digestion Flashcards
Where does swallowed food enter?
Gastrointestinal tract (GI tract)
What is in the GI tract?
Esophagus, stomach, small intestine, large intestine
What is the gut?
The portion of the GI tract running from the stomach to the anus
Where does digestion primarily take place?
In the Lumen of the gut
Along the way, secretions are added to ingested food by…
Secretory epithelial cells and accessory glandular organs (e.g. salivary glands, the liver, the gallbladder, and the pancreas)
What is the soupy mixture of food and secretions known as?
Chyme
What is the GI tract?
A long tube with muscular walls lined by secretory and transporting epithelium.
At intervals along the tract, rings of muscle function as sphincters to separate the tube into segments with distinct functions
What are the main digestive organs?
Oral cavity (mouth)
Pharynx (back of throat) and oesophagus (straight tube)
Stomach
Small intestine (duodenum, jejunum, ileum)
Large intestine (starts at caecum then goes to ascending colon, then transverse colon, descending colon, sigmoid colon)
What are the accessory digestive organs?
Do various things to help in process of digestion and absorption
- Teeth, tongue
- Salivary glands
- Peritoneum and Mesenteries (connective tissue that hold everything together)
- Liver (allows to detoxify and has own biliary system)
- gallbladder
- pancreas
Whats the general structure of the digestive tract
- Central point - lumen (highly variable diameter)
- surrounding lumen - mucosa (filled with loose connective tissue like lamina propria, muscularis mucosa)
- submucosa (majority of nerves, blood vessels, and glands are located here)
- Muscularis externa ( smooth muscle layer - two or three layers)
- Adventitia serosa (connective tissue, holds everything in place)
Mucosa
Epithelial lining
lymph vessels
lamina propria of loose connective tissues rich in blood
Submucosa
Dense connective tissues with blood and lymph vessels
specialized nerve plexi (e.g Meissner plexus)
Muscle (muscularis)
contains smooth muscle cells
Divides into 2 layers; internal (circular) and external (longitudinal) - could be 3 layers but it’ll complicate the system
Nerve plexi in this layer too (stimulates the muscles) - myenteric Auerbach’s plexus
Serosa (or adventitia)
Thin layer of loose connective tissue rich in blood and lymph vessels
Adipose and single squamous epithelium (mesothelium)
What is mastication
The manual breakdown of things in your mouth
salivary glands structure
- parotid gland (largest) - has serous cells (cell type that secretes a protein which helps break down carbohydrates)
- Submandibular gland - mucous (to help with lubrication) and serous
- Sublingual gland - mucous and serous
They all secrete saliva
What does saliva do?
- Begin digestion of starches/carbs
- keep mouth moist
- lubricates food
- helps prevent tooth decay
- controls bacterial flora of mouth
Oesophagus
Muscular tube (~25cm long in adult) that transports food and fluid from the pharynx through the throax and diaphragm to the stomach
What state is the lumen usually in in the oesophagus?
Usually collapsed except when swallowing food/liquid
What is at the lower part of the esophagus?
Cardiac gland
Provides rich source of mucous to provide a barrier between the acid and the lower part of the esophagus
Stomach structure
A distensible sac (highly flexible wall) that holds food while it is digested.
Acidifies food during digestion and contains mucous secreting cells to protect the mucosal surface.
Three layers of smooth muscle are found within the muscularis layer; inner oblique, middle circular, outer longitudinal layer
What are rugae?
Valleys and hills, a rough wall inside the stomach
help to increase surface area inside stomach
Where does stomach acid come from?
from Gastric glands in sub-mucosa.
The parietal cells in these gastric glands produce hydrochloric acid that gets funneled into the inside of the stomach.
Why do the walls of the small intestine have a huge absorptive surface area?
Due to microvilli; villi; plicae circularis (circular folds)
Small intestine: duodenum
Brunner’s glands (submucosa) - alkaline secretion to protect small intestine from stomach acid
Small intestine: Jejunum
More characteristic microvilli and villi. Absorption happens here.
Goblet cells - mucous secreting to get effective absorption
Paneth cells - antibacterial properties
Lacteals - lymphatic capillaries to digest fats
Small intestine: ileum
More lymphatic tissue (come in the form of Peyer’s patches)
Villi and microvilli present but are shorter because of transition into large intestine
Where can lipid breakdown happen?
Small intestine and liver
Where acn peptide breakdown happen?
Pancreas and small intestine
Where can carbohydrate breakdown happen
Small intestine
Large intestine functions
Functions to absorb fluid (water and electrolytes) to dry out chyme and create/store faeces
The mucosal lining is specialised to absorb water and electrolytes
Large intestine structure
Mucosa; crypts of Lieberkuhn (lines the mucosa) and goblet cells BUT absence of villi (not specialised for absorption of nutrients)
Muscle; circular and longitudinal layer come together to form muscular layer. Nerve (meisnners) plexus found between the 2 muscle layers
Serosa (outer); Appendices epiploicae (fat patches that hang off, they’re like jewels of fat due to excess fat in system)
Rectum and anal canal
“Rectum” - straight tube that directs faeces out of the body
Transitions from smooth muscle to skeletal muscle because of voluntary control
Junction between rectum and anal there are two sphincters (internal and external)
Internal anal sphincter
Involuntary control because built of smooth muscle
Is smooth muscle voluntary or involuntary, and where is it found?
Involuntary; found in digestive tract, blood vessel walls, uterus
Is smooth muscle striated?
No, not organised in any way. It has one central nuclei (as opposed to skeletal muscle which has many)
Does smooth muscle have sarcomeres and what does this mean?
No sarcomeres. It means they have to get their energy in different ways because of the poorly developed sarcoplasmic reticulum.
