a. the control of thyroid hormone secretion
The main control system of thyroid hormone is by feedback
The T4/T3 inhibit the TSH and TSH stimulate T4 and T3 secretion.
To a certain degree, T3/T4 also inhibit TRH secretion.
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.
TSH remains increased in the face of T4/T3 negative feedback
as long as cold-induced TRH persists.
b. transport of oxygen in blood
Some of the oxygen dissolve in the plasma but the vast majority
of oxygen carried in the blood is bout to haemoglobin.
Haemoglobin is the primary constituent of the erythrocyte
and combines reversibly with oxygen to form oxyhaemoglobin.
In normal whole blood, the concentration haemoglobin is about
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.
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
The amount of oxygen that is carried by Hb depends on the
partial oxygen dissociation curve.
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
c. calcium homeostasis
In the steady state, calcium intake should roughly equal
calcium loss via the gastorinterinal tract and the urine.
Calcium absorption from the gastrointestinal tract and kidney
is increased by
vitamin D3 (1,25(OH)2D) whose production is increased
by parathyroid hormone.
Parathyroid hormone also increases calcium resorption from
bone and calcium resorption from the urine
If plasma calcium is high, calcitonin is secreted to decrease