and then team of osteoblasts lays down new bone matrix. Remodelling is controlled by two mechanisms. firstly, osteoclasts and osteoblasts respond to the mechanical stresses put on bones by gravity and muscle tension. Secondly, two hormones, parathyroid hormone (PTH) and calcitonin respectively stimulate and inhibit osteoclast activity in order to regulate the release of calcium ions from bonematrix. This maintains constant levels of calcium, essential for muscle contraction and many other processes, in the blood.