The human brain is roughly 73 to 75 percent water by weight. The rest is a dense mix of fat, protein, carbohydrates, and dissolved minerals and salts. Despite weighing only about three pounds (around 2% of total body weight), the brain consumes about 20% of the body’s oxygen and calories, making it the most metabolically demanding organ you have.
Water, Fat, and Protein
Water is the brain’s dominant ingredient, making up nearly three quarters of its total mass. This water isn’t just filler. It serves as the medium for every chemical reaction in the brain, carries nutrients to cells, and helps flush out waste products. Even mild dehydration can impair concentration and short-term memory, which makes sense given how water-dependent brain tissue is.
Fat is the next largest component, accounting for about 10 to 12 percent of the brain’s total weight. When you remove the water and look only at the brain’s dry weight, fat makes up roughly half of what’s left. The brain contains more cholesterol than any other organ, and it relies heavily on specialized fats called phospholipids that form the membranes surrounding every brain cell. One particularly important fat is an omega-3 fatty acid (DHA) concentrated in cell membranes, where it helps keep them flexible and responsive to signals.
Proteins round out the major components. They serve as the building blocks for enzymes, receptors, and the chemical messengers (neurotransmitters) that brain cells use to communicate. Smaller amounts of carbohydrates and minerals fill in the rest.
The Cells Inside Your Brain
The brain contains roughly 86 billion neurons, the cells responsible for sending and receiving electrical signals. Each neuron connects to thousands of others through junctions called synapses, creating a communication network of staggering complexity. But neurons aren’t the only cells in the brain. Glial cells, which support, insulate, and nourish neurons, are at least as numerous and possibly several times more so, though the exact ratio is still debated in neuroscience.
Neurons have a few key parts. The cell body handles the neuron’s basic metabolic functions. Dendrites branch out like tree limbs to receive incoming signals from other neurons. And axons, which can range from a fraction of a millimeter to over a meter long in the spinal cord, carry outgoing signals to the next cell in the chain.
Gray Matter vs. White Matter
If you sliced the brain open, you’d see two visually distinct types of tissue. Gray matter, which has a grayish-pink color, is packed with neuron cell bodies, dendrites, and short axons. This is where the brain does its heavy computational work: processing sensory input, forming memories, making decisions.
White matter looks lighter because its axons are wrapped in myelin, a fatty coating that acts like insulation on an electrical wire. Myelin dramatically speeds up signal transmission, allowing distant brain regions to communicate quickly. White matter essentially functions as the brain’s internal wiring system, connecting different areas of gray matter to each other and linking the brain to the spinal cord and the rest of the body. Gray matter tissue contains more blood (about 4 to 6 percent of its volume) than white matter (1 to 3 percent), reflecting its higher energy demands.
Minerals That Keep Signals Flowing
The brain depends on a handful of dissolved minerals to generate and transmit electrical signals. Sodium and potassium are the most critical pair. Neurons fire by rapidly swapping these two ions across their cell membranes. When a neuron “fires,” positively charged sodium ions rush in, creating a brief electrical spike that travels down the axon. Potassium ions then flow out to reset the cell for the next signal. This cycle happens millions of times per second across your brain.
Calcium plays a different but equally important role. Inside neurons, calcium ions trigger the release of neurotransmitters into synapses, which is how one neuron passes a message to the next. Disruptions in calcium balance have been linked to cognitive decline during normal aging and may contribute to neurodegenerative diseases.
Zinc is present at unusually high levels in brain tissue. Most of it is locked into proteins where it supports cell metabolism, but free zinc also floats inside tiny pouches called synaptic vesicles, where it helps regulate signaling between neurons. Magnesium supports the enzymes that pump sodium, potassium, and calcium in and out of cells, making it essential for the whole electrical system to function properly.
How the Brain Fuels Itself
Your brain runs almost exclusively on glucose, the simple sugar your body extracts from food. Although the brain makes up just 2 percent of your body weight, it burns through about 20 percent of your total calorie intake and oxygen supply. Most of that energy goes toward maintaining the electrical gradients neurons need to fire. Pumping sodium and potassium ions back into position after each signal is expensive work, and billions of neurons doing it simultaneously adds up fast.
The brain has almost no energy reserves of its own. It depends on a constant supply of glucose and oxygen delivered through an extraordinarily dense network of blood vessels. Blood occupies roughly 4 to 6 percent of gray matter volume at any given moment, ensuring that the most active brain regions always have fuel on hand. When blood flow to a region drops even briefly, those neurons begin to malfunction within seconds, which is why strokes cause damage so quickly.
The Protective Layers
The brain itself is soft, with a consistency often compared to firm gelatin. It doesn’t protect itself. Instead, it sits inside a series of defensive layers. The outermost is the skull, obviously, but between bone and brain tissue lie three membranes collectively called the meninges.
The dura mater is the outermost membrane, a tough, thick layer of connective tissue that sits directly under the skull. Beneath it is the arachnoid mater, a thinner middle layer with web-like projections extending inward. The innermost layer, the pia mater, clings directly to the brain’s surface, following every fold and groove. Between the arachnoid and pia layers sits a space filled with cerebrospinal fluid, a clear liquid that cushions the brain against sudden impacts, delivers nutrients, and carries away metabolic waste. Your brain essentially floats in this fluid, which reduces its effective weight and acts as a shock absorber during everyday movement.