BONES Anatomy, Physiology
The Dynamic Nature and Functions of Human Bone
Bones are active, dynamic organs made of living connective tissue that constantly breaks down, regenerates, and repairs itself throughout a person’s lifetime.
A human skeleton is entirely replaced via cellular processes approximately every to years.
Beyond providing structural scaffolding and support for the "squishy sack of flesh" that is the human body, bones serve several vital functions: - Mineral Storage: They store calcium phosphate and other essential minerals required for neuron firing and muscle contraction. - Hematopoiesis (Blood Cell Production): Approximately (one trillion) blood cells are generated daily within the bone marrow. - Energy Storage: Bone marrow helps store energy in the form of fat. - Homeostasis Maintenance: Bones help regulate blood calcium levels. - Hormone Production: Bones produce the hormone osteocalcin, which regulates bone formation and offers protection against glucose intolerance and diabetes.
Bone loss in space is severe: A person in microgravity suffers bone loss every month. By contrast, an average elderly person on Earth experiences only bone loss per year.
Over a year-long mission like that of Kelly and Kornienko, an astronaut could lose up to of their bone mass.
While much of this loss is reversible, rehabilitation on Earth is a long, arduous process compared to fictional solutions like "Madame Pomfrey’s Skelegro potion" and can take years of hard work.
Skeletal Classification by Location and Shape
The average human body contains bones, ranging from the tiny stapes in the inner ear to the large femur in the thigh.
Anatomists divide the skeleton into two primary groups based on location: - Axial Skeleton: Located along the body’s vertical axis. It includes the skull, vertebral column, and rib cage. These bones act as the body's foundation, carrying other parts and providing protection for vital organs. - Appendicular Skeleton: Comprises the limbs and the structures that attach them to the axial skeleton, such as the pelvis and shoulder blades. These bones are primary facilitators of movement.
Bones are further classified by their shape: - Long Bones: Classic dog-bone shaped structures that are longer than they are wide. Examples include the tibia and fibula of the lower legs, as well as the bones in the fingers. - Short Bones: Cube-shaped bones. The transcript identifies the foot's talus and cuboid, as well as the wrist’s lacunae and scapulae (noting the speaker’s specific categorization). - Flat Bones: Thinner bones such as the sternum and scapulae, and the bones comprising the brain case. - Irregular Bones: Specialized, uniquely shaped bones like the vertebrae and the pelvis.
Gross Anatomy and Internal Structure of Bone
Despite variations in size and shape, all bones share a similar internal architecture consisting of two main layers: - Compact (Cortical) Bone: A dense, smooth-looking external layer. - Spongy Bone: A porous, honeycomb-like internal area.
Spongy bone is made of tiny cross-hatching supports called trabeculae, which help the bone resist stress.
Bone marrow is housed within the spongy bone and comes in two types: - Red Marrow: Responsible for producing blood cells. - Yellow Marrow: Used for energy storage as fat (often a high-calorie source for predatory animals).
Variation in tissue arrangement: - In short, flat, and irregular bones, the structure resembles a "spongy bone sandwich on compact bone bread." - In long bones (e.g., femur, humerus), spongy bone and red marrow are concentrated at the flared ends, known as the epiphyses. - The shaft of a long bone is called the diaphysis, which surrounds a hollow medullary cavity filled with yellow marrow.
Microanatomy and Cellular Composition of the Skeleton
Under a microscope, bone tissue is revealed to be a complex system of layered plates and tunnels.
Osteons: The basic structural units of bone. They are cylindrical, weight-bearing structures running parallel to the bone's axis. Their cross-sections resemble tree trunk rings.
Lamellae: Concentric tubes within an osteon. They are filled with collagen fibers. Fibers in neighboring lamellae run in different directions, creating an alternating pattern that helps the bone resist torsion (twisting) stress.
Central Canals: Tunnels running along the length of each osteon that hold nerves and blood vessels to provide nourishment to the tissue.
Lacunae: Tiny oblong spaces tucked between lamellae layers.
Osteocytes: Mature bone cells housed within the lacunae. They act as "construction foremen," monitoring and maintaining the bone matrix and passing commands to other cells.
Osteoblasts: "Bone building" cells (from the Greek for "bone" and "germ/sprout"). They construct bone by secreting a glue-like cocktail of collagen and enzymes that absorb calcium phosphate and other minerals from the blood. - In the embryo, bone starts as cartilage. Osteoblasts use this as a framework to lay down a matrix that is approximately mineral and protein. - Bones typically continue to grow and harden until approximately age .
Osteoclasts: "Bone breakers" responsible for bone resorption. They work in tandem with osteoblasts to allow for bone regeneration.
The Process of Bone Remodeling and Space-Induced Degradation
Bone Remodeling: A process analogous to home renovation where old structures are removed before new ones are installed. It involves a balance between osteoclasts and osteoblasts.
