Content Knowledge and Student Growth and Development · 14 min read
Major Human Body Systems
Physical education teachers need a working knowledge of human anatomy and physiology. Understanding body systems enables educators to design safe, effective programs and explain to students why certain exercises produce specific results.
The Skeletal System
The skeletal system provides the structural framework for the human body. It consists of 206 bones in adults, though infants are born with roughly 270 to 300 bony elements, many of which fuse during development.
Functions of the Skeletal System
The skeletal system serves five primary functions. First, it provides support by creating a rigid framework that holds the body upright and gives shape to soft tissues. Second, it offers protection for vital organs. The skull encases the brain, the rib cage shields the heart and lungs, and the vertebral column surrounds the spinal cord. Third, bones serve as attachment points for muscles, enabling movement through a system of levers. Fourth, the skeletal system stores minerals, primarily calcium and phosphorus, releasing them into the bloodstream when needed. Fifth, red bone marrow produces blood cells through hematopoiesis.
Classification of Bones
Bones are classified by shape into five categories. Long bones, such as the femur, humerus, and phalanges, are longer than they are wide and function primarily in movement. Short bones, including the carpals and tarsals, are roughly cube-shaped and provide stability with limited movement. Flat bones, such as the scapula, sternum, and cranial bones, protect organs and provide broad surfaces for muscle attachment. Irregular bones, including vertebrae and facial bones, have complex shapes that serve specialized functions. Sesamoid bones, like the patella, develop within tendons and protect them from excessive wear.
| Shape | Examples | Main Role |
|---|---|---|
| Long | Femur, humerus, tibia, phalanges | Levers for movement |
| Short | Carpals, tarsals | Stability with limited motion |
| Flat | Scapula, sternum, ribs, cranial bones | Protection; broad muscle attachment |
| Irregular | Vertebrae, facial bones | Specialized shapes and functions |
| Sesamoid | Patella | Protects tendon; improves leverage |
Bone Structure
Each bone contains two types of tissue. Compact bone forms the dense outer layer, providing strength and rigidity. Spongy bone, found at the ends of long bones and within flat and irregular bones, contains trabeculae arranged along lines of stress and houses red marrow.
Long bones have distinct regions. The diaphysis is the shaft, composed primarily of compact bone surrounding the medullary cavity. The epiphyses are the expanded ends, covered with articular cartilage where they form joints. The metaphysis is the flared region between the diaphysis and each epiphysis; in growing individuals, the epiphyseal (growth) plate lies at its border with the epiphysis.
Bone Growth and Remodeling
Bones grow through two processes. Interstitial growth occurs at the epiphyseal plate, where cartilage cells divide and are replaced by bone, increasing length. Appositional growth adds bone to external surfaces, increasing diameter.
Bone remodeling continues throughout life. Osteoblasts deposit new bone tissue while osteoclasts break down existing bone. This process responds to mechanical stress according to Wolff's Law, which states that bone adapts to the loads placed upon it. Physical activity stimulates bone formation, making weight-bearing exercise essential for skeletal health.
The Axial and Appendicular Skeleton
The axial skeleton includes 80 bones forming the central axis: the skull (22 bones), the hyoid bone and six auditory ossicles (7 bones), the vertebral column (26 bones), and the thoracic cage (25 bones including ribs and sternum). The vertebral column contains cervical (7), thoracic (12), lumbar (5), sacral (5 fused), and coccygeal (4 fused) vertebrae, with intervertebral discs providing cushioning.
The appendicular skeleton contains 126 bones forming the limbs and girdles. The pectoral girdle (clavicle and scapula) attaches the upper limbs, while the pelvic girdle attaches the lower limbs. The upper limb includes the humerus, radius, ulna, carpals, metacarpals, and phalanges. The lower limb includes the femur, patella, tibia, fibula, tarsals, metatarsals, and phalanges.
Joints and Articulations
Joints are classified structurally and functionally. Structural classification includes fibrous joints (connected by fibrous tissue, like skull sutures), cartilaginous joints (connected by cartilage, like intervertebral discs), and synovial joints (containing a fluid-filled cavity).
Functional classification describes movement potential. Synarthroses are immovable joints. Amphiarthroses allow slight movement. Diarthroses permit free movement and include most limb joints.
