In postmenopausal osteoporosis, decreased estrogen levels lead to increased bone resorption.

在绝经后骨质疏松中，雌激素的减少引发了骨吸收的增加。

So, bone resorption usually overtakes bone formation around the 8th decade of life.

因此，人到80岁之后，骨吸收能力通常压制骨生长能力。

Finally, vitamin D promotes calcium absorption in the gut, so it increases serum calcium, promoting bone formation and decreasing bone resorption.

最后，维生素D促进钙在肠内的吸收，因此增加了血清钙的水平，促进骨生长并减弱骨吸收。

Alright, as a quick recap, osteoporosis refers to decreased bone density, on account of increased bone resorption compared to bone formation.

好！快速回顾一下：骨质疏松意味着骨密度的降低，由自身骨吸收能力大于骨生长能力造成。

Parathyroid hormone is produced by the parathyroid glands in response to low serum calcium, and it increases bone resorption to release calcium into the bloodstream.

顾名思义，甲状旁腺激素在甲状旁腺产生，来应对血清中过低的钙水平，从而增加骨质的吸收来释放更多的钙进入血液。

Now, even though parathyroid hormone stimulates bone resorption, it's been found that intermittent injections with teriparatide activates osteoblasts more than osteoclasts, therefore increasing bone formation.

其实，即使甲状旁腺激素刺激骨吸收，研究发现间歇性注射特立帕肽激活了更多造骨细胞而非破骨细胞，从而增强骨生长。

This is caused by increased bone resorption, which is a process that breaks bones down, eventually fast enough that it outweighs the rate of new bone growth.

这是由骨骼逐渐溶蚀所引起，整个过程会造成骨折，最终则因速度太快，远超过骨骼的生长率。

But, for reasons that we don't understand yet, the osteoclasts actually increase their rate of bone resorption in low gravity, while the osteoblasts dial back on the bone formation.

但是，由于我们还不明白的原因，破骨细胞实际上在低重力下增加了骨吸收的速度，而成骨细胞则减少了骨的形成。

On the other hand, calcitonin is produced by the thyroid gland in response to high serum calcium, so it opposes the action of PTH - therefore promoting bone formation and decreasing bone resorption.

另一方面，当血清中钙含量较高时，甲状腺释放降血钙素，来制约甲状旁腺激素的作用，从而促进骨骼的形成并减缓骨骼的吸收。