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Esophageal Transit
When a swallowing is initiated for a brief period of 1.5 Seconds the upper esophageal sphincter opens to admit the bolus in to esophagus in a
jet.
A slower and more variable transit occurs through the esophageal body depending upon the physical properties of the bolus and the position
of the thorax when the swallow occurs. Liquids swallowed by People seated with the trunk upright takes about two or three seconds to the
level of the lower esophageal sphincter, moving mainly under the influence of gravity. When either or both the consistency of the bolus and the
position of the thorax reduce the effect of gravity, the intrinsic peristaltic mechanism in the esophagus must carry the bolus. In this case, it
moves at about 2 to 4 centimeters per second, requiring 5 to 7 seconds to traverse the distance to the lower esophageal sphincter.
Transit through the lower esophageal sphincter, like that through the upper esophageal sphincter, normally occurs only in the short interval
when it is open after a swallow. The sphincter opens about 2 to 3 seconds after the initiation of a swallow and remains open for 5 to10
seconds. Movement through this segment takes place in boluses. The passage of the bolus may be incomplete, leaving a part of it behind the
sphincter after the sphincter has closed. Such a residual, of course, is expelled by a second swallow or by the phenomenon of secondary
peristalsis

Gastric Transit: Filling and Emptying
Boluses usually enter the stomach at least several seconds apart
because effective swallowing cannot be induced more rapidly. After
a bolus first enters the stomach, the solid part remains in the
fundus (the region near enterance), layered at the periphery over the
mucosa while the more fluid parts flow more toward the antrum (the
region near exit). With subsequent boluses the solid components
tend to move into the proximal stomach as well, each coming to lie
in a layer on the luminal side of the solids from the last bolus.
Liquids pass around these solids to enter the distal stomach
directly.

Most species eat rapidly and at long intervals so that a large volume of food enters the stomach over a short time. The stomach relaxes to
accommodate this increase in volume with little or no increase in pressure, a process called receptive relaxation or accommodation. As a
result, the human stomach increases in size from its "empty" volume (the volume aspirated after a prolonged fast) of about 50 ml to a "filled"
volume of 1500 ml or more. It can expand at a rate of at least 100ml/min. Another reason for not to Gulp down your food too fast forcing your
stomach to expand at a higher rate.
The storage of the ingested solids in the proximal stomach lasts for a long time relative to the time required for their entry. Almost
immediately after entry begins, the secretion of gastric juice and mucus starts. This occurs especially from the mucosa of the proximal
stomach, so that the layering of one bolus over another must take place at the same time that gastric juice and mucus are beginning to
spread over and around the solid mass to flow into the distal stomach., The volume of gastric fluids secreted after a meal often exceeds that
of the ingested material, though their production takes a relatively long time. As a result, the quantity of fluid presented to the duodenum
following a meal can greatly surpass the volume ingested. May be this explains why one feels the expansion of the tummy is much more than
the amount of food consumed.
Contraction of the muscle of the proximal stomach provides most of the force required to empty the stomach. Having relaxed in the process
of gastric filling, this muscle then begins to regain its tone. Its slow sustained contraction presses the material stored in the saccular
proximal stomach (the region near enterance) into the conical distal stomach(the region near exit) where antral peristalsis kneads and stirs
it. Antral peristalsis reduces the solid material, suspended in large particles in the stored gastric content, to smaller and smaller particles.
The larger particles move at first toward the gastroduodenal junction (pylorus), carried in that direction by the peristaltic antral contractions,
only to be pushed back into more proximal regions because of the geometry of the antrum. The progressive narrowing of the conical antral
lumen leads finally to the development of a lumen ahead of the solid mass that is too small to receive it, and so the bolus must move back as
the antrum narrows further. This back-and-forth transit of large solid masses continues until grinding has reduced the masses to particles
with a diameter of about 1-2 mm, a dimension that allows them to escape through the pylorus into the duodenum, suspended in fluid. A
special term, gastric sieving, refers to this process of effective filtration.

