okay in this video we talk about bone remodeling so about five to seven percent of your bones mass is recycled each week in fact spongy bones replaced every three to four years and compact bones replace every ten years so if you think about yourself ten years ago you had a different compact bone then and your spongy bones room have been replaced several times since then as well now bone remodeling consists of both resorption and deposit remember bone deposit is mediated by osteoblasts and bone resorption is mediated by osteoclasts so we're talking about the building up and the breaking down of both now these occur at the surfaces of periosteum and endosteum and you find that these osteoclasts and blasts are located nearby and what we call remodeling units which are basically packets of blasting clasts that are nearby and they coordinate the remodeling process so bone deposit is when new bony matrix is deposited by osteoblasts we find that osteoblasts create new osteoid and we see that there's a band of unknown a matrix near these osteoblasts we find them too is that these eschewed seam gives rise to a calcification front where this is actually mineralized osteoid that had been created or laid down you know at some time prior but is now calcified because of the calcium binding proteins and collagen fibres that trap those calcium minerals now the trigger for bone deposit is confirmed and can include things like mechanical signals increased concentrations of calcium and phosphate matrix proteins that bind to calcium as well as appropriate amounts of alkaline phosphatase for mineralization now resorption is the breakdown of bone and this is a function mediated by osteoclasts we find that osteo classes to actually dig depressions or grooves as they break down the matrix and they break down the bony matrix by secreting lysosomal enzymes and protons onto the matrix that actually digest that material so acidity converts the calcium salts into soluble forms that can dissolve and then float away from bone and dissolve throughout the fluid of your body including the Seiler fluid and blood plasma so osteo class also figgus a toast demineralized matrix and dead osteocytes these digested products are trans side toast across the cell and released into the interstitial fluid then into blood and once resorption is complete osteo class undergo apoptosis or programmed cell death so these osteoclasts don't live very long however their activity is under hormonal control so we find that osteo class activation involves parathyroid hormone as well as immune T cell proteins that can stimulate osteoclasts to become more active so we find that remodeling occurs continuously but is regulated by genetic factors as well as you know hormonal factors so the hormonal controls of remodeling include negative feedback loops that regulate blood calcium levels so you find that if blood calcium levels are too high then that's going to stimulate bone deposit where calcium store within phone or if blood calcium levels are too low this is gonna stimulate bone resorption that way you can release some of the calcium from blood bone into the blood stream to your particular blood calcium levels back up now calcium functions and many important processes which is why it's one of those things that needs to be tightly homeostatic be regulated and we find that calcium is significant for nerve transmission muscle contraction coagulation of blood and nerve secretions as well as glandular secretions and cell division so it's extremely important in all these essential roles we find that most of the calcium of body like Dynamis nth of it is actually found in bone the rest of it is in solutions our body or in cells an intestinal absorption of calcium requires vitamin D so vitamin D is a hormone that stimulates the intestinal cells to be able to absorb calcium from the food you eat now the second thing that also controls remodeling is actually mechanical stress so we'll come back to mechanical stress a little bit but the other hormonal controls are remodeling include parathyroid hormone and calcitonin parathyroid hormones produced by the parathyroid glands just behind your thyroid gland on your neck and it's parathyroid hormone or PTH stimulates Ostia to resorb bone or break it down there for calcium is released in blood and it raises your blood calcium levels up once your blood calcium levels are at a normal limit parathyroid hormone secretion stops which is why it's a negative feedback loop calcitonin is the opposite in fact calcitonin is actually released by your thyroid gland also on your neck but these are released by the pair of follicular cells and these are released in response to high levels of calcium the effects are typically negligible but at high pharmacological doses it can lower blood calcium levels temporarily so calcitonin is synthesized and secreted by our bodies and it's been shown to lower blood calcium levels by stimulating bone deposit so you're removing calcium from blood and basically storing it in bone so this is showing this nice little negative feedback loop of blood calcium homeostasis we find that we want to maintain about nine to eleven milligrams per hundred mils in our body and if you get outside of this