GenEd: Lesson 13 Skeletal System | Free LET Reviewer and Drill

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General Education

 Lesson 13: Skeletal System

(Human Systems)  

What is the Skeletal System?

The skeletal system is a complex framework of bones, cartilage, ligaments, and tendons that provides structure and support for your entire body. The skeletal system is the structural foundation that gives your body shape and allows movement. It protects your vital organs, produces blood cells, and stores important minerals like calcium and phosphorus. Without the skeletal system, your body would collapse into a shapeless mass.

Your body's framework of bones that holds you up, protects your organs, and makes movement possible.

Main Functions of the Skeletal System

1. Protects and supports body parts
2. Enables movement
3. Produces blood cells in bone marrow
4. Stores calcium and phosphorus
5. Stores fat for energy

Adult VS Infant Skeleton

Image from nemfrog

Adult Human Skeleton:

• 206 bones in an adult human
• About 360 joints where bones connect
• Hundreds of muscles attached to bones
• Cartilage at the ends of bones
• Ligaments connecting bones to bones
• Tendons connecting muscles to bones

Infant Skeleton:

• Babies are born with about 270 bones
• Many of these are made of soft cartilage
• As the baby grows, cartilage is replaced by hard bone through ossification
• By adulthood, most cartilage has been replaced by bone, leaving 206 bones

An adult has 206 bones that form the skeleton, which supports the body, protects organs, and allows movement.

The skeletal system provides both protection and support.

Image from The Independent

Protection
Bones form protective cages and shields for vital organs:

1. Skull
• Protects the brain
• Surrounds and protects the eyes and ears
• Made up of 22 bones (in adults)
2. Ribcage (Thoracic Cavity)
• 12 pairs of ribs (24 ribs total)
• Protects the heart and lungs
• Also protects part of the liver
3. Vertebral Column (Spine)
• 33 vertebrae (individual bones of the spine)
• Protects the spinal cord
4. Pelvis
• Protects reproductive organs
• Protects intestines and bladder

Bones form protective shields around your most important organs like your brain, heart, lungs, and spinal cord.

Support
Bones support and hold up your body:

1. Vertebral Column (Spine)
• Supports the head, neck, and trunk
• Bears the weight of the upper body
2. Pelvis
• Supports the vertebral column
• Transfers weight to the legs
3. Long Bones of Legs
• Femur (thighbone), Tibia (shinbone), Fibula
• Bear all your body weight when standing or walking

Bones act like columns and beams that support your body weight and give you shape.

The skeletal system works with muscles to create movement.

Image from Elephango

How movement happens:

1. Muscles are attached to bones by tendons
2. When muscles contract, they pull on bones
3. Bones move at joints where they connect

Joints allow movement:

• Hinge Joints: Move in one direction (e.g., elbow, knee).
• Ball-and-Socket Joints: Move in many directions (e.g., shoulder, hip).
• Pivot Joints: Allow rotation (e.g., neck).
• Gliding Joints: Allow sliding movement (e.g., wrists, ankles).

Joints are connections between bones that allow them to move. Muscles pull on bones at joints, creating all your body movements.

Bone marrow is a soft tissue inside bones that produces blood cells.

Image from Healthline

What is Bone Marrow?
Bone marrow is a soft, gel-like substance found inside the hollow centers of bones.

Two types of bone marrow:

1. Red Bone Marrow
• Produces red blood cells (RBCs), white blood cells (WBCs), and platelets.
• Located in flat bones (pelvis, ribs, sternum, skull) and ends of long bones.
• Produces about 2 million red blood cells per second.
2. Yellow Bone Marrow
• Stores fat.
• Located in hollow centers of long bones.
• Can convert back to red bone marrow if needed.

Red bone marrow is like a factory inside your bones that produces new blood cells to replace old ones. Yellow bone marrow stores fat for energy.

Why This is Important

Red blood cells carry oxygen. White blood cells fight infection. Platelets stop bleeding. Without bone marrow, you would not have enough blood cells to survive.

Bones serve as a storage site for essential minerals.

Image from Open library publishing platform

Calcium Storage

• Bones contain about 99% of the body's calcium.
• Calcium is needed for strong bones, muscle contraction, nerve signals, blood clotting, and heart function.
• Calcium is released from bones when blood levels drop and absorbed when levels are adequate.

Bones store about 99% of your body's calcium and release it when your body needs it for muscles, nerves, and other functions.

Phosphorus Storage

• About 85% of body phosphorus is in bones and teeth.
• Phosphorus combines with calcium to make bones hard and is part of ATP (energy), DNA, and cell membranes.

Phosphorus works with calcium to make bones and teeth hard and strong.

Bones have a complex internal structure that makes them strong yet lightweight.

