Digestion and Absorption Flashcards
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Three main macronutrients?
Carbohydrate, Fat, Protein
What must be done for humans to use macronutrients for energy production?
They must be broken down into their constituent parts
What are carbs broken down into?
Monosaccharides
What are fats broken down into?
Fatty acids
What are proteins broken down into?
AAs
What other nutrient (not macronutrient) can humans derive energy from?
Alcohol
Main processes of gastrointestinal (GI) tract?
Digestion, absorption, secretion, motility
What two main group can the gastrointestinal system tract be broken up into?
GI tract and accessory organs
Order of GI tract?
Mouth–> pharynx–> oesophagus–> stomach–> small intestine–> large intestine
What are the accessory organs?
Salivary glands, liver, gallbladder, exocrine pancreas
Main layers of GI wall (ordered lumen outwards)?
Mucosa, submucosa, muscularis externa, serosa
Why is smooth muscle needed to line the GI tract?
One main function of GI tract is motility, as the food needs to be passed along the lumen
What makes up the mucosa?
Epithelium, lamina propia, muscularis mucosa
Role of epithelium in mucosa?
First ones that see the food, contain endocrine and exocrine cells
Why does the epithelium contain endocrine cells?
Can secrete hormones for signalling (i.e. nutrients in intestine detected= full signal)
Role of lamina propia in mucosa?
Loose connective tissue between the epithelium and muscularis mucosa
What makes up the submucosa?
Blood and lymphatic vessels, and the submucosal plexus
Role of blood vessels in submucosa?
Transport of nutrients once it has been absorbed
Role of lymphatic system in submucosa?
fats enter it before then entering the bloodstream
Structure of submucosal plexus?
Network of neurons
Role of submucosal plexus?
Neurons are used to activate the muscles in the GI tract
Components of the muscularis externa?
Circular muscle, myenteric plexus, longitudinal muscle
Role of circular muscle?
Its contraction causes the lumen to narrow
Structure of myenteric plexus?
Networkof thin axons and neurones that is innervated from autonomic nervous system
What is the myenteric plexus connected to?
Submucosal plexus–> both contain nerves
Role of longitudinal muscle in muscularis externa?
Its contraction causes the tract to shorten
What can be used to change the shape of the GI tract/lumen?
Circular muscle and longitudinal muscle
What is the serosa?
Connective tissue that surrounds the outer surface of the tract
How is the GI tract kept in place?
Sheets of connective tissue connect the serosa to the abdominal wall
Where does digestion begin?
mouth
What can cause secretion of enzymes in the mouth (saliva)?
Thought, sight or smell of food
What is the cephalic phase of digestion?
Secretion of saliva in response to thought, sight or smell of food
What is produced during the cephalic phase?
Saliva in the mouth, gastric juices and insulin
How is saliva secreted?
Secreted into acini of salivary duct
What can be done to saliva once it has left the acini?
The conc of it can be modified by adding/removing water/ions
What stages of digestion occur in the mouth?
Mechanical breakdown of food
Mixing of food with saliva
CHO chemical digestion
Antibacterial action
What is in the mouth that begins the chemical digestion of CHO?
Salivary amylase
Key components of saliva?
Bicarbonate
Mucus
Salivary amylase
Lysozyme
Role of bicarbonate in saliva?
Neutralises acid
Role of mucus in saliva?
Lubricates and protects from abrasion
Role of salivary amylase in saliva?
Breakdown CHO
Role of lysozymes in saliva?
Destroys certain bacteria (prevents tooth decay)
Why does salivary amylase not do much of the total starch digestion?
Food usually exits the mouth soon after chewing
Role of pharynx?
Conduction of food to oesophagus from mouth
Role of oesophagus?
Conduction of food to stomach from pharynx
Role of epiglottis?
Closes so food doesn’t go down trachea
What is the sphincter?
Ring of muscle at bottom of pharynx
Role of sphincter at bottom of pharynx?
prevents movement of air and stomach contents entering oesophagus
What is peristalsis
Movement of food towards stomach occurs due o progressive muscular contraction
How does peristalsis work?
Muscle behind food contracts while muscle in front relaxes
What is secondary peristalsis?
Repetition of contraction of muscle if a large part of food does not reach the stomach
Roles of stomach?
Mechanical breakdown of food
Secretion of HCl to kill bacteria
Secretion of some enzymes
What is chyme?
Partially digested food mixed w/ contents of stomach
What enzymes does the stomach secrete?
Pepsinogen and gastric lipase
Role of pepsiongen?
becomes pepsin, and then begins protein digestion
Role of gastric lipase?
Involved in fat digestion
Roles of small intestine?
Chemical digestion of all nutrients
Absorption of digestive end-products, water, ions and vitamins
Secretion of hormones
Secretion of bicarbonate rich fluid
What kind of enzymes digest chyme in the small intestine?
