GI Physiology- Overview of GI tract Flashcards
Describe the structure of the GI tract and its associated organs
About 5.5 m long.
Exocrine glands: secretion (e.g. salivary glands, liver, pancreas).
o There are three types of salivary glands: sublingual, parotid
and submandibular.
Stomach: storage and breakdown of food.
Small intestine: digestion and absorption.
Large intestine: storage (of faeces for 12-24 hours) and absorption.
About 1.3 m in length, 15 cm in diameter.
Accessory organs:
o Mouth: tongue and teeth aid maceration of food.
o Liver and pancreas aid digestion.
What is the general structure of the digestive canal
Mucosa: shows particular region-specific specialisation (e.g. mucus and secretory glands in the stomach, villi in the small
intestine and crypts in the large intestine). The mucosa is made up of 3 layers: epithelium, lamina propria and muscularis
mucosa. The epithelium forms the interface between the luminal contents and the gut wall, and has both digestive and
absorptive functions and also secretes locally acting and circulatory hormones.
Submucosa: connective tissue layer which contains blood and lymph vessels, nerve fibres and ganglia.
o Submucosal plexus is embedded in the submucosa.
Muscularis externa: circular and longitudinal smooth muscle that is responsible for propulsion of contents along the digestive tract (peristalsis) as well as mixing and storage functions.
o Myenteric plexus is located between the muscularis layers (circular and longitudinal).
Serosa: single layer of cells that separates the GI tract from the peritoneum.
What are the mucosal differences along the GI tract
Oesophagus: stratified squamous epithelium which acts as a
protective endothelium against abrasion.
Stomach: simple columnar epithelium, dotted with gastric pits
where HCl is secreted. 50% of the surface area of the stomach
is made up of gastric pits. Tubular glands can be simply or
branched depending on the region.
Small intestine: finger-like projections of villi with absorptive
cells called enterocytes increases the surface area for
absorption of nutrients. Simply columnar epithelium is designed for absorption.
Large intestine: simple columnar epithelium (for absorption) which is interspersed with mucous-producing goblet cells
which lubricates faeces.
What are the cell types in the intestinal epithelium
Villi are responsible for absorbing nutrients and electrolytes in the small intestine. They are sensitive to hypoxia and
irritancy so are replaced every 3-6 days.
Microvilli on the brush border of the villi further increase surface area.
Intestinal stem cells in the crypt of the villi differentiate into transit amplifying cells, and further differentiate whilst moving
up the villus to mature into one of four cells:
o Absorptive cell: enterocytes have microvilli on the apical surface which increases surface area for absorption.
o Goblet cell: protects the epithelium by releasing mucous when stimulated.
o Enteroendocrine cell: specific functions in each area of the GI tract.
Endocrine cell: sensory organ at the apical pole senses nutrients within the GI tact, causing the release of
hormones at the basal pole of the cell, where there are hormone-containing granules.
o Paneth cell: packed with granules containing antimicrobial agents.
What are the functions of the GI tract
Digestion and absorption of nutrients.
Absorption and retention of water and electrolytes.
Elimination of toxins
Maintenance of barrier function
Immunological barrier:
o Saliva contains lysozyme which breaks down bacterial cell
walls.
o Low pH in the stomach by the secretion of HCl kills the majority
of bacteria.
o Peyer’s patches (distal part of small intestine) are
immunological cells which can initiate an immunological
response in the mucosa.
Give an overview of GI function
Oral cavity and salivary glands: secrete amylase which initiates carbohydrate and starch digestion. Mucous is produced by
particular salivary glands which lubricates and protects the epithelium of the oesophagus.
Stomach: stores food for 2-4 hours and breaks down the food. Churning of the stomach mechanically breaks the food and
mixes it with digestive enzymes and HCl.
o 𝐇𝐂𝐥 secretion: breaks down connective tissue and muscle fibres.
o Pepsinogen: a prohormone which converts to pepsin which initiates protein digestion.
o Intrinsic factor: important for absorption of vitamin B12 which occurs in the ileum.
o Alkaline mucous secretion: protects the stomach
epithelium from the harsh acidic environment.
Small intestine: nutrient digestion and absorption, vitamin
absorption, ion and water absorption occurs.
o Calcium and iron ions: absorbed in the duodenum.
o Phosphate ions: absorbed in the jejunum.
Ileum of the small intestine: vitamin B12 absorption, bile salt
reabsorption (reprocessing of the bile salts back into the liver
for fat digestion and absorption), NaCl and water absorption.
Colon: electrolyte and water reabsorption. Rate of water
reabsorption determines how solid the faeces are. Potassium
and bicarbonate secretion. HCO3
− neutralises the acid
produced by the bacteria in the colon. The colon stores the
food contents for 12-24 hours.
Describe intestinal electrolyte handling
Most Na+ and Cl− ions entering the GI tract are absorbed by
the small intestine.
Exocrine secretion comes from the extracellular fluid. K+ mostly comes from the diet and low exocrine secretions are due to lower extracellular content of K+. The majority is absorbed in the small intestine, and the body has the capacity to secrete K + in the colon. The colon is a way in which the body can maintain K+ homeostasis by increasing its secretion rate when the kidneys are failing.
Small amounts of electrolytes are lost in faeces.
