27.2: Skin
• The skin is made of 2 layers: dermis & epidermis.
• The dermis is the deeper layer that contains epithelial, muscle, nerve, and connective tissues.
      Sweat glands cool your body.  Oil glands waterproof and lubricate your skin.
• The epidermis is the outer layer that contains melanin and keratin.
      Your skin color is determined by melanin (brown), blood (pink), and carotene (orange).
      Hair and nails are dead cells made primarily of keratin.
• Acne is caused by overactive oil glands.
• Skin cancer is caused by DNA mutations in the skin.  
     A leading cause of this is ultraviolet rays from the sun. The worst type, though rare, is malignant melanomas.

27.3: Bones
• Bones are dynamic structures that are continually being broken down and repaired.
• As an embryo develops, its cartilage is replaced by bone.
• Long bones (arm, leg) have a hard, dense outer layer over a core of marrow. 
• Fat is stored in marrow and blood cells are produced there.
• The skeleton contains 206 bones.  
• A fracture is a broken bone.
• Osteoporosis is a condition where bone loses minerals and becomes brittle & easily broken.  
     It occurs most commonly in older females.
• Functions of the bones:
     1 – support the body
     2 – enable the body to move
     3 – protect internal organs
     4 – store minerals
     5 – make blood cells
• Bones meet at joints and are held together by ligaments.
• A sprain is when ligaments are damaged.

27.4: Muscles
• There are 3 types of muscle tissue:
      1 – Smooth: in internal organs
      2 – Cardiac: in the heart
      3 – Skeletal: attached to the skeleton
• Movement of bones requires 2 sets of muscles, 1 to contract while the other relaxes.
• Muscles are connected to bones by tendons.
• Skeletal muscle can be slow-twitch (for endurance) or fast-twitch (for short-term events).
• Anaerobic exercise (high intensity for a short time; ex. sprinting) uses fast-twitch muscles.
• Aerobic exercise (steady low-intensity; ex. walking) uses more oxygen.
• Resistance exercise (ex. weight lifting) can increase muscle size & strength.
• Anabolic steroids taken to increase muscle size have dangerous side effects.
• Overuse of muscles can lead to a “pulled muscle” or can cause tendinitis.