2 Fisiologia y Mineralización Ósea

Understanding Bone Physiology and Mineralization

Structure of Bone

• The bone is composed of several layers, starting with the periosteum, which has two distinct layers: an outer fibrous layer and an inner cambium layer responsible for new bone formation.

• Beneath the periosteum lies cortical bone, which constitutes approximately 80% of total bone volume, followed by the endosteum that covers the internal surface of cortical bone. The remaining 20% is made up of trabecular or spongy bone.

Biological Activity in Bone

• The surface of bones is crucial for biological reactions related to both formation and resorption processes. This activity occurs predominantly in trabecular (spongy) bone due to its larger surface area compared to cortical bone despite its lower volume percentage.

• Trabecular bone's structure allows for greater biological activity because it provides more contact area for cellular interactions involved in remodeling processes.

Cellular Composition of Bone

• Bone consists of a specialized connective tissue with a mineralized matrix housing various cells: osteoblasts (bone-forming cells derived from fibroblasts) and osteoclasts (monocyte-derived cells responsible for bone resorption).

• Osteoblast activity leads to new bone formation but requires prior resorption by osteoclasts; this interplay is essential during the remodeling process where osteoblasts become encased in newly formed matrix and transform into osteocytes.

Components of Bone Matrix

• The organic component primarily consists of collagen fibers (95% type I), which are arranged laminar in mature bones, while immature bones exhibit a plexiform structure that matures over time. Additionally, there are proteoglycans like chondroitin sulfate and proteins such as osteocalcin present in smaller amounts.

• Inorganic components include calcium (95% of minerals), phosphorus, magnesium, fluoride, sodium, potassium, and zinc—each playing vital roles in maintaining physiological functions within the skeletal system. Understanding calcium metabolism is particularly important for grasping overall bone physiology.

Importance and Functions of Calcium

• Calcium serves multiple critical functions including skeletal growth and maintenance; without adequate calcium levels, new bone production would be impaired significantly. It also plays a role in neuromuscular excitability necessary for muscle contraction and heart rhythm regulation.

• Beyond structural roles, calcium contributes to enzymatic functions essential for blood coagulation as part of various clotting factors; it interacts with hormones like parathyroid hormone and calcitonin influencing its metabolic pathways within the body.

Calcium Homeostasis

• Only about 0.5% to 1% of total body calcium can exchange with plasma; serum calcium concentration typically ranges between 8.5 to 10.5 mg/dL—a critical range indicating normal physiological function where deviations can lead to conditions like hypocalcemia or hypercalcemia that may threaten life if severe enough.

• Serum calcium exists in three fractions: diffusible ionized fraction (47%, biologically active), complexed fraction (6%, bound to phosphates), and non-diffusible fraction (47%, bound mainly to albumins). A typical daily dietary intake should be between 600–1000 mg depending on individual needs for optimal health outcomes related to skeletal integrity and function.

Calcium Absorption and Dietary Impact

Understanding Calcium Absorption

• The absorption of calcium from the diet is only between 30% to 40%, with more effective absorption occurring in the duodenum, but larger quantities in the ileum due to its size.

• Calcium absorption can occur through facilitated diffusion, where a high concentration within the intestine allows for easier movement into the bloodstream.

Role of Vitamin D3

• Active transport of calcium relies on vitamin D3 (1,25-hydroxycholecalciferol), which sends proteins to bind calcium in the intestine and facilitate its entry into blood circulation.

• Despite dietary intake, actual calcium absorption remains limited due to various factors affecting excretion and bioavailability.

Dietary Influences on Calcium Excretion

• Daily digestive secretions contribute approximately 200 mg of calcium; however, dietary choices significantly influence fecal excretion rates. Omnivorous diets lead to varied mineral intake from both plant and animal sources.

• High fiber diets can increase fecal calcium excretion, leading to reduced overall absorption; this is particularly relevant for women consuming packaged high-fiber cereals aimed at weight management.

Controversies Surrounding Dairy Consumption

• There has been debate regarding cow's milk as a source of absorbable calcium; lactose present in milk may actually enhance digestive secretion and thus improve calcium absorption despite common misconceptions.

• Adequate daily requirements for adults are estimated between 500 mg to 600 mg, but increased intake (800 mg to 1000 mg) is recommended due to dietary factors that elevate calcium elimination rates.

Special Considerations for Certain Populations

• Specific groups such as growing children, pregnant women, lactating mothers, menopausal women, and individuals over 60 years old may require an additional 200 mg of calcium per day beyond their regular diet. This supplementation can include pharmacological sources if necessary.

