In March of 2015, American astronaut Scott Kelly and his Russian colleague Mikhail Kornienko, started an unprecedented mission in house. They started a one-year time period of service aboard the Worldwide House Station, the longest tour of responsibility ever served on the ISS. Now, I think about there’s all kinds of stuff to fret about once you’re packing for a year-long house voyage, like, say, “What number of books ought to I deliver? What number of pairs of underwear? Am I actually okay with pooping right into a suctioned plastic bag day by day for a yr? Will I stumble upon a derelict ship haunted by some stranded and insane astronaut from a forgotten mission, like in just about each house horror film ever? Will there be espresso?” Affordable questions, all, however in actuality, one other one you may wish to ask is: “Will I be capable to stroll after I get again dwelling?” We all know micro-gravity is tough on a physique, and this mission is essentially about testing the bodily results of being weightless for therefore lengthy. Astronauts typically expertise issues like hassle sleeping, puffy faces, and lack of muscle mass, however maybe essentially the most critical injury a microgravity surroundings causes is to the bones. And bones, nicely, they’re fairly clutch. Although they might look all dried up and austere, don’t be fooled — your bones are alive. ALIVE I let you know! They’re really as dynamic as any of your organs, and are product of lively connective tissue that’s consistently breaking down, regenerating, and repairing itself all through your lifetime. In reality, you mainly get a complete new skeleton each 7 to 10 years! In brief, your bones do far more than simply offering your squishy sack of flesh with help and scaffolding and the flexibility to maneuver round. Your bones are mainly the way you retailer the calcium, phosphate, and different minerals you have to hold neurons firing and muscular tissues contracting. They’re additionally essential to hematopoiesis, or blood cell manufacturing. Your entire new blood — and we’re speaking like, a trillion blood cells a day! — is generated in your bone marrow, which additionally helps retailer power as fats. Bones even assist preserve homeostasis by regulating blood calcium ranges and producing the hormone osteocalcin, which regulates bone formation and protects in opposition to glucose intolerance and diabetes. So, the massive buzzkill about life in house is that, up there, an individual suffers one to 2 % bone loss EVERY MONTH. By comparability, your common aged particular person experiences 1-2 % bone loss each YEAR. So for Kelly and Kornienko, that would imply shedding as much as 20 % over a yr in orbit. Given every little thing your bones do for you, that’s actually critical. And whereas most of that loss is reversible as soon as they’re again on earth, it’s not as simple as chugging a few of Madame Pomfrey’s Skel-E-Gro potion. Rehabilitation can take years of arduous work, and that’s simply after a couple of months in orbit… Which is why Kelly and Kornienko are heroes of science, and never only for students of anatomy and physiology in every single place, however for anyone who has bones. A mean human physique accommodates 206 bones, ranging in form and measurement from the tiny stapes of the inside ear to the large femur of the thigh. That’s quite a lot of bones to maintain tabs on, so anatomists typically divide these constructions first by location, into both axial or appendicular teams. As you may guess, your axial bones are discovered alongside your physique’s vertical axis — in your cranium, vertebral column, and rib cage. They’re form of like your basis, the stuff you possibly can’t actually stay with out — they carry your different physique elements, present skeletal help, and organ safety. Your appendicular bones are just about every little thing else, the bones that make up your limbs, and the issues that connect these limbs to your axial skeleton, like your pelvis and shoulder blades. These are the bones that assist us transfer round. From there, bones are typically categorised by their form, and fortuitously these names are fairly apparent. Lengthy bones are your classic-looking, dog-bone-shaped bones — the limb bones which are longer than they’re huge, like tibia and fibula of your decrease legs, but in addition the trio of bones that make up your fingers. Observe a few of these lengthy bones to your foot or hand, and also you’ll hit a cube-shaped quick bone, like your foot’s talus and cuboid, or your wrist’s lacunate or scaphoid. Your flat bones are the thinner ones, like your sternum and scapulae, and likewise the bones that make up your mind case. And your irregular bones are all of the weirdly-shaped issues like your vertebrae and pelvis, which are typically extra specialised and distinctive. However regardless of their variations in measurement, form, and finer perform, all bones have an identical inner construction. All of them have a dense, smooth-looking exterior layer of compact, or cortical bone round a porous, honeycomb-looking space of spongy bone. This spongy bone tissue is made up of tiny cross-hatching helps known as trabeculae that assist the bone resist stress. And it’s additionally the place you usually discover your bone marrow, which is available in two colours, purple and yellow. Crimson marrow is the stuff that makes blood cells, so try to be glad that you’ve got a few of that. And yellow marrow shops power as fats — when you occur to be a predatory animal, yellow bone marrow might be top-of-the-line sources of energy you could find. The association of those bone tissues, although, might be barely completely different, from one kind of bone to the following. In flat, quick, and irregular bones, for instance, these tissues kinda appear like a spongy bone sandwich on compact-bone bread. However in a few of your basic lengthy bones, just like the femur and humerus, the spongy bone and its purple marrow are concentrated on the ideas. These flared ends, or epiphyses bookend the bone’s shaft, or diaphysis, which — as a substitute of having spongy bone within the heart — surrounds a hole medullary cavity that’s filled with that yellow marrow. Now, though bone can look rock-solid, seize a microscope and also you’ll see that it’s really loaded with layered plates and laced with little tunnels. It’s intricate and kinda complicated in there, however the extra you zoom into the microanatomy of bones, the higher you possibly can see how they’re constructed and the way they perform, proper right down to the mobile degree. Let’s begin with the fundamental structural items of bone, known as osteons. These are cylindrical, weight-bearing constructions that run parallel to the bone’s axis. Look inside one and also you’ll see that they’re composed of tubes inside