Digestive Physiology Flashcards
four process of the digestive system
- secretion
- digestion
- motility
- absorption
anatomy of the digestive system
oral cavity
salivary glands
esophagus
gallbladder
liver
pancreas
stomach
small and large intestine
rectum
upper gastrointestinal tract
- begins the digestion of food
- minimal macronutrient absorption here
- mechanical and chemical digestion
saliva
Complex solution containing the enzyme salivary amylase (in babies also some lingual lipase**)
Also composed of water, mucus and ions
3 major glands secrete saliva and each gland secretes a different composition of fluid
- Parotid gland: watery saliva
- Submandibular gland: mixed saliva and mucus
- Sublingual gland: mainly mucus
stages of swallowing
After appropriate mechanical and chemical digestion, the bolus must be passed to the stomach via the esophagus
- Voluntary stage
Decisions for how much to chew and when to begin the process of swallowing
- Pharyngeal stage
Closing off of the nasal cavity and trachea, involuntary
- Esophageal stage
Movement of food down the esophagus, involuntary
mastication
the mechanical digestion of the good into a bolus
chemical digestion also occurs in the mouth
- enzyme salivary amylase: digests carbohydrates
- enzyme lingual lipase: digests fat *but doesnt begin until it reaches the stomach*
peristalsis
- co-ordinate contraction of the muscles in the esophagus
- involuntary control
- propels bolus toward the stomach (gravity not necessary)
- secondary paristalsis initiated bolus is lodged items in the esophagus
summary of mouth digestion
secretion: saliva of varying composition
digestion: chemical (amylase and lipase)
mechanical - mastication
motility: mastication in mouth peristalsis in esophagus
absorption: minimal
stomach
- acts as a reservior for bolus before it enters the intestine
- muscular contractions for mechanical breakdown
- bolus is liquefied to enhance enzymatic digestion
- secretion of 2-3 L of gastric juices
- gastric juices contributed by a number of cell types
basic stomach anatomy
esophagus
low esophageal sphincter
cardia
fundus
pylorus/pyloric sphincter
antrum
rugue
layers of the stomach
1. Mucosa
- single layer of cells that can be endocrine or exocrine
- large folds called rugue and also invaginations called pits
2. Submucosa
- contains a neural network (submucosal plexus)
- connective tissue to adhere mucosa to smooth muscle layer
3. Smooth muscle (muscularis externa)
- circular muscle, longitudinal muscle to change shape of the stomach
- contains a neural network (myeteric plexus)
4. Serosa
- external layer of densa connective tissue
exocrine & endocrine cells of the stomach
exocrine cells
1. Mucus neck cells
- secretes mucus and bicaronate
2. Chief cells
- secretes pepsinogen and gastric lipase
3. Parietal cells (oxyntic cells)
- secretes H+ and Cl- (HCl), intrinsic factor
endocrine cells
1. G cells
- secretes the hormone gastrin
exocrine and endocrine cells of stomach
exocrine cells
- Mucus neck cells
- secretes mucus and bicaronate - Chief cells
- secretes pepsinogen and gastric lipase - Parietal cells (oxyntic cells)
- secretes H+ and Cl- (HCl), intrinsic factor
endocrine cells
- G cells
- secretes the hormone gastrin
stomach mechanical digestion
Propulsion
Grinding
Retropulsion
bolus to chyme
stomach chemical digestion
secreted gastric juices contains hydrochloric acid (HCl)
lingual lipase *activated by HCl - lipid digestion begins
secreted pepsinogen converted to pepsin (because of HCl)
pepsin - protein digestion
secreted gastric lipase - lipid digestion
small intestine
- duodenum
- jejunum
- ileum
functions of acid in the stomach
- activation of lingual lipase - lipid digestion can occur
- activation of pepsin (from pepsinogen) - protein digestion can occur
- inactivation of salivary amylase - carbohydrate digestion stops
- kills microbes
- denatures (unwraps folded structure) proteins
- stimulus secretion of hormones
layers of the small intestine
- Mucosa
- contains mixed population of epithelial cells, small blood vessels and lymph vessels
- invaginations called crypts folds called villi - Submucosa
- contains a neural network (submucosal plexus) - Smooth muscle (muscularis)
- layers of smooth muscle (circular and longitudinal)
- contains a neural network (myenteric plexus) - Serosa
- thin layer of connective tissue
motility of the small intestine
segmentations
- special localized contractions for mixing chyme with digestive juices
- increases the interactions of particles of food in chyme with absorptive cells of the mucosa layer
peristalsis
- propels chyme from the pyloric sphincter towards the large intestine
cell types of the small intestine
- Absorptive cells
- epithelial cells with microvilli - Goblet cells
- secretes mucus - Intestinal gland cells
- secretes alkaline watery mucus - Paneth cells
- secretes lysozyme - S cells
- secretes secretin - CCK cells
- secretes cholecystokinin (CCK) stimulates release of bile - K cells
