- muscle-attachment and movement
- protecting soft tissue: skull, thorax
- calcium reservoir
- making blood cells in the bone marrow
The key is that there are two complementary hormones that control the amount of calcium circulating in the blood. You have a gland in your neck called the thyroid [prev], it is pink but it has cells of at least two distinct types, so distinct that someone named the minority type the parathyroid gland - not glands even though there are four clusters of parathyroid cells embedded in the thyroid but producing a hormone, memorably called PTH parathyroid hormone. These cells are not yellow despite being coloured so for clarity in the cartoon thyroid in a mnemonic picture at the top of this post. Other cells of the thyroid proper make another hormone called calcitonin. When blood calcium falls below a threshold, PTH is released. There are cells embedded in bone called osteoclasts which are sensitive to PTH and dissolve the calcium phosphate matrix and leach calcium into circulation. When blood calcium is too high calcitonin is released which triggers activity in another sort of bone cell called osteoblasts whose job it is to scavenge calcium and phosphate from the blood and build more bone with them. It is, like all normal Hum Phys, exquisitely finely balanced . . . until it isn't: clearly something has gone wonk if you suffer from osteoporosis. So this is what we know in the world of bullet-point learning:
|Blood calcium||Too low||Too high|
|Bone matrix||Crush Down||Build Up|
Q1. Describe how calcium balance is regulated in human physiology.
Q2. What are the four functions of bone?
In the belt-and-braces way that is so typical of the effect of hormones, PTH affects not only the osteoclasts in bone but also the gut which is thereby primed to absorb more calcium as it passes through. And remember that vitamin-D's other name is calciferol and that it's also involved in calcium balance. But for answering Summer Exam questions for Human Physiology, let's keep it cartoon simple.