Anatomy of the gastrointestinal (GI) tract
All diseases begin in the gut - Hippocraes (460 - 370 BC)
Components of the digestive system- from mouth to anus
GI tract does propulsion, digestion and absorption
Propulsion: moving food through the GI tract.
• It starts in the mouth with the mastication and the
tongue pushing food backwards (which initiates a
reflex that makes the phyringhial muscles contract).
• The soft palet lifts upwards, with the uvula (ugula);
the lower pharinx opens so that food can go down;
the larix lifts up (addam's apple in men), which closes the
epiglottis so that food doesn't go up in the nose or in the air ways.
• There is also peristalsis in the esophagus, in the stomach and in
the intestines
• Defecation process in the rectum
Mechanical digestion:
• chewing (mouth),
• churning (like what happens for butter) (stomach),
• segmentation (small intestine).
• The small intestine has the biggest amount of work to do, since there is all the
reabsorption
Absorption:
• Nutrients and water to blood vessels and lymph vessels (small intestine)
• Water to blood vessels (large intestine)
Water balance: volumes handled by the GI-tract
• The liver is so high bc there is a high bile production in it
• We have 9 L in the small intestine that are secreted, but
around 6,5-7,8 are reabsorbed. While it reabsorbs
water, it takes up also Na+, K+, and Cl-.
• Most goblet cells are in the colon (it's the one that
produces the most amount of mucus).
• It secretes HCO3- and reabsorbs water, which is
important bc even tho its work is small, if it doesn't do
it's work properly, that's how you end up with diarrhea
(that's why we associate it with water absorption
even tho it does the smallest amount of it).
• Water passes in between cells while ions go through
them.
,The gastrointestinal tract also has a large role in our body’s water balance
• Water is often absorbed via the paracellular route, while ions follow the
transcellular route, usually through channels or transporters
• Sodium enters the cell from the apical side often through a co-transporter
(eg.
Glucose-Na cotransport) and leaves the cell by active
transport at the basal side (Na,K-ATPase).
• Within the cell, sodium concentration is kept low,
facilitating transport via sodium co-transporters.
GI-tract: pH and transit time
• The transit times are very hard to determine, they vary per type of food and
person. If water passes through faster than food or not, it depends on the food
you eat, the sleep times you have, how active you are, and just on you.
• Fletcherism : it's a guy in 2d ww that thought that if
you chew your food from 30-40 times
you'd extract more from your food and
so you'd have to eat less food to get the
same amount of nutrients
• Esophagus is 25 cm long.
• Fondus is where the food is stored in the begining.
Food stays in the stomach until the pH reaches the
lowest point
• Whether it stays in the large intestine for longer or
not it depends on the person.
• pH-range is also variable, depending on food intake, type of food, digestive
period, etc.
Salivary glands
• Under the tongue there is the frenulum: a thin ligament that attaches the tongue
to the floor of the mouth.
• Left and right to it there are the openings to the sumandibular ducts and glands
(you can see where the saliva flows through the mouth)
What happens to the salivary glands, when you see food
• Submandibular gland: ducts end in caruncles left and right of the frenulum
under the tongue. Produces 60 to 70% of saliva in resting
state.
• Parotid gland: - best-known disease is mumps (orecchioni) –which leads to a
painful swelling of the parotid gland after an infection with the
paramyxo virus. Duct enters the oral
vestibule between cheek and gums at the
level of the second upper molar
(maxillary)
• Sublingual gland: 3-5% of salivary secretions, duct also
ends in sublingual caruncle (like
submandibular).
,Secretions of salivary glands
Composition of Saliva
Ions
• Na⁺, K⁺, Ca²⁺, Mg²⁺, PO₄³⁻, HCO₃⁻
• Thiocyanate (SCN⁻): antibacterial; converted to hypothiocyanite (OSCN⁻) by
sialoperoxidase using bacterial peroxide.
