Glossary of Parts

The Terminology You Never Asked For

The Frame (Skeletal & Muscular Systems)

Bone marrow

Soft tissue inside bones. Red marrow produces blood cells. Yellow marrow stores fat. Your bones are not sticks; they are factories.

Cartilage

Smooth, rubbery connective tissue that cushions joints and prevents bone-on-bone contact. Notably does not regenerate well in adults, which is why joint damage tends to be permanent.

Collagen

The most abundant protein in your body. Provides structural support to skin, bones, tendons, and ligaments. Production declines with age, and the anti-aging industry would like to sell it back to you.

Fascia

Connective tissue that wraps muscles, organs, and other structures like biological shrink wrap. Increasingly recognized as important in pain and movement.

Ligaments

Connect bone to bone. Stabilize joints. Heal slowly when damaged because of limited blood supply.

Osteoporosis

Loss of bone density, making bones fragile and fracture-prone. Common after menopause. Preventable (partially) with weight-bearing exercise and adequate calcium and vitamin D.

Sarcopenia

Age-related loss of muscle mass and strength. Begins around age 30. Significantly slowed by resistance training. The aging chapter discussed this at length.

Tendons

Connect muscle to bone. Transfer the force your muscles generate into movement. Prone to overuse injuries (tendinitis).

The Engine Room (Cardiovascular & Respiratory)

Alveoli

Tiny air sacs in the lungs where gas exchange occurs. You have roughly 480 million of them, providing a surface area approximately the size of a tennis court.

Aorta

The largest artery in the body, carrying oxygenated blood from the left ventricle to the rest of you.

Arteries

Blood vessels that carry blood away from the heart. Generally carry oxygenated blood (exception: pulmonary arteries).

Atherosclerosis

Buildup of plaque (fat, cholesterol, calcium, other debris) inside artery walls. The slow, silent process behind most heart attacks and strokes.

Atria

The two upper chambers of the heart. They receive blood — the right atrium from the body, the left atrium from the lungs.

Capillaries

The smallest blood vessels. One cell thick. Where the actual exchange of oxygen, nutrients, and waste occurs between blood and tissues.

Diaphragm

The dome-shaped muscle beneath the lungs that drives breathing. When it contracts, it pulls down, creating negative pressure that draws air in. You do this about 20,000 times per day without thinking about it.

Hemoglobin

The iron-containing protein in red blood cells that binds to oxygen and gives blood its red color. Also binds carbon monoxide much more readily than oxygen, which is why carbon monoxide poisoning works.

Platelets

Cell fragments that clump together to form blood clots. Essential for stopping bleeding. Problematic when they form clots inside intact blood vessels.

Veins

Blood vessels that carry blood toward the heart. Contain one-way valves to prevent backflow, because blood returning from your feet has to travel uphill.

Ventricles

The two lower chambers of the heart. They pump blood — the right ventricle to the lungs, the left ventricle to the entire body. The left ventricle is the hardest-working chamber and the one most commonly involved in heart failure.

The Processing Plant (Digestive System)

Bile

Produced by the liver, stored in the gallbladder, released into the small intestine to emulsify fats. Basically biological dish soap.

Enzyme

A protein that speeds up a specific chemical reaction. Digestive enzymes break food into absorbable components. Your body uses thousands of different enzymes for thousands of different purposes.

Esophagus

The tube connecting your throat to your stomach. Moves food via peristalsis (coordinated muscle contractions), which is why you can swallow while standing on your head, though there is rarely a good reason to do so.

Gallbladder

Small organ that stores concentrated bile. Can develop gallstones. Can be removed without major consequence, making it one of the few organs whose absence you may not notice.

Gut microbiome

The trillions of bacteria, fungi, and other microorganisms that live in your intestines. They help digest food, produce vitamins, train your immune system, and influence your mood. You are, in a meaningful sense, an ecosystem.

Liver

The largest internal organ. Performs over 500 functions including detoxification, protein synthesis, bile production, and metabolizing drugs. Remarkably capable of regeneration. Nevertheless not indestructible.

Pancreas

Produces digestive enzymes (exocrine function) and hormones including insulin and glucagon (endocrine function). Performs double duty across two bodily systems, which is either efficient or concerning depending on your perspective.

Peristalsis

Involuntary, wave-like muscle contractions that move food through the digestive tract. Your gut is always doing this, whether you are aware of it or not.

The Filtration Plant (Kidneys & Urinary)

Bladder

Muscular sac that stores urine. Capacity approximately 16 ounces (473 mL), though the urge to urinate begins well before it is full.