Synovial joints are further classified by movement type. Hinge joints (elbow, knee) allow flexion and extension. Ball-and-socket joints (shoulder, hip) permit movement in all planes. Pivot joints (atlantoaxial) allow rotation. Saddle joints (thumb carpometacarpal) permit biaxial movement. Condyloid joints (wrist) allow flexion, extension, abduction, and adduction. Gliding joints (intercarpal) permit sliding movements.
| Joint Type | Example | Movement Allowed |
|---|---|---|
| Hinge | Elbow, knee | Flexion and extension (one plane) |
| Ball-and-socket | Shoulder, hip | Movement in all planes, including rotation |
| Pivot | Atlantoaxial (C1-C2) | Rotation around one axis |
| Saddle | Thumb carpometacarpal | Biaxial movement (thumb opposition) |
| Condyloid | Wrist (radiocarpal) | Flexion, extension, abduction, adduction |
| Gliding (plane) | Intercarpal joints | Short sliding movements |
The Muscular System
The muscular system enables movement, maintains posture, generates heat, and protects internal structures.
Types of Muscle Tissue
Three types of muscle tissue exist. Skeletal muscle attaches to bones and produces voluntary movement. It appears striated under microscopy due to organized sarcomere arrangement. Cardiac muscle forms the heart wall, contracting involuntarily and rhythmically. It is striated but branched, with intercalated discs allowing synchronized contraction. Smooth muscle lines hollow organs and blood vessels, contracting involuntarily to move substances through the body. It lacks striations and has a spindle-shaped cellular structure.
Skeletal Muscle Structure
Skeletal muscles have hierarchical organization. The whole muscle is surrounded by epimysium. Within this, fascicles (bundles of fibers) are wrapped in perimysium. Individual muscle fibers (cells) are covered by endomysium. Each fiber contains myofibrils, which contain sarcomeres, the functional units of contraction.
Sarcomeres contain thick filaments (myosin) and thin filaments (actin). During contraction, myosin heads attach to actin, pull the thin filaments toward the sarcomere center, and release. This sliding filament mechanism shortens the muscle.
Muscle Fiber Types
Skeletal muscle contains three fiber types. Type I (slow-twitch) fibers contract slowly, resist fatigue, and rely on aerobic metabolism. They predominate in postural muscles and endurance athletes. Type IIa (fast-twitch oxidative) fibers contract quickly, have moderate fatigue resistance, and use both aerobic and anaerobic pathways. Type IIx (fast-twitch glycolytic) fibers contract very quickly, fatigue rapidly, and rely on anaerobic metabolism. They predominate in power athletes.
| Feature | Type I | Type IIa | Type IIx |
|---|---|---|---|
| Contraction speed | Slow | Fast | Very fast |
| Fatigue resistance | High | Moderate | Low |
| Main energy pathway | Aerobic (oxidative) | Aerobic and anaerobic | Anaerobic (glycolytic) |
| Force output | Low | Moderate to high | High |
| Typical activity | Posture, distance running | 800-m run, repeated sprints | Shot put, 40-yard dash |
Training can shift fiber characteristics. Endurance training increases oxidative capacity in all fibers. Resistance training increases fiber size (hypertrophy) and may shift some Type IIx fibers toward Type IIa characteristics.
Muscle Actions
Muscles produce different types of contractions. Concentric contractions shorten the muscle while generating force (lifting a weight). Eccentric contractions lengthen the muscle while generating force (lowering a weight). Isometric contractions generate force without changing length (holding a weight stationary).
Muscles work in groups with defined roles. The agonist (prime mover) produces the primary movement. The antagonist opposes the agonist's action. Synergists assist the agonist. Stabilizers hold joints steady during movement.
Major Muscle Groups
Physical educators must know major muscles and their actions. In the upper body, the pectoralis major adducts and medially rotates the shoulder. The latissimus dorsi extends, adducts, and medially rotates the shoulder. The deltoid abducts the shoulder. The biceps brachii flexes the elbow and supinates the forearm. The triceps brachii extends the elbow.
In the core, the rectus abdominis flexes the spine. The external and internal obliques rotate and laterally flex the spine. The transverse abdominis compresses the abdomen and stabilizes the spine. The erector spinae group extends the spine.
In the lower body, the gluteus maximus extends and laterally rotates the hip. The gluteus medius and minimus abduct and medially rotate the hip. The iliopsoas flexes the hip. The quadriceps group (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius) extends the knee. The hamstring group (biceps femoris, semitendinosus, semimembranosus) flexes the knee and extends the hip. The gastrocnemius and soleus plantarflex the ankle. The tibialis anterior dorsiflexes the ankle.
The Cardiovascular System
The cardiovascular system transports oxygen, nutrients, hormones, and waste products throughout the body.
Heart Structure
The heart is a muscular pump with four chambers. The right atrium receives deoxygenated blood from the body via the superior and inferior vena cava. The right ventricle pumps blood to the lungs through the pulmonary arteries. The left atrium receives oxygenated blood from the lungs via the pulmonary veins. The left ventricle pumps blood to the body through the aorta.
Valves ensure unidirectional flow. The tricuspid valve separates the right atrium and ventricle. The bicuspid (mitral) valve separates the left atrium and ventricle. The pulmonary and aortic semilunar valves guard the exits from the ventricles.
