FIS2601 - Digestion, Endocrine Control And Metabolism
Institution
University Of South Africa (Unisa)
Book
Guyton and Hall Textbook of Medical Physiology
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FIS2601 - Digestion, Endocrine Control And Metabolism
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FIS1601 NOTES
QUESTION 2
2.1 Give an illustration of the chemical reactions for glycogenesis. (2)
Glucose + ATP hexokinase→ Glycose-6-phosphate + ADP
2.2 Name factors that depolarize the membrane in the intestinal smooth muscle. (4)
When the potential becomes less negative, which is called depolarization of the membrane, the muscle fibers become
more excitable. Factors that depolarize the membrane—that is, make it more excitable—are
(1) stretching of the muscle,
(2) stimulation by acetylcholine released from the endings of parasympathetic nerves, and
(3) stimulation by several specific gastrointestinal hormones
2.3 Name various hormones that play a role in protein metabolism. (6)
Insulin, GH, and IGF-I play a dominant role in the day-to-day regulation of protein metabolism. In humans’ insulin
appears to act primarily to inhibit proteolysis while GH stimulates protein synthesis.
• Growth Hormone Increases the Synthesis of Cellular Proteins
• Insulin Is Necessary for Protein Synthesis.
• Glucocorticoids Increase Breakdown of Most Tissue Proteins.
• Testosterone Increases Protein Deposition in Tissues
• Estrogen, the principal female sex hormone, also causes some deposition of protein.
• Thyroxine Increases Metabolism of Cells.
,2.4 During the gastrointestinal activity, blood flow in the submucosa increases as much as eight-fold. Outline the
mechanisms involved in increased blood flow. (8)
First, several vasodilator substances are released from the mucosa of the intestinal tract during the digestive process.
Most of these substances are peptide hormones, including cholecystokinin, vasoactive intestinal peptide, gastrin, and
secretin. These same hormones control specific motor and secretory activities of the gut.
Second, some of the gastrointestinal glands also release into the gut wall two kinins, kallidin and bradykinin, while they
secrete other substances into the lumen. These kinins are powerful vasodilators that are believed to cause much of the
increased mucosal vasodilation that occurs along with secretion.
Third, decreased oxygen concentration in the gut wall can increase intestinal blood flow at least 50 to 100 percent;
therefore, the increased mucosal and gut wall metabolic rate during gut activity probably lowers the oxygen
concentration enough to cause much of the vasodilation. The decrease in oxygen can also lead to as much as a fourfold
increase of adenosine, a well-known vasodilator that could be responsible for much of the increased flow.
Thus, the increased blood flow during increased gastrointestinal activity is probably a combination of many of the
aforementioned factors plus still others yet undiscovered.
QUESTION 3
3.1 How does glucagon lead to glycogenolysis? [10]
Glycogenolysis means the breakdown of the cell’s stored glycogen to re-form glucose in the cells. The glucose can then
be used to provide energy. Glycogenolysis does not occur by reversal of the same chemical reactions that form glycogen;
instead, each succeeding glucose molecule on each branch of the glycogen polymer is split away by phosphorylation,
catalyzed by the enzyme phosphorylase. Under resting conditions, the phosphorylase is in an inactive form, and thus
glycogen remains stored. When it is necessary to re-form glucose from glycogen, the phosphorylase must first be
activated. This activation can be accomplished in several ways, including activation by epinephrine or by glucagon.
Activation of Phosphorylase by Epinephrine or by Glucagon. Two hormones, epinephrine, and glucagon can activate
phosphorylase and thereby cause rapid glycogenolysis. The initial effect of each of these hormones is to promote the
formation of cyclic AMP in the cells, which then initiates a cascade of chemical reactions that activates the
phosphorylase.
Glucagon is a hormone secreted by the alpha cells of the pancreas when the blood glucose concentration falls too low. It
stimulates formation of cyclic AMP mainly in the liver cells, which in turn promotes conversion of liver glycogen into
glucose and its release into the blood, thereby elevating the blood glucose concentration.
3.2 Describe the effect of parathyroid hormone on bone, kidney, and intestine. [10]
Your parathyroid gland releases parathyroid hormone (PTH) when your body detects low calcium levels in your blood.
Parathyroid hormone regulates calcium levels in your blood by affecting the following parts of your body:
• Bones: PTH stimulates bone resorption, causing release of calcium into the extracellular fluid.
