Skeletal System

The skeletal system serves as the body's structural framework, supports organs, provides attachment points for muscles, stores minerals, and produces blood cells.

Functions of the Skeletal System

1. Support: Provides structure and shape to the body.

2. Protection: Protects internal organs (e.g., the skull protects the brain, ribcage protects the heart and lungs).

3. Movement: Serves as a system of levers that work with muscles to create movement.

4. Mineral Storage: Stores essential minerals like calcium and phosphorus.

5. Blood Cell Production (Hematopoiesis): Produces blood cells in the bone marrow.

6. Fat Storage: Stores fat in yellow marrow, primarily for energy reserves.

Types of Bones

The human body has 206 bones, classified into five main types:

1. Long Bones: Found in the arms and legs (e.g., femur, humerus). They are longer than they are wide.

2. Short Bones: Cube-like bones found in the wrists and ankles (e.g., carpals, tarsals).

3. Flat Bones: Protect organs and provide attachment sites for muscles (e.g., skull, ribs, sternum).

4. Irregular Bones: Complex shapes (e.g., vertebrae, hip bones).

5. Sesamoid Bones: Develop within tendons (e.g., patella or kneecap).

Bone Structure

• Compact Bone: Dense, strong bone tissue that makes up the outer layer of bone, providing strength and structure.

• Spongy Bone: Less dense bone found at the ends of long bones and inside other bones. Contains red bone marrow, where hematopoiesis occurs.

• Bone Marrow:

  • Red Marrow: Found in spongy bone; site of blood cell production.

  • Yellow Marrow: Found in the shaft of long bones; stores fat.

Bone Cells

1. Osteoblasts: Bone-building cells that synthesize the bone matrix.

2. Osteoclasts: Bone-resorbing cells that break down bone tissue.

3. Osteocytes: Mature bone cells that maintain bone tissue.

Bone Growth and Remodeling

• Ossification: The process of bone formation that occurs throughout childhood and adolescence.

• Bone Remodeling: A continuous process where old bone is replaced by new bone, involving both osteoclasts and osteoblasts.

Joints

Joints are places where bones meet, allowing movement.

1. Fibrous Joints: No movement (e.g., sutures of the skull).

2. Cartilaginous Joints: Limited movement (e.g., vertebrae).

3. Synovial Joints: Freely movable joints that contain synovial fluid to reduce friction (e.g., knees, elbows).

Muscular System

The muscular system enables movement, maintains posture, and generates heat.

Types of Muscle Tissue

1. Skeletal Muscle:

   • Voluntary: Under conscious control.

   • Striated: Appears striped due to sarcomeres.

   • Function: Moves bones and maintains posture.

2. Smooth Muscle:

   • Involuntary: Not under conscious control.

   • Non-striated: Found in internal organs like the digestive tract, blood vessels, and bladder.

   • Function: Moves substances through organs, regulates blood vessel diameter.

3. Cardiac Muscle:

   • Involuntary: Only found in the heart.

   • Striated: Has a unique branching structure with intercalated discs that allow synchronized contraction.

   • Function: Pumps blood throughout the body.

Muscle Structure

• Muscle Fiber: The basic unit of a muscle, containing myofibrils, which are made up of repeating units called sarcomeres.

• Sarcomere: The functional unit of contraction, consisting of actin (thin filaments) and myosin (thick filaments).

Muscle Contraction

• Sliding Filament Theory: Explains how muscles contract. Actin and myosin filaments slide past each other, shortening the sarcomere, which shortens the muscle fiber, causing contraction.

• Neuromuscular Junction: The point where a motor neuron communicates with a muscle fiber, releasing the neurotransmitter acetylcholine to trigger contraction.

• ATP: Required for muscle contraction and relaxation.

Types of Muscle Contractions

1. Isotonic: Muscle changes length (e.g., lifting a weight).

   • Concentric: Muscle shortens during contraction.

   • Eccentric: Muscle lengthens during contraction.

2. Isometric: Muscle tension increases without changing length (e.g., holding a plank).

Blood

Blood is a connective tissue that transports substances throughout the body and is vital for homeostasis.

Composition of Blood

1. Plasma (55% of blood volume):

   • Mostly water (90%), and contains nutrients, hormones, waste products, proteins (e.g., albumin, globulins, fibrinogen).

2. Formed Elements (45% of blood volume):

   • Red Blood Cells (RBCs) or Erythrocytes:

     • Function: Transport oxygen from the lungs to tissues and carry carbon dioxide back to the lungs.

     • Hemoglobin: Protein that binds oxygen.

     • Life Span: Approximately 120 days.

   • White Blood Cells (WBCs) or Leukocytes:

     • Function: Part of the immune system that helps fight infections.

     • Types:

       • Neutrophils: Fight bacteria.

       • Lymphocytes: B-cells (produce antibodies) and T-cells (attack infected cells).

       • Monocytes: Become macrophages and engulf pathogens.

       • Eosinophils: Combat parasites and are involved in allergic reactions.

       • Basophils: Release histamine during allergic reactions.

   • Platelets or Thrombocytes:

     • Function: Essential for blood clotting. They form a temporary plug at the site of a blood vessel injury.

Blood Types

The ABO system and Rh factor determine blood types.

1. ABO Blood Types:

   • Type A: Has A antigens on RBCs and anti-B antibodies in plasma.

   • Type B: Has B antigens and anti-A antibodies.

   • Type AB: Has A and B antigens and no antibodies. Universal recipient.

   • Type O: Has no antigens but both anti-A and anti-B antibodies. Universal donor.

2. Rh Factor:

   • Rh+: Has Rh antigen.

   • Rh-: Lacks Rh antigen.

Blood Functions

1. Transportation: Carries oxygen, nutrients, hormones, and waste products.

2. Regulation: Helps maintain body temperature, pH, and fluid balance.

3. Protection: WBCs protect against infection, and platelets help in blood clotting.

Blood Circulation

Blood is circulated through the body by the heart via blood vessels:

1. Arteries: Carry oxygen-rich blood away from the heart to tissues.

2. Veins: Carry oxygen-poor blood back to the heart.

3. Capillaries: Tiny vessels where the exchange of gases, nutrients, and waste occurs between blood and tissues.

Pathway of Blood through the Heart

1. Deoxygenated Blood enters the right atrium from the superior and inferior vena cava.

2. Blood moves to the right ventricle and is pumped to the lungs via the pulmonary arteries for oxygenation.

3. Oxygenated Blood returns to the left atrium through the pulmonary veins.

4. Blood moves to the left ventricle and is pumped through the aorta to the rest of the body.

Summary for Exam

• Skeletal System: 206 bones; supports, protects, allows movement, stores minerals, and produces blood cells. Key cells are osteoblasts, osteoclasts, and osteocytes.

• Muscular System: Includes skeletal, smooth, and cardiac muscles. Contraction is through the sliding filament theory, involving actin and myosin.

• Blood: Composed of plasma and formed elements (RBCs, WBCs, and platelets). Blood types are determined by ABO and Rh systems.