Explain the splanchnic circulation
Blood supply to the GI tract is via the splanchnic circulation.
Represents 20-25% of cardiac output. This is 1.5 ml of blood
per minute (fasting) and can increase to 3 ml of blood per min
during digestion. It is a large reservoir as it holds 30% o blood
volume under normal conditions.
From the aorta comes the celiac artery which supplies the
stomach, spleen, pancreas and small intestine.
The superior mesenteric artery and inferior mesenteric artery
supply the small and large intestine.
Arteries become smaller and smaller so they can penetrate
through the villi and air recovery of nutrients.
70% of blood entering the liver is generated via movement of
blood through these organs and are transported by the portal
vein through the liver for further processing.
30% of blood form the hepatic artery supplies oxygen to the liver.
How do endocrine, neurocrine and pracrine secretions impact GI motility
Activation of enteric neurons release acetylcholine which stimulates the parietal cell to secrete of HCl. Gastrin released from G cells in the stomach:
o Endocrine effect: directly
increase HCl secretion form
parietal cells.
o Paracrine effect: causes the release of histamine from enterochromaffin like
cells (ECL). Histamine has a paracrine effect which increases parietal cell HCl secretion.
When HCl is too high, signals stimulate somatostatin-releasing cells to release somatostatin to reduce HCl secretion.
What are the endocrine hormones secreted by the GI tract and how do they work
Gastrin: regulates acid secretion in the stomach and induces endocrine
and paracrine effects.
Cholecystokinin (CCK): released from the duodenum in response to fat
digestion products. In response to fatty acids:
o Increase gall bladder contraction to increase the delivery of bile
salts to the duodenum which allows processing of fats.
o Decrease GI motility to slow down the digestive process to
optimise fat absorption.
o Increase pancreatic exocrine secretions to aid digestion.
Secretin: released in response to low pH in the duodenum and stimulates
pancreatic exocrine secretion (bicarbonate in response to low pH in the
duodenum) to protect the duodenum.
GLP-1: role in glucose homeostasis, with drugs being developed to treat
diabetes.
Describe the nervous control in the GI tract
There are 3 types of nervous control in the GI tract
Central nervous system
Autonomic nervous system
Local nerve reflexes: enteric nervous system
- Central nervous system
1.1. Conditioned (acquired) Pavlovian reflexes
Association of a neutral stimulus with presentation of food.
Cerebral cortex involved.
1.2. Unconditioned reflexes GI region Cell type Hormone Stomach G cells Gastrin Pancreas β-cells α-cells Insulin Glucagon Duodenum S cells K cells Secretin GIP Ileum and colon L cells GLP-1 and 2 PYY
- Autonomic nervous system
2.1. Parasympathetic
Increases exocrine secretion
Increases motility
Increases GI blood flow
Vagus nerve innervates the oesophagus, stomach and small intestine.
Sacral parasympathetic nerve originating from the spinal cord
innervates the large intestine, rectum and anus.
2.2. Sympathetic
Decreases motility
Decreases GI blood flow
No major control of exocrine secretion
Enteric nervous system:
Myenteric plexus: motor functions and is found between different types of smooth muscle so plays a major part in motor function.
Submucosal plexus: sensory function and is located within
the mucosa so has a greater sensory function.
ENS has been called the gut brain as it contains more
neurones than the spinal cord.
If the nerves from the ANS are severed, the ENS can
function independently.
Describe the function of the enteric nervous system including the short and long reflex
Control of movements of the GI tract.
Control of fluid and electrolyte transport.
Mucosal protection (e.g. mucus secretion).
Short and long reflexes innervate the ENS. Receptors within the wall of the GI tract: osmoreceptors or mechanoreceptors are activated and
activate the:
o Long reflex: activation of receptors initiate long reflex
pathways via vagal nerves to activate the CNS. They can feedback and activate enteric neurons within the GI tract via the
ANS.
o Short reflex: activation of receptors directly stimulate the ENS
and these can cause local, or short reflexes activating their target cells (smooth muscles or secretory cells).
Describe where exocrine secretions occurs from in the GI tract
Exocrine secretions originate form glands that have ducts.
Exocrine glands secrete digestive juices accounting for 3.5 L of fluid entering the GI tract:
o Salivary glands
o Liver
o Pancreas
The stomach contains gastric glands that secrete HCl, pepsinogen and mucous.
Duodenum contains Brunner’s glands that produce mucus rich alkaline secretion.
What are the principles of exocrine secretion and control (with reference to both acinar and duct cells in a gland)
A gland consists of:
o Acinar cell:
Primary secretion caused by active secretion of electrolytes into the lumen of the acinus.
Following active secretion of electrolytes, there is passive water movement forming a primary secretion which is
isotonic in nature.
o Duct cell:
The secretion is modified by ion exchange processes as it
moves along the duct which produces a final secretion which is hypotonic.
What is in salivary secretions
1-1.5 L of salivary is secreted per day.
Parotid gland: serous (water secretion).
Other salivary glands: mucous secretion.
Secretion is always hypotonic with a pH of 6.5-7.5.
Functions, facilitates:
o Speaking
o Chewing
o Swallowing
o Tasting
Contains α-amylase: initiates carbohydrate digestion.
Anti-bacterial properties