Hormonal Regulation of Calcium Levels

Calcitonin Functionality

• Calcitonin is produced by C cells in the thyroid gland and functions by reducing bone-derived calcium release into circulation; bones act as reservoirs for stored calcium essential for bodily functions.

Parathyroid Hormone Dynamics

• In cases of hypocalcemia (calcium levels below normal), parathyroid hormone (PTH) increases bone resorption releasing more calcium into circulation while also enhancing renal reabsorption processes to minimize urinary loss.

Mechanisms of Action

• PTH acts on three key areas: kidneys (enhancing tubular reabsorption), bones (mobilizing stored calcium), and intestines (stimulating vitamin D3 synthesis). This triad helps normalize serum calcium levels effectively through multiple pathways including renal conservation and intestinal absorption enhancement.

Understanding Calcium and Phosphorus Regulation in the Body

Hormonal Regulation of Calcium Levels

• In cases of hypercalcemia, the pituitary gland signals the parathyroid glands to reduce parathyroid hormone (PTH) production.

• The increase in calcitonin from thyroid C cells counteracts PTH effects, leading to decreased calcium mobilization from bones and reduced renal absorption of calcium.

• This hormonal interplay results in normocalcemia by lowering active vitamin D3 synthesis, which decreases intestinal calcium absorption.

Importance of Phosphorus in the Body

• An adult body contains approximately 700 grams of phosphorus, with 85% stored in bones; it plays a crucial role in cellular functions and energy metabolism.

• Phosphorus is vital for cell membranes (as phospholipids), DNA structure, enzyme regulation, and ATP energy transfer processes.

Dietary Intake and Absorption of Phosphorus

• Normal dietary intake provides about 1400 mg/day of phosphorus; deficiencies are rare except in patients on parenteral nutrition.

• About 60% of phosphorus is absorbed through passive diffusion or active transport stimulated by vitamin D3.

Plasma Phosphorus Levels

• Normal plasma phosphorus levels range from 2.2 to 4.4 mg/dL; levels can be higher in children but normalize into adulthood.

• Daily fecal elimination averages around 500 mg due to low physiological requirements for phosphorus.

Response to Low Serum Phosphorus

• Hypophosphatemia triggers increased synthesis of active vitamin D3, enhancing intestinal absorption and mobilization from bones while inhibiting further PTH release.

• Renal mechanisms for regulating serum phosphorus take longer than 48 hours to adjust independently from PTH actions.

Magnesium's Role in Bone Health

• Approximately 50% of magnesium is found in bone; it is not interchangeable with extracellular fluid but plays a significant role alongside calcium and phosphorus.

Magnesium and Its Importance in Nutrition

The Role of Magnesium in the Body

• Magnesium balance is influenced by dietary intake, intestinal absorption, and renal excretion. It remains unclear if parathyroid hormone (PTH) regulates magnesium levels.

• Processed foods, caffeine, and excessive sugar can lower magnesium levels. For instance, 287 molecules of magnesium are required to metabolize one glucose molecule.

• Foods high in phytic acid (e.g., sesame seeds, peas, barley) hinder magnesium absorption. Certain medications like diuretics and insulin also affect magnesium levels negatively.

Effects of Magnesium Deficiency

• Individuals who exercise heavily may experience reduced magnesium levels; thus, they should ensure adequate intake for calcium and vitamin C assimilation.

• Magnesium has a calming effect on the central nervous system and aids in nerve impulse transmission. A deficiency can lead to insomnia or anxiety.

• Magnesium plays a crucial role in ATP production and protein synthesis; it helps maintain potassium concentration within cells.

Health Implications Related to Magnesium

• Adequate magnesium increases synovial fluid viscosity, essential for joint health; low viscosity can lead to cartilage degradation and osteoarthritis.

• Recommended daily intake ranges from 300 to 350 mg. Absorption rates vary between 30% to 60%, depending on diet composition.

Factors Affecting Magnesium Absorption

• Acidic pH enhances magnesium absorption; hence it's advised to take supplements on an empty stomach when gastric pH is low.

• The intestines regulate magnesium absorption based on bodily needs while kidneys manage its elimination.

Symptoms of Magnesium Deficiency

• Mild deficiency may cause symptoms like anorexia, confusion, fatigue, irritability, or muscle twitching. Severe deficiency could result in cardiovascular issues or hallucinations.

Bone Physiology: Modeling and Remodeling

Understanding Bone Growth

• Bone modeling involves changes during growth that maintain its characteristic shape; bones originate from either membranous or cartilaginous molds.

