Digestive System Flashcards
What are the 3 primary functions of the digestive system
To digest food into absorbable nutrients, move (absorb) nutrients, water, and electrolytes from the GI lumen into blood and ISF using transport mechanisms, and repel foreign invaders via secretions
What are the 3 kinds of ingested macromolecules
Carbohydrates, proteins, and lipids (fats)
Describe digestion
Mechanical (chewing and churning) and chemical (enzymatic) breaks down food into absorbable units
What 6 secretions happen in the digestive system and where (4 places)
Saliva, mucus, digestive enzymes, bile, H2O, and ions all for chemical digestion and lubrication, coming from salivary glands, the pancreas, liver, and epithelial cells of the stomach and small intestine
What is absorption in the digestive system
Nutrients moved into the blood or lymph mainly in the small intestine using brush borer cells (enterocytes) on villi, but also some of ions and H2O in large intestine
What is motility
The movement of material through the GI tract via smooth muscle contractions
Where does carbohydrate digestion begin
The mouth via amylase from saliva (glucose polymers like starch and glycogen are broken down into disaccharides)
Where does carbohydrate chemical digestion continue
In the small intestine, epithelial cells secrete disaccharidase that breaks down disaccharides into monosaccharides, and the pancreas secretes amylase as well
What is the absorbable end unit of carbs
Monosaccharides
What glucose transporters move glucose and galactose in the small intestine
Apical entry: SGLT (Na+-linked cotransporter)
Basolateral exit: GLUT2
What glucose transporters move fructose in the small intestine
Apical entry: GLUT5
Basolateral exit: GLUT2
How do glucose, galactose, and fructose enter the blood in the small intestine
Simply diffuse through fenestrated (leaky) capillaries
Where does chemical digestion of proteins occur
The stomach (HCl denatures proteins and peptidases digest into di&tri peptides, and oligopeptides) and continues in small intestine (peptidases from epithelium and pancreas)
What triggers the pancreas to release peptidases into the small intestine
CCK
What do endopeptidases do
Break the peptide bond internally (in the middle)
What do exopeptidase do
Break the peptide bond on the terminal end (can be aminopeptidase or carboxypeptidase)
What are the absorbable units for proteins
Di-peptides, tri-peptides, oligopeptides, and free amino acids
How are free amino acids absorbed
Secondary active cotransport with Na+
How are di- and tri-peptides absorbed
Secondary active cotransport with H+
How are oligopeptides absorbed
Transcytosis (moving in vesicles through the cell)
What are the absorbable units of lipids
Free fatty acids and monoglycerides
Where are lipids chemically digested
In the small intestine (CCK triggers release of bile that forms emulsions and pancreas secretes lipase that breaks triglycerides into monoglycerides and free fatty acids)
Where is bile made and stored, and secreted
Made in the liver, stored in the gal bladder, secreted into the duodenum of the small intestine
How does bile work
Bile salts break down the fat droplets into smaller ones (emulsions then micelles) which increases the surface area for enzymatic digestion by lipase
How are fats absorbed
Via simple diffusion through the enterocytes (brush border) of lumen of the small intestine, cholesterol is actively transported to speed up absorption (too slow because of size)
Once monoglycerides and free fatty acids enter the smooth ER, what happens
They are reassembled into triglycerides which will combine with cholesterol and proteins to form large droplets (chylomicrons)
How are chylomicrons absorbed
Leave enterocytes into interstitial fluid where they are exocytosed into lacteals (lymphatic system) and venous blood
What are the characteristics of the smooth muscle of the GI tract
Circular (diameter) and longitudinal (length) muscle connected via gap junctions
What causes phasic contractions in the GI tract
Spontaneous slow wave potentials created by the interstitial cells of Cajal (pacemaker cells)
What are the 3 characteristics of the enteric nervous system
Neurons in the GI was that control motility and secretion, can work independently (short reflexes) or with CNS (long reflexes) via ANS
Where can GI peptides act
in digestive system to increase or decrease motility and secretion or outside the GI tract to influence hunger/satiety, food intake, and insulin/glucagon secretion
What 4 GI peptides influence motility and secretion
Cholecystokinin (bile), motlin (migrating complex), gastrin (HCl and pepsin), and secretin (gastric secretion to neutralize acid)
What GI peptides influence hunger/satiety and food intake
Cholecystokinin (satiety) and gherlin (hunger)
What GI peptides influence insulin/glucagon secretions (via pancreas)
Incretins
What parts of the body does the cephalic phase deal with
Oral cavity, esophagus, and stomach
What parts of the body does the gastric phase deal with
Stomach and intestines
What parts of the body does the intestinal phase deal with
Small intestine, large intestine, and rectum
What triggers the cephalic phase
