Anatomy of the Gastrointestinal system

What are the three accessory organs to the GI tract?

Liver

Gall bladder

Pancreas

What are the three main salivary glands?

• Parotid

• Submandibular

• Sublingual

What types of saliva do the Submandibular, Parotid and Sublingual salivary glands secrete?

Submandibular - Both serous and mucus saliva

Parotid - Serous saliva

Sublingual - Mucous saliva

Where in the saliva glands is saliva produced?

The acini

How is saliva initially produced in the saliva glands?

Via active filtration of ions from the blood.

Has a similar composition to extracellular fluid

Where is the composition of saliva modified?

In the ducts of the saliva glands.

Modification makes it appropriate for requirements.

How does parasympathetic stimulation of the saliva glands affect saliva composition/volume?

Parasympathetic simulation produces a large volume of watery saliva.

How does sympathetic stimulation of the saliva glands affect saliva composition/volume?

Sympathetic simulation produces a small volume of mucous saliva.

List some functions of saliva.

Lubrication (due to mucin content)

Digestion (due to presence of α-amylase)

Protection of oral mucosa

Antibacterial (due to antimicrobial thiocyanate)

Speech

Absorption in the mouth

Why is food broken down into smaller pieces by our teeth?

To increase surface area for action of digestive enzymes.

What are the muscles of the tongue and cheeks called?

Buccinator muscles.

What is the lower jaw bone called?

Mandible

What is the upper jaw bone called?

Maxilla

What is the joint which moves when mastication (chewing) occurs?

The Temporomandibular Joint (TMJ)

What are the four muscles of mastication called?

Temporalis

Masseter

Lateral pterygoid

Medial pterygoid

What muscle is this?

Temporalis muscle

What muscle is this?

Masseter muscle

What muscle is this?

Medial pterygoid

What muscle is this?

Lateral pterygoid

Which of the four muscles of mastication are the major effector?

Temporalis muscle

Which muscles of mastication are responsible for lateral/side-to-side movement?

Medial and lateral pterygoids

What two stages can swallowing be divided into?

Pharyngeal stage and Oesophageal stage.

How is the swallow reflex triggered?

Food is voluntarily swallowed and the tongue pushes the bolus (food) posteriorly

Food contacts the back of the pharynx and the swallow reflex is triggered.

How is food prevented from entering the trachea?

Following the swallow reflex, the larynx is elevated and moves the epiglottis over the trachea

Which muscles move the larynx in order to close off the trachea?

Suprahyoid muscles.

At the junction between the oesophagus and stomach, there is an anatomical sphincter to prevent reflux of stomach contents

True or False.

False.

There is no anatomical sphincter at the junction. Instead there are bands of muscle from the diaphragm (lower oesophageal sphincter) which increase tension produced by the oesophageal wall.

This is also referred to as a physiological/functional sphincter rather than an anatomical sphincter.

What type of epithelium lines the oesophagus?

Stratified squamous epithelium.

What is a hiatus hernia?

The upper part of the stomach squeezes through a gap between the oesophagus and diaphragm (hiatus).

This is usually asymptomatic and only presents when it causes reflux of stomach contents.

Where does the stomach lie?

Just below the diaphragm, to the left of the abdominal cavity.

What does the stomach open into?

Opens into the duodenum (proximal part of small intestine)

What is the name of the orifice where the stomach opens into the duodenum?

Pyloric orifice

What are the different structures of the stomach shown in the image?

1) Abdominal oesophagus

2) Cardial notch

3) Fundus

4) Cardia

5) Greater curvature

6) Body

7) Pyloric antrum

8.) Lesser curvature

9) Angular incisure

10) Pyloric canal

11) Pyloric sphincter

12) Pyloric constriction

13) Pyloric orifice

14) Duodenum

Which parts of the stomach secrete acid?

Fundus and body

What cell type secretes acid in the stomach?

Parietal cells.

What cell type secretes pepsin precursor, pepsinogen?

Peptic cells (Also called Chief cells)

Which part of the stomach secretes endocrine molecules which control gastric secretion and gastric motility?

