a. the control of thyroid hormone secretion
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The main control system of thyroid hormone is by feedback
mechanisms.
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The T4/T3 inhibit the TSH and TSH stimulate T4 and T3 secretion.
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To a certain degree, T3/T4 also inhibit TRH secretion.
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TRH however, increase the set-point for T4/T3 negative feedback.
For example, if exposed to cold temperature, TRH is released, stimulating
TSH release. This increases T4/T3 which increases basal metabolic rate
and heat production.
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TSH remains increased in the face of T4/T3 negative feedback
as long as cold-induced TRH persists.
b. transport of oxygen in blood
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Some of the oxygen dissolve in the plasma but the vast majority
of oxygen carried in the blood is bout to haemoglobin.
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Haemoglobin is the primary constituent of the erythrocyte
and combines reversibly with oxygen to form oxyhaemoglobin.
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In normal whole blood, the concentration haemoglobin is about
15g /dL.
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When blood is exposed to high oxygen pressure, all the haemoglobin
combines with oxygen to form oxyhaemoglobin. Under these conditions, the
haemoglobin is said to be fully saturated.
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Fully saturated Hb can accommodate about 1.39 ml of oxygen
per gram of Hb. Thus blood with a Hb concentration of 15 g/dL has an oxygen
capacity of about 20.8 ml/dL of blood or 20.8 volume percent
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The amount of oxygen that is carried by Hb depends on the
partial oxygen dissociation curve.
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Under normal conditions, the PO2 level found in
the lungs results in blood being about 97% saturated. In this case, when
arterial blood has a Hb concentration of 15g/dl, the oxygen content is
about 20ml/dL
c. calcium homeostasis
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In the steady state, calcium intake should roughly equal
calcium loss via the gastorinterinal tract and the urine.
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Calcium absorption from the gastrointestinal tract and kidney
is increased by
vitamin D3 (1,25(OH)2D) whose production is increased
by parathyroid hormone.
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Parathyroid hormone also increases calcium resorption from
bone and calcium resorption from the urine
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If plasma calcium is high, calcitonin is secreted to decrease
resorption.
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