Calcium Homeostasis

Calcium Homeostasis

Calcium is in the plasmaThe liquid component of blood. portion of your blood.  The watery portion.  Calcium is a cation with 2 fewer electronsNegatively charged subatomic particles found in atoms. than the atomThe smallest units of matter that retain the properties of an element. has and therefore, that is why we use the abbreviation Ca+2.  So, when depositionThe process of bone matrix formation by osteoblasts. by osteoblasts happens, calcium is taken from the blood and tied up in your bones.  This process would lower the calcium in your blood lower than 8.5 mg/dL, which is an average target for calcium.  Remember different hospitals might have different ranges depending on the lab and test they use.  When your calcium levels fall, a hormone is released into your blood called the parathyroid hormone(PTH): Raises blood calcium by stimulating bone resorption. or PTH.  You have 4 little parathyroid glands in your posterior neckNarrow region just below the head; common fracture site. region that make this hormone PTH.  These glands are actually attached to the surface of the thyroid gland Endocrine gland regulating metabolism through thyroid hormones., which wraps around your trachea.  This is why it called the para-thyroid.  The prefix para means “to the side.” 

In the past, we didn’t know the parathyroid glands were there. We would have people go in for thyroidectomies. Surgeons often removed part of the thyroid gland with the parathyroid glands attached. The patient would return after surgery with hypocalcemiaLow calcium levels in the blood, causing muscle spasms and weakness.. They were no longer making PTH to help raise the calcium level in the blood. 

Homeostasis of Blood Calcium

PTH doesn’t actually do anything to the blood.  PTH is just a signal.  The parathyroid glandSmall glands on the thyroid that secrete PTH to regulate calcium. sits around and monitors the calcium in the blood.  When it goes down, they make PTH, which then travels to three organs: the small intestine, the kidney, and bone.  The small intestine absorbs more calcium, the kidney refuses to release calcium into urineThe liquid waste excreted by the kidneys..  Both of these raise blood calcium. 

OsteoclastsBone-resorbing cells that break down bone matrix. respond to PTH and begin osteolysis to break down bone, releasing calcium into the blood to raise the level. 

At the same time, the thyroid gland itself is monitoring calcium.  When it goes too high, these cellsThe basic structural and functional units of life. in the thyroid called C cells, release a hormone called calcitoninA hormone from the thyroid that lowers blood calcium levels by inhibiting osteoclasts..  Calcitonin, like PTH, then travels to the same three organs. However, it instructs the organs to do the opposite of PTH.  Calcitonin instructs the small intestine to stop absorbing calcium. It also tells the kidney to flush as much calcium as it can into the urine.  Both of these actions lower the calcium in blood.  Bone responds by ceasing osteoclast activity.  This allows the osteoblasts to continue to grab calcium from blood and deposit it in remodeling.  Since there is no balancing osteoclast activity, this also serves to lower the blood calcium.