The antrum

What three molecules are secreted by the antrum in order to control gastric secretions/motility?

Gastrin

Histamine

Somatostatin

What type of epithelium lines the stomach?

Columnar epithelia which contain tight junctions between cells to prevent tissue damage from acid secretions

In addition to circular and longitudinal layers of smooth muscle, the stomach also contains ______ layer of smooth muscle which allows _______ of the stomach.

Oblique (sloping) layer of smooth muscle which allows distension of the stomach.

What are the three parts of the small intestine?

Duodenum

Jejunum

Ileum

The duodenum can be further divided into four parts. What are these parts?

Superior duodenum

Descending duodenum

Horizontal duodenum

Ascending duodenum

Where does the superior duodenum travel and at what level?

Travels slightly superiorly and posteriorly at the level of L1

Where does the descending duodenum travel and at what level?

Travels inferiorly over part of the kidney at level of L3

Where does the horizontal duodenum travel and at what level?

Travels medially to the left and crosses the aorta at the level of L3

Where does the ascending duodenum travel and at what level?

Travels superiorly on the left of the aorta at the level of L2

It then becomes the jejunum.

Which part of the duodenum receives further digestive secretions?

The descending duodenum.

Receives from the liver/gallbladder and pancreas.

How does the descending duodenum receive digestive secretions from the liver/gallbladder and pancreas?

Liver/gallbladder - Via the common bile duct

Pancreas - Via the main pancreatic duct

What vertebral level does the jejunum begin?

L2 vertebrae

The jejunum becomes the ileum at an anatomically indistinct junction.

True or False?

True.

They ‘join’ at an anatomically, indistinct junction.

How are the jejunum and ileum anchored to the posterior body wall?

They are anchored by the mesentery.

What parts of the small intestine are the main sites of absorption?

Jejunum and Ileum

What is the major function of the large intestine?

Absorption of water and ions (some nutrients absorbed in the proximal colon)

What are the three bands of longitudinal smooth muscle in the large intestine known as?

Taeniae coli.

What parts can the large intestine be divided into?

Caecum

Colon

• Ascending colon

• Transverse colon

• Descending colon

• Sigmoid colon

Rectum

Anal canal

What is the caecum and where does it begin?

First section of the large intestine. Pouch of the large intestine and contains the vermiform appendix.

Begins at the ileocecal junction (connection between Ileum and Caecum)

Where does the ascending colon travel?

Superiorly from the right lower to right upper quadrant

Then forms a 90° bend to the left and into the transverse colon.

What is the 90° bend between the ascending and transverse colon called?

Right colic flexure.

Where does the transverse colon travel?

From right upper to left upper quadrant

Then forms a 90° bend down and into the descending colon.

What is the 90° bend between the transverse and descending colon called?

Left colic flexure

Where does the descending colon travel?

From the left upper to left lower quadrant.

Then becomes the sigmoid colon (S-shaped)

What does the sigmoid colon connect?

The descending colon with the rectum.

Where do the rectum and anal canal sit?

Within the pelvic cavity, posterior to the:

• Vagina in females

• Prostate in males.

What are some of the livers many functions?

• Glucose storage

• Protein, lipoprotein and cholesterol synthesis

• Digestion: Production of bile and bile salts

• Storage of fat soluble vitamins

• Toxin and drug metabolism/excretion

How many lobes does the liver have?

Four lobes

• Right

• Left

• Caudate

• Quadrate

What are the four lobes of the liver?

Right lobe

Left lobe

Caudate lobe

Quadrate lobe

What is the gallbladder?

Fluid filled sac which stores and concentrates bile.

What three parts make up the gallbladder?

Fundus

Body

Neck

Where does the neck of the gallbladder lead to?

Into the cystic duct which transports bile to and from the gallbladder.

What are some of the roles of bile?

• Produces an alkaline pH to decrease acidity of gastric contents released from the stomach

• Decreasing pH also facilitates emulsion of fats by bile salts

• Toxic waste products from the liver may also be excreted by the kidneys in the bile

What cell type produces bile?