Kidneys

Paired organs that filter blood, regulate fluid balance, control electrolytes, manage blood pressure, and produce hormones. Filter approximately 45 gallons (170 liters) of blood per day. You can survive with one. You cannot survive with none.

Nephron

The functional unit of the kidney. Each kidney contains about 1 million of them. They do the actual filtering. You cannot grow new ones.

The Messengers (Endocrine System)

Adrenal glands

Sit on top of the kidneys. Produce cortisol, adrenaline (epinephrine), and other hormones. Responsible for the stress response. Not responsible for “adrenal fatigue,” which is not a recognized medical diagnosis.

Cortisol

The primary stress hormone. Raises blood sugar, suppresses the immune system, and increases alertness. Useful in emergencies. Damaging when chronically elevated.

Estrogen

Primary female sex hormone. Affects bone density, cardiovascular health, mood, and dozens of other systems. Declines sharply at menopause.

Hormones

Chemical messengers produced by glands, transported by blood, and received by target cells with matching receptors. They are not a router that needs rebooting.

HPA axis

Hypothalamus-Pituitary-Adrenal axis. The neuroendocrine system that controls the stress response. When chronically activated, it produces widespread physiological damage. Stress is not “just in your head” — it runs through this system and affects everything.

Insulin

Hormone produced by the pancreas that allows cells to absorb glucose from the blood. Insufficient insulin (Type 1 diabetes) or cellular resistance to insulin (Type 2 diabetes) results in dangerously elevated blood sugar.

Testosterone

Primary male sex hormone, also present in smaller amounts in women. Affects muscle mass, bone density, mood, and libido. Declines gradually in men after age 30.

Thyroid

Butterfly-shaped gland in the neck that produces hormones regulating metabolism, energy, and body temperature. When overactive (hyperthyroidism) or underactive (hypothyroidism), the effects are systemic and significant.

The Defense Network (Immune System)

Antibodies

Proteins produced by B cells that bind to specific pathogens and mark them for destruction. The basis of vaccine-induced immunity.

Antigens

Molecules on the surface of pathogens (or other foreign substances) that the immune system recognizes as “not self.”

B cells

White blood cells that produce antibodies. Part of the adaptive immune system.

Cytokines

Signaling proteins released by immune cells that coordinate the immune response. Responsible for many symptoms of illness (fever, fatigue, aches). When overproduced, they cause a “cytokine storm,” which can be fatal.

Inflammation

The immune system’s first response to injury or infection. Redness, heat, swelling, pain. Acute inflammation is protective. Chronic inflammation is destructive.

Lymph nodes

Small structures throughout the body that filter lymph fluid and house immune cells. Swell when you are fighting an infection — this is normal and means they are working.

T cells

White blood cells that directly kill infected cells (killer T cells) or coordinate the immune response (helper T cells). Trained in the thymus.

Thymus

Organ behind the sternum where T cells mature. Active in childhood, begins shrinking in adolescence, substantially diminished by middle age. A declining training academy.

The Wiring (Nervous System)

Amygdala

Almond-shaped structure in the brain involved in processing fear and other emotions. Responsible for the fight-or-flight response. Fast but not always accurate.

Autonomic nervous system

The division of the nervous system that controls involuntary functions: heart rate, digestion, breathing, blood pressure. Divided into sympathetic (accelerator) and parasympathetic (brake).

Axon

The long projection of a neuron that transmits electrical signals to other cells. Can be very long — the sciatic nerve axons run from your lower spine to your feet.

Central nervous system (CNS)

Brain and spinal cord. The command center.

Cerebellum

“Little brain” at the back of the skull. Coordinates movement, balance, and motor learning. Involved in making physical actions smooth rather than jerky.

Dopamine

Neurotransmitter involved in reward, motivation, movement, and learning. Not “the pleasure chemical” — it is more accurately the wanting chemical. Heavily involved in addiction.

Myelin

Fatty sheath that insulates axons and speeds up signal transmission. Damage to myelin (as in multiple sclerosis) disrupts nerve communication.

Neuron

A nerve cell. The basic functional unit of the nervous system. You have roughly 86 billion in your brain alone.

Neurotransmitter

Chemical messenger that transmits signals across synapses between neurons. Major examples: serotonin, dopamine, norepinephrine, GABA, glutamate, acetylcholine.

Parasympathetic nervous system

“Rest and digest.” Slows heart rate, promotes digestion, conserves energy. The brake pedal.

Peripheral nervous system

All the nerves outside the brain and spinal cord. Connects the CNS to the rest of the body.

Prefrontal cortex

Front part of the brain responsible for planning, decision-making, impulse control, and social behavior. The last region to fully mature (mid-twenties). The part of the brain that asks “is this a good idea?” and is frequently overruled.

