Download Digestive System Notes Mouth / Oral Cavity: Pharynx: Epiglottis

Digestive System Notes
Anatomy is the study of structures. Physiology is the study of functions.
Functions of the Digestive System:
• Ingest food
• Secretes (enzymes, bile, HCl) to assist in digestion
• Digest food
• Absorbs food to make energy and to help us grow and repair ourselves.
• Eliminates indigestible waste
Four Steps of Digestion
1. Ingestion of food in mouth
2. Mechanical & Chemical digestion
3. Absorption of molecules
4. Elimination of indigestible substances
Mechanical digestion
– Large pieces of food become smaller ones without creating a new product
– Examples: Chewing of food in the mouth & Churning and mixing of food in the stomach
Chemical digestion
– Enzymes chemically break down macromolecules into new smaller products that can be
absorbed. Example: polymer → monomer
Mouth / Oral Cavity:
Functions
1. Ingestion
2. To begin digestion: Mechanical & Chemical
Structures – teeth, salivary glands and tongue
1. Teeth - Begins mechanical digestion to increase surface area
(SA) of food for enzymes to act upon.
2. Salivary Glands: ducted glands that produce saliva, which:
1) Liquefies food
2) Contains salivary amylase and begins chemical
digestion (optimal @ pH 7)
3) Lubricates and softens the BOLUS of food.
4) Enzymes in saliva kill bacteria
SALIVARY AMYLASE
Starch ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ maltose
3. Tongue: 3 functions
• Contains taste buds
• Moves the food around in the mouth
• Pushes the BOLUS of the food to the back of the throat to the ‘swallow reflex centre’.
Pharynx:
Structure: The back of the throat. Opens to the respiratory and digestive systems
Function: Swallowing: when food is placed on the “reflex centre”, the following things happen:
• the soft palate covers nasopharynx
• the epiglottis covers the trachea
• peristalsis of the esophagus begins
Epiglottis:
•
A flap of tissue that covers the trachea (lungs) when we swallow food
Esophagus:
Structure: A muscular tube that connects the pharynx to the stomach. No
digestion occurs here.
• At the beginning of stomach, there is a ring of muscle tissue called the cardiac sphincter that stops
food from re-entering the esophagus.
Function
• Food moves through the esophagus by PERISTALSIS which is a slow, rhythmic contraction that
pushes the BOLUS along. Peristalsis continues down the length of the entire digestive tract.
Stomach
Structure: a “J” shaped organ with 3 muscle layers and it begins and ends with sphincters called cardiac
sphincter and pyloric sphincter.
Functions:
• Churns food and liquefies it (mechanical digestion).
• This process is aided by the ridges or rugae in the stomach layer
• Begins the chemical digestion of proteins with gastric glands secretions.
Gastric Juices: Contain: pepsinogen, 3M hydrochloric acid and mucous.
Function:
• Hydrochloric acid (HCl) released when proteins enters stomach.
• creates a pH of 2.5 which kills bacteria
• transforms pepsinogen into an active hydrolytic enzyme PEPSIN, which begins protein digestion
Hydrochloric Acid
Pepsinogen ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Pepsin
Protein
⎯⎯⎯⎯⎯⎯⎯⎯⎯→ smaller polypeptides
• Mucous secretions help protect stomach wall from HCl
• An ULCER can result if acid penetrates the mucous layer and pepsin begins to digest stomach cells.
• Bolus leaves the stomach is an acidic liquid called CHYME.
• The pyloric sphincter at the base of the stomach will meter out the chyme into the duodenum at a
slow, controlled rate.
Accessory Organs: There are 3 that we look at: Pancreas, Liver and Gall Bladder.
The pancreas is a dual organ.
1. ENDOCRINE GLAND which makes hormones insulin and glucagon.
2. EXOCRINE GLAND which make the enzymes to digest carbs, fats, proteins and nucleic acids.
Pancreas: endocrine gland
Insulin and Glucagon are made by specialized cells
• high blood sugar → insulin secreted
• insulin removes glucose from the blood by:
1. causes the liver to store it as glycogen
2. promotes formation of fats
3. causes cells to absorb glucose
• low blood sugar → glucagon secreted
• glucagon adds glucose back into the blood by causing the liver to break down glycogen &
release glucose
Pancreas: exocrine gland:
The pancreatic juices include: (memory trick: SALT + N):
1. Sodium Bicarbonate (NaHCO3) - a base that is released to neutralize the stomach acid
Sodium bicarbonate
Chyme pH 2.5 ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ pH 8.5
2. Pancreatic Amylase - an enzyme that converts starch to maltose.
Pancreatic Amylase
Starch
⎯⎯⎯⎯⎯⎯⎯⎯⎯→
Maltose
3. Lipase - an enzyme that converts lipids into fatty acids and glycerol
Lipase
Lipids ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Fatty Acids & Glycerol
4. Trypsin - an enzyme that converts small protein chains into dipeptides and tripeptides.
Trypsin
Small polypeptides ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Di, Tri peptides
5. Nucleases - enzymes that convert nucleic acids (DNA and RNA) into nucleotides.
Nucleases
Nucleic Acids ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Nucleotides
Small Intestine: Structures:
There are 3 regions:
1. Duodenum: chemical digestion from pancreatic juices
2. Jejenum: digestion and absorption
3. Ileum: absorption of the nutrients
•
•
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increased rate of absorption due to its highly convoluted walls with a very large
SA.
folds in layer of small intestine called VILLI. Villi contains:
•
Blood capillaries for nutrient absorption (sugars, nucleotides & aa’s)
•
Lacteals (lymph capillaries) for glycerol & fatty acids
villi also have smaller folds called MICROVILLI.
