Human Body Systems: Skin and Skeletal Structures
Skin
Structure and Function
The skin can be categorized into two main types: thick skin (found on the palms of the hands, fingertips, and soles of the feet) and thin skin (covering most of the body and containing hair follicles, sebaceous glands, and arrector pili muscles).
The skin serves several crucial functions:
- Protection: Shields the body from pathogens and environmental factors.
- Sensation: Contains nerve endings for tactile and thermal perception.
- Body Temperature Regulation: Facilitates sweating for cooling.
- Fluid Balance: Prevents excessive water loss.
- Lipid Storage: Stores fat in the hypodermis.
- Excretion: Eliminates salts and waste products.
Layers of the Skin
The skin comprises three primary layers:
- Epidermis: The outermost layer, responsible for water retention, pathogen defense, and temperature regulation.
- Dermis: The middle layer, housing nerve endings, hair follicles, sweat glands, sebaceous glands, apocrine glands, lymphatic vessels, and blood vessels. It provides nourishment and waste removal for both itself and the epidermis.
- Hypodermis: The deepest layer, connecting the skin to underlying bone and muscle, and supplying it with blood vessels and nerves.
Epidermal Layers and Cell Types
The epidermis consists of five layers: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum.
Key cell types within the skin include:
- Keratinocytes: Produce keratin, providing protection against heat, water loss, microbes, and chemicals.
- Langerhans cells: Migrate from red bone marrow and contribute to immune defense against microbes.
- Melanocytes: Produce melanin, which protects against UV radiation.
- Merkel cells: Function as touch receptors.
Skeletal System
Functions
The skeletal system performs several essential roles:
- Protection: Safeguards internal organs.
- Framework: Provides shape and support to the body.
- Movement: Anchors muscles to enable movement.
- Blood Cell Production: Generates white blood cells, red blood cells, and platelets in the bone marrow.
- Mineral Storage: Stores essential minerals, including calcium, phosphorus, zinc, potassium, iron, magnesium, manganese, and trace amounts of copper, selenium, and strontium.
- Triglyceride Storage: Stores energy reserves in the form of triglycerides.
Axial Skeleton
The axial skeleton consists of 80 bones, encompassing the skull, ribcage, ossicles (middle ear), hyoid bone (throat), sternum (center chest), and vertebral column (spine).
- Skull (22 bones): Cranial bones (8), parietal bones (2), temporal bones (2), frontal bone (1), occipital bone (1), ethmoid bone (1), and sphenoid bone (1).
- Ossicles (6 bones): Malleus (2), incus (2), and stapes (2).
- Hyoid bone (1 bone)
- Vertebral Column (26 bones): Cervical vertebrae (7), thoracic vertebrae (12), lumbar vertebrae (5), sacrum (1, formed from 5 fused vertebrae), and coccyx (1, formed from 4 fused vertebrae).
- Thoracic Cage (25 bones): Sternum (1) and ribs (24).
Appendicular Skeleton
The appendicular skeleton comprises 126 bones, divided into six major regions:
- Pectoral Girdles (4 bones): Left and right clavicle (2) and scapula (2).
- Arm and Forearm (6 bones): Left and right humerus (2), ulna (2), and radius (2).
- Hands (54 bones): Left and right carpals (16), metacarpals (10), proximal phalanges (10), middle phalanges (8), and distal phalanges (10).
- Pelvis (2 bones): Left and right os coxae (2).
- Thigh and Leg (8 bones): Femur (2), tibia (2), patella (2), and fibula (2).
- Feet and Ankles (52 bones): Tarsals (14), metatarsals (10), proximal phalanges (10), middle phalanges (8), and distal phalanges (10).
Bone Cells
- Osteoclasts: Break down bone tissue.
- Osteoblasts: Form new bone tissue.
- Osteocytes: Maintain bone tissue.
- Osteoprogenitor cells: Contribute to bone growth and repair.
Bone Types
Compact bone: Dense and heavy, forming the outer surfaces of bones.
Spongy bone: Porous and less dense than compact bone, typically found within the interior and containing red bone marrow.