Anatomy of Bones + Calcium
Cholla Cactus Skeleton
Tigua, Piro, and Pueblo Land ABQ, NM
Bones
We tend to think of bones as hard, unchangeable fixtures in the body, but they’re not! Bones are porous structures that are constantly being built, broken down, and regenerated, depending on the body’s overall needs. There are a lot of factors at play, when it comes to bone health. Many of us learn early on that the bones make up the skeletal system, and are taught to view that system in isolation, when in fact numerous systems are working together. My aim here is to get you to view the body as a more holistic organism and have a basic understanding of how it works so you can be empowered and take a more active role in managing your health, particularly as it relates to proper nutrition and movement.
This short and sweet discussion looks at how the body manages calcium in relation to bone tissue. Please view this as a stepping stone for further study, and not a complete analysis of anatomy and physiology.
An Oversimplified Explanation of Bone Anatomy
The body’s bones are principally made of the protein collagen and the mineral calcium phosphate. The hard outer layer of bone is called the periosteum, that part of bone that muscles, tendons, and ligaments attach to. Underneath this tough layer is a city of blood vessels and lymphatic vessels. The inside of a bone is quite porous and where blood cells and platelets (for clotting) are made. For a quick and fun animated look inside, click here.
Bone has several important functions. It creates the structure muscles need to pull on to allow for movement. In addition to structure and movement, 95% of the body’s blood cells are created in the bone marrow—the soft spongy material inside bones. Bones also protect the major organs: the brain is encased in the skull, the heart, lungs, kidneys, other vital organs are protected within the ribcage, and the spine is encapsulated in backbone. Bone also plays a critical role in regulating the body’s calcium levels, which is what we’re discussing here.
In the Western model, the human body has 206 bones. The softest bone in the body is the clavicle (collarbone), the strongest is the femur (thigh bone). In the Eastern model, Charaka, author of one of ancient India’s key medical texts, claims there are 360 bones in the human body. This different perspective considers teeth, nails, and more as bones.
Major Players Involved in Calcium Regulation
Calcium: Mineral needed for building healthy bones and teeth.
Intestines: Stimulated by calcitrol/D3 to absorb calcium from digesting food.
Kidneys: Stimulated by PTH to preserve calcium from the urine and to release calcitrol (D3, a biologically active form of Vitamin D).
Osteoblasts: Cells that, when stimulated, absorb and break down bone tissue to release into the bloodstream in an effort to increase low calcium levels.
Osteoclasts: Cells that, when stimulated, build bone from calcium (and other minerals) removed from the bloodstream as a result of sufficient/high calcium levels in the body.
Ovaries: Produce and release estrogen. During perimenopause and menopause, the ovaries stop producing estrogen, which plays a role in inhibiting bone resorption. Low levels of estrogen stimulate the osteoclasts to utilize calcium in the bone to raise overall calcium levels, i.e., break down the bones and distribute calcium into the bloodstream.
Parathyroid gland: Regulates blood calcium levels and helps the body control levels of calcium entering the bloodstream from the bones, kidneys, and intestines. If it senses blood calcium levels are low, the chief cells of the parathyroid release PTH into the system.
Parathyroid hormone (PTH): Tells osteoclasts to increase overall blood calcium levels by breaking down bone tissue and releasing it into the bloodstream. Increases when blood calcium levels fall and decreases once they rise.
Thyroid gland: Parafollicular cells in the thyroid release calcitonin if there is a high concentration of calcium detected in the bloodstream. Calcitonin temporarily inhibits the osteoclasts (absorbers) and stimulates the osteoblasts (builders) to pull calcium from the blood and use it to build bone.
Vitamin D: D2 obtained from plants. D3 produced in the body when the skin is exposed to UVB radiation (we don’t “soak up” vitamin D, we make it!)
***
As you can see, a LOT of systems are involved in the regulation of calcium!
In short, the parathyroid and thyroid glands govern the process of regulating calcium in the body by sensing overall blood calcium levels. If those levels are perceived as too low, as when the ovaries are no longer producing estrogen in menopause or in the case of a diet deficiency, the chief cells of the parathyroid gland will release parathyroid hormone (PTH) to:
Inhibit the osteoblasts (bone builders) and stimulate the osteoclasts (bone absorbers) to increase blood calcium content by breaking down bone and sending it into the bloodstream;
Stimulate the kidneys to conserve calcium by drawing it out from the urine. PTH will also stimulate the kidneys to release calcitrol/D3 (the biologically active form of vitamin D), and then…
Calcitrol/D3 stimulates the intestines to absorb calcium from digesting food.
On the other hand, if the calcium levels are optimal or too high, the parafollicular cells in the thyroid release calcitonin. This hormone, in turn, flips the narrative by inhibiting the osteoclast cells (absorbers) and stimulating the osteoblast cells (builders) to remove calcium from the blood and get to work on building bone. If the body didn’t address excess calcium, there’d be a greater risk of kidney stones, lethargy, GI distress, and even heart problems.
***
While bones are often seen as hard, solid masses of tissue that form a rigid skeleton, they are much more than that. Bones are living tissues that are constantly at work building, remodeling, and creating. Taking into consideration bone health, particularly where it comes to aging and osteoporosis, should be a part of our everyday diet and lifestyle. Just as with so many health-related issues, what we eat and how we move matters.
Sources
https://www.britannica.com/science/bone-anatomy/Types-of-bone-formation
http://www.iamj.in/posts/2020/images/upload/4801_4805_1.pdf
https://oxsci.org/lymph-vessels-exist-within-bone/