Water
• Major component; amount depends on autonomic activity
• Parasympathetic activation → watery saliva
• Sympathetic activation → thick, mucus-rich saliva
Proteins
• ≈50 different proteins present.
• α-Amylase (ptyalin): most abundant; digests starch → maltose, maltotriose,
dextrins.
• Mucins: lubrication; give saliva its viscosity.
• Lysozyme: antibacterial against gram-positive bacteria.
• Sialoperoxidase: produces bactericidal hypothiocyanite.
• Proline-rich proteins & statherins: keep calcium phosphate dissolved; prevent
precipitation.
pH
• Slightly alkaline; varies with flow rate.
• α-Amylase works optimally at slightly alkaline pH.
Supersaturation and Tooth Protection
• Saliva is supersaturated with calcium phosphate.
• Supersaturation = containing more dissolved material than normally possible at
equilibrium.
• Prevents demineralization of teeth (dentin = calcium apatite).
• Special proteins (proline-rich proteins, statherins) prevent crystals from
forming and depositing on teeth.
• This protects teeth from acidic foods/drinks and supports remineralization.
• Chewing gum stimulates saliva flow → helps clean teeth and maintain mineral
balance.
Antibacterial Defense
Lysozyme
• Attacks peptidoglycan in gram-positive bacteria.
Sialoperoxidase System
• Converts thiocyanate (SCN⁻) → hypothiocyanite (OSCN⁻) using hydrogen
peroxide (H₂O₂) produced by bacteria.
• OSCN⁻ / HOSCN equilibrium:
• At pH 5.3 → more HOSCN (stronger bactericide).
• At pH 7 → ≈98% OSCN⁻, 2% HOSCN.
• Antimicrobial mechanism:
• Oxidizes bacterial -SH groups.
• Disrupts membrane transport → loss of glucose transport, leakage of K⁺,
amino acids, peptides.
• Found in saliva, milk, tears, mucus.
General
• Combined antimicrobial components → infections in the mouth are rare, even
after surgery.
, Digestive Functions
Carbohydrate Digestion
• Begins in the mouth.
• α-Amylase breaks down starch/glycogen into:
• Maltose (2 glucose)
• Maltotriose (3 glucose)
• Dextrins (>3 glucose)
• Longer chewing → food tastes sweeter due to early sugar release.
• Digestion continues in the stomach until the food bolus is penetrated by acid →
enzyme is inactivated.
• Up to 75% of starch/glycogen digestion may occur before inactivation.
Minor Role of Lipase
• Present in saliva (lingual lipase), though less significant orally.
Mechanical and Protective Roles
Lubrication
• Mucins and water lubricate:
• Food bolus → easier swallowing.
• Oral cavity → essential for speech.
Teeth Cleaning
• Saliva washes away food particles.
• This is why chewing gum is often considered beneficial (stimulates saliva).
Protection
• Mineral protection via calcium phosphate.
• Antimicrobial protection via lysozyme, OSCN⁻ system.
• Creates a bactericidal environment → reduces oral infections.
Ptyalin (⍺-amylase)
Ptyalin (salivary α-amylase) comes from Greek Ρτυω, meaning “I spit.”
• Hydrolyzes starch into:
• Maltose (2 glucose)
• Maltotriose (3 glucose)
• Dextrins (>3 glucose): low molecular weight carbohydrates.
• Mixtures of D-glucose polymers with ⍺-(1→ 4) or ⍺-(1→ 6)
glycosidic bonds
• Only a small amount is hydrolyzed in the mouth; longer chewing
releases a slightly sweet taste.
• Digestion is effective because chewing mixes α-amylase with food.
• Digestion continues in the stomach until:
• The pH drops below ≈4 (when gastric acid reaches the bolus),
• At which point α-amylase is inactivated.
• Hydrolyzes up to 30–40% of ingested starches (depending on how long the bolus
stays above pH 4).
Stomach pH kinetics:
α-amylase remains active only in the center of the bolus until acid diffuses into it.
