(a) Parathyroids are under the feedback control of blood calcium level.
(b) A fall in blood calcium level stimulates them to secrete parathormone which restores blood calcium level.
(c) It increases blood calcium by drawing calcium from the bones into the plasma, by increasing calcium absorption in the digestive tract, and by reducing loss of calcium in the urine.
(d) It lowers blood phosphate by increasing elimination
of phosphate in the urine