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What is the primary function of the digestive system?
transferring molecules from external to internal environment
What are the 4 basic digestive processes?
motility, secretion, digestion, and absorption
Motility
muscular contractions of the digestive tract
What are the 2 types of motility?
propulsive and mixing
Propulsive motility
propel or push contents through the tract
Mixing motility
mixes food with digestive juices and exposes molecules to absorbing surfaces
Secretion
digestive juices are moved into the digestive tract by the exocrine glands
Digestion
biochemical breakdown of foodstuffs into absorbable units (monomers)
Absorption
movement of absorbable units, water, electrolytes, and vitamins from the digestive tract to the blood
What are the 2 divisions of the digestive system?
accessory digestive organs and the digestive tract
Accessory digestive organs
salivary glands, liver and gallbladder, exocrine pancreas
Digestive tract
oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, anus
What are the 4 layers of the digestive tract?
mucosa, submucosa, muscularis, and serosa
Mucosa
protective surface that is specialized in some areas for secretion and absorption
Submucosa
connective tissue which allows for distensibility and elasticity
Submucosal plexus
regulates gut activity
Muscularis
smooth muscle that produces propulsive and mixing movements
Serosa
outer connective tissue that secretes fluid to prevent friction with surrounding viscera
What are the 4 factors that maximize digestion and absorption?
autonomous smooth muscle function, intrinsic nerve plexus, extrinsic nerves, gastrointestinal hormones
Autonomous smooth muscle function
some intestinal cells are pacemaker cells that continuously fluctuate membrane potentials
Intrinsic nerve plexus
networks of nerve fibers contained within the digestive tract
Extrinsic nerves
innervate digestive organs through the parasympathetic and sympathetic divisions
Gastrointestinal hormones
endocrine glands are located in various portions of the digestive tract
Smooth muscle cells
affects motility
Exocrine gland cells
secretion of digestive juices
Endocrine glands cells
secretion of hormones
What are the functional parts of the digestive system?
tongue and taste buds which stimulate salivary, gastric, pancreatic, and bile secretion
Secretion in the mouth
saliva begins carbohydrate digestion which facilitates swallowing
What is saliva production regulated by?
conditioned salivary reflex and autonomic influence
Pharynx and esophagus
uses propulsion motility move the bolus from the mouth through the esophagus to the stomach
What does mucus act as?
lubricant and a protective layer
What are the 2 stages of swallowing?
oropharyngeal and esophageal stage
Oropharyngeal stage
bolus moves from mouth through the pharynx to the esophagus through propulsive motility
Esophageal stage
bolus moves through esophagus to the stomach through propulsive motility
What are the 3 stomach regions?
fundus, body, and antrum
Fundus of the stomach
a thin muscle layer above the esophageal opening
Body of the stomach
thin muscle layer that stores food
Antrum of the stomach
thick muscle layer involved in mixing food
What are the 3 main functions of the stomach?
stores ingested food until emptied into small intestine, secretes hydrochloric acid and enzymes, and mixing motility pulverizes food and mixes with secretions
What do parietal cells secrete?
hydrochloric acid which activates pepsinogen
What do chief cells secrete?
pepsinogen
What does pepsinogen do?
converts to pepsin which initiates protein digestion
G cells
secrete the hormone gastrin in response to proteins in the stomach
Enterochromaffin-like cells (ECL)
release histamines which acts on parietal cells to speed up HCl secretion
D cells
release somatostatin which inhibits secretions by parietal cells, G cells, and ECL cells
Cephalic phase
thinking about smelling, or tasting the food before food arrives
Gastric phase
protein stimulates secretion of hormone gastrin which promotes secretion of HCl pepsinogen
Intestinal phase
inhibitory effects from small intestine which slows gastric secretion to regulate emptying
What are digested in the stomach?
carbs and protein
What can be absorbed in the stomach?
ethyl alcohol and aspirin can be but not food or water
What does the food mix with in the duodenum?
small intestine secretions, exocrine secretions, and liver secretions from bile ducts
What does the biliary system contain?
liver, gallbladder, and associated ducts
Proteolytic enzymes
digest proteins into small peptide chains and amino acids
Pancreatic amylase
digests polysaccharides into disaccharides
Pancreatic lipase
only secretion to digest fat
Aqueous alkaline secretion
neutralizes chyme and promotes pancreatic enzyme action
What are the functional parts of the small intestine?
duodenum, jejunum and ileum
Duodenum
does a lot of the digestion and absorption
Jejunum and ileum
finishes the process and aids in mixing
Segmentation of duodenum
initiated by distension from chyme
Segmentation of ileum
result of gastrin secretion from chyme goes into stomach
Migrating motility complex
when segmentation stops between meals and occurs after most absorption has occurred
Aqueous salt and mucus solution
provides lubrication and protection and provides H2O for hydrolysis
Na+ absorption
passive in between epithelial cells and active through epithelial cells
Vitamin absorption
mostly passive absorption
Electrolyte absorption
iron and calcium absorption are regulated
Carb and protein absorption
absorbed via secondary active transport into cells
Fat absorption
passively absorbed through micelles
Functional parts of the large intestine
cecum, colon, appendix, and rectum
Haustral contractions
slow mixing movements which shuffle contents back and forth
Mass movements
happens 3-4 times a day usually after meals where parts of the colon contract simultaneously
Anabolism
synthesis of larger macromolecules from smaller monomers which requires energy input
Catabolism
breakdown of macromolecules into monomers though hydrolysis and oxidation after anabolism if needed
Energy input
comes from ingested food and is released when molecule bonds are broken
Energy output
comes from the use of energy either through internal work or external work
External work
energy used to produce mechanical work
Internal work
activities necessary to sustain life
Essential nutrients only in food
amino acids, fatty acids, vitamin D and vitamin C
What stimulates hunger?
neuropeptide
What suppresses appetite?
melanocortins
Alpha cells
glucagon
Beta cells
insulin
What are the functional glands of the endocrine system?
pituitary, thyroid and parathyroid, adrenal, pancreas, gonads, thymus
Where do non-polar hormones bind to?
can pass through plasma membrane and bind to the receptor within the organ