Small Intestine

Question 1

What are the sections of the small intestines and what are their functions?

Answer 1

• Duodenum
○ secretes HCO3- to neutralise acid from the stomach
○ Secretes CCK and secretin (and somatostatin) into blood (secretin is sensitive to acid so tell stomach to stop releasing acid)
- CCK is an acid inhibitor, stimulates release of pancreatic juices, goes to gall bladder to stimulate relace of bile and inhibits gastric emptying
- Secretin inhibits acid secretion goes to pancreas, simulates release of pancreatic juices and stimulates release of intestinal/pancreatic/biliary HCO3-
○ Has layer of enzymes called enterokinase which activates the enzyme trypsinogen to the active trypsin. Trypsin then will activate the other enzyme
- This stops self-digestion of the pancreas
-
• Jejunum
○ Secrets more digestive juices
○ Breaks down food (increase surface area for attachment o enzymes)
and mixes with bile and enzymes
• Ilium
○ Longest part of small intestines ( maximise absorption)
○ Adds water to food to liquefy it and ensure it can travel through small intestine easily
○ Has villi and microvilli to increace surface area for absorption
○ absorption of bile
• Mesentery
○ Lots of blood vessels for diffusion of monomers into blood and to maintain concentration gradient
○ Blood to and from small intestine

Question 2

Describe how small intestine increases amount of absorption and how absorption occurs.

Answer 2

• Enterocytes have a brush border villi on their surface
• These villi have micro villi to increase their surface area for diffusion
• villi have good blood supply and circulations to maintain concertation gradient
• Capillaries inside microvilli drain into bigger blood vessels and products of digestion are transported around the body
Lacteal/lymphatic capillary also inside villi for absorption of lipids. The lacteal drains into the lymphatic system which joins the circulatory system at the subclavian vein.

Question 3

Why isn’t the small intestine digested by enzymes?

Answer 3

• No real reason but theorised:
○ Mucus membrane protects the lining of the small intestine (and could provide antimicrobial properties)
○ Tight junctions of the enterocytes so enzymes can pass in between and digest lining
○ Compartmentalisation of digestive enzymes in lumen
• Ischaemic gut could lead to autodigestion
• If lots of blood lost blood will be directed away from intestine (non-vital) and towards the heart and brain (vital). This means no/little oxygen to intestine which means no mucus is produced and could lead to digestion of the small intestine lining. Enzymes could also get into blood and digest other parts of the body.

Question 4

What are the secretions of the small intestine?

Answer 4

• H2O is secreted from the small intestine to wet and liquify food and ensure easy travel of the food in intestine (almost all this is reabsorbed)
• Mucus o protect lining from enzymes and provide immune functions
• Digestive enzymes
• CCK, secretin and somatostatin at duodenum
• Alkaline solutions to neutalise acid and form a concertation gradient in the small intestine
○ HCO3-, Na+ and Cl- (to create NaCl)

Question 5

Describe Water addition and reabsorption in the small intestine.

Answer 5

1. Salts (Na+ and Cl- and HCO3- are secreted into the small intestine by crypts. Also large polymers are broken into small monomer (this increases concertation gradient inside small intestine)
   
   2. The water potential inside the small intestine is lower than the water potential of the cells and blood so water is moved into the small intestine by osmosis.
   3. As the products of digestion and salts are absorbed a the water potential of small intestine increases above that of the cells and blood so water moved down a water potential gradient into the cells/blood.