Human Skeleton System UPSC & Other Exams

Ragini Chopra (MSc, Biological Sciences, Science Faculty UPSC)

Biology UPSC, Human Skeleton System UPSC

Human Skeleton System

The vertebrate body structure is supported by a skeleton. The skeletal system forms the framework of the human body, providing a rigid structure that:

• Supports the body and maintains its shape.
• Protects vital organs (e.g., brain, heart, lungs).

The human body’s structure is composed of interconnected connective tissues.

Connective Tissue: In the human body, connective tissue serves to connect one organ to another. It is a widespread group of tissues found in every organ. The primary functions of connective tissues are to combine, cover, and support organs, keeping them in the correct position.

1. Connective Tissues of Human Skeletal:

1.1 Bone

• Primary structural components
• Hard connective tissue
• Provides structural support
• Produces blood cells
• Stores minerals
• Thick and long bones contain hollow, pit-like spaces called marrow cavities. The substance found in these cavities is known as bone marrow. Bone marrow, inside bones, produces blood cells.
  • Red marrow (blood cell production – RBC, WBC and Platelets)
  • Yellow marrow (fat storage)
  • Note – In early embryonic development, the liver is the primary site of blood cell formation.
• Matrix: Bone is a solid, tough, and strong connective tissue composed of fibers and matrix. Its matrix is made of a protein called ossein and is rich in calcium and magnesium minerals. The strength of bones primarily comes from these mineral components. The bone matrix is structured in concentric rings called lamellae. Osteoblasts are bone-forming cell, osteoclasts resorb or break down bone (bone eating bone), and osteocytes are mature bone cells. An equilibrium between osteoblasts and osteoclasts maintains bone tissue.
• Bones Composition: Bone composition primarily consists of minerals like calcium phosphate and calcium carbonate (65%), collagen protein fibers (35%), and approximately 25% water, along with key elements such as calcium, phosphorus, and cellular structures.
• Bone Membrane: A double-layered membrane called the periosteum surrounds the bone.

1.2 Cartilage

• Flexible connective tissue
• Provides support, flexibility, and shock absorption at joints
• Covers bone ends
• Reduces friction
• Provides cushioning
• Matrix: Its matrix is primarily composed of proteins and becomes somewhat rigid due to the presence of calcium salts.

Bone vs Cartilage

Bone is a more complex, mineralized tissue with active blood cell production

Cartilage is a more flexible tissue with limited nutritional supply

1.3 Ligaments

• Connect bones to bones, stabilizing joints.
• Provides stability
• Limits excessive movement
• Elastic and strong

1.4 Tendons

• Connects muscle to bone
• Transmits muscular force
• Enables movement
• Made of tough collagen
• Muscles attached to bones via tendons enable movement. When muscles contract, they pull on the bones, causing joint movement.

2. Divisions of the Human Skeleton

The adult human skeleton usually consists of 206 named bones. These bones can be grouped in two divisions: axial skeleton and appendicular skeleton.

The 80 bones of the axial skeleton form the vertical axis of the body. They include the bones of the head, vertebral column, ribs and sternum (breastbone).

The appendicular skeleton consists of 126 bones and includes the free appendages and their attachments to the axial skeleton. The free appendages are the upper and lower limbs (extremities), and their attachments which are called girdles.

2.1 Appendicular Skeleton (126 bones)

Shoulder (Pectoral) girdles

• Clavicle (2)
• Scapula (2)

Pelvic Girdle

• Coxal, innominate, or hip bones (2)

Upper Limbs (Extremity)

• Humerus (2)
• Radius (2) – Front forearm
• Ulna (2) – Back forearm
• Carpals (16)
• Metacarpals (10)
• Phalanges (28)

Lower Limbs (Extremity)

• Femur (2)
• Tibia (2) – Front leg
• Fibula (2) – Back leg
• Patella (2) – knee bone
• Tarsals (14)
• Metatarsals (10)
• Phalanges (28)

2.2 Axial Skeleton (80 bones)

2.2.1  Skull (28)

Cranial Bones

• Parietal (2)
• Temporal (2)
• Frontal (1)
• Occipital (1)
• Ethmoid (1)
• Sphenoid (1)

Facial Bones

• Maxilla (2) – upper jaw – fixed
• Mandible (1) – the lower jaw – can move
• Zygomatic (2)
• Nasal (2)
• Platine (2)
• Inferior nasal concha (2)
• Lacrimal (2)
• Vomer (1)

Hyoid (1)

• Hyoid

Ear Bones (Auditory Ossicles)

MIS

• Malleus (Hammer) (2) — attached to the eardrum.
• Incus (Anvil) (2) — in the middle of the chain of bones.
• Stapes (Stirrup) (2) — attached to the membrane-covered opening that connects the middle ear with the inner ear (oval window).

