “displaced” part of the CNS, concerned with regulating GI function
Explain the autonomously of the Enteric nervous system.
The ENS can function autonomously without extrinsic connections. The myenteric (Auerbach’s) plexus is primarily involved with the contraction of motility and innervates the longitudinal and circular smooth muscle layers. The sub mucousal (Meissner’s) plexus mainly innervates the glandular epithelium, intestinal endocrine cells and submucosal vessels to control intestinal secretion
Describe the parasympathetic division of the enteric nervous system
Origin in the brain stem and sacral segments of the spinal cord (S2-S4)
Parasympathetic ganglia located near the target organs
- Excitatory
- Vagus(Upper GIT) and pelvic nerves/splanchnic nerves (Lower GIT)
- Preganglionics synapse on plexus-long
- Post-ganglionic synapse on smooth muscle, secretory cells and endocrine cells-short
- Sensory component respond to stretch, pressure, temperature and osmolarity
- Neurotransmitters-ACh, Gastric releasing peptide and substance P
Describe the sympathetic division of the enteric nervous system
Origin in the thoracolumbar segments (T1-L2) Sympathetic ganglia located near the central nervous system
- Inhibitory
- Preganglionic (cholinergic) synapse in the prevertebral ganglion-short
- Postganglionic (adrenergic) synapse on plexus- long
- Plexus relay info to target cells
- Neurotransmitters are norepinephrine and neuropeptide Y
What is the function of the intrinsic pathway?
Forms an interconnection with extrinsic,
- Uses local reflexes
- Can function in absence of extrinsic stimulation- ‘mini brain’
Consists of messenteric plexus (Auerbach) and submucosal plexus(Meissner’s)
What is the function of the myenteric plexus?
Regulates intestinal smooth muscle causing tonic and rhythmic contractions (motility)
What is the function of the submucosal plexus?
Intestinal secretions and absorptive surface area
- blood vessels
- receives sensory information from chemoreceptors and mechanoreceptors
What are the neurocrine reflexes/responses that involve GI peptides?
Neurocrines:
- Peptides contained within the nerves of the gut
- ACh, substance P (smooth muscle contraction),
- Norepinephrine, vasoactive intestinal peptide (relaxation)
What are the endocrine responses/reflexes that involve GI peptides?
- Hormones synthesized by cells in the gastric and intestinal epithelium and released into the blood stream
- Glucose dependent Insulinotropic peptide (GIP), Secretin, Cholecytostokinin and Gastrin
What are the paracrine reflexes/responses that involve GI peptides?
- Peptides that act locally within the same tissue that secretes them
- Histamine and somatostatin
How do hormones affect the GIT?
Endocrine cell of the GI tract secrete hormones that act at long distances
This leads to portal circulation —> liver—> systemic circulation and bind to the target cell
This goes for secretin, CCK, gastrin and GIP
How do paracrines regulate the GIT?
Endocrine cell of the GI tract diffuse, they act at short distances, and bind to the target cell
This goes for histamine and somatostatin
How do neurocrines regulate the GIT ?
Neuron of the GI tract release neurocrines from nerve endings to act at short distances
Includes
Vasoactive intestinal peptide
Gastrin releasing hormone (GRP)
Describe gastrin effects
(+) secretion- AA in stomach, stomach distension, Gastrin releasing peptide, ACh (Vagus)
(-) secretion- acid pH under 1.5, somatostatin
Site of secretion: G cells of antrum, duodenum, jejunum
Target cells: entero-chromaffin cell (ECL) parietal cells
Action: stimulates gastric acid
Increased gastric mucosal growth
Describe the effects of Cholecystokinin(CCK) effects
(+) secretion-FA, AA, distention
(-) somatostatin
Site of secretion: I cells of duodenum, jejunum and ileum
Target of cells:secretion in circulation
Action: -motility of gall bladder
- reduced delivery of chyme from stomach
- increased pancreatic secretion
- increased satiety
Describe the effects of. Secretin in the GIT
(+) Acid in Sl
(-) secretion: somatostatin
Site of secretion: S cells of duodenum, jejunum and ileum
Target cells: pancreas, stomach
Action: decreased gastric acid
- increased HCO3- secretion by pancreas
- neutralizes acid delivered to duodenum
Optimum pH for intestinal enzymes
Describe the effects of Glucise dependent insulin-tropic polypeptide (GIP) on the GIT
(+) secretion- FA, AA, CHO
(-) secretion- neuropeptides
Site of secretion- K cells of duodenum and jejunum upper GI
Target cells- B cells of pancreas
Action: stimulates insulin release
Inhibits gastric acid secretion
What is the incretin effect?
GIP and GLP-1 enhances insulin secretion in response to oral carbohydrates
The total amount of insulin secreted is greater when glucose is administered orally then when administered intravenously
This is known as the incretin effect, the enhanced response to oral glucose interpreted as an indication that the presence of glucose in the G.I. tract stimulates hormonal release and other effects that enhance the sensitivity of the beta cell to glucose
Gastric inhibitory peptide , glucagon like peptide
Describe the effects of somatostatin on the GIT
(+) secretion- protons in the lumen
(-) secretion- vagal stimulation
Site of secretion- D cells of stomach and pyloric atrium, duodenum and islets
Target cells- potent inhibitor
Action:
- decreased pancreatic /gastric secretion
- decreased motility
- decreased gall bladder contraction
- decreased nutrient absorption
- vasoconstrictor
Describe the effects of histamine on the GIT
(+) secretion- food in the stomach
(-) secretion -protons in the stomach
Site of secretion-Entero-chromaffin like cell ECL, mast cells
Target cells- specific cells
Action: stimulates gastric acid secretion
-Enhances vascular vasodilation
Describe the effects of the vasoactive intestinal peptide
(+) secretion- released by action potential
(-) secretion- somatostatin
Site of secretion- neurons in the mucosa and smooth muscle of the GIT
Target cells- smooth muscles including lower esophageal sphincter
Action: relaxation of the GI smooth muscle
- pancreatic HCO3- secretion
- inhibits gastric proton secretion
Describe the effect of Gastrin releasing peptide
(+) secretion-vagus nerve
(-) secretion- acid in the stomach
Site of secretion- post ganglionic fibers of Vagus nerve that innervate the G cells
Target cells- G cells
Action- stimulates Gastrin secretion
Describe the integration and overlap of control modes for gastric acid secretion
- Neural ACh release stimulate potentiation/ gastric parietal cell to release HCl
- Neurocrines cell releases GRP which stimulates gastri
Gut hormone stimulates potentiation/gastric parietal cell which leads to HCl release - Paracrine cells release histamine that stimulates potentiation/gastric parietal cell which leads to HCl release
- End organ responses involve integration overlap of signaling modes- - neural, hormone and paracrine
Explain the phases of digestion
- Cephalic phase - vagus nerve stimulates secretion even before food is swallowed as a result of sensory and mental input
- Gastric phase- food stretches the stomach and activates myenteric and vagovagal reflexes. These reflexes stimulate gastric secretion . Histamine and Gastrin also stimulate acid and enzyme secretion
- Intestinal phase- intestinal Gastrin briefly stimulates the stomach, but then secretin, GIP, CCK, and the enterogastric reflex inhibit gastric secretion and motility while the duodenum processes the chyme already in it. Sympathetic nerve fibers suppress gastric activity, while vagal (parasympathetic ) stimulation of the stomach is now inhibited
