Digestion and absorbtion
Physiology of digestion
Cooking food can be viewed as first step
in digestion. It denatures-unfolds protein and
softens tough connective tissue in meat and
fibers in plants such as broccoli.
Major organs of digestion
Mouth Esophagus
Pharyngs Stomach
Laryngs Small intestine
Large intestine
To view and learn more about these organs
of digestive system click here
Auxiliary organs- help digestion
Tongue Liver
Teeth Pancreas
Salivary glands
Click here to see an animated movie on digestion and take quick quiz. An excellent
tool for better comprehension especially for those of you, who do not know anything about
digestion.
Chevying food is the initial part of digestive process. This mechanical process
mixes food with saliva and forms bolus.
Saliva consists of-enzymes-salivary amylase
-Lysozyme-breaks down bacteria
-Mucous-lubricates food
In the mouth we have beginning of starch digestion . Salivary amylase breaks down
starch into dextrins and maltose. No other chemical digestion takes place
in the mouth.
From mouth bolus continues through PHARYNGS. In the Pharyngs bolus
passes over EPIGLOTTIS -valve like flab of tissue that covers the entrance to
the trachea-the windpipe.
When food is swallowed it travels through Esophagus-structure that connect
mouth and stomach. Before food enters stomach, it passes through first of the five
SPHINCTERS (structure that controls flow of food throughout GI tract). We call
this particular sphincter LOWER ESOPHAGEAL SPHINCTER or CARDIAC SPHINCTER.
The Lover esophageal sphincter prevents back-flow of food from stomach back
into esophagus. This sphincter, as well as all the others, are controlled by either
hormonal or neural stimulation.
To see organs of GI tract interactively click here
STOMACH
Stomach is a place where food is further processed to be more available for
absorbtion in small intestine. In the stomach bolus is mixed with stomach secretions
and becomes CHYME.
Stomach has several secretions that are important for proper digestive function.
They are:
-HCL-hydrochloric acid -PH 2
-Pepsinogen-inactive form of enzyme Pepsin that helps to digest protein
-Mucous-forms thick layer over the stomach walls to prevent auto digestion
-Gastric lipase-enzyme breaking down short and medium fatty acids
-Intrinsic factor-necessary for absorbtion of vitamin B12
-Gastrin-hormone that- stimulates release of HCL and psinogen.
-closes Lover esophageal sphincter after food enters stomach
-relaxes pyloric sphincter when food is ready to pass into small
intestine.
-it is activated through our visual sense, smell of food or chewing
of food.
In the stomach there is only a little bit of absorbtion, mostly only water, alcohol,
medication and some fats (short and medium f.a.) are absorbed here.
Click here to see histology of stomach
Small intestine
From stomach food passes into small intestine. Stomach is connected with small
intestine through PYLORIC SPHINCTER that allows only small amount of food to
enter the small intestine at a time.
We have three parts of small intestine
1) Duodenum-at this point bile and pancreatic enzyme enter small intestine
through common Bile duct
2) Jejunum
3) Illeum
The most digestion of food is completed in duodenum and upper portion of
jejunum.
Large intestine
Mass of digested food passes from small intestine into large intestine through
another sphincter, we call this sphincter ILLEO-CECAL VALVE.
Large intestine consists of
-cecum
-ascending colon
-transverse colon
-descending colon
-sigmoid colon
Very little digestion takes place here. From here residue of food leaves
body as feces.
Now lets have a look on how food is actually digested. Before we do so lets identify
few terms that we will be referring to throughout or journey through GI tract
Enzyme-small protein like catalysts that increase rate of digestion
-mostly made by pancreas
- are PH sensitive
-substrate specific
-have suffix "ASE"
Click here to see how enzymes work- click on play buttom when page loads
Hormone- a substance that is produced at one place, then secreted into blood
stream after which it travels to its targeted place where it performs it's
action
It can be- protein based -insulin
- fat based -estrogen
PERISTALSIS - Movement of food along the GI tract achieved by contraction and
relaxation of longitudinal muscles.
SEGMENTATION- Contraction of circular muscles along GI that divides mass into
smaller pieces, and allows for mixing of intestinal content.
We said that in mouth only CHO is partially broken down to Dextrins. During the
process of MASTICATION Stomach prepares itself for upcoming food. Gastrin is
released and gives signal to increase production of HCL and Pepsinogen. Once food
reaches stomach PH of stomach reaches PH 2 and pepsinogen now converts into its active
form PEPSIN. Pepsin will break down protein into smaller chain of amino acids
that we call Peptones. Peptones are still too large for absorption,
however, and have to wait for their complete digestion in small intestine.
High acidity of stomach disables salivary amylase. That is why at this point there
is no digestion of CHO in the stomach. It stops here.
Gastric Lipase- will act upon small and medium chain fatty acids and
break them down to glycerol, f.a. , and mono-glycerol. These can
be absorbed directly into blood stream from the stomach wall in
limited amount. Most of them is however absorbed in the small
intestine.
Hormone gastrin besides stimulating release of HCL and
also contracts Lower esophageal sphincter to
prevent back flow of food into esophagus, but also relaxes
pyloric sphincter to release small amount of CHYME into duodenum.
Duodenum.