Long, feedforward reflexes from brain (sight, smell, or taste of food) trigger saliva secretion (ANS control) from salivary glands (chemical digestion) and mastication/chewing begins (mechanical digestion), and triggers swallowing reflex
What 5 things is saliva made of
Water, ions, mucus, enzymes (amylase), and antibodies to moisten, lubricate, and digest food
What is the swallowing reflex (4 steps)
Integrated by the medulla oblongata, the tongue pushes bolus (food) against the soft palate, it passes through the closed airway (epiglottis) while breathing is inhibited and the food moves into the esophagus and stomach via peristaltic contractions
What is aspiration
Fluid or food entering the lungs during ingestion
Describe your upper esophageal sphincter at rest vs. during the cephalic phase
At rest, it is tonically contracted, when you swallow it relaxes
What happens during the integration of the cephalic and gastric phase reflexes
Before food arrival, long reflex of cephalic phase and PS ANS travels through vagal nerve to increase secretion and motility, then food entry initiates short reflexes that activate gastric phase
What are the 2 stimuli for the gastric phase
Distention (stretch) of the stomach and presence of peptides and amino acids in the gastric lumen
What are the 3 functions of the gastric phase of digestion
Storage of food (upper stomach relaxes and expands), digestion (lower stomach), and protection (destroyed pathogens with swallowed with food)
Describe digestion during the gastric phase (4 things)
Chemical and mechanical digestion turn food into chyme (watery mixture), digests proteins, secretes HCl, enzymes, paracrine signals, and hormones, and regulates entry into the small intestine via peristaltic contractile waves
How is acid secreted in the stomach during the gastric phase
Parietal cells (gastric cells) secrete hydrochloric acid using a proton pump (primary active for H+), regulated by gastrin (stimulated by peptides and amino acids - short reflex)
What are 3 functions of hydrochloric acid
To denature proteins for enzyme access, to activate a peptidase (chief cells secrete pepsin), and to kill pathogens
What protection occurs during the gastric phase
Mucus and bicarbonate buffer are continuously secreted by gastric mucous cells to form a barrier of gastric mucosa
What are the 4 things chyme does once it enters the small intestine
Slows chyme delivery (via ENS and GI peptides like secretin, CCK, and incretins), decreases stomach acid and increases SI bicarbonate (secretin breaks down acids), increase digestive enzymes and bile (CCK to break down fats and proteins), and increase insulin (incretins to break down carbs)
What are the 2 kinds of incretins
GIP (gastric inhibitory peptide) and GLP-1 (glucagon-like peptide-1)
What happens during the intestinal phase in the small intestine
Chyme mixes with enzymes for chemical digestions (enterocytes secrete disaccharides and peptidases), exocrine pancreas secretes amylase and peptidases, and the gallbladder secretes bile
Where does most of the bicarbonate come from in the intestinal phase
The pancreas, some also comes from duodenum
What does the exocrine pancreas secrete
Acing cells secrete digestive enzymes and duct cells secrete sodium bicarbonate (NaHCO3)
What does the endocrine pancreas secrete
Alpha islet cells secrete glucagon and beta islet cells secrete insulin, both to regulate blood glucose levels
What 3 things happen in the large intestine during the intestinal phase
Water is concentrated for secretion and most water in chyme is absorbed, bacteria in the colon break down undigested carbs and proteins via fermentation, and some absorption of ions, minerals, vitamins, and water
What happens in the rectum during the intestinal phase
Defecation (waste material eliminated from digestive tract)
What triggers the defecation reflex and what does it do
Distention of rectum creates urge to defecate, then internal anal sphincter relaxes (ENS and PSNS control) and external anal sphincter contracts (skeletal muscle, somatic control)
What are the two key hypothalamic control centers in the digestive system
Appetite center (always on) and satiety center (inhibits feeding center)
What 3 input signals go to the appetite and satiety centers
Neural inputs (cerebral cortex and limbic system), peptide hormones (GI tract), and adipocytokines (adipose tissue)
What are adipocytokines
Chemical signals released by adipose tissue involved in energy homeostasis and regulation of glucose and lipid metabolism
What does neuropeptide Y (NPY) do
Released from the brain, it increases food intake and energy storage (fat)
What does gherlin do
Secreted by empty stomach, increases hunger
What does leptin do
Made by adipocytes, as fat stores increase, more leptin is secreted to decrease food intake (negative/inhibitory feedback for NYP)
What do CCK and incretin (GLP-1) do to the hypothalamic feeding center
Released from the duodenum in response to fats and carbs in chyme, they promote satiety and decrease food intake
What is metabolism
Chemical processes occurring in a living organism to maintain life (all anabolic and catabolic reactions)
What are wastes from nutrient conversion
Urea, CO2, H2O, etc.