PTH has two effects to mobilize calcium and phosphate from bone. One is a rapid phase that begins in minutes
and increases progressively for several hours. This phase results from activation of the already existing bone
cells (mainly the osteocytes) to promote calcium and phosphate release. The second phase is a much slower
one, requiring several days or even weeks to become fully developed; it results from proliferation of the
osteoclasts, followed by greatly increased osteoclastic resorption of the bone itself, not merely release of the
calcium phosphate salts from the bon
, • Kidneys: PTH increases reabsorption of calcium and decreases phosphate reabsorption by the renal tubules,
leading to decreased excretion of calcium and increased excretion of phosphate
Administration of PTH causes rapid loss of phosphate in the urine because of the effect of the hormone to
diminish proximal tubular reabsorption of phosphate ions. PTH also increases renal tubular reabsorption of
calcium while it diminishes phosphate reabsorption. Moreover, it increases reabsorption of magnesium ions and
hydrogen ions while it decreases reabsorption of sodium, potassium, and amino acid ions in much the same way
that it affects phosphate. The increased calcium reabsorption occurs mainly in the late distal tubules, the
collecting tubules, the early collecting ducts, and possibly the ascending loop of Henle to a lesser extent.
• Small intestine: PTH is necessary for conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol,
which, in turn, increases calcium absorption by the intestines.
PTH enhances the absorption of calcium in the intestine by increasing the production of activated vitamin D.
Vitamin D activation occurs in the kidney. PTH up-regulates 25-hydroxyvitamin D3 1-alpha-hydroxylase, the
enzyme responsible for 1-alpha hydroxylation of 25-hydroxy vitamin D, converting vitamin D to its active form
(1,25-dihydroxy vitamin D). This activated form of vitamin D increases the absorption of calcium (as Ca2+ ions)
by the intestine via calbindin.
Once your parathyroid glands release PTH when you have low blood calcium levels, the PTH is only active in your body
for a few minutes. When your blood calcium levels rise, your parathyroid glands stop releasing PTH.
QUESTION 4
Apply your knowledge to the following scenarios:
4.1 Patient A has abnormal increased levels of cortisol. Using your accumulative knowledge of endocrinology, what
are the different types of stress that increase cortisol release? (5)
, As your body perceives stress, your adrenal glands make and release the hormone cortisol into your bloodstream. Often
called the “stress hormone,” cortisol causes an increase in your heart rate and blood pressure. It’s your natural “flight or
fight” response that has kept humans alive for thousands of years.
Normal levels of cortisol also are released when you wake up in the morning or exercise. These levels can help regulate
your blood pressure and blood sugar levels and even strengthen your heart muscle. In small doses, the hormone can
heighten memory, increase your immune system, and lower sensitivity to pain.
The danger of a fast-paced culture, however, is that many of us are constantly in high-stress mode. If your body
experiences chronic stress, you may begin to feel unpleasant and even dangerous effects, such as:
• Fatigue
• Irritability
• Headaches
• Intestinal problems, such as constipation, bloating or diarrhea
• Anxiety or depression
• Weight gain
• Increased blood pressure
• Low libido, erectile dysfunction or problems with regular ovulation or menstrual periods
• Difficulty recovering from exercise
• Poor sleep
When the adrenal glands release cortisol into your bloodstream, the hormone triggers a flood of glucose that supplies
an immediate energy source to your large muscles. It also inhibits insulin production so the glucose won’t be stored but
will be available for immediate use.
Cortisol narrows the arteries, while another hormone, epinephrine, increases your heart rate. Working together, they
force your blood to pump harder and faster as you confront and resolve the immediate threat
Hormone levels return to normal as you swerve to miss an oncoming car, find out that your child has only a few scrapes
or meet the deadline for your presentation
Increased blood sugar levels. Insulin typically helps the cells convert glucose to energy. As your pancreas struggles to
keep up with the high demand for insulin, glucose levels in your blood remain high and your cells don’t get the sugar
they need to perform at their best.
Weight gain. As your cells are crying out for energy, your body may send signals to the brain that you are hungry and
need to eat. Studies have demonstrated a direct association between cortisol levels and calorie intake in populations of
women. False hunger signals can lead you to crave high-calorie foods, overeat and thus gain weight. Unused glucose in
the blood is eventually stored as body fat.
Suppressed immune system. Cortisol’s positive action to reduce inflammation in the body can turn against you if your
levels are too high for too long. The elevated levels may suppress your immune system. You could be more susceptible
to colds and contagious illnesses. Your risk of cancer and autoimmune diseases increases, and you may develop food
allergies.
Digestive problems. When your body reacts to a threat, it shuts down other less critical functions, such as digestion. If
the high-stress level is constant, your digestive tract can’t digest or absorb food well. It’s no coincidence that ulcers
occur during stressful times and people with colitis or irritable bowel syndrome report better symptom control when
they get their stress under control.
Heart disease. Constricted arteries and high blood pressure can lead to blood vessel damage and plaque buildup in your
arteries. They could be setting the stage for a heart attack or stroke.
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