The human body is a living organism made up of roughly 30 trillion cells working together across multiple levels of organization, from individual atoms all the way up to interconnected organ systems. It maintains itself through constant internal regulation, burns energy even at complete rest, and hosts nearly as many bacterial cells as human ones. Here’s what it’s actually made of and how it all fits together.
What the Body Is Made Of
At the most basic level, the human body is a collection of chemical elements. Oxygen accounts for about 65% of your body mass, mostly because it’s a major component of water and is locked into the structure of proteins, fats, and carbohydrates. Carbon makes up roughly 18.5%, hydrogen about 9.5%, and nitrogen around 3.3%. Calcium contributes about 1.5%, concentrated mainly in bones and teeth. Dozens of other elements, including iron, potassium, sodium, and zinc, make up the remaining fraction, each playing outsized roles despite their tiny quantities.
Water is the body’s most abundant molecule. In normal-weight adult men, total body water sits around 62% of body weight. For adult women, it’s closer to 54%, largely because women tend to carry a higher proportion of body fat, which contains less water than muscle. After age 60, water content drops in both sexes: to about 57% in men and 50% in women. Children maintain water percentages similar to adult men, around 62%.
Six Levels of Organization
Biologists describe the body as being organized into six levels of increasing complexity. The simplest is the chemical level: atoms of elements like carbon and oxygen combine into molecules such as water, proteins, and sugars. These molecules are the building blocks of every structure in the body.
Next is the cellular level. A cell is the smallest independently functioning unit of a living organism. Each human cell is enclosed by a flexible membrane and filled with a water-based fluid containing tiny specialized structures called organelles that carry out specific jobs, from generating energy to copying DNA.
Groups of similar cells form tissues, the third level. Your body has four primary tissue types: muscle, nerve, connective, and epithelial (the sheets of cells lining surfaces and cavities). When two or more tissue types combine into a distinct structure with a specific job, that structure becomes an organ, the fourth level. Your heart, for example, is built from muscle tissue, nerve tissue, and connective tissue all working in concert.
Organs that cooperate to perform a major function form an organ system, the fifth level. And all organ systems functioning together make up the organism, the sixth and highest level: you, a single living being capable of independently performing every function necessary for life.
The 11 Major Organ Systems
The body runs on 11 organ systems, each handling a different set of responsibilities:
- Cardiovascular: pumps blood throughout the body, delivering oxygen and nutrients to cells while removing carbon dioxide and waste.
- Respiratory: brings air into the body, adds oxygen to the blood, and removes carbon dioxide.
- Nervous: directs both intentional and automatic actions, and enables thinking, self-awareness, and emotions.
- Musculoskeletal: provides structural support and allows movement.
- Integumentary (skin): acts as a barrier between the internal body and the outside environment, and helps regulate temperature.
- Digestive: extracts nutrients from food and excretes solid waste.
- Endocrine: produces hormones, chemical messengers carried in the blood that direct the activities of other systems.
- Urinary: filters waste products from the blood and excretes them as urine.
- Reproductive (male): produces sperm and male hormones.
- Reproductive (female): produces eggs and provides the environment for a fertilized egg to develop.
These systems don’t work in isolation. Breathing brings in oxygen, the cardiovascular system distributes it, the nervous system adjusts breathing rate based on demand, and the endocrine system can override all of them during a stress response. The body’s real complexity lies in how these systems overlap and communicate.
Cells, Bones, and Skin by the Numbers
The average adult body contains about 30 trillion human cells. But you’re also host to roughly 38 trillion bacteria, most of them living in the gut. That means bacterial cells slightly outnumber human cells at a ratio of about 1.3 to 1. These microbes aren’t freeloaders; they help digest food, train the immune system, and produce vitamins your own cells cannot make.
A newborn enters the world with 275 to 300 bones, many of which are made of cartilage, a tough but rubbery material. As a child grows, smaller bones gradually fuse together and harden through a process called ossification. By adulthood, the total count has settled to 206 bones.
Skin is the body’s largest organ. In an average adult, it covers 1.5 to 2 square meters of surface area and weighs between 3.5 and 10 kilograms (roughly 7.5 to 22 pounds), depending on body size. It serves as the first line of defense against infection, physical damage, and ultraviolet radiation, while also helping regulate body temperature through sweating and changes in blood flow.
How the Body Regulates Itself
The body doesn’t simply match its surroundings. When the outside temperature drops, you don’t cool down to match; instead, multiple systems kick in to keep your core temperature stable. Blood vessels near the skin constrict to reduce heat loss, muscles contract rapidly (shivering) to generate warmth, and tiny muscles at the base of hair follicles contract to create goose bumps, a leftover reflex from when our ancestors had thicker body hair.
When you overheat, the reverse happens. Blood vessels near the skin dilate to release more heat into the air, and sweat glands push moisture onto the surface, where evaporation pulls heat away from the body. This kind of self-correction is called homeostasis, and it applies to nearly everything: blood sugar, blood pressure, hydration, pH, and oxygen levels are all kept within tight ranges by similar feedback loops running constantly in the background.
Where the Body Spends Its Energy
Even when you’re lying perfectly still, your body burns a significant amount of energy just to stay alive. This baseline is called your basal metabolic rate. What’s surprising is how unevenly that energy is distributed. The brain, which makes up only about 2% of body weight, burns roughly 240 calories per kilogram per day. The heart and kidneys are even more demanding at about 440 calories per kilogram per day each, though their small size keeps the absolute totals lower. The liver uses around 200 calories per kilogram daily.
Skeletal muscle, by contrast, burns only about 13 calories per kilogram per day at rest. But because muscle makes up a large share of total body mass, it still accounts for a meaningful portion of resting energy use. Fat tissue is the least metabolically active at roughly 4.5 calories per kilogram per day, which is one reason body composition affects how many calories you burn at rest.
The Body Is Still Being Mapped
You might assume that anatomy was fully charted centuries ago, but questions about the body’s structure still come up. In 2016 and 2018, the mesentery (a fold of tissue anchoring the intestines) and the interstitium (fluid-filled spaces within connective tissues) were both described in popular press as newly discovered organs. Anatomists pushed back, noting that neither structure meets the traditional definition of an organ, which requires two or more tissue types performing a special function. Both structures are real and important, but calling them organs overstates the case. The debate is a reminder that even something as familiar as the human body still generates genuine scientific disagreement about how to classify what’s inside it.