← Human Body

World 1 · Cells & Tissues

1What anatomy studies 2The cell 3Inside the cell 4Energy in the cell 5From cells to tissues 6The four tissue types 7Organs and systems 8Staying in balance

World 2 · Skeletal System

1The skeleton 2Bone as living tissue 3The axial skeleton 4The appendicular skeleton 5Joints 6Cartilage and ligaments 7Growth and repair

World 3 · Muscular System · PRO

1Three kinds of muscle 2Inside a skeletal muscle 3How muscles move bones 4Muscle energy and fatigue 5Posture and core muscles 6Muscle and the whole body

World 4 · Cardiovascular System · PRO

1The heart 2The cardiac cycle 3Blood vessels 4Lab: build the circulatory system 5Blood 6Pulse and blood pressure

World 5 · Respiratory System · PRO

1The airway 2The lungs and alveoli 3The mechanics of breathing 4Gas exchange and transport 5Controlling your breath 6Lab: build the respiratory system

World 6 · Digestive System · PRO

1The digestive system 2Mouth to stomach 3The small intestine 4The liver, pancreas, and gallbladder 5The large intestine 6Nutrients and metabolism 7Lab: build the digestive system

World 7 · Urinary System · PRO

1The kidneys 2The nephron 3Urine formation and balance 4Ureters, bladder, and urethra 5Lab: build the excretory system

World 8 · Nervous System · PRO

1The nervous system 2Neurons 3Synapses 4The brain 5The spinal cord and reflexes 6The autonomic nervous system 7Lab: build the nervous system

World 9 · Endocrine System · PRO

1Hormones and glands 2The pituitary gland 3The thyroid 4Adrenal glands and the pancreas 5Hormone feedback loops 6Two ways to communicate

World 10 · Immune & Lymphatic System · PRO

1The lymphatic system 2Innate immunity 3Adaptive immunity 4Inflammation and fever 5Immune memory 6Fluid balance and the whole body

World 11 · Integumentary System & Senses · PRO

1The skin 2Hair, nails, and glands 3Regulating temperature 4Sight 5Hearing and balance 6Taste and smell 7Touch and pain

World 12 · Reproductive System · PRO

1The reproductive system 2The male reproductive system 3The female reproductive system 4The menstrual cycle 5Puberty 6Fertilization and early development 7One connected body

World 13 · Structure, sections & movement · PRO

1Assemble the key bones 2Open four heart chambers 3Trace the double circulation 4An antagonistic muscle pair 5Read an upper-arm section 6Inside a long-bone shaft

World 14 · Lungs & breathing · PRO

1Boyle’s law in the chest 2Inhale and exhale 3When the pleural seal breaks 4Gas exchange in an alveolus

World 15 · Digestive journey · PRO

1Where protein digestion starts 2Fat, lipase and bile 3Why amylase stops in the stomach 4Villi and surface area

World 16 · Neurons & action potential · PRO

1Threshold: all or none 2The refractory period 3Myelin and saltatory conduction 4Synapses: summation and TTX

World 17 · Neuroanatomy · PRO

1Brain regions and their jobs 2Cranial nerves 3The visual pathway 4Long tracts and where they cross

World 18 · Medical imaging · PRO

1A body made of voxels 2CT numbers and the window 3MRI contrast: T1 and T2 4How CT builds a slice

World 19 · Practice Exams · PRO

1Cells & Tissues Basics 2Skeleton & Muscles 3The Heart & Circulation 4Respiratory System 5Blood & Immunity 6Nervous System Basics 7The Brain & Spinal Cord 8Reflexes & Reactions 9The Senses Overview 10Sight: The Eye 11Hearing & Balance: The Ear 12Touch, Taste & Smell 13Digestive Tract 14The Liver & Accessory Organs 15Kidneys & Urinary System 16Endocrine Glands 17Skin & the Integumentary System 18Cell Biology Deep Dive 19Tissues in Depth 20The Skeleton in Depth 21Joints & Movement 22Muscles in Depth 23Reproductive System 24The Lymphatic System 25Homeostasis & Feedback 26Growth, Development & Aging 27Nutrition & Metabolism 28Blood Vessels & Blood Pressure 29Anatomical Terms & Body Planes 30Whole-Body Mixed Review

World 3 · Muscular System · 7 min · PRO

How muscles move bones

Pulling in pairs

PRO · Human Body

World 3: Muscular System is part of PRO

The first world of Human Body is free. PRO opens the other 119 lessons in 17 worlds — simulations, quizzes and all.

Three kinds of muscle
Inside a skeletal muscle
Muscle energy and fatigue
Posture and core muscles
Muscle and the whole body

PRO includes

All 26 labs — every world, every lesson
Practice exams and certificates

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← Inside a skeletal muscle Muscle energy and fatigue →

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