Introduction

The skeletal system physiology focuses on the functioning of bones, joints, and supporting tissues that maintain posture, facilitate movement, protect vital organs, and serve as a storehouse for minerals like calcium and phosphorus. Bones are not lifeless structures — they are dynamic, living tissues that continuously remodel throughout life.

This chapter explains how bones grow, maintain themselves, adapt to mechanical stress, and repair damage. Understanding bone physiology helps NHPC students grasp the basis of orthopedic diseases, fractures, metabolic disorders, and calcium imbalance in clinical practice.

[Diagram Placeholder: Overview of skeletal physiology – growth, remodeling, repair, and mineral regulation]

1. Bone Growth and Development

Bone growth begins even before birth and continues until early adulthood. The process through which bones are formed is called ossification or osteogenesis.

There are two main types of ossification:1. Intramembranous Ossification: Formation of flat bones (like skull and clavicle) directly from mesenchymal tissue.2. Endochondral Ossification: Formation of most long bones (like femur, humerus) from a cartilage model.

[Diagram Placeholder: Endochondral ossification process – cartilage model replaced by bone]

Stages of Endochondral Ossification

1. Cartilage Model Formation: During fetal development, mesenchymal cells differentiate into chondrocytes, forming a cartilage model.2. Bone Collar Formation: Osteoblasts form a thin layer of bone around the cartilage.3. Primary Ossification Center: Blood vessels invade the cartilage in the diaphysis (shaft), bringing osteoblasts that replace cartilage with bone.4. Secondary Ossification Center: Develops in the epiphyses (ends of bones) after birth.5. Epiphyseal (Growth) Plate: A layer of cartilage that allows bone lengthening during childhood and adolescence.6. Epiphyseal Plate Closure: Around 18–25 years, cartilage is replaced by bone, ending bone growth in length.

Exam Focus: NHPC often asks where bone growth in length occurs – it’s at the epiphyseal plate (growth plate).

2. Bone Remodeling

Bone remodeling is a lifelong process of bone renewal involving resorption (breakdown) and formation (rebuilding). This process ensures that bones remain strong and adaptable to stress.

Phases of Bone Remodeling:1. Activation: Osteoclasts are activated to resorb (break down) bone tissue.2. Resorption: Osteoclasts dissolve bone matrix, releasing calcium and phosphate into the bloodstream.3. Reversal: Osteoclasts undergo apoptosis (death), and osteoblasts are recruited.4. Formation: Osteoblasts lay down new bone matrix and mineralize it.5. Quiescence: Bone surface becomes inactive until the next remodeling cycle.

Did You Know? About 10% of your bone mass is remodeled every year!

Bone remodeling is influenced by hormones, physical activity, nutrition, and age. Weight-bearing exercises stimulate bone formation, while inactivity leads to bone loss.

3. Bone Repair (Fracture Healing)

When a bone breaks, the body initiates a well-coordinated healing process that involves several stages.

Stages of Fracture Healing

1. Hematoma Formation (0–3 days): Blood vessels rupture, forming a clot (hematoma) around the fracture. This brings inflammatory cells.2. Fibrocartilaginous Callus Formation (3–14 days): Fibroblasts and chondrocytes produce collagen and cartilage to bridge the gap.3. Bony Callus Formation (2–6 weeks): Osteoblasts replace cartilage with spongy bone.4. Bone Remodeling (Months): The bone reshapes itself to restore its original structure and strength.

[Diagram Placeholder: Stages of fracture healing]

Clinical Insight: Healing is slower in older adults and in cases of poor nutrition, infection, or diabetes.

4. Hormonal Regulation of Bone Metabolism

Bone physiology is tightly regulated by several hormones that control calcium and phosphate balance, bone growth, and remodeling.

1. Parathyroid Hormone (PTH):- Secreted by the parathyroid glands.- Increases blood calcium by stimulating osteoclasts to break down bone.- Enhances calcium reabsorption in kidneys and activates vitamin D.

2. Calcitonin:- Secreted by the thyroid gland.- Lowers blood calcium by inhibiting osteoclast activity.

3. Vitamin D (Calcitriol):- Increases calcium absorption from the intestine.- Promotes bone mineralization.

4. Growth Hormone and Sex Hormones:- Growth hormone stimulates bone lengthening.- Estrogen and testosterone promote bone strength but also regulate growth plate closure.

Exam Focus: PTH increases blood calcium; Calcitonin decreases blood calcium — remember this contrast for MCQs!

5. Calcium and Phosphorus Balance

Calcium and phosphorus are the two major minerals stored in bone. Around 99% of body calcium and 85% of phosphorus are in bones and teeth.

Functions of Calcium:- Muscle contraction- Nerve impulse transmission- Blood clotting- Enzyme activity

Functions of Phosphorus:- Component of ATP (energy molecule)- Forms part of DNA and RNA- Buffers blood pH

Homeostasis: When blood calcium drops, PTH increases calcium release from bones. When calcium is high, calcitonin reduces it.

[Diagram Placeholder: Calcium homeostasis feedback loop between bones, kidneys, and intestine]

6. Factors Affecting Bone Health

Several internal and external factors influence bone growth, density, and strength.

1. Nutrition: Adequate intake of calcium, phosphorus, and vitamin D is vital.**2. Hormones:** Growth hormone, thyroid, and sex hormones regulate growth and density.**3. Physical Activity:** Weight-bearing exercise strengthens bones.**4. Age:** Bone mass peaks in early adulthood and decreases with age.**5. Disease:** Conditions like rickets, osteoporosis, and osteomalacia weaken bones.

Clinical Tip: Sunlight exposure helps the body make Vitamin D — essential for calcium absorption and bone strength.

7. Disorders Related to Bone Physiology

1. Osteoporosis: Loss of bone density, common in postmenopausal women due to reduced estrogen.2. Rickets: Soft bones in children caused by vitamin D deficiency.3. Osteomalacia: Soft bones in adults due to defective mineralization.4. Hypercalcemia: Excess blood calcium due to overactive parathyroid glands.5. Hypocalcemia: Low calcium leading to muscle cramps and spasms.6. Paget’s Disease: Abnormal bone remodeling causing deformities.

8. Bone and Exercise

Physical activity plays a major role in maintaining bone health. Weight-bearing exercises such as walking, running, and resistance training stimulate osteoblast activity and increase bone mass.

Did You Know? Astronauts lose bone mass in space because of zero gravity — bones need stress to stay strong!

9. Effects of Aging on the Skeletal System

With age, bone remodeling slows down. Osteoblast activity decreases, leading to reduced bone density and brittle bones. Cartilage also becomes thinner and less elastic, contributing to joint stiffness.

Consequences of Aging:- Decreased bone mass and height.- Increased fracture risk.- Delayed healing after injury.- Degenerative joint diseases (like osteoarthritis).

Summary Table

| Process | Main Cells | Hormones Involved | Key Function ||----------|-------------|------------------|---------------|| Bone Formation | Osteoblasts | Growth hormone, Estrogen | Builds bone || Bone Resorption | Osteoclasts | PTH | Releases calcium || Mineralization | Osteocytes | Vitamin D, Calcitonin | Hardens matrix || Remodeling | Osteoblasts + Osteoclasts | PTH, Estrogen | Maintains bone health |

Conclusion

The skeletal system is not just a rigid framework but a living, dynamic organ system. Its physiology involves constant interaction between cells, hormones, minerals, and mechanical forces. For NHPC students, understanding these processes helps explain how the body maintains calcium balance, repairs injuries, and adapts to stress — forming the foundation for clinical practice in orthopedics and general medicine.