Lecture 22 - The Small intestine

Question 1

Mucosae is modified in the stomach for secretion

Answer 1

Close to the surface and in the neck of these glands we find goblet cells that provide the nice alkaline mucus coat to protect the epithelium from acid and enzymes but deeper down in these glands we will find endocrine cells which secrete hormones into the blood vessels which are running in the lamina propria and also parietal cells which are secreting acid and intrinsic factor and we are going to find chief cells that secrete pepsinogen which is the inactive precursor of the enzyme pepsin which is going to digest proteins

Question 2

Zymogen

Answer 2

Another word for an enzyme

Question 3

Location of the small intestine

Answer 3

Three main regions
Duodenum approx 25 cm
Jejunum approx 2.5 m - majority of absorption of nutrients occurs at the jejunum
Ileum approx 3.6 m - this part empties into the first part of the large intestine

Don’t Jump In - pneumonic to remember it by

Total length is approximately 6m - overall very long and it is one of the factors that contribute to the massive surface area we have in the small intestine

Question 4

Diameters of the small and large intestine

Answer 4

Called the small intestine because it is small in diameter about 3-4 cm compared to the large intestine which has a diameter of about 7cm

Question 5

Mesentry and the small intestine

Answer 5

Initial segment of the duodenum is retroperitoneal - behind the peritoneal cavity because it does not need to sit inside the cavity in order to carry out its function, other parts of the small intestine are in the peritoneal cavity because they want to move around and this is known as intraperitoneal as movement needs to be allowed for peristalsis and segmentation to occur

Small intestine held in place by mesentery - helps to anchor and prevent them from getting tangled with each other

Allows movement, but prevents small intestine from getting tangled

Mesentry contains arteries, veins, nerves and lymphatics
Mesentry artery is important for supplying the small intestine with blood and the mesentery vein is faint to be draining deoxygenated blood from the small intestine

Visceral peritoneum comes together and forms a fold or a double layer and this will anchor the bit of the small intestine to the body wall and this is called a mesentery

Question 6

What is delivered to the small intestine?

Answer 6

Chyme which is acidic

Need to…
Protect epithelium - mucus secreting cells AND glands in submucosa (specifically in the duodenum)
Neutralise pH - Bicarbonate from pancreas (secreted by the duct cells in the pancreas)

Glands in the submucosa are specific to the duodenum and you don’t have them in the other sections because you do not need as much protection in other regions

Question 7

Modifications that allow the small intestine to do its function

Answer 7

Circular folds

Villi

Question 8

Pilcae Circulares (circular folds)

Answer 8

Permanent large folds of the small intestine

Core of submucosa with overlying mucosa

Unlike the rugae, these are permanent and do not flatten out because the connective tissue components between the two are different

Increases the surface area of the tube but they also are important in spiralling the food material as it goes through the lumen and slowing its passage down through the lumen to increase the amount of time for digestion and absorption to take place

Question 9

Villi

Answer 9

Numerous villi, found on the surface of the plicae circulares

Muscularis mucosal (allows villi to wiggle back and forth - so that they can be exposed to the lumenal contents)

Core of each villi is made up of FCT (lamina propria)

Contains vessels 
Lymph lacteal (carrys products of fat digestion) - without these the fat would clog the capillaries so use lymphatic lacteal system instead  
Capillary network (products of protein and carbohydrate digestion) 

Blood leaving the villus will be deoxygenated because the epithelial cells will be using oxygen but it will also be nutrient rich because it will contain products of protein and carbohydrate digestion

In the epithelial layer there will be lots of simple columnar epithelial cells carrying out the process of absorption and are sometimes referred to as enterocytes

Capillary network is surrounding a central lymphatic lacteal, makes sense that the vessels are extending up towards the surface because they are closer to the site of absorption

Question 10

Mesentric blood vessels and lymphatic drainage

Answer 10

Nutrient rich, deoxygenated blood absorbed in the small intestine enters into the mesenteric veins

These drain into the hepatic portal vein - GI tract to the liver and the liver can detoxify the blood before it goes to the rest of the body

Lymph lacteals ultimately drain into the cisterna chyli, thoracic duct and then the left subclavian vein

Question 11

Fat laden lymph pathway

Answer 11

Fat laden lymph fluid from the lacteals will eventually drain into the cistern chyli and from here this fluid will move into the thoracic duct which will move up and eventually drain into the left subclavian vein and this is where it will reenter the blood vascular system

Question 12

Blood flow pathway for the GI tract

Answer 12

Nutrient rich, deoxygenated blood comes from the intestines and it is going to travel through the mesenteric veins which are going to drain into the hepatic portal vein which is going to take the blood to the liver so it can be processed and detoxified before the blood makes its way back up to the heart

Question 13

Increasing the small intestine surface area

Answer 13

Long - 6m in length
Fold the submucosa - permanent folds = plicae circulares
Fold the mucosa (not the muscular mucosa) = villi
Microvili

Question 14

Epithelium of the small intestine

Answer 14

Epithelium of the small intestine is simple columnar
Microvilli on apical surface - increases surface area further
Microvilli is also known as a brush border

Question 15

Microvilli brush border

Answer 15

Glycocalyx
Glycoproteins - branched filaments
Tether enzymes and will be involved in contact digestion

Contact digestion - involves enzymes - attached to the brush border

Question 16

Junctions between epithelial cells

Answer 16

The plasma membrane is a barrier … it is selectively permeable, has proteins - channels, transporters
This is a phospholipid bilayer - things that are lipid soluble such as the products of fat digestion are going to be able to simply diffuse through the lipid bilayer but the products that are water soluble are not going to be able to simply diffuse and will need assistance from transmembrane proteins

Tight junctions
Very tight
Small molecules may diffuse through

Question 17

Enterocytes

Answer 17

Absorptive cells

Question 18

Goblet cells

Answer 18

Secrete mucus for protection

Question 19

Stem cells

Answer 19

Make all cell types

Question 20

Paneth cells

Answer 20

Granules, antibacterial enzymes

Immune role

Question 21

Endocrine cells

Answer 21

Secrete hormones

Question 22

Epithelium of the small intestine

Answer 22

Enterocytes, goblet cells, stem cells, panted cells, endocrine cells

Question 23

Permanent folds of the small intestine with a core of submucosa?

Answer 23

Pilcae Circulares