- secretes glucose dependent insulinotrophic peptide (GIP) stimulates release of insulin
microvilli
- also called brush border (fuzzy appearance)
- increases surface area for absorption of nutrients
- cells on the microvilli have enzymes called brush border enzymes
- final digestion of some nutrients to allow for absorption
brush border enzymes
- Lactase
- Sucrase
- Maltase
- Aminopeptidase
- removes one amino acid from the end of a protein - Dipeptidase
- cuts a dipeptide into two single amino acids - Enteropeptidase
- cuts trypsinogen into trypsin
large intestine
- completes absorption, usually water
- highly populated by bacteria, benefical for completing nutrient extraction via fermentation if any nutrients remain
motility of the large intestine
i. Gastroileal reflex
- presence of food in the stomach stimulates the opening of the ileocecal valve (neural reflex)
ii. Haustral churning
- mixing of large intestine contents from one haustrum to the next
- allows for optimal absorption of mostly water from the lumen contents
iii. Peristalsis & mass peristalsis
- unidirectional movement of lumen contents out of the large intestine
pancreas
epithelial cell clusters called acinar cells and cells that form the ducts (ductal cells) make exocrine secretions
exocrine secretion into ducts converge to form the pancreatic duct, which joins to the common bile duct, and secretions go into the duodenum
exocrine secretions of the pancreas
from ductal cells
i. bicarbonate - neutralizes the acid from the stomach
from acinar cells
ii. pancreatic amylase - digestion of carbohydrates
iii. pancreatic lipase - digestion of lipids
iv. trypsinogen → trypsin → protein digestion
v. chymotrypsinogen → chymotrypsin → protein digestion
vi. procarboxypeptidase → carboxypeptidase - digestion of proteins
vii. prophospholipase → phospholipase - digestion of phospholipids
viii. procolipase → colipase - aids in lipid digestion but not an enzyme
lumen of small intestine
pancreatic duct - pancreatic secretions (include inactive zymogens)
→ trypinogen → trypsin (activated by enteropeptidase in brush border)
→ zymogens - chymotrypsinogen, procarboxypeptidase, prophospholipase, procolipase
→ activated enzymes (activated by trypsin) - chymotrypsin, carboxypeptidase, phospholipase, colipase
endocrine secretions of the pancreas
islets of langerhans
- insulin - from beta cells
- glucagon - from alpha cells
- somatostatin - from delta cells
blood flow of liver
blood → Aorta
→ Hepatic artery - supplies oxygen to liver
→ Digestive tract artery - to capillaries of the intestines
capillaries of intestines connect to the Hepatic portal vein with the nutrients
→ the blood then goes to the liver (capillaries of liver)
→ reconnect as Hepatic vein to the inferior vena cava
liver
made up of mostly hepatocytes (liver cell type)
many functions including secretion of bile which is important for lipid digestion
blood flow to liver has a special arrangement
- oxygen rich blood artery
- nutrient rich blood portal system
hepatocytes
hepatocytes secrete bile into a vessel called a bile canaliculus (plural - bile canaliculi)
these small vessels gather together and join to form a hepatic duct, which eventually forms the common hepatic duct
blood from the hepatic artery and blood from the hepatic portal vein merge together into vessels called sinusoids
sinusoids join together to form the central vein, and then the hepatic vein
the gallbladder
the liver makes bile and stores bile in the gallbladder
gallbladder and common hepatic duct connect to bile duct
the common bile duct connects to the duodenum with a sphincter of Oddi
pancreatic duct joins to the common bile duct and also share the sphincter of Oddi
regulation of gastrix motility and secretions
hormonal and neural communication organized into phases of regulation
phase named by the location of initation in the tract
cephalic phase
stimulus - sight, smell and tase of food
neural control - through medulla oblongata, activation of the submucosal plexus neurons (secretions) and myenteric plexus neurons (motility)
increased secretions from:
- salivary glands (saliva), stomach (ie HCl) and intestine (ie mucus)
increased motility of:
- stomach and small intestine
gastric phase
stimulus - presence of a bolus in the stomach causing stretching, presence of amino acids
neural control - sensory infomation to the submucosal plexus (secretions) and to the myeteric plexus (motility)
hormonal control - gastrin (G cells)
both cause:
increased secretions from:
- stomach (ie HCl) and intestine (ie mucus)
increased motility of:
- stomach & increased gastric emptying
intestinal phase
stimulus - presence of chyme in the intestine
neural control - sensory information to the submuscosal plexus (secretions), and myenteric plexus (motility)
hormonal control - secretin (S cells), CCK (CCK cells), GIP (K cells)
increased secretions from:
- intestine (ie mucus) and pancreas
- bicarbonate from ductal cells *secretin
- digestive enzymes from acinar cells *CCK
