In response to lowered blood calcium levels, the parathyroid gland secretes more parathyroid hormone (PTH). PTH instructs the kidneys to increase production of the vitamin D hormone (calcitriol)l , and to reasorb more calcium from the urine. PTH also instructs cells in the bone to resorb bone and release calcium into the blood. The vitamin D hormone (calcitriol) also instructs the kidneys and bones to increase reabsorption and resorption of calcium. Additionally, calcitriol instructs the intestine to increase absorption of calcium.多重下拉选择题