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Topic_13_-_Digestion.docx-Topic 13 – Digestion  What
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Topic_13_-_Digestion.docx-Topic 13 – Digestion  What
Page 4
Lipids (or fats)
: Made up of hydrophobic carbon-based molecules such as
fatty acids
(and maybe nucleic acids)
The digestion of each macronutrient takes place in a particular location in the
digestive tract
Locations of Macronutrient Digestion

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Structure and function of some sub-regions of the digestive tract (also called the
gastrointestinal tract or the gut)
The Stomach
The wall of the stomach contains a thick layer of smooth muscle fibers, and the
epithelium that lines the stomach includes many (exocrine) glands that secrete
proteases or HCl
A sphincter is a ring of smooth muscle surrounding a passage from one structure
to another
Smooth vs. Skeletal Muscles
Skeletal Muscles
are organized in the muscles that are attached to bones, and they
move parts of the body
Smooth Muscles
are primarily in the walls of hollow organs (such as stomach,
blood vessels, iris of eye, etc.)
Force is generated when myosin in thick filaments binds cyclically to actin
in thin filaments
ATP has the same role as in skeletal muscle fibers
plays a critical role in regulating the binding of myosin to
(these are features of Smooth Muscle):
Each fiber is very small and includes only one nucleus

Page 7
Fibers are innervated by autonomic neurons, not by spinal motor neurons,
and in most smooth muscle tissues, both branches of the autonomic
nervous system innervate each fiber
In some smooth muscle tissues, fibers are coupled to one another via gap
junctions, so excitation spreads from one muscle fiber to another,
producing a wave of contraction that travels along the tissue
No sarcomeres, so they aren’t striped (that’s why they’re called smooth)
The Small Intestine
The wall of the small intestine includes layers of smooth muscle in which the
fibers are coupled together via gap junctions
Peristaltic contraction of the muscles moves food along the digestive tract from
the esophagus to the anus
The small intestinal tract also has digestive features
The intestines are lined with epithelial tissue that has two levels of folding: villi
and microvilli
The villi
are the big folds of the intestine lining
The microvilli
are the tiny folds that look just like villi, but within the villi
The Liver
Excretion of hydrophobic molecules

Page 8
Activity in neurons from both branches of the autonomic nervous system affects
function of digestive organs
In addition, a complex set of hormones modulates the function of these organs
Paracrine Control
is a subtype of endocrine control where it has local effects
(affects things that are near where it is secreted)
Activity in neurons from both branches of the autonomic nervous system affects function
of the digestive organs
At the same time, a complex set of hormones modulates the function of these
Either stimulation or inhibition
CCK (cholecystokinin) from the duodenum
Secretin from the duodenum
Gastrin from the stomach

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