normal limit like if your blood calcium levels get too low your parathyroid glands can I believe parathyroid hormone which travels in your bloodstream and stimulates osteoclast to become more active once these osteoclasts are more active they can resorb more bone breakdown the matrix release that calcium in your blood and then get your blood calcium levels back up once calcium homeostasis is returned to like normal physiological limits then we find that parathyroid hormone released stops so you stopped stimulating osteoclasts so other hormonal controls of bone remodeling include things like leptin and serotonin leptin is actually a hormone released by adipose tissue but it plays a role in bone density regulation by inhibiting osteoblasts so you find that if someone's leptin levels are too high for extended period of time and you're going to inhibit bone deposits which can lead to things like osteoporosis now serotonin is a neurotransmitter that regulates mood sleep but it's also something that can interfere with osteoblast activity so most of their body serotonin is made by your gut neurons and it's actually involved with coordinating digestive activities but it's secreted in the blood after a meal so a meal after a meal serotonin levels rise in the blood and it may inhibit bone turnover after me so bone calcium is locked when new calcium is flooding in the bloodstream so if your serotonin levels are too high it actually prevents calcium from being you know essentially released from bone now other factors that can affect remodeling aren't just hormones but also responses to mechanical stress in fact bones will reflect the stresses that they encounter so that bones that are stressed when weight bears on them and muscles pull on them and essentially they're gonna regrow and remodel in a way that reflects the type of stress that's been introduced to that bone in fact we call this Wolf's law so Wolf's law tells us that bones grow and remodel in response to the demands placed on them stress is usually off center so bones tend to bed bending compresses to one side and stretches on the other side so the diathesis will be thicker on the side where the bending stresses are the greatest which makes sense because you want the bone to be able to resist that stress and the bone can be hollow because more compression and tension cancel each other out in the center of that bone so we find is that the medullary cavity stresses and tensions cancel out which explains why there is no spongy bone in the medullary cavity so this show an example of stress that's placed on your femur so because of your body's weight here you find that the lines of stress in your femur kind of at an angle and you see this compression force on the medial side and tension force in the lateral side but these these stresses in tensions cancel out in this midpoint here which explains why there's no spongy bone there you have a medullary cavity because there's nothing to resist here however the compact bone in these regions is much thicker in order to resist these tension forces in compression forces that are placed on the bone in fact the more body weight and the more stress you place on this bone the thicker those bones are going to get in order to resist those stresses so Wolf's law explains the hardness of bone so you find that with handedness the the hand they use the most whether you're right-handed or left-handed results in thicker and stronger bones that correspond with that hand so right-handed individuals will have thicker bones on the right side of the body because they're more likely to use their right hand for a lot of tasks now curved bones are thickest where they're most likely to buckle trabeculae or basically the spongy bone and these bones can form trusses along lines of stress enlarged bony projections occur where heavy active muscles attach in fact when muscles get larger bones get larger where those muscles pull so weight lifters have enormous thickening zet muscle attachment sites and this is useful for muscles because they have more bone to attach to and the bones can actually resist those stresses placed on them now the bones of a fetus and bedridden are usually featureless because they lack stress on the bones and the bones didn't grow or remodel in response to that particular stimulus so they're practically featureless now mechanical stresses cause remodeling by producing electrical signals from the bones that formed we'll go into more detail on what these electrical signals are when we get to the nervous system but we know that the movement of fluids with lots of ions and it creates a separation of charge and these oppositely charged regions are electrical signals that osteocytes can use to help stimulate remodeling so you find that fluid flows within canaliculi this can stimulate remodeling in a way that allows for the bones to grow and remodel and to basically resist the stresses placed on them you know hormonal controls determine weather and wind remodeling occurs and this can also affect blood calcium levels but mechanical stresses determine where it occurs you know specifically where in the bone will that bone get thinner or thicker and it's based on the mechanical stresses or demands placed on it