Lecture 21 - The Stomach and Pancreas Flashcards
The esophagus
Tube that connects the pharynx to the stomach
Posterior to the trachea
Muscosa - epithelium
Stratified squamous epithelium
Protection from abrasion
Muscularis - modified to propel food down to the stomach
Mucosa and submucosa are highly folded which gives the oesophagus the capacity to stretch/expand to accommodate the food bolus as it travels down to the stomach
Location of the stomach
J shaped organ
Located at the base of the oesophagus
Esophagus passes through the diaphragm (esophageal hiatus = hole/break in the diagphragm that allows the oesophagus to open up into the stomach)
Lower esophageal sphincter (LES) prevents reflex - to allow the bolus into the stomach the lower oesophageal sphincter has to relax but once the bolus has entered the stomach the LES can contract to prevent reflex of acidic material back up into the oesophagus - reflect of acidic material can cause damage to the epithelial lining of the oesophagus which can lead to the sensation of heart burn
Gross structure of the stomach
4 main parts = Cardia Fundus Body Pylorus (pyloric antrum)
Important sphincter in this region of the stomach is called the pyloric sphincter which regulates the passage of the lumenal contents from the stomach into the first part of the small intestine called the duodenum - regulating the process of gastric emptying
4 main parts of the stomach
4 main parts =
Cardia - The small region which the oesophagus opens up into
Fundus - Proximal region located up towards the diaphragm and it is an area where you find lots of gastric glands and gases produced as a byproduct of digestion will be able to rise up to here
Body - central portion of the stomach that makes up the bulk of it
Pylorus (pyloric antrum) - gatekeeper
Lesser curvature
Concave notch that is more medial in the stomach
Greater curvature
Convex area that is a little more lateral in the stomach
Omentum
Way to remember = O equals organ to organ
A double layer of peritoneum that connects one organ to another
Greater omentum
Stomach to transverse colon
Contains a significant cluster of immune cells
A few blood vessels running through here but also lots of adipose so the greater momentum is going to provide some cushioning anteriorly
Lesser omentum
Stomach to the liver
You find important verse structures like blood vessels that are faint to and from the liver
Modifications of the gut tube for the stomach
The 4 layers of the gut tube is modified to carry out specific functions
Muscularis = motility
Thicker in the distal region of the stomach than in the proximal region such as the fundas because we actually need a lot more contractility to happen distally to aid in mechanical and chemical digestion therefore thickest distally
3 layers
Oblique (inner)
Circular (middle)
Longitudinal (outer)
3 layers of the stomach
3 layers
Oblique (inner)
Circular (middle)
Longitudinal (outer)
The layers allows the stomach to contract and churn in many different directions
Internal surface of the stomach
Rugae are temporary folds that allow for the expansion of the stomach
These have an important storage function
Spincters are also important in the storage function - sometimes have to keep food material in the stomach for 2-3 hours to allow for mechanical and chemical digestion to take place and in order t=for this we need the pyloric sphincter and the lower oesophageal sphincter to be closed
Rugae
Rugae are temporary folds that allow for the expansion of the stomach
Rugae are important for the storage function of the stomach, because when the stomach is empty it can collapse and shrink to a small size and after a big meal the rug can flatten out in order to allow the stomach to expand and increase its volume to accommodate the arrival of the food material
Have a folded core of submucosa with overlying mucosa
Submucosa is the core of the rugae - it is because of the connective tissue elements of the submucosa that the rugae are able to flatten out and accomodate the food material
Mucosa is also folded into simple gastric glands but unlike the rugae, these glands of mucosa are permanent structures so even when the rugae are flattened out these glands remain in tact
Gastric glands - modification of the gut tube
Mucosa
Simple columnar epithelium
In-folding increases surface area for secretion (1.5L/day). When you infold into the lamina propriety underneath you form simple gastric glands.