All diseases begin in the gut - Hippocraes (460 - 370 BC)
Components of the digestive system- from mouth to anus
GI tract does propulsion, digestion and absorption
Propulsion: moving food through the GI tract.
• It starts in the mouth with the mastication and the
tongue pushing food backwards (which initiates a
reflex that makes the phyringhial muscles contract).
• The soft palet lifts upwards, with the uvula (ugula);
the lower pharinx opens so that food can go down;
the larix lifts up (addam's apple in men), which closes the
epiglottis so that food doesn't go up in the nose or in the air ways.
• There is also peristalsis in the esophagus, in the stomach and in
the intestines
• Defecation process in the rectum
Mechanical digestion:
• chewing (mouth),
• churning (like what happens for butter) (stomach),
• segmentation (small intestine).
• The small intestine has the biggest amount of work to do, since there is all the
reabsorption
Absorption:
• Nutrients and water to blood vessels and lymph vessels (small intestine)
• Water to blood vessels (large intestine)
Water balance: volumes handled by the GI-tract
• The liver is so high bc there is a high bile production in it
• We have 9 L in the small intestine that are secreted, but
around 6,5-7,8 are reabsorbed. While it reabsorbs
water, it takes up also Na+, K+, and Cl-.
• Most goblet cells are in the colon (it's the one that
produces the most amount of mucus).
• It secretes HCO3- and reabsorbs water, which is
important bc even tho its work is small, if it doesn't do
it's work properly, that's how you end up with diarrhea
(that's why we associate it with water absorption
even tho it does the smallest amount of it).
• Water passes in between cells while ions go through
them.
,The gastrointestinal tract also has a large role in our body’s water balance
• Water is often absorbed via the paracellular route, while ions follow the
transcellular route, usually through channels or transporters
• Sodium enters the cell from the apical side often through a co-transporter
(eg.
Glucose-Na cotransport) and leaves the cell by active
transport at the basal side (Na,K-ATPase).
• Within the cell, sodium concentration is kept low,
facilitating transport via sodium co-transporters.
GI-tract: pH and transit time
• The transit times are very hard to determine, they vary per type of food and
person. If water passes through faster than food or not, it depends on the food
you eat, the sleep times you have, how active you are, and just on you.
• Fletcherism : it's a guy in 2d ww that thought that if
you chew your food from 30-40 times
you'd extract more from your food and
so you'd have to eat less food to get the
same amount of nutrients
• Esophagus is 25 cm long.
• Fondus is where the food is stored in the begining.
Food stays in the stomach until the pH reaches the
lowest point
• Whether it stays in the large intestine for longer or
not it depends on the person.
• pH-range is also variable, depending on food intake, type of food, digestive
period, etc.
Salivary glands
• Under the tongue there is the frenulum: a thin ligament that attaches the tongue
to the floor of the mouth.
• Left and right to it there are the openings to the sumandibular ducts and glands
(you can see where the saliva flows through the mouth)
What happens to the salivary glands, when you see food
• Submandibular gland: ducts end in caruncles left and right of the frenulum
under the tongue. Produces 60 to 70% of saliva in resting
state.
• Parotid gland: - best-known disease is mumps (orecchioni) –which leads to a
painful swelling of the parotid gland after an infection with the
paramyxo virus. Duct enters the oral
vestibule between cheek and gums at the
level of the second upper molar
(maxillary)
• Sublingual gland: 3-5% of salivary secretions, duct also
ends in sublingual caruncle (like
submandibular).
,Secretions of salivary glands
Composition of Saliva
Ions
• Na⁺, K⁺, Ca²⁺, Mg²⁺, PO₄³⁻, HCO₃⁻
• Thiocyanate (SCN⁻): antibacterial; converted to hypothiocyanite (OSCN⁻) by
sialoperoxidase using bacterial peroxide.
