HUMAN BODY
The human brain contains about 86 billion neurons.
The brain uses about 20% of the body’s oxygen.
The brain weighs about 1.3–1.4 kg in adults.
The brain itself cannot feel pain.
Nerve impulses can travel up to 400 km/h.
The brain is about 60% fat.
The cerebrum is the largest part of the brain.
The cerebellum controls balance and coordination.
The brainstem controls breathing and heartbeat.
The brain remains active during sleep.
Most dreams occur during REM sleep.
The hippocampus is essential for memory formation.
The brain contains billions of synapses.
Neuroplasticity allows the brain to reorganize itself.
The spinal cord connects the brain to the body.
Reflex actions can occur without brain involvement.
The left hemisphere is associated with language.
The right hemisphere is linked to spatial abilities.
The brain depends primarily on glucose for energy.
Brain cells can be damaged after 5 minutes without oxygen.
The nervous system includes the central and peripheral systems.
The brain generates measurable electrical signals.
Dopamine influences reward and motivation.
Serotonin affects mood and sleep.
The brain regulates body temperature.
Gray matter contains neuron cell bodies.
White matter contains nerve fibers.
The brain shrinks slightly with aging.
Learning strengthens neural connections.
Alcohol slows brain function.
The brain processes visual information in the occipital lobe.
The frontal lobe controls decision-making.
The temporal lobe processes sound.
The parietal lobe processes sensory input.
The brain is protected by the skull.
Cerebrospinal fluid cushions the brain.
The human brain develops fully in the mid-20s.
Stress can alter brain structure.
Sleep is essential for memory consolidation.
The nervous system controls voluntary and involuntary actions.
❤️ Heart & Blood (41–80)
The heart is a muscular organ.
The heart beats about 100,000 times per day.
It pumps about 5 liters of blood per minute at rest.
The average adult has about 5 liters of blood.
Blood vessels stretch about 100,000 km in total length.
Red blood cells carry oxygen.
Red blood cells live about 120 days.
White blood cells fight infection.
Platelets help blood clot.
Blood plasma is mostly water.
The heart has four chambers.
The left ventricle is the strongest chamber.
The heart has its own electrical conduction system.
Arteries carry blood away from the heart.
Veins return blood to the heart.
Capillaries allow exchange of gases and nutrients.
Hemoglobin binds oxygen.
Iron is essential for hemoglobin production.
Blood pressure measures force against artery walls.
High blood pressure can damage organs.
Blood types include A, B, AB, and O.
O negative is the universal donor type.
AB positive is the universal recipient type.
The spleen filters blood.
The fetus’s heart starts beating around week 4.
Exercise strengthens the heart muscle.
Cholesterol travels in the bloodstream.
A heart attack occurs due to blocked blood flow.
Veins contain valves to prevent backflow.
Capillaries are one cell thick.
The heart is enclosed in the pericardium.
The pulse reflects heart rate.
Resting heart rate averages 60–100 bpm.
Athletes often have lower resting heart rates.
Blood carries hormones.
The circulatory system includes arteries, veins, and capillaries.
Carbon dioxide is transported to the lungs for removal.
Blood donation is generally safe.
Clotting involves a complex protein cascade.
The heart pumps continuously throughout life.
🫁 Lungs & Respiration (81–110)
Humans have two lungs.
The right lung has three lobes.
The left lung has two lobes.
Lungs contain about 300 million alveoli.
Gas exchange occurs in the alveoli.
The diaphragm controls breathing.
Adults breathe 12–20 times per minute.
Oxygen diffuses into the bloodstream in the lungs.
Carbon dioxide is exhaled.
The lungs’ surface area is about 70 square meters.
Sneezing can reach speeds over 150 km/h.
Coughing protects airways.
Smoking damages lung tissue.
The lungs are covered by the pleura.
Breathing is automatic but can be controlled voluntarily.
Asthma narrows airways.
Hiccups are diaphragm spasms.
Yawning may help regulate oxygen levels.
Exercise increases lung efficiency.
The fetus does not use lungs before birth.
The nose filters incoming air.
Cilia in airways trap particles.
Mucus traps dust and microbes.
The respiratory center is in the brainstem.
Oxygen binds to hemoglobin in red blood cells.
Carbon monoxide binds hemoglobin more strongly than oxygen.
Deep breathing increases oxygen intake.
The voice is produced in the larynx.
Air passes through the trachea to the lungs.
The lungs help regulate blood pH.
🍽 Digestive System (111–140)
Digestion begins in the mouth.
Saliva contains digestive enzymes.
The esophagus moves food via peristalsis.
The stomach produces hydrochloric acid.
Stomach acid has a pH of about 1–2.
The small intestine is about 6 meters long.
Most nutrient absorption occurs in the small intestine.
The large intestine absorbs water.
The liver is the largest internal organ.
The liver detoxifies chemicals.
The pancreas produces insulin.
The gallbladder stores bile.
Digestion can take 24–72 hours.
Gut bacteria aid digestion.
The appendix may support immune function.
Swallowing involves coordinated muscle action.
Taste buds detect sweet, sour, salty, bitter, and umami.
Fiber supports bowel movement.
Vomiting is a protective reflex.
Chewing improves digestion.
Enzymes break down macromolecules.
Lactase digests lactose.
The stomach lining renews frequently.
Hunger is regulated by hormones.
Ghrelin stimulates appetite.
Leptin signals fullness.
The gut communicates with the brain.
Dehydration slows digestion.
The colon houses trillions of microbes.
Bile helps digest fats.
🦴 Bones, Muscles, Skin & Other Systems (141–200)
Adults have 206 bones.
The femur is the strongest bone.
Bones store calcium and phosphorus.
Bone marrow produces blood cells.
The body has over 600 muscles.
Skeletal muscles enable voluntary movement.
Cardiac muscle controls heartbeats.
Smooth muscle lines internal organs.
Tendons connect muscle to bone.
Ligaments connect bone to bone.
Joints allow movement.
Synovial fluid lubricates joints.
Skin is the largest organ.
Skin has three main layers.
Melanin determines skin color.
Sweat glands regulate temperature.
Hair and nails are made of keratin.
Fingernails grow faster than toenails.
Skin renews about every 4 weeks.
Goosebumps result from tiny muscle contractions.
Body temperature averages 37°C (98.6°F).
Fever is an immune response.
Vitamin D is produced in skin via sunlight.
Scars are formed from collagen.
The immune system defends against pathogens.
Antibodies are specific to antigens.
The lymphatic system supports immunity.
Kidneys filter blood.
Each kidney contains about 1 million nephrons.
The bladder stores urine.
Urine removes waste products.
Hormones are secreted by endocrine glands.
The thyroid regulates metabolism.
The adrenal glands release stress hormones.
Insulin regulates blood sugar.
The pancreas has both digestive and endocrine roles.
Sperm is the smallest human cell.
The ovum is the largest human cell.
Fertilization forms a zygote.
Cells divide through mitosis.
Meiosis produces reproductive cells.
Apoptosis is programmed cell death.
Cancer involves uncontrolled cell division.
The body maintains homeostasis.
Collagen is the most abundant protein in the body.
Water makes up about 60% of body weight.
The human body contains trillions of cells.
Blood carries nutrients and oxygen.
The endocrine system regulates hormones.
The reproductive system enables reproduction.
The skeletal system provides structure.
The muscular system enables movement.
The nervous system controls body functions.
The respiratory system exchanges gases.
The digestive system processes food.
The circulatory system transports substances.
The urinary system removes waste.
The immune system protects the body.
The integumentary system includes skin, hair, and nails.
All body systems work together to sustain life.