Cardiac Cycle and Electrical Conduction
The cardiac cycle includes systole (contraction) and diastole (relaxation). During atrial systole, the atria contract, completing ventricular filling. During ventricular systole, the ventricles contract, ejecting blood. During diastole, chambers relax and fill passively.
The heart's electrical system controls rhythm. The sinoatrial (SA) node, the natural pacemaker, initiates impulses. The impulse spreads through the atria to the atrioventricular (AV) node, which delays it briefly. The bundle of His and Purkinje fibers then distribute the impulse through the ventricles.
Blood Vessels
Arteries carry blood away from the heart. They have thick, muscular walls to withstand high pressure. Arteries branch into smaller arterioles, which regulate blood distribution through vasoconstriction and vasodilation.
Capillaries are microscopic vessels where exchange occurs. Their thin walls (one cell thick) permit diffusion of gases, nutrients, and wastes between blood and tissues.
Veins return blood to the heart. They have thinner walls and contain valves to prevent backflow. Veins merge from smaller venules into larger vessels.
Blood Pressure and Circulation
Blood pressure measures the force exerted on arterial walls. Systolic pressure (the higher number) reflects ventricular contraction. Diastolic pressure (the lower number) reflects ventricular relaxation. Normal resting blood pressure for adults is below 120/80 mmHg.
During exercise, systolic pressure increases to meet oxygen demands while diastolic pressure remains relatively stable or decreases slightly due to vasodilation in working muscles.
Cardiovascular Adaptations to Exercise
Regular aerobic exercise produces beneficial adaptations. The heart undergoes eccentric hypertrophy, mainly an enlarged left ventricular chamber with modest wall thickening, which increases stroke volume. Resting heart rate decreases as the heart pumps more blood per beat. Maximal cardiac output increases. Capillary density in trained muscles increases, improving oxygen delivery. Blood volume expands, and trained muscles extract more oxygen from the blood (a wider arteriovenous oxygen difference).
The Respiratory System
The respiratory system facilitates gas exchange, bringing oxygen into the body and removing carbon dioxide.
Respiratory Anatomy
The upper respiratory tract includes the nose, nasal cavity, pharynx, and larynx. These structures warm, humidify, and filter incoming air. The lower respiratory tract includes the trachea, bronchi, bronchioles, and alveoli.
The trachea divides into two primary bronchi, one for each lung. Bronchi branch repeatedly into smaller bronchioles, ending in alveolar ducts and alveoli. Alveoli are tiny air sacs surrounded by capillaries where gas exchange occurs. The lungs contain roughly 300 to 500 million alveoli, providing enormous surface area.
Mechanics of Breathing
Ventilation involves inspiration and expiration. During inspiration, the diaphragm contracts and flattens while external intercostals elevate the ribs. This expands the thoracic cavity, decreasing pressure and drawing air in. During quiet expiration, the diaphragm and intercostals relax, and elastic recoil of the lungs expels air passively. Forced expiration involves contraction of internal intercostals and abdominal muscles.
Lung volumes include tidal volume (normal breath, approximately 500 mL), inspiratory reserve volume (additional air that can be inhaled), expiratory reserve volume (additional air that can be exhaled), and residual volume (air remaining after maximal expiration). Vital capacity is the maximum air that can be moved in one breath.
Gas Exchange
Gas exchange occurs through diffusion driven by partial pressure gradients. Oxygen diffuses from alveoli (high partial pressure) into capillary blood (lower partial pressure). Carbon dioxide diffuses from blood (high partial pressure) into alveoli (lower partial pressure).
Hemoglobin in red blood cells carries most oxygen. Each hemoglobin molecule can bind four oxygen molecules. Carbon dioxide is transported dissolved in plasma, bound to hemoglobin, and as bicarbonate ions.
Respiratory Responses to Exercise
During exercise, ventilation increases to meet metabolic demands. Breathing rate and tidal volume both increase. Minute ventilation (the product of rate and tidal volume) can increase from 6 L/min at rest to over 100 L/min during maximal exercise.
The ventilatory threshold marks the point where ventilation increases disproportionately to oxygen consumption, reflecting increased reliance on anaerobic metabolism and accumulating lactate.
The Nervous System
The nervous system controls and coordinates body functions, processes sensory information, and generates movement.
Nervous System Organization
The central nervous system (CNS) includes the brain and spinal cord. The peripheral nervous system (PNS) includes all nerves outside the CNS.
The PNS has somatic and autonomic divisions. The somatic nervous system controls voluntary skeletal muscle movement. The autonomic nervous system controls involuntary functions and has sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) branches.