Once chyme enters duodenum, hormone secretin is stimulated by its presence, and is
released into blood stream. It reaches pancreas and stimulates release of
BICARBONATE that will neutralize the highly acidic Chyme bringing its PH to 5-7.
At the same time hormone GIP-gastric inhibitory peptide is released and it tells
Pyloric sphincter that enough Chyme is present for the moment. It closes the pyloric
sphincter and also tells stomach to reduce production of HCL and Pepsinogen. GIP
regulates the interval of relaxing and contracting of pyloric sphincter based on
the representation of protein, fat and carbohydrates in the chyme.
Presence of lot of -fat causes pyloric sphincter to stay closed for a longer
period of time.
-Protein has intermediate effect
-CHO causes pyloric sphincter to be opening in fairly quick
intervals
From the wall of small intestine hormone CCK-cholecystokinin is released. It
reaches gall bladder and causes it's contraction to release bile, but also pancreas to
release enzymes for Charbohydrate digestion- Pancreatic amylase
Protein digestion- protein splitting enzymes- trypsin
-chymotrypsin
-carboxytripsin
Fat digestion -pancreatic lipase
But it also relaxes another sphincter we didn't mentioned yet-SPHINCTER OF ODI
(located in common bile duct) that allows mixed bile and pancreatic enzymes to enter
duodenum.
When pancreatic amylase enters duodenum it will resume CHO digestion. Dextrins will
be broken down into disacharides and monosacharides From the wall of small intestine
enzymes - maltase
-lactase
-sucrase
are released to break down these disacharides into monosacharides. At this point they can
be absorbed.
When protein splitting enzymes enter duodenum, they are activated by
enterokinase and turn into their active form. They will act upon peptones and
break them down to peptides and a.a. These can be absorbed into the wall of small
intestine where they will be finally all broken into single a.a for absorption by amino
peptidase.
When bile enters duodenum it will together with lecithin- produced also in the
small intestine- emulsify (break down) large globules of TG (long chain f.a).into smaller
globules of TG, to make them more available for break down by Pancreatic Lipase. When
that happens, we end up with glycerol, mono-glycerol, and f.a.. This is packaged into
micelle, and shifted into the wall of small intestine. Here it is repackaged into
lipoprotein -chilomicrone for distribution throughout our body.
ABSORPTIONS
Most absorption takes place in the small intestine. There is only little bit
of absorption in the stomach and large intestine. The wall of small intestine is folded
to increase surface area. Each fold has finger like projections that we call VILLI.
Each villi ( you can imagine them like fingers sticking out of the fold) is made up
of numerous absorptive cells-cells that do the actual absorption. We call these
absorptive cells also ENTEROCYTES. Each of these cells contain a microvili that is
covered with hair like projections that we call GLYCOKALYX. Through them we absorb our
nutrients.
All the absorptive cells are produced in the crypts deep in mucous lining of small
intestine. They travel on the surface of Villi where they are eventually slaughed off
and replaced by new ones every 2-5 days
Click here to see histology of small intestine
We distinguish between four types of absorption
a) Passive
b) Facilitated
d) Active
e) Endocytosis-phagocytosis-cell "eating"
-pinocytosis-cell" drinking"
Passive-no energy or carrier for transport of the nutrient into the absorptive
cell is needed.
- We need high concentration of the nutrient in the GI(difference in
concentration makes this transport possible)
- nutrients transported-water most fats, some minerals.
Facilitated-carrier needed, no energy needed
- made possible because of difference in concentration of nutrient in GI
-nutrient absorbed-Fructose
Active-Carrier and energy is needed (ATP)
-nutrients in low concentration can be absorbed -against concentration gradient
-nutrients absorbed-glucose, galactose, amino acids, and other nutrients
Endocytosis- Infants' absorption of antibodies from mothers milk.
Portal and lymphatic circulation
We can divide the villi in the small intestine into 2 different circulatory systems
a) Portal
b) Lymphatic
The choice of circulatory systems that will be used for nutrient absorption depends
on nutrient's solubility in water or organic solvent.
Portal
Nutrients soluble in water will enter portal circulatory system(protein, CHO, short
and medium chain f.a.-butter) they will enter portal vein that collects nutrients from
the little capillaries running through the intestinal villi, and will go to liver for
processing before they enter general circulation.
Lymphatic system
Carries either fat soluble vitamins A, D, K, E, or long chain fatty acids -
chylomicrones. They are too big to pass through the small capillaries. They enter
thoracic duct, and travel all the way up close to the heart until they enter general
circulation at the subclavian vein.
Enterohepatic circulation
This is a path that is used for recycling of bile . 98% of bile is recycled ,and
returns back to the liver to be reused .
ABSORPTION OF NUTRIENTS
a)Protein
- Peptides and amino acids are actively absorbed
- I the liver we can make from amino acids-protein, glucose, energy, fat.
B)Fat
- fatty acids and monoglycerides -passively absorbed
-short and medium chained -less than 12 carbons-they are water soluble =go to portal
vein.
- long chained amino acids-12 or more carbons-reform TG and transport via
lymphatic system.
C) Carbohydrates
-Fructose=facilitated
-Glucose and galactose=active
-portal vein-liver-glycogen, fat , energy, release to blood.
For more detailed information on pathophysiology of digestion click here
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