Hepatocytes found in the liver.

What are sinusoids in the liver?

Structure which runs between layers of hepatocytes in allow close contact between hepatocytes and portal blood supply

How is bile collected from the hepatocytes?

Products of hepatocyte function are removed via small channels found between cells called canaliculi.

Canaliculi then drain into bile ducts.

Bile ducts → Right/Left Hepatic ducts → Common hepatic duct → Common bile duct

What does the unity of the common hepatic duct and the cystic duct form?

The unity forms the common bile duct.

Where does the main pancreatic duct join the common bile duct?

They join at the hepatopancreatic ampulla which opens into the duodenum.

Where does the pancreas sit?

Lies horizontally across the posterior abdominal wall.

Sits posterior to the stomach.

What are the five regions which make up the pancreas?

• Uncinate process

• Head

• Neck

• Body

• Tail

______ tissues of the pancreas release pancreatic juice.

Exocrine tissues.

What are the two components of pancreatic juice?

Alkaline secretion

• High bicarbonate and low enzyme content

• Helps neutralise acidic gastric contents

Enzyme rich secretion

• Contains major digestive enzymes

• Enzymes secreted as pre-enzymes which are activated in the gut

Why are digestive enzymes secreted by the pancreas released as pre-enzymes?

Pre-enzymes become activated in the gut.

This prevents the enzymes from digesting the pancreas.

Sympathetic stimulation of the pancreas causes ________

Decreased secretions

Parasympathetic stimulation of the pancreas causes ________

Increased secretions

How are exocrine glands similar to salivary glands?

Both contain acinus and ducts.

Pancreatic enzymes are secreted in the acinus and modified in the ducts

What controls entry of bile and pancreatic juice into the descending duodenum?

The Hepatopancreatic Sphincter (of Oddi)

What is the Peritoneum?

Layer of connective tissue which covers the walls and viscera of the abdomen.

What is the purpose of the peritoneum?

Acts to anchor floppy abdominal organs to the posterior body wall.

This prevents them from squashing each other or moving too much when we jump

Also carries blood supply to organs.

What is the greater omentum?

A major reflection or ‘flap’ of the peritoneum which covers the abdominal contents.

This is the first structure visible if you were to remove the anterior abdominal wall.

What is the mesentery?

A major reflection or ‘flap’ of the peritoneum which originates from the posterior abdominal wall.

It surrounds the majority of the small intestines.

What is the peritoneal cavity?

The continuous space around all the abdominal organs as a result of the peritoneum ‘vacuum packing’ the abdominal organs

What are the six classifications of epithelium found in the GI tract?

Squamous

• Simple Squamous

• Stratified Squamous

Cuboidal

• Simple Cuboidal

• Stratified Cuboidal

Columnar

• Simple Columnar

• Stratified Columnar

How thick is a simple epithelial layer?

1 cell thick

How thick is a stratified epithelial layer?

>1 cell thick

What is a pseudostratified epithelial layer?

A 1 layer thick epithelium which pretends to be 2 layers thick

What shape are squamous cells?

Flat cells.

What shape are columnar cells?

Column-like cells.

Long and thin

What shape are cuboidal cells?

Square shaped (cubes)

What four layers make up the wall of the digestive tract?

• Mucosa

• Submucosa

• Muscularis externa

• Serosa

The mucosa layer of the digestive tract can be further divided into three component layers. What are they?

Epithelium

• Thin layer of cells which line the lumen

Lamina propria

• Layer of loose connective tissue

Muscularis mucosa

• Thin layer of smooth muscle cells which cause localised contractions of the mucosa

What is the submucosa of the digestive tract wall?

Layer of dense connective tissue which contains the submucosal plexus which is a component of the enteric nervous system

What is the enteric nervous system?

Branch of the autonomic nervous system which is entirely separate from the sympathetic and parasympathetic systems.

Can operate independently of the CNS

What two major plexi make up the enteric nervous system?

Myenteric and submucosal plexi

• Old names: Meissner’s and Auerbach’s plexi