The chemical composition of the gastric contents regulates the bulk-emptying rate, the flow being slowed especially by lipids (read as fatty
food). Excessive acidity and osmotic pressure in the gastric effluent can also retard emptying. The responsible chemoreceptors(chemical
sensors) lie in the duodenal mucosa and excite an inhibitory reflex that traverses vagal pathways. The reflex probably retards both the
restoration of tone in the proximal stomach and antral peristalsis. Thus, the stomach empties a fluid that constitutes a suspension of solid
particles 1 to 2 mm in diameter through the pylorus.

Transit in the Small Intestine
The small intestine receives about 6 to 12 liters of fluid daily. It
delivers 1.0 to 1.5 liters daily to the colon. In most of the cases of
diarrhea, the portion affected is small intestine. This explains why
you discharge a lot during diarrhea, even when you have eaten so
little.
The transit time through the whole organ, from pylorus to cecum,
varies enormously. Clearly, a great deal of bulk mixing takes place
in the small intestine. Likewise, an intense microcirculation of
intestinal contents across the mucosal surface must occur, for the
stirring of the unstirred layer at this surface is essential for the
optimal absorption of nutrients.

Transit in the Biliary System
Bile passes continuously from its source, the liver, into the biliary system at about 1 liter per day. During fasting about 25% of the bile secreted
passes directly into the duodenum through the sphincter of Oddi, while the rest (75%) enters the gallbladder due to the slight resistance to
flow produced by the sphincter of Oddi. The pressure gradient across the sphincter rarely exceeds about 5 to 6 centimeters of water. The
gallbladder thus fills slowly under low pressure, probably expanding in a process resembling that of receptive relaxation in the stomach, so
that its intra-luminal pressure rises negligibly with volume.
With a meal, three changes alter this pattern of flow. The volume of bile production from the liver doubles, resistance to transit at the
sphincter of Oddi falls, and the gallbladder contracts. The muscle of the filled gallbladder contracts very slowly to reduce its volume (usually
about 35 milliliters) by 50 to 75 % in 30 to 60 minutes after the meal. The gallbladder never empties completely, however, about 10 % of the
volume remaining in the organ. The pressure produced in the common bile duct by the contraction of the gallbladder never exceeds the
maximal secretion pressure of the liver, about 10 centimeters of water. After emptying, the flow pattern slowly returns to that which
characterizes fasting. The 0.5 to 1.0 milliliters per minute rate of transit of bile through the common duct in fasting, can double or triple under
the influence of meal-enhanced bile secretion and postprandial gallbladder contraction.

Transit in the Large Intestine and and rectum
The movement of intestinal contents through the ileocecal valve rarely actually stops for very long except in prolonged fasting, but it
fluctuates widely, rising in phase with meals. The actual daily volume, often quoted as an average of 1.0 to 1.5 liters, may occasionally be less
than that. The minimal daily output from an established ileostomy is often as low as 800 milliliters. This may be considered the obligatory
volume of input to the large intestine. In diarrhea due to defective absorption by the small intestine, the volume of flow into the colon may
exceed the normal by a factor of 10 to 15.
Just as in the case of transit in the small intestine, there are only bulk flow term descriptions of large intestine. Fluid remains in the cecum
and ascending colon for a prolonged period while the absorption of water reduces its volume. There is extensive mixing of the content during
its prolonged stay in this region of the colon. Beyond about the level of the hepatic flexure, however, the flow is faster on an average. From
here to the rectum, movement is highly intermittent, the fecal mass remaining stationary for tens of minutes to hours and then rapidly moving
antegrade (forward) for a variable distance. Such an episode of rapid transit is called the mass movement. The fecal mass thus comes to the
rectum in boluses, at intervals. The rectum and a variable length of the left colon above it, can store the fecal mass in a variable volume for a
variable time. The daily stool volume varies greatly, the average in Western societies often being stated to be about 150 ml. It is about twice
that in underdeveloped countries.

The fluid escapes from the stomach in boluses because of the
rhythm of antral peristalsis. This is the reason old wise grandma
always told you to chew your food well to avoid Motility. The
well-chewed food goes faster through the stomach. Shorter the
time spent by the food in your stomach less likely it is to cause
motility.
Bulk emptying follows a single exponential curve with a half time of
about 45 minutes for a standard meal. The curve below explains
why doctors advise with acidity not to go bed with in two hours of
eating a meal. After 2 hours there is only about 16% of food is still
normally left in your stomach.