Digestive system part two Flashcards
The Small & Large Intestine
Some foods travel quickly from the stomach to the small intestine leaving you hungry. Others move slowly keeping you full.
Carbohydrates, proteins and lipids are digested in the small intestine with the help of hormones and enzymes.
3-5 hours are required to process the contents of a meal in the small intestine.
In the small intestine, nutrients from digested food is absorbed.
Small Intestine
Most digestion and absorption occurs here.
A long tube about 2.5cm in diameter.
Can be up to 7m in length.
Lipids and carbohydrates, as well as any remaining proteins are digested here.
Made up of 3 sections: Duodenum, Jejunum and Ileum.
3 Sections of the small intestine
Duodenum- the first 25-30 cm and where most enzymes are added and digestion in the small intestine begins.
Jejunum- digestion continues and some nutrients are absorbed.
Ileum- the last section, where the majority of the nutrients are absorbed
Inside the Small Intestine
The inner surface provides max surface area for efficient nutrient absorption.
The inner layer is folded into ridges and has many small fingerlike projections called villi.
To increase the surface area even more, the epithelial cells of the villi is covered in microvilli.
Together, villi and microvilli increases the surface area of the small intestine by 500 times.
Each villi is filled with a network of tiny blood vessels called capillaries. Digested nutrients enter the bloodstream through capillaries.
Digested fats are transported through small vessels called lacteals which are taken to the lymphatic system then the bloodstream.
Chemical Digestion in the Small Intestine
The pyloric sphincter controls the passage from the stomach to the small intestine.
When the food in the stomach has been mixed with gastric juices and the proteins are partially digested, the pyloric sphincter periodically relaxes and releases small portions of chyme into the duodenum.
This slow and steady release prevents overloading and allows time for thorough digestion.
Most of the enzymes required for digestion are added in the duodenum.
This requires help from the pancreas, liver and gallbladder.
Pancreas
A long, flat gland located between the stomach and the duodenum.
Roles: secrete enzymes and secrete hormones that regulate absorption and storage of glucose.
The pancreas secretes amylase which continues the digestion of starch that was started in the saliva.
When fat-rich chyme enters the duodenum a hormone called cholecytokinin (CCK) is secreted by the duodenum and released in the bloodstream. This signals the pancreas to secrete many substances, including ones that control the pH of the intestine and enzymes needed for lipid, carb and protein digestion.
The secretions enter the duodenum via the pancreatic duct.
Pancreas 2
When chyme enters the small intestine it is very acidic and the hormone prosecretin is present in the small intestine is converted into its active form secretin which causes the liver to make more bile and the pancreas to secrete lipid and protein enzymes.
The pancreas also secretes bicarbonate ions (HCO3-) to neutralize the acidic chyme.
Secretin protects the small intestine from stomach acid and acts as a digestive rate regulator.
Pepsin is only activated in acidic conditions so it is not active in the small intestine.
Pancreas releases trypsinogen which is an inactive form of the protein digesting enzyme trypsin, Once in the duodenum the enzyme enterokinase converts it into active trypsin.
Trypsin continues the work of pepsin in the stomach breaking down proteins into amino acids.
pancreas 3
Fats that enter the duodenum are acted on by lipases- enzymes secreted by the pancreas.
Lipases break down lipid chains into shorter chains and individual fatty acid molecules.
Fats in chyme are present in larger globules and lipases cannot penetrate these so the liver must become involved.
Liver & Gall Bladder
Largest internal organ of the body, located underneath the diaphragm.
For digestion, it is considered a gland because it produces and secretes bile- which emulsifies fat and breaks it into tiny droplets called micelles.
Fat globules can be broken down into micelles so that lipases can act on them.
Bile is produced in the liver but stored in the gall bladder until food enter the duodenum. When fat enters the duodenum the gallbladder contracts and squeezes out the bile.
Blood from the capillaries in the intestines go to the liver before going back to the heart. The liver removes and breaks down toxins like alcohol.
The liver is also involved in producing and storing various nutrients like glycogen and fat soluble vitamins.
Small Intestine Absorption
Proteins, carbs, and lipids are absorbed in the jejunum and ileum once broken down. Vitamins, minerals and water are also absorbed here.
Nutrients pass through the villi/microvilli and then enter the bloodstream where they are transported by capillaries to the tissues of the body.
The mechanisms for transport can be classified as passive or active transport.
Passive Transport
the movement of materials across a cell membrane without the use of energy from the cell. Includes: diffusion, osmosis and facilitated diffusion.
Diffusion- particles will move from an area of higher concentration to an area of lower concentration until they are evenly distributed. Known as moving down the concentration gradient. Ex) amino acids into the bloodstream
Osmosis- the diffusion if water molecules across a selectively permeable membrane. Ex) water from food
Facilitated Diffusion- the diffusion of molecules across a membrane using transport proteins. These proteins are in the cell membrane and attach to molecules on one side and release them on the other. Each protein is shaped for specific substances. Ex) Carbohydrates
Active Transport
materials are moved across a cell membrane from an area of lower concentration to an area of higher concentration using energy provided by the cell.
Special transport proteins move the materials across the membrane.
Used to transport molecules that are too large to pass through the membrane or who have a strong or uneven electrical charge.
Large Intestine
The small intestine joins the large intestine a few cm from the end. The short upper end of the large intestine creates a pouch called the cecum- a dead end that receives the processed material from the small intestine.
The appendix is a finger-like projection from the cecum.
Colon- the longest part of the large intestine and has 4 segments: ascending colon, transverse colon, descending colon and the sigmoid colon.
The Rectum is the last 20cm of the large intestine which holds the waste products of digestion until they are eliminated through the anus.
Large Intestine 2
Digestion is complete and most of the nutrients have been absorbed by the time the digested material reaches the large intestine.
Most of the water is absorbed here. The fluid comes from water in the diet, saliva, and gastric juices.
Vitamins B & K, sodium and chloride ions are also absorbed.
It may take 4-72 hr for undigested material to pass through the large intestine.
More than 500 species of bacteria normally inhabit the large intestines. Some are important partners and produce vitamins in exchange for food.
The action of bacteria can produce gas (carbon dioxide, methane and hydrogen sulfide). The amount of gas is affected by the type of food.
Large Intestine cont’d
Digestion is complete and most of the nutrients have been absorbed by the time the digested material reaches the large intestine.
Most of the water is absorbed here. The fluid comes from water in the diet, saliva, and gastric juices.
Vitamins B & K, sodium and chloride ions are also absorbed.
It may take 4-72 hr for undigested material to pass through the large intestine.
More than 500 species of bacteria normally inhabit the large intestines. Some are important partners and produce vitamins in exchange for food.
The action of bacteria can produce gas (carbon dioxide, methane and hydrogen sulfide). The amount of gas is affected by the type of food.