How are absorbed nutrients stored
Glycogen (mostly liver and muscle) or triglycerides in adipose tissue
Which macromolecule provides the most energy
Fat
What 3 processes are involved in ATP production
Glycolysis (cytosol), citric acid/Kreb cycle (mitochondria), and electron transport system (mitochondria)
Where do carbohydrates enter the production of ATP
Glycolysis as glucose, then broken into 2 3-carbon chains)
Where do lipids enter the production of ATP
Glycerol enters glycolysis, fatty acids go through beta-oxidation to become acyl units (2 carbons) which then enter the Kreb cycle as acetyl CoA
Where do amino acids enter the production of ATP
At various points
What is glycogenesis
Production of glycogen
What is lipogenesis
Production of fat
What is lipolysis
Breakdown of fat into fatty acids
What is glycogenolysis
Breakdown of glycogen into glucose
What is gluconeogenesis
Making glucose from amino acids
Which state of digestion of anabolic and which state is catabolic (fed vs fasted)
Fed/absorptive is anabolic and fasted/post-absorptive if catabolic
What happens to amino acids during the fed state
Amino acids are made into proteins and excess is converted into fat
What happens to proteins in the fasted state
Proteins broken into amino acids (only during extended fasting), amino acids are deaminated for glycolysis, or made into glucose (gluconeogenesis)
What is created when amino acids are deaminated for glycolysis
Ammonia (NH3) is made, goes to urea
What happens to carbs during the fed state
Carbs are used for ATP (glycolysis) or stored as glycogen (glycogenesis) in the liver, excess converted into fat
What happens to lipids during the fed state
Lipids are converted into fat in adipocytes (lipogenesis)
What happens to liver glycogen during the fasted state
Liver glycogen broken down to glucose (glycogenolysis) to use in glycolysis
What happens to lipids during the fasted state
Triglycerides are broken down to glycerol and fatty acids by lipases which are used to produce ATP
What is the dominant hormone in the fed state and why
Insulin because of high blood glucose and amino acid levels, resulting in secretion of insulin from beta-cells of the pancreas
What is the dominant hormone in the fasted state and why
Glucagon because low blood glucose levels result in secretion of glucagon from alpha-cells of the pancreas
Describe the 3 characteristics of insulin
Anabolic hormone that promotes glucose uptake, glucose metabolism (glycolysis), and energy storage as glycogen and fat (glycogenesis and lipogenesis) to decrease plasma glucose and increase fat storage
What kinds of glucose uptake are insulin-sensitive and why
Adipose tissue and resting skeletal muscle because GLUT4 (carrier protein) is either in the membrane of hepatocytes (liver) or not to aid in facilitated diffusion
What factors influence insulin secretion
Feedforward effects of GI hormones, PSNS activity (increased during and after a meal), and SNS activity (inhibits beta-cells of pancreas)
What is the endocrine response to hypoglycemia
Glucagon promotes the breakdown of glycogen (glycogenolysis) and makes it into glucose (gluconeogenesis) to increase plasma glucose
What does homeothermic mean
Homeostatically regulate internal temperature within narrow range
What does hyperthermia cause
Denatures enzymes
What does hypothermia cause
Chemical reactions are too slow
What is the average oral temperature
98.6 degrees F (~96-99.99) with daily fluctuations of about 1.8 degrees
What is heat loss equal to
Internal heat production from metabolism and movement + external heat input
What are the four mechanisms of heat exchange
Conduction, convection, evaporation, and radiation
What is conduction
Gain or loss of heat through contact
What is convection
Gain or loss of heat by bulk flow of air or water
What is radiation
Gain or loss of heat via gradient energy (warm surfaces emit and absorb electromagnetic waves)
What is evaporation
Loss of heat from skin and respiratory tract
What are the 2 unregulated sources of heat production
Metabolism (~75% of nutrient energy), and “waste heat” generated by voluntary muscle contractions
What are the 2 kinds of regulated heat production
Shivering thermogenesis (involuntary tremors in skeletal muscles caused by hypothalamic thermoregulatory center) and non-shivering thermogenesis
What receptors feed into the hypothalamic thermoregulatory center
Central thermoreceptors sense changes in core body temp and peripheral thermoreceptors sense changes in environmental temperature
What are 3 characteristics of non-shivering thermogenesis
Important for thermoregulation in newborns, only in brown adipose tissue, and involves mitochondrial coupling
What is brown adipose tissue/brown fat
Higher density of mitochondria and less lipids
What is mitochondrial uncoupling
Energy from electron transport system released as heat instead of making ATP
What are the bodies 3 thermoregulatory reflexes
Vascular (vasoconstriction or vasodilation), sweat secretion, and/or shivering
How does the body respond to decreased temperature
Hypothalamic thermoregulatory neurons activate sympathetic neurons causing cutaneous vasoconstriction and possibly non-shivering thermogenesis, while somatic motor neurons activate shivering thermogenesis
How does the body respond to increased temperature
Hypothalamic thermoregulatory neurons activate sweat glands which secrete sweat, and cutaneous vasodilation
What kinds of neurons are used for in the SNS for thermoregulaiton
Sympathetic cholinergic neurons release ACh onto muscarinic receptors
What is the therm-neutral zone
Range of environmental temps in which thermoregulation requires only vascular adjustments (don’t need to sweat or shiver)
When does the body’s set point for temperature change
During fever due to pyrogens (from macrophages in WBCs) that act on the hypothalamus or postmenopausal hot flashes (changes in hormone levels act on hypothalamus)
What are 3 benefits to having a fever
Slow down bacterial and viral growth, increases WBCs, and reduces severity of illness and shortens illness duration