2.2.2 Vertebral Column (26)

• Cervical vertebrae (7)
• Thoracic vertebrae (12)
• Lumbar vertebrae (5)
• Sacral (1) – (made up of 5 small joints)
• Coccyx (1)

2.2.3 Thoracic Cage (25)

• Sternum (1)
• Ribs (24)

3. Key Functions of the Skeletal System:

Support: The skeleton provides a rigid framework that supports the body and maintains its shape.  

Protection: It safeguards vital organs such as the brain (skull), heart and lungs (rib cage), and spinal cord (vertebral column).  

Movement: Bones act as levers, allowing movement in conjunction with muscles and joints.  

Blood Cell Production: Bone marrow within certain bones produces red and white blood cells, as well as platelets.  

Mineral Storage: Bones serve as a reservoir for essential minerals like calcium and phosphorus, which can be released into the bloodstream as needed.  

Endocrine Regulation: Bones play a role in hormone production and regulation.  

4. Types of Bones

Long Bones: Longer than they are wide, such as the femur (thigh bone) and humerus (upper arm bone).

Ulna, Radius, Tibia and Fibula

Short Bones: Roughly cube-shaped, like the carpals in the wrist and tarsals in the ankle.

Flat Bones: Thin and relatively flat, such as the skull bones and ribs.  

Irregular Bones: Have complex shapes, like the vertebrae and facial bones.

5. Types of Body Joints

Connections between bones that enable body movement and flexibility.

Allow skeletal parts to move

Connect different bone structures

Facilitate various motion ranges

Absorb shock

Fixed (Immovable) Joints

• Skull bones
• No movement
• Connected by tough connective tissue

Partially Movable Joints

• Spine vertebrae
• Limited movement
• Connected by cartilage

Freely Movable (Synovial) Joints

• Knee, shoulder, hip
• Allows wide range of motion
• Contains synovial fluid for lubrication

Main Joint Types:

• Ball and Socket (hip, shoulder)
• Hinge (elbow, knee)
• Pivot (neck rotation)
• Gliding (wrist)
• Saddle (thumb)

6. Types of Skeletons

There are two primary types of skeletons based on their location within the body:

Exoskeleton:

• An exoskeleton is located on the outer surface of the body.
• It originates from the embryonic ectoderm or mesoderm.
• The cuticle or dermis of the skin is often transformed into an exoskeleton.
• Exoskeletons provide protection for internal organs and are typically inert (non-living).
• Examples include scales in fish, the upper shell in turtles, feathers in birds, and hair and nails in mammals.
• They can also provide protection from environmental extremes such as cold and heat.

Endoskeleton:

• An endoskeleton is located inside the body.
• It originates from the embryonic mesoderm.
• Endoskeletons are characteristic of all vertebrates.
• They form the main structural framework of the vertebrate body.
• Endoskeletons are typically covered with muscles.

7. Common Skeletal Disorders

Osteoporosis: Decreased bone density, leading to increased fracture risk. Age-related disorder characterized by decreased bone mass density (BMD). Peak bone density typically occurs around age 30.

Osteoarthritis: Degeneration of joint cartilage, causing pain and stiffness.  

Rheumatoid Arthritis: Autoimmune disease causing inflammation in joints. RA is an autoimmune disease where the body’s immune system mistakenly attacks the lining of the joints, leading to inflammation, swelling, and stiffness.

Gout: Inflammation of joints due to accumulation of uric acid crystals.

Fractures: Breaks in the bone.

8. Important facts related to Bones

Longest Bone:

• Femur (thigh bone)
• Length: ~19 inches
• Located in leg

Shortest Bone:

• Stapes (ear bone)
• Length: ~3 millimeters
• In middle ear

Strongest Bone:

• Femur
• Can support 30x body weight
• Thick, dense structure

Weakest Bone:

• Clavicle (collar bone)
• Most frequently fractured bone
• Thin, S-shaped structure
• Vulnerable to breaking during impacts

Height:

Height is primarily determined by growth plates (epiphyseal plates) located at the ends of long bones. These growth plates are areas of cartilage where new bone tissue is formed through a process called ossification. During childhood and adolescence, these plates are actively producing new bone tissue, allowing for height increase.

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