Glands are permanent
Glands are not in the submucosa or muscular
We need
Acid and enzymes for digestion
Mucous for protection
Hormones for regulation
Cells of the gastric glands
Mucous epithelial cells
Parietal cells
G cells
Chief cells
Mucus epithelial cells
Secrete mucus for protection
These are close to the surface, in the neck of the gastric glands are the mucus secreting cells and they provide an alkaline mucus layer that protects the epithelial lining of the stomach from the acid and enzymes that will be secreted
Parietal cells
Secrete acid and intrinsic factor
Particularly hydrochloric acid and intrinsic factor which is important for vitamin B12 absorption
Parietal cells produce acid
Pump ions (H+) - to secrete HCl, we need to be able to pump ions against their concentration gradient and to pump ions against their concentration gradient we are going to need a lot of energy in the form of ATP so they have an abundance of mitochondria to provide this energy to power the proton pumps
Abundant mitochondria
Central nucleus
Folded structure to increase surface area - if we increase the membrane surface area we will be able to fit a lot more ion pumps in so that we can carry out a lot more acid secretion
G cells
Secrete hormones (gastrin) Secretes hormones into the blood and gastrin is important for regulating the motility and secretatory events in the stomach
Chief cells
Secrete pepsinogen (an inactive precursor of pepsin, pepsin is important for protein digestion)
Chief cells produce enzymes
They have an abundant rough endoplasmic reticulum because this is the protein producing organelle and this therefore makes enzymes
Apical zymogen granules (contains enzymes) - zymogen is another word for enzyme, and these little granules are important for transporting the enzymes towards the apical membrane where they can be exocytosed and secreted into the lumen
Basal nucleus - since apical aspect is so packed full of zymogen granules other organelles like the nucleus get forced to the basal aspect of the cell
Zymogen
Another word for an enzyme
Regulation of stomach function
Endocrine control
Endocrine cells in mucosa
Gastrin and Ghrelin secreted into the bloodstream (Ghrelin is sometimes referred to as the hunger hormone)
Hormones are secreted into the blood stream and there are blood vessels located in the lamina propria close by to the gastric glands
Neural control
Enteric nervous system (ENS) - local reflexes (primary control)
CNS modulates ENS function - long neural reflexes
What do we need to control the passage from the stomach to the small intestine?
Need controlled release of the digested material (chyme) into the small intestine therefore there is the pyloric sphincter
Big difference in diameter between the stomach and the first region of the small intestine, the duodenum, and this means that you cannot just unload all of the contents from the stomach into the duodenum at once and we actually need to control the rate of gastric emptying and release the material into the small intestine at a rate that it can handle which brings us back to the gatekeeper function of the pyloric sphincter
Chyme
Digested material that is released into the small intestine, it is quite acidic
Pyloric sphincter
Thickened region, contracts to prevent movement from the stomach to the duodenum and can relax to allow the passage of lumenal contents
Stomach to small intestine
Acidic chyme enters the small intestine
Requires: (by now all the mechanical digestion has probably been done so it is now just going to require some further chemical digestion using enzymes) Further digestion (enzymes) Protection from acidic chyme - mucous and a way to neutralise the acid (bicarbonate)
Mucus is provided by the glands in the submucosa of the duodenum. Enzymes and bicarbonate provided by the pancreas (the bicarbonate neutralises the acid)
Pancreas location
Head in C shaped duodenum (wider part of the pancrease)
Tail to spleen
Posterior to the stomach
Duct into duodenal lumen
Reteroperitoneal - sits behind the peritoneum i.e. peritoneum is on the anterior side
Has important endocrine secretions such as insulin and the exocrine secretions are carried through the duct into the lumen of the duodenum
Duodenum location
Most of the duodenum is actually reteroperitneal because it doesn’t need to carry out any movement to fulfil its function as all it needs to do is receive chyme from the stomach and to receive secretions coming from the pancreas (enzymes and the bicarbonate)
Pancreas
Bile duct meets the pancreatic duct at the entrance to the hepatopancreatic ampulla
Duodenal papilla - projects into the duodenal lumen
Release controlled by hepatopancreatic sphincter
Endocrine and exocrine functions
Endocrine
Pancreatic islet alpha cells secrete glucagon
Pancreatic islet beta cells secrete insulin
Exocrine
Acinar cells secrete digestive enzymes - enzymes get secreted into the ducts and travel along and the duct is also lined with cells which produce the bicarbonate which helps neutralise the acidity of the chyme coming into the small intestine
Ducts cells secrete bicarbonate
Secretions into the lumen in the GI tract are technically secretions to the outside of the body because the lumen is just an extension of the outside world hence it is exocrine
Pancreatic acinar cells
Structure
Apical zymogen granules
Basal nucleus
Abundant rER
Function
Secrete enzymes
Similar to the chief cells of the stomach and salivary acinar cells (same function as trying to produce enzymes)
What structures allow storage in the stomach?
Sphincters
Rugae
What do you need for mechanical digestion in the stomach?
Smooth muscle - additional oblique layer in the stomach
Spincter (material being pushed up against a closed sphincter helps to churn it)
What is needed for chemical digestion in the stomach?
Secretion of strong chemicals (acids, enzymes)
Protective epithelium
