like many other tissues in our body such as our skin bone forms before birth but is continually renewing itself thereafter bone remodeling is the ongoing replacement of old bone tissue with new bone tissue so bone remodeling involves bone resorption so not reabsorption resorption which is the removal of minerals and collagen fibers from bone by osteoclasts it also involves bone deposition which is the addition of minerals and collagen fibers to Bone by osteoblasts in other words bone resorption results in the destruction of the bone extracellular matrix while bone deposition results in the formation of new bone extracellular matrix at any given time about 5% of the total bone mass in the body is being remodeled the renewal rate for compact bone is about 4% per year for spongy bone it's about 20% per year remodling also takes place at different rates in different regions of the body depending on several factors including injury exerise hormones and diet remodeling is an important on goinging process as new bone is more resistance to stress and Fracture than his old bone so normal bone metabolism which includes growth in children and bone remodeling in adults depends on several factors these include the adequate dietary intake of minerals and vitamins sufficient levels of several hormones and the presence or absence of mechanical stress so large amounts of calcium and phosphorus are needed well bones are growing and remodeling alongside smaller amounts of magnesium fluide and magnes these minerals are deposited into the extracellular Matrix to calcify or Harden bone tissue when the concentration of these minerals in the blood is adequate the minerals combine to form the crystals of hydroxyapatite which are then deposited into the network of collagen fibers when the concentration of these minerals in the blood is low particularly for calcium the body will release parathyroid hormone which acts to increase the activity of osteoclast to break down bone and remove calcium from The extracellular Matrix returning it back to the blood now the body will preference adequate levels of calcium in the blood over needing calcium to build bone therefore it's really important during particularly childhood and after an injury that adequate levels of these minerals are consumed alongside the minerals that we just mentioned several vitamins are also important for bone growth and remodeling vitamin A for example stimulates the activity of osteoblasts without osteoblasts new osteoid or that bone extracellular matrix cannot be produced vitamin C is needed for the synthesis or the production of collagen fibers which we know is the most prevalent protein fiber in the bone it forms the Bone's really strong structural support and vitamin D helps build bone by increasing the absorption of calcium from Foods in the gastrointestinal tract into the blood and we've already established that without adequate levels of calcium the body will not preference building bone over other activities such as the contraction of muscle tissue and heart tissue which require the presence of calcium during childhood the hormone most prevalent to or most important to bone growth and remodeling is the growth hormone growth hormone stimulates osteoblasts and it promotes cell division at the epipal plate so bones can get longer at puberty the secretion of hormones know as sex hormones causes a dramatic effect on bone growth the sex hormones include estrogen and testosterone which are primarily produced by the ovaries and the testes respectively these hormones are responsible for the increased osteoblast activity and the production of the bone extracellular matrix that causes the sudden growth spurt that we see in the teenage years sex hormones are also responsible for the closure of growth plates at the end of adolescence during adulthood sex hormones contribute to bone remodeling by slowing the resorption of own old bone and promoting the deposition of new bone following menopause in females estrogen levels decrease dramatically which puts older females and an increased risk of disease such as osteoporosis which is characterized by low bone density or lower amount of calcified extracellular matrix in that bone tissue we then have parathyroid hormone as we just spoke about and calcitonin which regulate the levels of calcium in the blood so parathyroid hormone is released when blood calcium levels drop it stimulates osteoclast activity breaking down the bone extracellular matrix to return the calcium to the blood calcitonin is released in response to high levels of calcium in the blood and it inhibits the activity of osteoclasts so that calcium remains in the bone weightbearing exercise or mechanical stress on a bone will also increase the rate of bone remodeling this process or principle is termed Wolf's Law which states that bone tissue will adapt to the mechanical loads placed upon it this law works on the principle that bone tissue is dynamic and constantly undergoing a process of remodeling and that the bone and that the tabula of spongy bone in particular will remodel over time in response to its mechanical demands so increased mechanical stress on a bone will stimulate Osteo blast activity and the deposition of new bone in areas experiencing the greater stress conversely reduced stress can lead to increased bone resorption so for example astronauts who go to space and spend long times uh long periods of time in no gravity have shown to have reduced bone density upon their return to Earth weight lifters show Improvement in bone density in response to training and the bones in the arms of tennis players in the arm that they hold a tennis bracket with have shown to have high bone density than that of their opposite arm