Human Body Systems
Skeletal System: The Framework
The human adult skeleton consists of 206 bones — a framework that supports the body, protects internal organs, enables movement, produces blood cells, and stores minerals (especially calcium and phosphorus).
Divisions of the Skeleton
Axial Skeleton (80 bones):
- Skull (cranium + facial bones)
- Vertebral column (33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, 4 fused coccygeal)
- Rib cage (12 pairs of ribs: 7 true, 3 false, 2 floating)
Appendicular Skeleton (126 bones):
- Upper limbs: Shoulder girdle (2 clavicles + 2 scapulae), arm (humerus), forearm (radius + ulna), hand (carpals, metacarpals, phalanges)
- Lower limbs: Pelvic girdle (2 hip bones + sacrum), thigh (femur), leg (tibia + fibula), foot (tarsals, metatarsals, phalanges)
Types of Joints
| Joint Type | Movement | Example |
|---|---|---|
| Ball and Socket | All directions (multiaxial) | Shoulder, Hip |
| Hinge | One plane (monaxial) | Elbow, Knee |
| Pivot | Rotation around a single axis | Atlas-Axis (neck rotation) |
| Gliding | Sliding movement | Wrist bones, ankle bones |
| Saddle | Biaxial (two planes) | Thumb (carpometacarpal) |
| Fixed/Immovable | No movement | Skull sutures |
Synovial joints: Joints with a fluid-filled cavity (synovial fluid) — allow free movement. Ball-and-socket and hinge joints are types of synovial joints.
Bone and Cartilage
- Bones: Rigid, vascular (have blood supply), can heal when broken
- Cartilage: Flexible, avascular (no blood supply) — found in ear, nose, trachea, joints (smooth cartilage for joint movement)
- Ligaments: Connect bone to bone (at joints)
- Tendons: Connect muscle to bone — very strong, non-elastic
Growth: Bones grow in length at the epiphyseal plate (growth plate) — cartilage that hardens into bone. Growth stops after puberty when the plate fuses.
Disorders: Osteoporosis (bone density loss — common in postmenopausal women), arthritis (joint inflammation), rickets (childhood calcium/vitamin D deficiency — soft bones), fractures.
Muscular System
Types of Muscle Tissue
Skeletal (Striated) Muscle:
- Attached to bones — responsible for voluntary movement
- Striated (striped) appearance
- Multi-nucleated
- Contraction: Fast and strong
Smooth Muscle:
- Found in internal organs (stomach, intestines, blood vessels)
- Not striated (smooth)
- Involuntary — controlled by autonomic nervous system
- Contraction: Slow and rhythmic
Cardiac Muscle:
- Found only in the heart (myocardium)
- Striated but branching
- Involuntary — intrinsic rhythm (pacemaker)
- Can contract without nervous stimulation
- Cannot be consciously controlled
How muscles work: Muscles contract when they receive a signal from motor neurons. Muscles work in pairs — agonist (prime mover) contracts while antagonist relaxes. For example:
- Biceps (agonist) contracts to flex arm → Triceps (antagonist) relaxes
- Triceps contracts to extend arm → Biceps relaxes
ATP: Muscle contraction requires ATP — produced by aerobic and anaerobic respiration. During heavy exercise, muscles produce lactic acid (oxygen debt).
Nervous System
Central Nervous System (CNS)
Brain (protected by skull + three meninges):
- Cerebrum: Largest part, controls voluntary actions, intelligence, memory, speech — divided into left and right hemispheres (left controls right body, right controls left body)
- Cerebellum: Coordinates movement, balance, posture
- Medulla Oblongata: Controls autonomic functions (heart rate, breathing, blood pressure)
- Hypothalamus: Controls body temperature, hunger, thirst, sleep
Spinal Cord (protected by vertebral column):
- Carries messages between brain and body
- Responsible for reflex actions (rapid, automatic responses — not involving the brain)
Peripheral Nervous System (PNS)
Somatic NS: Controls voluntary actions — messages from CNS to skeletal muscles.
Autonomic NS: Controls involuntary functions — further divided into:
- Sympathetic NS: “Fight or flight” — increases heart rate, dilates pupils, slows digestion
- Parasympathetic NS: “Rest and digest” — decreases heart rate, constricts pupils, promotes digestion
Neurons (Nerve Cells)
Structure of a neuron:
- Cell body (soma): Contains nucleus and most organelles
- Dendrites: Receive signals (input)
- Axon: Conducts signal away from cell body (output)
- Myelin sheath: Fatty layer around some axons — speeds up signal transmission (white matter)
- Axon terminals: Transmit signal to next neuron or effector
Types of neurons:
- Sensory neurons: Carry signals from sense organs to CNS
- Motor neurons: Carry signals from CNS to muscles/glands
- Interneurons: Connect sensory and motor neurons (in brain and spinal cord)
Reflex Arc
A reflex is a rapid, automatic response to a stimulus that doesn’t require conscious thought:
Stimulus → Sensory receptor → Sensory neuron → Spinal cord (interneuron) → Motor neuron → Effector (muscle/gland)Examples: Knee-jerk reflex, pulling hand away from hot object. These are evolutionary adaptations for survival — fast responses.
