PHGY 210 Digestion March 9, 2012
Lecture 24
Right:
The environment surrounging the individual cells remains relatively
constant no matter what changes take place in the external
environment.
o No matter what whether its more/less concentrated, at a
higher altitude, with more/less pressure
o For the organism as a whole, the condition surrounding the
individual cells remains relatively constant principle of homeostasis
o Milieu interieur; the internal environment as opposed to the external environment
i. Internal environment which surrounds the individual cells
o Respiratory system: The internal environment requires that oxygen be made available to the cells but
the relative concentrations remain essentially constant
i. But the [CO2] will also be kept constant
ii. Necessary for the maintenance of the respiratory gas exchanges
o Renal System: maintains the total body water constant, the distribution constant within the body, the
various electrolytes and constituents of the body fluids
2) The physiology of the digestive system is also directed at maintaining a relatively constant environment making
available the necessary supplies for the individual cells
The necessary supplies in this case refer to the various nutrients
Where do these nutrients come from?
o We injest or take into the digestive tract nutrients in the form of foods
o Foods exist in the external environment
o We introduce the food into the oral cavity or the mouth
o From there it is transported along the length of the digestive tract
o It is converted to smaller particles and progressively into smaller and smaller molecules until a
level of molecular size and characteristics will be attained which allows it to be transferred into
the internal environment
o The internal environment begins with the blood and the various body fluids, the ISF surrounding
the individual cells
o These nutrients may undergo further processing and theyre ultimately distributed to all the
cells of the body where they are providing a energy and raw materials which are going to be
utilized for growth and repair and for function and regulation
The homeostasis of nutrients
o How are they made available and how are they utilized
o Exactly what the nutrients are and what role they will play in the body is studied as part of
nutrition
What are essential, what quantities are, etc
o The processing and the distribution, and utilization of the nutrients constitutes field of
biochemistry and metabolism
o Were going to focus on the actual physiology which deals with what happens?
How are foods which are ingested converted to absorbable molecules
How are they absorbed so as to enter the internal environment?
1 PHGY 210 Digestion March 9, 2012
Lecture 24
3) GIT structure
Need to appreciate structure before can fully understand the function of the digestive system
The simplest form of a digestive tract is the earthworm.
o We can identify a tube which begins at one end of the animal and ends at the other end of the
animal
o Hollow tube
o Has a lumen (the cavity within this tube)
o It communicates with the external environment at both ends of the animal
Communicates at anterior and posterior end
o In the course of evolution that leads to higher complexity, perhaps more efficiency, as you move
up the animal kingdom, getting to mammals and including humans, we retain many of these
features
We retain the tubular nature
Our digestive tract is a hollow tube, has lumen, and it too communicates with the
external environment
There are 2 stages of development which are incorporated in the more advanced animals
o Growth of the digestive system
o its differentiation
4) Growth 1
Growth manifests at two levels of the tract
It is greatly elongated
o Earthworm - from one end to the other end of the animal
o In humans its many times the length of the trunk where the organs are located
IN a human, in our typical physiological male of 70kg, the height is usually 1.5m 1.7m
The digestive tracts is several times that length. Its 4.5m.
ANATOMY: says the digestive tract is 9m
o Anatomists look at the cadaver and the cadaver has an elongated track because the muscular
elements in the wall of the digestive tracts are fully relaxed
o In the intact individual, living individual, the tract is half that length because there is
considerable amount of tone within the muscular elements of the wall of the digestive tract
All of the 4.5m must be folded and accommodated primarily in the abdominal cavity
5) Growth 2
nd
The 2 aspect of growth that takes place is an enormous increase in the internal surface area
The central cavity is the lumen.
The lining of the lumen is many times larger in surface area that the outer surface of the tube as a whole
o the inner lining of the digestive tube is 600x larger than the outer lining.
2
It reaches about 200 to 250m
o tennis court available for surface area has to be a lot of transport across that membrane
because you need surface area for efficiency of transport
Many in-folds/in-pushings and out-folds/out-pushings, which allow for this tremendous
increase surface area
6) In addition to growth, we see differentiation in the course of evolution
2PHGY 210 Digestion March 9, 2012
Lecture 24
We start out with a simple tube of the earthworm but by the time we get to humans we see that the
simple tube is now being converted to a very complex and highly differentiated sequence of
interconnected organs each of which in the course of evolution has become specialized to perform
certain aspects of the digestive process most efficiently
Started with oral cavity (mouth), pharynx, esophagus, stomach, small intestine, large intestine, rectum
and the anal opening
Specialized digestive tract seen higher up in the animal kingdom
The abdominal cavity accommodates all this huge area and huge length of the digestive tract which has
to be folded
7) Wall Structure
If you take a cross section anywhere between the esophagus and the rectal region, there are 4 typical
layers of tissue that can be identified
o despite of the fact that the esophagus and the small or large intestine have different functions,
you can still identify the 4 distinct layers
o Quantitative and qualitative changes, but the 4 layers are always there
Outside of the tube: Serosa
o Thin but very tough layer of connective tissue that surrounds the tube as a whole and its
continuous
o In the abdominal cavity the serosa is continuous with the lining of the abdominal or peritoneal
cavity
Under the serosa, working our way towards the central cavity of lumen, we find a muscular layer known
as the muscularis externa
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