Summary
Highlights
Dr. Locklin introduces the skeletal system, covering fundamental aspects such as calcium homeostasis and bone classification. He outlines the primary functions of the skeletal system: support, providing structural integrity; protection, particularly by the axial skeleton safeguarding vital organs; movement, facilitated by joints and an understanding of the appendicular skeleton; and hematopoiesis, the production of blood cells within red bone marrow.
The skeletal system plays a crucial role in storing fat and bone salts, primarily calcium and phosphorus. These salts form hydroxyapatite crystals, contributing to bone hardness and rigidity. Collagen fibers interwoven within the bone matrix provide toughness and elasticity, preventing shattering. It's emphasized that calcium stored in bones isn't solely for bone strength but serves as a crucial reserve for various bodily functions.
Calcium homeostasis, specifically maintaining blood calcium levels, is vital for preventing damage to essential physiological systems. The body, containing 2.2 to 4.4 pounds of calcium (99% stored in the skeleton), strictly regulates blood calcium between 9 and 11 mg per 100 mL of blood. This narrow range is critical because calcium ions are essential for muscle contractions, nerve impulses, neurotransmitter release, and enzymatic reactions.
When blood calcium levels drop (hypocalcemia), parathyroid hormone (PTH) is released from the parathyroid glands. PTH triggers stromal stem cells to produce RANKL, which activates osteoclast precursors into full-blown osteoclasts. These osteoclasts break down bone tissue, releasing calcium into the bloodstream. Additionally, PTH acts on kidneys to decrease calcium loss in urine and stimulates calcitriol production, promoting calcium absorption in the small intestines.
In cases of high blood calcium (hypercalcemia), calcitonin is released from thyroid gland cells. Calcitonin decreases osteoclast activity, thus reducing calcium release from bones into the blood. It also promotes increased calcium uptake by bones and accelerates calcium deposition, effectively moving excess calcium from the blood into bone tissue to restore balance.
The 206 bones in the human body are categorized into four types based on shape: long bones, short bones, flat bones, and irregular bones. All bones feature compact bone on the exterior for density and protection, and cancellous (spongy) bone internally for lightness and red bone marrow production.
Long bones, such as femurs or tibias, have a shaft (diaphysis) and swollen ends (epiphyses). Key structures include the periosteum (dense irregular connective tissue on the surface for ligament/tendon attachment), the endosteum (lining the internal cavities), and the medullary cavity within the diaphysis, which stores fat and makes the bone lighter. Articular cartilage (hyaline cartilage) covers epiphyses at joints, acting as a shock absorber. Epiphyseal plates (growth plates) are present in younger individuals, allowing bones to lengthen.
Short bones, like carpals in the wrist, are roughly cube-shaped. Flat bones, such as those in the skull (e.g., frontal, parietal), are broad and flat, resembling a spongy bone sandwich between layers of compact bone. Irregular bones, like vertebrae or the sphenoid bone, have complex shapes that don't fit into the other categories.
Beyond the 206 primary bones, some individuals may have sesamoid bones, which are rounded bones that develop in tendons subjected to stress (e.g., the patella, which everyone has, and others in hands/feet). Wormian bones are small, extra bones found within skull sutures, particularly the lambdoidal suture, and are highly genetic. These additional bones are not typically included in the count of 206.