2. Write short notes on:
a. the control of thyroid hormone secretion
b. transport of oxygen in blood
c. calcium homeostasis 
a. the control of thyroid hormone secretion 
  • The main control system of thyroid hormone is by feedback mechanisms. 
  • 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 15g /dL. 
  • 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 about 20ml/dL 

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 resorption. 

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