Digestive System, Lecture 1 Flashcards
Digestive System Overview
functions - material to build and fuel
- ingestion with breakdown to absorbable from (products of digestion)
- absorption to internal environment
- elimination of remaining material, metabolic end products/wastes
-> have to get most things done at level of digestion
-> absorb some peptide if they are small enough
What is the Digestive System is Composed of?
composed of:
- gastrointestinal (GI) tract - muscular tube; mouth to anus
- accessory organs/structures - make contributions to GI tract
◦ ex. organ like pancreas will have passageways
that lead into GI tract
◦ ex. teeth make contribution by breaking down
foods for digestion
6 Component Processes of Digestive System
ingestion - entry to mouth (only at mouth)
motility - movement - mixing/propulsion (movement of GI tract)
- propulsion: moving through tube
- mixing: add things from accessory structures, we have to mix them up and when we add enzyme we gave to mix it up wit materials to get most effective breakdown
- sometimes we get more mixing then moving because we want a particular breakdown of the foods
secretion - contributions to the GI tract (tends to be lots of water but also, enzymes and bicarbonate)
digestion - breakdown - mechanical (physical) / chemical (break bonds in structure) (taking macromolecules down to something we can actually digest)
absorption - entry to body - blood/ lymphatic (out of the GI tract to blood stream or also to lymphatic system)
- initially to lymphatic but goes to blood after with some things
defecation - elimination (other end of ingestion - exit)
- can have multiple processes from one structure (ex. stomach)
- actually has to be absorbed to enter the body, otherwise it just stays in the tube (absorption gets us into body)
Gastrointestinal Regulation
control by volume and composition, rather than nutritional state
stimuli - receptor:
- distension (stretch): mechanoreceptors (large amount food coming into stomach that is going to stretch stomach as large volume entered)
- osmolarity: osmoreceptors (high or low osmotic pressure)
- specific concentrations or acidity: chemoreceptors (that detects acidity levels of detects fats, carbohydrates, amino acids (concentration or volume rising))
responses: hormonal and neural
key targets for responsesL
- smooth muscle cells (layers of muscles surrounding GI tract) - circular (around the tube - can constrict a certain layer) or longitudinal muscle contractions (run down length of layer)
- epithelial cells (layers = epithelium; inner layer of GI tract):
◦ enteroendocrine cells - hormonal secretions
(release hormones)
◦ exocrine cells - enzyme secretions (release
enzymes)
◦ mucous cells - mucus secretions (release mucus)
Gastrointestinal Regulation - Neural (CNS)
central nervous system (CNS; brain and spinal cord)
- afferent information sent to CNS from receptors (Into the CNS)
◦ somatic sensory - muscles
◦ visceral sensory - organs
◦ special sensory - eyes, ears, etc.
efferent information sent to effectors from CNS:
◦ somatic motor - muscles (telling muscles to
contract or relax)
◦ autonomic motor (or autonomic nervous system;
ANS) - sympathetic (increase HR) parasympathetic
(decrease HR), enteric - variable targets (nervous
system that can form a loop of its own) (capable
doing decision making on its own level, does not
need to send signals to spinal cord or brain)
‣ enteric (or enteric nervous system, gut brain,
2nd brain)
* GI tract’s own local neural control with
more neurons than spinal cord
Gastrointestinal Regulation - Neural
enteric nerve plexuses: myenteric and submucosal (neural signals are being regulated)
◦ ability to relay info down and up the GI tract with
plexus
connections/communications
- within plexus and between plexuses
- from receptors and CNS to plexuses
- myenteric signal especially to circular and longitudinal muscle layers (important to the muscles)
- submucosal signal especially to especially to epithelium cell types (epithelial cells) - more likely but both still communicate with everything
short reflexes (intrinsic; enteric)
- within the GI tract (not going up to CNS)
- stimuli - receptors - plexus - effector (maybe smooth muscle or gland) - response
- we can get response without sending signal up to CNS
long reflexes (extrinsic; CNS)