Endocrine System
The endocrine system uses hormones (chemical messengers) to coordinate body functions. Hormones are secreted by ductless glands directly into the bloodstream.
Major Endocrine Glands
| Gland | Hormone | Function |
|---|---|---|
| Pituitary (Master gland) | Many (GH, TSH, FSH, LH, prolactin) | Controls other glands, growth, reproduction |
| Thyroid | T3, T4, Calcitonin | Metabolism, energy, calcium regulation |
| Parathyroid | PTH (Parathyroid hormone) | Increases blood calcium |
| Pancreas | Insulin, Glucagon | Blood sugar regulation |
| Adrenal (Adrenal cortex) | Cortisol, Aldosterone | Stress response, salt balance |
| Adrenal (Adrenal medulla) | Adrenaline (Epinephrine) | “Fight or flight” response |
| Ovaries (female) | Estrogen, Progesterone | Female sexual characteristics, menstruation |
| Testes (male) | Testosterone | Male sexual characteristics |
| Pineal | Melatonin | Sleep-wake cycle (circadian rhythm) |
| Thymus | Thymosin | Immune system development |
Key Hormone Functions
Thyroid hormones (T3/T4):
- Regulate metabolism, heart rate, digestion, brain development
- Hypothyroidism: Weight gain, fatigue, cold intolerance
- Hyperthyroidism: Weight loss, rapid heartbeat, heat intolerance
- Goiter: Enlarged thyroid — due to iodine deficiency
Insulin and Glucagon (Pancreas):
- Insulin (beta cells): Lowers blood sugar → tells liver and muscles to take up glucose
- Glucagon (alpha cells): Raises blood sugar → tells liver to release stored glucose
- Diabetes mellitus:
- Type 1: Autoimmune destruction of beta cells → insulin deficiency (requires insulin injection)
- Type 2: Insulin resistance → body doesn’t respond to insulin (linked to obesity, lifestyle)
Adrenaline:
- Released by adrenal medulla during stress or danger
- Increases heart rate, blood pressure, pupil dilation — prepares body for “fight or flight”
Excretory System
Excretion is the removal of metabolic waste products (urea, CO₂, water, salts) from the body.
Human Excretory System
Kidneys (most important excretory organs):
- Bean-shaped, ~10 cm long, located in the lower back
- Nephron: Functional unit of kidney (~1 million nephrons per kidney)
- Urine formation: Blood is filtered → reabsorption of useful substances → excretion of waste
- Filtration: Glomerulus (capillary network) filters blood under high pressure → filtrate (water, glucose, amino acids, urea) enters Bowman’s capsule
- Reabsorption: Useful substances (glucose, water, amino acids) reabsorbed in the tubule
- Secretion: Additional waste (H⁺, K⁺, drugs) secreted into tubule
- Final output: Urine (water + urea + excess salts + pigments)
Ureter: Carries urine from kidney to bladder.
Urinary bladder: Stores urine (~300–500 mL); when full, stretch receptors send signal to brain.
Urethra: Carries urine out of the body. In males, it also carries semen.
Functions of the Kidneys
- Excretion: Removes urea, uric acid, creatinine (nitrogenous waste)
- Osmoregulation: Maintains water and salt balance
- pH regulation: Maintains blood pH (7.35–7.45)
- Blood pressure regulation: Via renin-angiotensin system
- Erythropoietin production: Stimulates RBC production
Dialysis: Used when kidneys fail — machine filters blood mechanically (kidney machine).
CTET Exam Focus
- Skeletal: 206 bones, axial + appendicular, ball-and-socket (shoulder), hinge (elbow), pivot (neck), synovial joints
- Muscular: Skeletal (voluntary, striated), smooth (involuntary), cardiac (heart, involuntary, striated)
- Nervous: CNS (brain, spinal cord) + PNS (somatic + autonomic); neurons (sensory, motor, interneuron); reflex arc
- Endocrine: Major glands and hormones (insulin, thyroxine, adrenaline, growth hormone)
- Excretory: Kidneys (nephron = functional unit), glomerular filtration, reabsorption, urine formation; dialysis as kidney failure treatment
Content adapted based on your selected roadmap duration. Switch tiers using the selector above.