Mechanisms of Bone Growth

• Longitudinal bone growth occurs at the growth plate (fisis), where new bone forms. However, bones must also grow in width for structural integrity.

Structural Changes During Growth

• Within the endosteum (inner layer), old bone is resorbed while new bone is added at the periosteum (outer layer), allowing for increased thickness as well as medullary cavity expansion.

Bone Growth and Remodeling Processes

Understanding Bone Growth

• Bone growth involves not only the increase in size of the bone but also the expansion of the marrow. The process of remodeling ensures that bones are constantly renewed, meaning that the bone you have now is different from what you had ten years ago.

Importance of Remodeling

• Continuous stress on bones, like bending a metal rod, can lead to fatigue and fractures. For example, the femur bears significant weight and undergoes angular loads; thus, remodeling is essential to prevent breaks.

Coupling in Bone Remodeling

• Coupling refers to the sequential action between osteoclasts (which absorb bone) and osteoblasts (which form new bone). This interaction forms a remodeling unit consisting of one osteoclast and one osteoblast.

Mineralization Process

• Newly formed bone does not immediately mineralize; it takes about 10 days for this process to occur after formation. The transition zone between mineralized and unmineralized bone is known as the mineralization front.

Turnover Rate and Balance

• The turnover rate indicates how much bone is renewed over time, influenced by the number of active remodeling units. A balance occurs when destruction equals formation—this equilibrium is crucial for maintaining healthy bones.

Bone Balance Throughout Life

Positive Bone Balance in Youth

• In childhood, there’s a positive balance where more bone is formed than destroyed. This phase continues until skeletal maturity.

Equilibrium Phase

• From ages 20 to 40, there’s an equilibrium where bone formation matches destruction—resulting in a zero balance.

Negative Balance Post-Age 40

• After age 40, especially post-menopause for women, there’s a negative balance with annual losses around 6% to 7%. Women experience accelerated loss (up to 3% annually) during menopause before stabilizing at similar rates as men.

Calcium Storage and Osteoporosis

Calcium Bank Concept

• It’s vital for individuals under 40 to build calcium reserves in their bones since future losses will occur. Waiting until later life to take calcium supplements may not be effective due to absorption issues.

Osteoporosis Visualization

• Osteoporosis can severely degrade vertebral structures resembling honeycombs due to excessive loss of density—a critical concern as we age.

Factors Influencing Bone Remodeling

General Factors Affecting Remodeling

• Hormones such as parathyroid hormone (PTH), vitamin D3 metabolites, and calcitonin play significant roles in calcium absorption and retention within bones.

Specific Influences During Menopause

• Estrogens significantly impact calcium levels; their decline during menopause leads to increased calcium loss from bones. Other factors include thyroid hormones and glucocorticoids which should be used cautiously due to their effects on calcium levels.

Mechanical Stress Effects on Bones

Piezoelectric Effect

• Mechanical overload induces piezoelectric effects where mechanical energy converts into electrical energy within bones. This change enhances calcium absorption through physical activity—emphasizing exercise's importance for maintaining strong bones throughout life.

Impact of Space Travel on Astronauts' Bone Health

Calcium Loss in Microgravity

• Astronauts experience significant calcium loss during extended space missions due to the near-zero gravity environment, which can lead to multiple fractures upon return to Earth.

• To counteract this calcium deficiency, astronauts must undergo a quarantine period with a calcium-rich diet to restore their bone health.

Bone Healing Process

• The healing process for any tissue, including bones, follows specific stages: trauma (injury), bleeding, hematoma formation, and coagulation leading to new bone development.

• The initial steps involve blood accumulation forming a hematoma that organizes into a clot where osteoblasts deposit and begin forming new bone tissue.

Stages of Bone Consolidation

• After the formation of the hematoma and subsequent clotting, the process known as "consolidación ósea" occurs, where new bone is formed and later remodeled.

• Remodeling is crucial as it shapes the newly formed bone into its original structure while also creating a medullary canal.

Factors Influencing Tissue Healing

• Two key types of factors are involved in tissue healing: mitogenic factors (cell proliferation through mitosis) and morphogenic factors (cell differentiation into various tissues like cartilage or blood vessels).

• Examples of mitogenic factors include PDGF (Platelet-Derived Growth Factor) and FGF (Fibroblast Growth Factor), which play roles in cellular multiplication.

Understanding Morphogenic Factors

• Morphogenic factors are less critical for memorization but include growth factors that assist in differentiating cells necessary for forming various tissues during healing processes.

• These growth factors contribute significantly to vascular formation and overall tissue regeneration following injury.