The Process of Resorption: 1. Osteocytes detect stress, strain, or mechanical stimuli (e.g., the impact of running or the weightlessness of space). 2. Osteocytes release chemical signals to direct osteoclasts to the site of damage (e.g., a microscopic fracture in the femur). 3. Osteoclasts secrete a collagen-digesting enzyme and an acidic hydrogen ion () mixture to dissolve calcium phosphate. 4. Components are released back into the blood. 5. After the old tissue is cleared, osteoclasts undergo apoptosis (self-destruction). 6. Before terminating, they signal osteoblasts to begin rebuilding the bone.
Stimulated Strength: Carrying extra weight or exercising stimulates osteoclasts to break down bone so it can be remade stronger. Exercising builds both muscle and bone.
Space Flight Implications: - Astronauts must exercise at least hours per week in space, but this cannot fully prevent bone degradation. - In microgravity, osteocytes receive less "loading stimuli" due to the lack of weight. - For reasons not yet fully understood, microgravity causes osteoclasts to increase the rate of bone resorption while osteoblasts decrease bone formation. - This imbalance leads to a net loss of bone mass.
Introduction to Joints and Functional Classifications
Joints are the meeting places between two or more bones. There are more joints in the human body than bones because many bones are part of multiple joints (e.g., in the hands and feet).
Body movements occur when muscles contract across joints, moving one bone toward another.
Joints are classified by two methods: what they are made of (structural) and what they do (functional).
Functional Classifications: - Synarthroses: Non-moving joints, such as the sutures between the bones of the cranium. - Amphiarthroses: Slightly movable joints, such as the point where the two pubic bones meet in the pelvis. These absorb shock from walking and facilitate childbirth. - Diarthroses: Fully movable joints, primarily found in the limbs (e.g., knee and elbow joints).
The Structural Classification of Joints
Structural classifications are based on the material binding the bones together: - Fibrous Joints: Connected by dense fibrous connective tissue; mostly immovable (e.g., skull sutures). - Cartilaginous Joints: Bones united by cartilage; lack a joint cavity and move very little. - Synovial Joints: Freely movable joints that characterize most joints in the body. They enable activities like sports, yoga, and dancing.
Features of Synovial Joints: - Bones are separated by a fluid-filled joint cavity. - They contain synovial fluid, a viscous, egg-white-like substance that acts as a lubricant (grease on a hinge). - Without this fluid, friction from movement could wear out joint surfaces or even generate enough heat to "cook" surrounding tissue.
Synovial Joint Movements and Skeletal Physiology
Synovial joints come in six configurations allowing diverse movements: - Gliding (Plane) Joints: One flat bone surface glides over another. Examples include movements between the carpal bones of the wrist or the distal ends of the radius and ulna. Seen in simple hand-waving motions. - Angular Movements: Change the angle between two bones. - Flexion: Decreasing the joint angle (e.g., bending the arm). - Extension: Increasing the joint angle. - Hyperextension: Continuing extension beyond the normal anatomical position. - Abduction: Moving a limb away from the body. - Adduction: Moving a limb toward the body. - Circumduction: A combination of flexion, extension, abduction, and adduction that allows a limb (like the forearm) to move in a circle while the joint (elbow) stays stable. - Hinge Joints and Condylar (knuckle) joints facilitate these angular motions. - Rotational Movements: Turning a bone around its own axis. - Ball and Socket Joints: Found in the hip and shoulder; they allow high flexibility but are inherently unstable and prone to dislocation. - Special/Unique Movements: - Opposition: Touching the thumb to fingertips, facilitated by the saddle joint, making thumbs "infusible" (likely intended as "opposable"). - Supination: Rotating the forearm so the thumb moves anteriorly (forward). - Pronation: Rotating the forearm so the thumb moves posteriorly (backward). - Pivot Joints: Such as the one between the radius and ulna, allow for these rotations.
Details on the Axial and Appendicular Divisions
Axial Skeleton ( bones): - Skull: Composed of cranial and facial bones ( total). It protects the brain and facilitates sensory functions (sight, smell, hearing) and eating. - Vertebral Column: Consists of irregular bones. The top bone is the atlas vertebra, named after the Greek god Atlas. It provides central support for the upper body and protects the spinal cord. - Thoracic Cage: Composed of pairs of ribs and the sternum. It protects internal organs and provides attachment points for muscles in the back, chest, shoulders, and neck.
Appendicular Skeleton: - Includes the upper and lower limbs and the pectoral and pelvic girdles that attach limbs to the axial skeleton at the shoulder and thigh. - Limb arrangement follows a pattern common in many animals (evolutionary "one bone, two bones, lots of bones, and digits" setup).