Pancreatic or brush border enzymes
Roles of colon?
Absorption of ions and water
Transformation of chyme into faeces
Storage of faeces
Role of rectum?
Storage of faeces
Role of gastric pit?
Increase SA of stomach
Cell types in gastric pit?
Mucus cells
Parietal cells
Chief cells
Enteroendocrine cells
Role of mucus cells in gastric pit?
secretes mucus–> alkaline solution to protect stomach lining
Role of parietal cells in gastric pit?
Secretes HCl and intrinsic factor
Role of intrinsic factor?
Necessary for vitamin B12 absorption
Role of chief cells in gastric pit?
Secretes pepsinogen
How is pepsinogen activated?
By being in acidic conditions
Role of enteroendocrine cells?
secretes gastrin
Role of entero-chromafin-like cells (ECL)?
Release histamine
Role of D cells?
secretes somatostatin
Important for H+ secretion
What is potentiation?
action of three substances combined is greater than the sum of the individual effects
Negative feedback of HCl secretion?
HCl secretion causes more somatostatin to be secreted. Somatostatin inhibiits the secretion of HCl as well as the secretion of histamine
Role of gastrin?
Stimulates ECL cell
Role of histamine in stomach?
Stimulates parietal cell to release HCl
Role of ACh in stomach?
Stimulates parietal cell to release HCl
Activation of pepsinogen into pepsin?
Chief cells secrete pepsinogen, parietal cells secrete HCl
HCl activates it to become pepsin
Role of pepsin?
Breaks down large proteins into smaller peptide fragments
How does the stomach blend food?
Thick muscle in antrum–> pushes food up to mix it w/ chyme
Muscle at top of stomach pushes food down
What happens to salivary amylase in the stomach and why?
Denatures as pH too low
What substances are absorbed in the stomach?
Alcohol and caffeine
Role of villi and microvilli in intestine?
Increase SA
What makes up the majority of the SA of the GI system?
Small and large intestines (mainly small)
Roles of pancreas?
Secretion of pancreatic juice
Secretion of enzymes
What enzymes does the pancreas secrete?
Proteases and pancreatic lipase
Role of gall bladder?
Storage and concentration of bile
Role of liver?
Secrete bile
Processing of absorbed nutrients
Components of bile needed for digestion?
Bile salts, phospholipids and HCO3 -ion
Components of bile that have been removed from the blood?
Cholesterol, bile pigments and trace metals
Role of bile salts?
Emulsify fats so they can have a larger SA
Why do bile salts emulsify fats?
larger SA=larger area for lipases to work on
Why can bile salts break down fat globules?
They have a hydrophobic (nonpolar) side and hydrophilic (polar) sides
What is the significance of bile salts having hydrophobic and hydrophilic regions?
It can bind to both fats and water
Products of breakdown of fat globules using bile salts?
Micelles
What happens to micelles after their formation?
They are broken down into monoglycerides and fatty acids by lipases
Why are micelles broken down into monoglycerides and fatty acids?
So they can be absorbed across the intestinal lining
Role of smooth ER in lipid absorption?
It reforms triglycerides out of monoglycerides and fatty acids
Role of golgi apparatus in lipid absorption?/
Packages triglycerides as chylomicrons
Difference in time taken to absorb between fats and CHOs?
Fats take longer to digest and absorb
Where does protein absorption occur?
Lining of the intestines
Role of brush boarder peptidases in protein absorption?
Breaks down small peptides into AAs
What are AAs absorbed alongside in the intestines?
Na+
Where can small peptides be broken into AAs?
Lumen of intestine (by brush boarder peptidases) and peptidases in the intestinal epithelial cell
How do AAs exit the lining of the intestines?
Via AA transporters
Where do AAs go after leaving the intestinal epithelial cell?
Interstitial fluid
Two methods of small intestine motility?
Peristalsis and segmentation
What method of motility is more common in the small intestine?
Segmentation
How does segmentation work?
Rhythmic contraction of relaxation of longitudinal muscles
What is the purpose of segmentation??
Greater mixing of material
Net flow through small intestine
What is the gastroileal reflex response?
An increase in segmentation as a response to gastric emptying
What is the migrating myoelectric complex (MMC)?
Moves undigested material to the large intestine
Prevents bacteria from remaining in the small intestine for too long
Roles of large intestine?
Storage of material prior to defecation
Digestion of undigested material (via bacteria)
Fluid absorption, which concentrates fecal matter
Role of ileocecal sphincter?
Opens to allow chyme through when ileum contacts
Closes when large intestine distends to prevent backflow
When and why does the ileocecal sphincter open?
When ileum contracts, to allow chyme through
When and why does the ileocecal sphincter close?
When large intestine distends, to prevent backflow
