The human brain is roughly 73 to 75 percent water. The remaining quarter of its weight comes from fats, proteins, and a smaller fraction of salts and carbohydrates. That mix of ingredients, organized into billions of cells and wrapped in protective membranes, creates the most complex structure we know of in the natural world. Here’s what it actually looks like when you break it down layer by layer.
Water, Fat, and Protein
By sheer mass, water dominates. Nearly three-quarters of brain tissue is water, which is why even mild dehydration can affect concentration, mood, and reaction time. The brain’s water isn’t just filler; it’s the medium that carries dissolved nutrients, ions, and signaling molecules between cells.
Fat is the next largest component, making up 10 to 12 percent of total brain weight. That makes the brain one of the fattiest organs in the body, and the fat it contains is highly specialized. Cholesterol and phospholipids form the membranes that wrap around every single brain cell. A particular type of fatty insulation called myelin coats the long fibers that carry electrical signals. Myelin is unusual because its dry weight is 70 to 85 percent lipid, far more than a typical cell membrane. Within myelin, roughly 40 percent of the lipid is cholesterol, another 40 percent is phospholipids, and the remaining 20 percent is a class of fats called glycolipids. This lipid-heavy composition is what allows electrical signals to travel quickly through long nerve fibers.
Proteins round out the picture. They serve as the structural scaffolding of cell membranes, the machinery for neurotransmitter production, and the receptors that allow cells to communicate with each other. A network of proteins also exists outside of cells, forming a mesh called the extracellular matrix. This mesh, made largely of proteoglycans, collagens, and laminins, holds cells in place, guides the growth of new connections, and helps regulate signaling between brain regions.
Grey Matter and White Matter
If you slice a brain in half, you’ll notice two distinct types of tissue. The outer surface and certain deeper structures appear grayish-pink. This is grey matter, and it’s where most of the brain’s processing happens. Grey matter is packed with the cell bodies of neurons, along with short branching fibers called dendrites that receive incoming signals. It has a pinkish tint because it lacks myelin, exposing the blood-rich tissue underneath.
White matter sits deeper inside the brain and gets its pale color from myelin, the fatty insulation described above. White matter is essentially the brain’s wiring system. It consists of long nerve fibers (axons) bundled together, carrying messages between different grey matter regions and between the brain and the rest of the body. Think of grey matter as the processing centers and white matter as the cables connecting them.
Neurons: The Signaling Cells
Neurons are the cells most people think of when they picture the brain. Each neuron has a cell body, branching dendrites that receive signals, and a long axon that sends signals onward. When a neuron fires, an electrical impulse races down its axon and triggers the release of chemical messengers (neurotransmitters) at the junction with the next cell. This electrochemical relay is how the brain processes everything from vision to memory to movement.
The commonly cited number is around 86 billion neurons in the adult brain, though they aren’t evenly distributed. The cerebellum, a fist-sized structure at the back of the brain responsible for coordinating movement, contains the majority of neurons despite being only about 10 percent of total brain volume. The cerebral cortex, the wrinkled outer layer responsible for thought, language, and perception, has fewer neurons but far more complex wiring between them.
Glial Cells: The Other Half
For decades, scientists believed glial cells outnumbered neurons by ten to one. More recent counting methods put the real ratio much closer to one-to-one: at least 50 percent of all cells in the human brain are glia. That still means tens of billions of glial cells, and they do far more than provide structural support.
Astrocytes are the most abundant glial type in most brain regions. They’re star-shaped cells that sit at the junctions between neurons, forming what scientists call a “tripartite synapse,” a three-way connection between two neurons and one astrocyte. In this role, astrocytes help regulate the chemical environment around synapses, supply neurons with the raw materials needed to make neurotransmitters, and clean up signaling chemicals after a message has been sent. They also help form the blood-brain barrier, the selective filter that prevents most substances in the bloodstream from entering brain tissue.
Oligodendrocytes are the cells that produce myelin. Each oligodendrocyte can extend its membrane to wrap around segments of multiple axons at once, creating the insulating layers that allow electrical signals to travel up to 100 times faster than they would on uninsulated fibers. When oligodendrocytes are damaged, as in multiple sclerosis, signal transmission slows or stops entirely.
Microglia make up roughly 5 to 15 percent of all brain cells and serve as the brain’s immune system. They constantly scan their surroundings, clearing out dead cells, pathogens, and cellular debris. But they aren’t only cleanup crews. Microglia also influence how synapses form and mature, pruning unnecessary connections during development and responding to local neural activity through a wide range of neurotransmitter receptors.
Blood Supply and Energy Demands
The brain weighs about 3 pounds, roughly 2 percent of body weight, yet it consumes around 20 percent of the body’s total energy at rest. Nearly all of that energy comes from glucose delivered by the bloodstream. A dense network of blood vessels threads through every part of the brain, and any interruption in blood flow, even for a few minutes, can cause permanent damage to the affected area.
The blood-brain barrier, formed partly by astrocytes wrapping around blood vessels, is selective about what enters the brain from the bloodstream. Small molecules like oxygen and glucose pass through easily. Larger molecules, most bacteria, and many drugs are blocked. This protection is essential, but it also makes delivering medication to brain tissue a significant challenge in treating neurological diseases.
How It All Fits Together
What makes the brain remarkable isn’t any single ingredient but how these components are organized. Neurons and glia are woven together into circuits, with grey matter hubs connected by white matter highways. The whole system floats in cerebrospinal fluid, a clear liquid that cushions the brain, removes waste, and circulates nutrients. Three layers of protective membranes called meninges sit between the brain and the skull.
At its most basic, the brain is water, fat, and protein arranged into cells that communicate through electricity and chemistry. That deceptively simple recipe produces every thought, sensation, memory, and movement you’ll ever experience.