Instead of the sarcoplasmic reticulum, they have “dense bodies”
Has spindle shaped fibres
Do smooth muscle contain gap junctions and what do these do?
Yes. For communicating with surrounding cells; transfer of ions across membrane
Why is the smooth muscle ideal for the GI tract?
walls
Allows to be flexible with the wall to allow different diameters for chyme etc. to move forward
How can the smooth muscle operate as one
Provides “syncytial” movement - when one cell is activated the next one is also activated
Rhythmically active
Fewer nerve endings are required in order to contract
Types of contractions in the gut
Phasic contractions and Tonic contractions
What is a phasic contraction in the gut?
Works in phases (e.g relaxation and contraction phase)
useful because don’t want to contract the entire GI tract at once.
Phasic contractions work to move bolus through.
Slow wave because don’t have to react quickly to do peristalsis.
What is a tonic contraction in the gut?
Contracted for a long period of time (E.g sphincters)
Motility in the digestive system
Motility moves chyme along the tract & mixes food (exposure to digestive enzymes)
What is smooth muscle controlled by?
The nervous system and paracrine signals
What is the order of food - faeces
- Food + saliva = bolus
- Bolus + acid = chyme
- Chyme > faeces
Where does the enteric nervous system exist in the digestive system?
The GI tract. It doesn’t work in isolation (e.g. CNS input that can effect it). Digestion on its own is largely effected by autonomic nervous system and the enteric nervous system
Patterns of contraction
Peristalsis (waves) - how we can move chyme throughout gut (2-25cm/sec)
Segmentation - helps to mix chyme in the gut (through contractions, no net forward movement)
Convolution - how small intestine opens up villial folds to increase surface area of absorption
Mass movement - Large intestine; includes mixing. Stores faeces in rectum. 3-4/day
What does submucosal plexus do
> Known as meissner plexus
Give stimuli of pain to signal
myenteric plexus sits within muscular layers (box plexus). drives the motility
Where do short reflexes originate from?
Originate in the ENS and in the absence of CNS control.
Respond to things happening in the gut
What are cephalic reflexes?
Long reflexes that originate in the CNS
What does the ENS respond to?
Higher centre inputs (long reflexes)
Hormonal inputs (e.g secretin, motilin)
Gi secretions; whats the point, what do they do/help with
When released in lumen, theyre gunna react with bolus or chyme to help with digestion
Also released into the blood to signal.
Gastric secretions; mucous cells, parietal cells, chief cells, D cells, G cells
Mucous cells provide both mucous for lubrication as well as alkaline bicarbonate (buffers gastric acid to prevent damage to epithelium)
Mucus also provides physical barrier between lumen and epithelium - tonic secretion; irritation of mucosa releases this.
Parietal cells secrete gastric acid (digest proteins) which activates pepsin and kills bacteria
Chief cells secrete pepsin which digests proteins. It also secretes gastric lipase which digests fats
D cells secrete somatostatin which inhibits gastric acid secretion
G cells secrete Gastrin which stimulates gastric acid secretion.
Pancreas secretes……which is a….for an enzyme
Zymogen - precursor for an enzyme
Enzymes - pepsin and pepsinogen type enzymes
Bicarbonate
Liver structure; vein and arteries and gall bladder
> Excretes harmful substances as bile
Hepatic artery brings oxygenated blood from tissues to the liver
Hepatic portal vein carries blood from GI tract from the spleen to the liver (could be deoxygenated or not. It’s going to liver to be detoxified).
Gall bladder is storage site for bile
> common bile duct carries bile from gall bladder to intestine (duodenum)
What is bile
High in bile salt which helps digest lipids
Filled with detergents and pigments
Can be secreted directly into duodenum to allow digest lipids
Where does Carbohydrate absorption happen? What does it require?
> in small intestine
> requires transporters to cross from lumen into blood
> All the sugars have their own transporters
What is GLUT2?
a hexose transporter
What is SGLT?
Sodium-glucose 2A symporter
transports 2 things
What is GLUT5?
Fructose transporter
How can protein digestion happen if it’s a full chain of amino acids?
endopeptidases
> Proteins are chains of amino acids
> Enzymes need to break the peptide bonds between the amino acids
> Does this by Endopeptidases (cleave the peptide bonds using pepsin, trypsin, chymotrypsin) and Exopeptidases (e.g aminopeptidase)
What is an endopeptidase?
Digests internal bonds between amino acids within long protein structure.
Breaking things into smaller peptides
What is an exopeptidase?
Cleaving things into much smaller structures (e.g cleaving into amino acids and small peptides).
Digest terminal peptide bonds to release amino acids
Protein absorption and how are they transported
Amino acids, Di/tripeptides, small peptides
Depending on the size, there’s different strategies for protein absorption
amino acids: reliant on co-transportation of sodium
Di/Tri-peptides: H+ cotransport
Small peptides: Endo/exocytosis transcytosis across cell membrane
Lipid digestion
Triglycerides are cleaves to mono/diacylglycerides and free fatty acids.
Cholesterol esters cleaves to AA’s and cholesterol
Phospholipids cleaved by phospholipase A to fatty acids
Why are bile salts needed for lipid emulsification?
To reduce surface tension of lipids and make them easier to digest
What do lacteals do
They are lymph vessels in the small intestine and absorb lipids
Iron absorption
Iron comes in the form of Fe2+
Requires specific transporters to get it into the extracellular fluid or blood.
Calcium absorption
Need ATP to move calcium through the channels. 3 sodium’s required to take a calcium out the channel
How does sodium enter the cell? How about water and potassium?
- through sodium channels
- cotransport with chloride
- through protein pumps
whereas water and potassium movement is paracellular (taken up and absorbed by the walls of the cells)