- insulin from beta cells *GIP **endocrine pancreas
intestinal phase inhibits the gastric phase
decreased secretions from: stomach (ie HCl)
decreased motility of: stomach & decreased gastric emptying
carbohydrate digestion (chemical)
salivary amylase - starch → maltose
pancreas
pancreatic amylase - starch → maltose
lactase → lactose into glucose + galactose
sucrase → sucrose into glucose + fructose
maltase → maltose into x2 glucose
carbohydrate enzyme mechanisms
starch - amylase → maltose + maltotriose
maltose - maltase → 2 glucose
lactose - lactase → glucose + galactose
sucrose - sucrase → glucose + fructose
carbohydrate absorption
fructose uniporter
protein enzyme mechanisms
pepsin, trypsin, chymotrypsin
endopeptidase → cut into 3 - bond peptide and 4 bond peptide
aminopeptidase → cuts the amino terminus side of the polypeptide
carboxypeptidase → cuts the carboxy terminus side of the polypeptide
protein absorption
amino acid + sodium symporter
di-, tri-peptide + hydrogen symporter
with peptidase in the cell di-, tri-peptides are turned into single amino acids
sodium + hydrogen antiporter (sodium enters cell)
the amino acids cross into the blood by amino acid uniporter
Na/K ATPase (sodium leaves cell into extracellular fluid)
lipid sources
triglycerol/triglycerides - glycerol and 3 fatty acids
fatty acids are variable in length (4-24 carbons), 18 carbons most common
can be saturated (double bond) or unsaturated (no double bond)
bile salts
bile solution - has bile salts that coat the fat droplet (keep lipids separated)
triglycerides - lipase with colipase (breaks down triglycerides into)
diglycerides, monoglycerides and free fatty acids
fat absorption
micelles (lipid droplets surrounded by bile salts) break into fatty acids and monoglycerides
fatty acids and monoglycerides can diffuse across the intestinal cell membranes
once inside the intestinal cells, fatty acids and monoglycerides reform into triglycerides
chylomicrons are the re-packaging of lipids within cells into carriers to allow for transport in the body (first lymphatic vessels by exocytosis, eventually blood)
lipid absorption
lipid droplet + bile salt → micelles
micelles are broken down into fatty acids (monoglyceride)
fatty acids can diffuse into cell
then are repackaged into vesicles and exocytosis into a lymphatic vessel
glycolysis, citric acid and electron transport system to create ATP
in cytoplasm a little ATP and pyruvic acid is created by glycolysis (10 steps)
in the mitochondria
pyruvic acid + CoA is turned into Acetyl CoA
Acetyl CoA is then used in the citric acid cycle to make some CO2 and some ATP
H+ & high energy electrons are used in the electron transport system which creates ATP
glucose uptake
cells of the body take glucose from the blood to make ATP
glucose uniporters (present in the membranes of most body cells) move glucose from a region of high concentration (blood) into a region of low concentration (cell)
glycogenesis
glycogenesis - storage of glucose
some cells have a large capacity to store glucose as glycogen
- skeletal muscle
- liver
some cells, like the brain cannot store glycogen
(fed state)
glycogenolysis
glycogenolysis - breakdown of glycogen
glycogen is converted Glucose 6-phosphate to be used for the production of ATP (step 1 of glycolysis)
liver is unique because it can continue to form glucose which can then be released into the circulation
(fasted state)
gluconeogenesis
gluconeogenesis - formation of new glucose
liver can create new glucose molecules from non-carb sources
from amino acids*, lactic acid and glycerol (part of triglycerides)
lipolysis
lypolysis - breakdown of triglycerides into glycerol and fatty acids
lipogenesis
lipogenesis - formation of triglycerides from non-lipid sources
liver and adipose cells can make triglycerides from glucose and amino acids
glucose absorptive (fed) state
stored in skeletal muscle
glycogenesis
glucose → glucose-6-phosphate → glycogen
most tissues
glucose uptake
liver
glycogenesis
lipogenesis - from extra glucose → triglyceride
adipose tissue
triglyceride from liver goes to circulation and into adipose tissues
fatty acids & glycerol absorptive (fed) state
liver
fatty acids & glycerol from circulation → triglycerides → back into circulation
adipose tissue
fatty acids & glycerol from circulation → triglycerides + triglycerides from circulation
amino acid absorptive (fed) state
skeletal muscle
amino acids from circulation to proteins (protein anabolism)
adipose tissue
extra amino acids from circulation to fatty acids → glycerol → tryglycerides
post-absorptive (fasted) state
liver
glycogen → glucose → circulation
lactic acid from skeletal muscle → glucose
nervous tissue
glucose → ATP
ketones → ATP
other tissues
glucose → ATP
fatty acids → ATP
heart
ketones → ATP
skeletal muscle
glycogen → ATP + lactic acid
proteins → (protein catabolism) amino acids → glucose (liver)
adipose tissue
triglycerides → glycerol + fatty acids
glycerol → liver → glucose
fatty acids → liver → ketones (ketongenesis)