Water
• Major component; amount depends on autonomic activity
• Parasympathetic activation → watery saliva
• Sympathetic activation → thick, mucus-rich saliva
Proteins
• ≈50 different proteins present.
• α-Amylase (ptyalin): most abundant; digests starch → maltose, maltotriose,
dextrins.
• Mucins: lubrication; give saliva its viscosity.
• Lysozyme: antibacterial against gram-positive bacteria.
• Sialoperoxidase: produces bactericidal hypothiocyanite.
• Proline-rich proteins & statherins: keep calcium phosphate dissolved; prevent
precipitation.
pH
• Slightly alkaline; varies with flow rate.
• α-Amylase works optimally at slightly alkaline pH.
Supersaturation and Tooth Protection
• Saliva is supersaturated with calcium phosphate.
• Supersaturation = containing more dissolved material than normally possible at
equilibrium.
• Prevents demineralization of teeth (dentin = calcium apatite).
• Special proteins (proline-rich proteins, statherins) prevent crystals from
forming and depositing on teeth.
• This protects teeth from acidic foods/drinks and supports remineralization.
• Chewing gum stimulates saliva flow → helps clean teeth and maintain mineral
balance.
Antibacterial Defense
Lysozyme
• Attacks peptidoglycan in gram-positive bacteria.
Sialoperoxidase System
• Converts thiocyanate (SCN⁻) → hypothiocyanite (OSCN⁻) using hydrogen
peroxide (H₂O₂) produced by bacteria.
• OSCN⁻ / HOSCN equilibrium:
• At pH 5.3 → more HOSCN (stronger bactericide).
• At pH 7 → ≈98% OSCN⁻, 2% HOSCN.
• Antimicrobial mechanism:
• Oxidizes bacterial -SH groups.
• Disrupts membrane transport → loss of glucose transport, leakage of K⁺,
amino acids, peptides.
• Found in saliva, milk, tears, mucus.
General
• Combined antimicrobial components → infections in the mouth are rare, even
after surgery.
, Digestive Functions
Carbohydrate Digestion
• Begins in the mouth.
• α-Amylase breaks down starch/glycogen into:
• Maltose (2 glucose)
• Maltotriose (3 glucose)
• Dextrins (>3 glucose)
• Longer chewing → food tastes sweeter due to early sugar release.
• Digestion continues in the stomach until the food bolus is penetrated by acid →
enzyme is inactivated.
• Up to 75% of starch/glycogen digestion may occur before inactivation.
Minor Role of Lipase
• Present in saliva (lingual lipase), though less significant orally.
Mechanical and Protective Roles
Lubrication
• Mucins and water lubricate:
• Food bolus → easier swallowing.
• Oral cavity → essential for speech.
Teeth Cleaning
• Saliva washes away food particles.
• This is why chewing gum is often considered beneficial (stimulates saliva).
Protection
• Mineral protection via calcium phosphate.
• Antimicrobial protection via lysozyme, OSCN⁻ system.
• Creates a bactericidal environment → reduces oral infections.
Ptyalin (⍺-amylase)
Ptyalin (salivary α-amylase) comes from Greek Ρτυω, meaning “I spit.”
• Hydrolyzes starch into:
• Maltose (2 glucose)
• Maltotriose (3 glucose)
• Dextrins (>3 glucose): low molecular weight carbohydrates.
• Mixtures of D-glucose polymers with ⍺-(1→ 4) or ⍺-(1→ 6)
glycosidic bonds
• Only a small amount is hydrolyzed in the mouth; longer chewing
releases a slightly sweet taste.
• Digestion is effective because chewing mixes α-amylase with food.
• Digestion continues in the stomach until:
• The pH drops below ≈4 (when gastric acid reaches the bolus),
• At which point α-amylase is inactivated.
• Hydrolyzes up to 30–40% of ingested starches (depending on how long the bolus
stays above pH 4).
Stomach pH kinetics:
α-amylase remains active only in the center of the bolus until acid diffuses into it.