Serotonin

Neurotransmitter involved in mood, sleep, appetite, and digestion. Most of it is produced in the gut, not the brain, which is one of the reasons the gut-brain connection matters.

Sympathetic nervous system

“Fight or flight.” Increases heart rate, diverts blood to muscles, dilates pupils, releases adrenaline. The accelerator.

Synapse

The tiny gap between neurons across which neurotransmitters transmit signals. The junction where one nerve cell communicates with the next.

The Sensors (Senses)

Cochlea

Spiral-shaped structure in the inner ear that converts sound vibrations into electrical signals. Contains approximately 15,000 hair cells that do not regenerate once damaged.

Cornea

The transparent front surface of the eye. Does most of the focusing work. Contains no blood vessels.

Nociceptors

Sensory receptors that detect potentially damaging stimuli and send pain signals. They detect threat, not damage — an important distinction covered in the wiring chapter.

Presbyopia

Age-related hardening of the lens that makes it difficult to focus on close objects. Affects virtually everyone by their mid-forties. This is why menus get further away.

Retina

The light-sensitive layer at the back of the eye containing rods (low-light, black-and-white vision) and cones (color, detail vision). Converts light into neural signals.

The Continuation Department (Reproductive)

Cervix

The lower, narrow portion of the uterus that connects to the vagina. Screened for cancer via Pap smear.

Ovaries

Paired organs that produce eggs (ova) and hormones (estrogen, progesterone). Contain a finite supply of eggs present from birth.

Prostate

Gland that produces a component of seminal fluid. Tends to enlarge with age. Screened for cancer via PSA test (a complicated decision — see Appendix B).

Testes

Paired organs that produce sperm and testosterone. Located outside the body because sperm production requires a temperature slightly below core body temperature.

Uterus

Muscular organ in which a fertilized egg implants and a fetus develops. Capable of expanding from the size of a fist to accommodating a full-term baby. One of the most mechanically impressive organs in the body.

General & Cross-system Terms

Adenosine

A molecule that accumulates in the brain during waking hours and promotes sleepiness. Caffeine works by blocking adenosine receptors, hiding the signal rather than eliminating the need.

ATP (adenosine triphosphate)

The molecule your cells use as their primary energy currency. Produced mainly in the mitochondria.

Autoimmune disease

A condition in which the immune system attacks the body’s own tissues. Examples: Type 1 diabetes, rheumatoid arthritis, lupus, multiple sclerosis, celiac disease.

Blood-brain barrier

A selective membrane that controls what enters the brain from the bloodstream. Protects the brain from many toxins and pathogens. Also prevents many drugs from reaching the brain, which is one of the challenges of treating neurological diseases.

BMR (basal metabolic rate)

The number of calories your body burns at rest just to maintain basic functions. Accounts for 60 to 70 percent of your total daily energy expenditure.

Calorie

A unit of energy. Technically a kilocalorie (kcal). The amount of energy needed to raise the temperature of 1 kilogram of water by 1 degree Celsius. Your body runs on roughly 2,000 of them per day, though individual needs vary widely.

Circadian rhythm

The approximately 24-hour internal clock that regulates sleep-wake cycles, hormone release, body temperature, and other functions. Primarily driven by light exposure.

DNA (deoxyribonucleic acid)

The molecule that carries your genetic instructions. A double helix. Present in nearly every cell. 99.9 percent identical between any two humans on earth.

Melatonin

Hormone produced by the pineal gland in response to darkness. Signals the brain that it is time to sleep. Not a sedative — a timing signal.

Metabolism

The sum of all chemical reactions in your body that sustain life. Includes both building molecules (anabolism) and breaking them down (catabolism).

Mitochondria

Organelles inside cells that produce ATP (energy). Often called “the powerhouse of the cell,” which is the one thing everyone remembers from biology class and is, to the mitochondria’s credit, accurate.

Telomeres

Protective caps at the ends of chromosomes that shorten with each cell division. When critically short, the cell can no longer divide. Loosely associated with biological aging.

This glossary covers the major terms you will encounter in this manual. It does not cover every term — the human body contains too many parts and processes for any single glossary to be truly comprehensive. If you encounter a term not listed here, the relevant chapter is the best place to find its explanation in context. Alternatively, a reputable medical resource (not the first result of a panicked 2 a.m. search) will serve you well.

Your body is made of all of these parts and runs all of these processes simultaneously, without supervision, while you are busy thinking about what to have for lunch. It is, by any reasonable standard, extraordinary. Now you know what some of the pieces are called.