Neurons and Neural Communication
Neurons are specialized cells that transmit electrical signals. Each neuron has a cell body, dendrites (receiving extensions), and an axon (transmitting extension). Neurons communicate at synapses through neurotransmitters.
Motor neurons carry signals from the CNS to muscles. Sensory neurons carry information from receptors to the CNS. Interneurons process information within the CNS.
Motor Control
Movement involves complex neural processes. The motor cortex initiates voluntary movement. The cerebellum coordinates movement timing and accuracy. The basal ganglia regulate movement initiation and inhibition. The spinal cord contains motor neuron pools and reflex circuits.
Motor units consist of a motor neuron and all muscle fibers it innervates. Small motor units (few fibers per neuron) allow fine control. Large motor units (many fibers per neuron) produce powerful but less precise movements.
Proprioception
Proprioceptors provide information about body position and movement. Muscle spindles detect muscle length and stretch velocity. Golgi tendon organs detect muscle tension. Joint receptors detect joint position and movement. The vestibular system detects head position and acceleration.
This proprioceptive information enables balance, coordination, and movement control without visual guidance.
The Endocrine System
The endocrine system uses hormones to regulate metabolism, growth, development, and responses to stress and exercise.
Major Endocrine Glands and Hormones
The pituitary gland produces growth hormone, which stimulates tissue growth and protein synthesis. The thyroid gland produces thyroid hormones, which regulate metabolism. The adrenal glands produce cortisol (regulates metabolism and stress response), aldosterone (regulates fluid balance), and catecholamines (epinephrine and norepinephrine, which mediate fight-or-flight responses).
The pancreas produces insulin, which lowers blood glucose by promoting uptake into cells, and glucagon, which raises blood glucose by stimulating liver glycogen breakdown. The gonads produce testosterone (stimulates muscle growth, bone density, and male characteristics) and estrogen (regulates female characteristics and bone health).
Hormonal Responses to Exercise
Exercise triggers hormonal changes. Catecholamine release increases heart rate, blood pressure, and energy mobilization. Growth hormone secretion increases, promoting protein synthesis. Cortisol release increases during prolonged or intense exercise, mobilizing energy stores. Insulin levels decrease during exercise while glucagon increases, maintaining blood glucose. Testosterone increases acutely after resistance exercise, supporting muscle adaptation.
Energy Systems
Understanding energy systems is fundamental to exercise programming.
ATP: The Energy Currency
Adenosine triphosphate (ATP) provides immediate energy for muscle contraction. When ATP is hydrolyzed (broken down), energy is released for cellular work. Muscle cells store limited ATP, so continuous regeneration is necessary during exercise.
The Phosphagen System
The phosphagen (ATP-PCr) system provides immediate energy for high-intensity, short-duration activities lasting up to 10-15 seconds. Creatine phosphate donates a phosphate group to regenerate ATP rapidly. This system does not require oxygen and does not produce lactate, but stored creatine phosphate runs low within seconds. Activities using this system include sprints, jumps, and throws.
Glycolysis
Glycolysis breaks down glucose to pyruvate, producing ATP. Fast glycolysis (anaerobic) converts pyruvate to lactate, producing 2 ATP per glucose molecule. This system dominates during high-intensity activities lasting 15 seconds to 2 minutes. The hydrogen ions (acidosis) that accumulate during rapid glycolysis contribute to fatigue; lactate itself is not the main cause and is later used as fuel.
Slow glycolysis occurs when oxygen is sufficient. Pyruvate enters the mitochondria for aerobic metabolism rather than being converted to lactate.
Oxidative Phosphorylation
The aerobic system produces ATP through oxidative phosphorylation in mitochondria. It uses carbohydrates, fats, and sometimes proteins as fuel. This system yields about 30-32 ATP per glucose molecule (older texts cite 36-38) and far more per fat molecule, and body fat stores are large enough to fuel very long, low-intensity activity. It dominates during low-to-moderate intensity activities lasting more than 2-3 minutes.
Energy System Interaction
All three systems contribute to energy production simultaneously, with dominance depending on intensity and duration. During a 400-meter sprint, the phosphagen system provides initial energy, glycolysis dominates mid-race, and the aerobic system contributes throughout and supports recovery. Understanding this interaction enables appropriate training design.
| System | Fuel | Dominant When | PE Examples |
|---|---|---|---|
| Phosphagen (ATP-PCr) | Stored ATP and creatine phosphate | Maximal effort, up to about 10-15 s | Sprint start, vertical jump, throw |
| Fast glycolysis | Glucose and glycogen (no oxygen) | Very hard effort, about 15 s to 2 min | 400-m run, long volleyball rally |
| Oxidative (aerobic) | Carbohydrate, fat, some protein | Low to moderate effort, beyond 2-3 min | Mile run, PACER early levels, hiking |
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