• The absorption through microvilli involves active transport and requires much energy (ATP).
Function:
• To complete the digestion of all of the nutrient types (organic molecules - polymers):
a) Proteins
b) Carbohydrates
c) Nucleic Acids
d) Lipids
• To begin the absorption of nutrients (monomers):
a) Amino acids (into the blood stream)
b) Glucose and other monomers of carbs (into the blood stream)
c) Nucleotides (into the blood stream)
d) Fatty acids and Glycerol (into the lacteal = lymphatic system)
The Walls of the Duodenum contain glands that produce and release
intestinal juices
Intestinal Juices contain: Peptidases, Nucleosidases and Maltase
1. Peptidases digest the tri and di-peptides into amino acids.
Peptidases
Di, Tri peptides ⎯⎯⎯⎯⎯⎯⎯⎯→ Amino Acids
2. Nucleosidases digest nucleotides into sugar, phosphate & base
Nucleosidases
Nucleotides ⎯⎯⎯⎯⎯⎯⎯⎯→ sugar, P & N-base
3. Maltase digests the maltose into 2 glucose molecules
Maltase
Maltose ⎯⎯⎯⎯⎯⎯⎯⎯→ Glucose + Glucose
Other disaccharides? Sucrase digests sucrose into its monomers
Sucrase
Sucrose ⎯⎯⎯⎯⎯⎯⎯⎯→Glucose + Fructose
AND Lactase digests lactose into its monomers.
Lactase
Lactose ⎯⎯⎯⎯⎯⎯⎯⎯→ Glucose + Galactose
The Other 2 Accessory Organs: Liver and Gall Bladder
(Pancreas already discussed)
Liver:
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•
•
largest internal organ.
blood from the villi travels via the hepatic portal vein to the liver.
liver acts as a ‘gatekeeper’ to the blood by keeping levels of various nutrients in the blood constant.
Hepatic Portal System:
• Absorbed nutrients from the small intestine are taken to the liver by the hepatic portal vein.
• Liver stores excess glucose and glycogen, releasing it as needed.
What happens to the blood in the liver?
1. Detoxifies harmful substances, e.g. turns alcohol into fatty acids. Over time this can cause scarring of
the liver tissue which gives rise to cirrhosis
2. Regulates the blood glucose level by signalling pancreas to release the required hormone.
If blood sugar is high:
Insulin
Glucose ⎯⎯⎯⎯⎯⎯⎯⎯→ Glycogen
If blood sugar is low:
Glucagon
Glycogen ⎯⎯⎯⎯⎯⎯⎯⎯→Glucose
3. Deamination of amino acids. If necessary the liver can convert amino acids into glucose to maintain
glucose concentration of the blood plasma. This process produces urea that is removed by the
kidneys in the production of urine.
4. Destroys old red blood cells and recycles Hemoglobin.
5. Produces bile (stored in gall bladder) that emulsifies fats by breaking fats into smaller pieces which
increases the surface area of fats for digestion by lipase.
6. Produces blood plasma proteins. For example, blood clotting proteins (fibrinogen and prothrombin)
and another protein called albumin which helps to maintain the osmotic pressure of the blood
Gall Bladder: attached to the liver
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Sac shaped structure that stores excess Bile which breaks fat blobs into smaller pieces so they are
easier to digest
Appendix:
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•
Has lymph tissue to help fight infection
Subject to inflammation = appendicitis which means it may need to be removed
Large Intestine:
Structures:
The large intestine is large in diameter, but is shorter than the small intestine.
It consists of:
1. colon
2. rectum
3. anus
Functions:
• Main Job: Absorption of the water and salts And stores indigestible material
until it can be eliminated
• It also has anaerobic bacteria, E.Coli, that:
1. eat the wastes and produce useful things that we need to survive (ie: vitamin K and amino
acids)
2. produce growth factors (proteins that stimulate cell growth)
3. these bacteria produce waste of their own (methane gas) Phew!
Rectum:
By the end of the large intestine wastes are transformed into pasty ‘feces’ which build up in rectum until
message is sent to central nervous system to stimulate bowel movement.
Anus:
Sphincter muscle through with feces exits the body.
Rectum & Anus:
If wastes moves through the large intestine too quickly:
• the intestines don’t have time to absorb enough water, the feces are liquefied, and you have diarrhea.
If feces moves through too slowly:
•
then too much water is absorbed and the feces become hard and you are constipated.