tubes, so {that a} cross-section of an osteon appears to be like just like the rings of a tree trunk. Every considered one of these concentric tubes, or lamellae, is stuffed with collagen fibers that run in the identical course However when you examine the fibers of a neighboring lamella — both on the within or exterior of the primary one — you’ll see that they run in a distinct course, creating an alternating sample. This bolstered construction helps your bone resist torsion stress, which is like twisting of your bones, which they expertise quite a bit, and I encourage you to not think about what a torsion fracture of considered one of your bones may really feel like. Now, bone wants nourishment like another tissue, so working alongside the size of every osteon are central canals, which maintain nerves and blood vessels. After which, tucked away between the layers of lamellae are tiny rectangular areas known as lacunae. As tiny as they’re, these little gaps are the place the actual work of your skeletal system will get finished, as a result of they home your osteocytes. These are mature bone cells that monitor and preserve your bone matrix. They’re like the development foremen of your bones, passing alongside instructions to your skeleton’s two principal workhorses: the osteoblasts and the osteoclasts. Osteoblasts — from the Greek phrases for “bone” and “germ” or “sprout” — are the bone-building cells, and so they’re really what assemble your bones within the first place. Within the embryonic section, bone tissue typically begins off as cartilage, which gives a framework in your bones to develop on. When osteoblasts are available, they secrete a glue-like cocktail of collagen, in addition to enzymes that take in calcium, phosphate, and different minerals from the blood. These minerals type calcium phosphate, which crystallize on the cartilage framework, finally forming a bone matrix that’s about one-third mineral, two-thirds protein. Out of your time within the womb till you’re about 25, your osteoblasts hold laying down extra collagen and extra calcium phosphate, till your bones are totally grown and utterly hardened. So whereas your osteoblasts are the bone-makers, your osteoclasts are the bone-breakers — which is a form of violent picture. Possibly consider them as like a bone-breaker-downer. Though the 2 sorts of cells do actual reverse jobs, they’re not mortal enemies. In reality, I’m pleased to report that they get alongside fabulously, and create an ideal stability that permits your bones to regenerate. It’s like if you wish to renovate your own home, you’ve gotta rip out all these busted cupboards and the musty carpeting earlier than you possibly can deliver within the good hardwood flooring and customized counter tops. These cells work in a kinda related method, in a course of that I’d argue is much less worrying than dwelling enchancment — it’s known as bone transforming. The supervisors of this course of are these osteocytes, which kick issues off after they sense stress and pressure, or reply to mechanical stimuli, just like the weightlessness of house, or the influence of working on pavement. So, say you’re out working and one thing occurs — nothing to be alarmed about! — however out of the blue the osteocytes in your femur detect a tiny, microscopic fracture, and provoke the transforming course of to repair it up. First, the osteocytes launch chemical alerts that direct osteoclasts to the positioning of the injury. After they get there, they secrete each a collagen-digesting enzyme, and an acidic hydrogen-ion combination that dissolves the calcium phosphate, releasing its parts again into the blood. This tear-down course of is known as resorption. When the outdated bone tissue is cleaned out, the osteoclasts then endure apoptosis, the place they mainly self-destruct earlier than they will do any extra injury. However earlier than they auto-terminate, they use the hormone hotline to name over the osteoblasts, who are available and start rebuilding the bone. The ratio of lively osteoclasts to osteoblasts can fluctuate drastically, and when you stress your bones quite a bit, by damage, by carrying further weight, or simply regular train, these osteoclasts are going to be swinging their little wrecking balls continuous, breaking down bone so it may be remade. On this method, exercising stimulates bone transforming — and finally bone energy — so when you’re figuring out, you’re constructing bone in addition to muscle. Which brings us again to our two space-heroes-slash- guinea-pigs, Scott Kelly and Mikhail Kornienko. House crews typically have to train no less than 15 hours every week to decelerate the method of bone degradation, however even that may’t totally stave lack of bone density. In microgravity, osteocytes aren’t getting a lot loading stimuli, as a result of much less gravity means much less weight. However, for causes that we don’t perceive but, the osteoclasts really enhance their charge of bone resorption in low gravity, whereas the osteoblasts dial again on the bone formation. As a result of there’s extra bone breaking than bone making happening, every little thing is out of stability, and out of the blue folks begin experiencing 1 to 2 % month-to-month loss in bone mass. So, along with offering astronauts with oxygen and water and meals and safety from radiation and an surroundings that can hold them mentally steady, it seems that we even have to determine the right way to hold their our bodies from consuming their very own skeletons. However no less than in the present day we realized in regards to the anatomy of the skeletal system, together with the flat, quick, and irregular bones, and their particular person preparations of compact and spongy bone. We additionally went over the microanatomy of bones, significantly the osteons and their inside lamella. And at last we obtained an introduction to the method of bone transforming, which is carried out by crews of osteocytes, osteoblasts, and osteoclasts. Particular because of our Headmaster of Studying Thomas Frank for his help for Crash Course and free schooling. And thanks to all of our Patreon patrons who make Crash Course attainable by their month-to-month contributions. Should you like Crash Course and also you wish to assist us hold making cool new movies like this one, you possibly can try patreon.com/crashcourse This episode was co-sponsored by The Midnight Home Elves, Fatima Iqbal, and Roger C. Rocha Crash Course is filmed within the Physician Cheryl C. Kinney Crash Course Studio. This episode was written by Kathleen Yale, edited by Blake de Pastino, and our marketing consultant, is Dr. Brandon Jackson. Our director is Nicholas Jenkins, the editor and script supervisor is Nicole Sweeney, our sound designer is Michael Aranda, and the graphics group is Thought Café.