Image from ScienceDirect

Bone Layers (From Outside to Inside)

1. Periosteum: Tough outer membrane with blood vessels and nerves.
2. Compact Bone: Hard, solid outer layer that provides strength.
3. Cancellous (Spongy) Bone: Porous inner layer containing red bone marrow.
4. Medullary Cavity: Hollow center containing yellow bone marrow.

Bones have a hard outer layer, a spongy middle layer with spaces, and a hollow center—making them strong but not too heavy.

Types of Bones

Image from IPTA

Bones are classified by their shape and function:

1. Long Bones: Longer than wide (e.g., femur, humerus). Support weight and allow movement.
2. Short Bones: Cube-shaped (e.g., carpals, tarsals). Provide strength and flexibility.
3. Flat Bones: Thin and broad (e.g., skull, ribs, pelvis). Provide protection and muscle attachment.
4. Irregular Bones: Complex shapes (e.g., vertebrae, mandible).
5. Sesamoid Bones: Small bones in tendons (e.g., patella/kneecap).

Bones come in different shapes, each designed for specific functions like protection, support, or movement.

Major Bones of the Human Skeleton

Image from Rainbow Resource Center

Skull (Cranium)

• 22 bones that protect the brain, eyes, ears, and nose.
• Major bones: Frontal, Parietal, Temporal, Occipital, Mandible (jaw), Maxilla.

The skull is 22 bones that form your head and protect your brain.

Vertebral Column (Spine)

• 33 vertebrae protecting the spinal cord, supporting the head, and allowing flexibility.
• Regions: Cervical (7), Thoracic (12), Lumbar (5), Sacral (5 fused), Coccygeal (4 fused).

The spine is 33 bones stacked on top of each other that protect your spinal cord and allow you to bend and twist.

Ribcage (Thoracic Cavity)

• 12 pairs of ribs (24 total) protecting the heart and lungs.
• Attach to thoracic vertebrae in back and sternum (breastbone) in front.

The ribcage is 24 ribs that form a protective cage around your heart and lungs.

Pelvis

• A ring-shaped structure protecting reproductive organs, bladder, and intestines.
• Bears upper body weight; different in males vs. females.

The pelvis is a ring of bones that protects organs and connects your spine to your legs.

Limb Bones

• Upper Limb: Humerus (upper arm), Radius & Ulna (forearm), Carpals (wrist), Metacarpals (hand), Phalanges (fingers).
• Lower Limb: Femur (thigh, longest bone), Tibia & Fibula (lower leg), Patella (kneecap), Tarsals (ankle), Metatarsals (foot), Phalanges (toes).

Your upper and lower limbs have long bones for structure and small bones for flexibility.

Cartilage

A flexible tissue found in joints and other areas.

• Hyaline Cartilage: Covers ends of bones for smooth joint movement.
• Elastic Cartilage: Found in ears and nose for flexibility.
• Fibrocartilage: Tough shock absorber in intervertebral discs.

Cartilage is a smooth, flexible tissue that covers bone ends, reducing friction at joints so they can move smoothly.

Ligaments and Tendons

• Ligaments: Connect bone to bone. Provide joint stability (e.g., ACL in knee).
• Tendons: Connect muscle to bone. Transmit force to move bones (e.g., Achilles tendon).

Ligaments are tough cords that tie bones together at joints. Tendons are tough cords that attach muscles to bones so muscles can move bones.

Bone Growth and Repair

Bone Growth

• In children, bones lengthen at the growth plate (epiphyseal plate), where cartilage is replaced by bone.
• Growth plates close in late teens/early 20s.

When you are young, cartilage at the ends of bones is replaced by bone, making bones longer and stronger.

Bone Repair Stages:

1. Inflammation: Blood clot forms.
2. Soft Callus: Cartilage forms a temporary connection.
3. Hard Callus: Cartilage is replaced by new bone.
4. Remodeling: Bone is reshaped and strengthened.

When you break a bone, your body forms cartilage to hold it together, then gradually replaces the cartilage with new bone.

Bone Health and Osteoporosis

Image from Multiple Sclerosis Foundation

Maintaining Healthy Bones

• Nutrition: Adequate calcium (dairy, greens) and phosphorus (meat, nuts). Vitamin D (sunlight, fatty fish) for calcium absorption.
• Exercise: Weight-bearing exercise (walking, running, strength training) stimulates bone strengthening.
• Avoid: Smoking, excessive caffeine/alcohol.

Osteoporosis

• A condition where bones become weak and brittle due to loss of minerals.
• Causes: Low calcium/vitamin D, sedentary lifestyle, aging, hormonal changes.
• Prevention: Good nutrition, regular exercise, avoiding harmful habits.

Osteoporosis is when bones lose calcium and become weak and brittle, breaking easily.

Skeletal System Quiz: click here

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