- neural path extending outside GI tract
- afferent - receptors to CNS
- efferent autonomic - CNS to plexuses (sympathetic or parasympathetic)
Gastrointestinal Regulation - Hormonal
enteroendocrine cells
- mainly stomach and small intestine
- one surface exposed to GI tract lumen; stimuli encountered
- when stimulated, opposite cell surface from GI tract lumen releases hormones into blood
- travels into blood to target cells where it binds and generates a response
- can be more than one target location
- responses can be excitatory and/or inhibitory
- key hormones:
◦ gastrin
◦ secretin
◦ cholecystokinin (CCK)
◦ somatostatin
my notes:
- stimulus detected on one side (lumen) and hormone released on other side as hormone is not going into lumen, it is going into bloodstream (most you release in the lumen)
- it will bind to target cells and get a response (has to circulate to find it)
Gastrointestinal Regulation - Cephalic Phase
- stimuli: signal to salivary centre in medulla oblongata
- response: salivary glands increase salivation (saliva production)
- parasympathetic and sympathetic stimulate salivation (not antagonistic)
◦ parasympathetic:
‣ provide basic amount to keep tissues moist
‣ stimulation: abundant, watery saliva rich in
enzymes (increase production, large volume)
‣ triggers - sight, smell, taste, hunger (could be
fairly small increase if there is not actually
food present, can only get so much
production)
◦ sympathetic
‣ stimulation: smaller volume thick saliva rich in
mucus (drier mouth feel) (tends to have a lot
more mucus in comparison to water)
‣ triggers - dehydration/stress
* start point can be from any stimuli, pathway would be fairly the same
GI Tract - Mouth (what does saliva compose of?)
water, mucus, bicarbonate and lysozymes and enzymes (start chemical digestion)
GI Tract - Mouth (saliva composition - water)
water (99%): moistens food and tissues / taste (can not taste much when the mouth is particularly dry) / rinses away residues to help prevent bacterial growth
GI Tract - Mouth (saliva composition - mucus)
mucus: lubricates food, mouth, pharynx to facilitate swallowing (going to be critical in forming something you can swallow, mucus makes it easier material to swallow)
GI Tract - Mouth (saliva composition - bicarbonate)
bicarbonate: neutralizes food acids (environmental: mouth slightly acidic) -> can neutralize acidity
GI Tract - Mouth (saliva composition - lysozymes)
kill bacterial to help prevent bacterial growth
GI Tract - Mouth (enzymes - start chemical digestion)
- salivary amylase: polysaccarides to maltose; operates best in slighty acidic environment - from mouth into fundus in stomach
◦ lingual lipase - triglycerides to monoglyceride and fatty acids; operates best in a highly acidic environment - body and antrum of stomach where activity peaks (when it hits stomach, peak activity is reached as it likes highly acidic) (before not in optimal environment at the mouth, seeing as the mouth is not highly acidic) - we want the mouth to be slightly acidic, because one of the enzymes prefers it (salivary amlylase will be its best in this environment - optimal conditions)
- salivary amylase breaks polysaccharides to maltose
- an enzyme will have minimal activity in an environment they cannot work their best in
- lingual lipase does work at mouth, but better in stomach
- salivary amylase, on the other hand, it at its highest activity (the stomach environment would properly deactivate it)
GI Tract - Mouth (mastication)
mastication - chewing - mechanical digestion
- skeletal muscles plus activity tongue, lips and cheeks
- voluntary aspects, but also involves involuntary (food pushing against certain structures) mastication patterns and reflexes (rhythmic jaw movements)
- triggered by pressure of food against mouth structures
my notes:
* largely involving a lot of muscles
* involuntary - automatic functions, some pressure receptors (pushing against teeth, for example)
Goals of Mastication
goals:
- physical breakdown - more surface area exposed (easier for enzymes to do their job if there is larger surface area)
- mixing with enzymes added in mouth (start chemical digestion)
- forming bolus (semisolid mass) for swallowing - easy to swallow
- limited absorption (farther along in system) - some vitamins and minerals, electrolytes, alcohol and drug
