An organ is made of two or more types of tissue working together to perform a specific function. Those tissues are, in turn, made of cells, and cells are built from molecules like proteins, fats, and water. So the short answer is: an organ is layers of organized tissue held together by a structural scaffold, threaded with blood vessels and nerves, and saturated with water.
To really understand what you’re looking at when you look at an organ, it helps to zoom in and out through several levels of organization.
The Four Tissue Types That Build Every Organ
The human body has four basic tissue types, and every organ uses at least two of them in combination.
- Epithelial tissue covers surfaces and lines internal passages. It forms the outer layer of your skin, the lining of your stomach, and the inner walls of your blood vessels.
- Connective tissue supports and binds everything together. This category is broad: it includes bone, cartilage, fat, blood, and the tough fibrous sheets that wrap around organs.
- Muscle tissue contracts to produce movement. Skeletal muscle moves your bones, while smooth muscle lines organs like the stomach and intestines to push contents through.
- Nervous tissue carries electrical signals. Nerve cells (neurons) run through virtually every organ, relaying information to and from the brain.
What makes each organ unique isn’t a special fifth material. It’s the specific ratio and arrangement of these same four tissues. The heart is mostly muscle tissue with a lining of epithelial tissue, a framework of connective tissue, and a dense network of nerves. The liver, by contrast, is dominated by specialized epithelial cells that filter blood, supported by connective tissue and laced with blood vessels.
Parenchyma vs. Stroma: The Two Halves of Every Organ
Anatomists divide organ tissue into two categories. The parenchyma is the functional tissue, the cells actually doing the organ’s job. In the liver, that’s the cells filtering toxins. In the lungs, it’s the thin-walled air sacs exchanging oxygen and carbon dioxide. Parenchyma usually makes up the bulk of the organ.
The stroma is everything else: the connective tissue framework, blood vessels, nerves, and ducts that support and supply the parenchyma. Think of stroma as the scaffolding and plumbing of a building, while the parenchyma is the people inside doing the work. Neither functions without the other.
The Extracellular Matrix: The Glue Between Cells
Cells don’t just float next to each other inside an organ. They’re embedded in a non-cellular material called the extracellular matrix, which exists in every tissue and organ in the body. This matrix provides physical scaffolding and also sends chemical signals that help cells know where they are and what to do.
The matrix is made of two main classes of molecules. The first is fibrous proteins. Collagen is the most abundant, making up roughly 30% of all protein in the body. It provides tensile strength, the resistance to being pulled apart. Elastin fibers sit alongside collagen and give tissues the ability to snap back after being stretched, which is critical in organs like the lungs and arteries that expand and contract constantly. A third protein, fibronectin, helps cells attach to the matrix and organize themselves properly.
The second class is proteoglycans, large sugar-coated molecules that absorb water and form a gel-like substance filling the spaces between fibers. This hydrated gel cushions cells, allows nutrients to diffuse through the tissue, and resists compression. Together, the protein fibers and gel create a flexible, resilient meshwork that holds the organ’s shape while allowing it to move and deform as needed.
Blood Vessels and Nerves Run Through Everything
No organ can function without a blood supply and nerve connections woven into its interior. Blood vessels branch into progressively smaller channels until they become capillaries, thin enough for oxygen and nutrients to pass through their walls and reach individual cells. This network penetrates deep into even the densest tissues. In bone, for example, arteries enter through small openings and branch through internal canals that run throughout the hard tissue.
Nerve fibers follow a similar pattern, often running alongside blood vessels and sharing the same entry points into the organ. In bone tissue, nerves penetrate through the same canals as blood vessels, with most nerve endings arranged around arteries and capillaries while some scatter into the marrow. This pairing of nerves and blood vessels is a recurring theme across organs: the two systems are physically and functionally coupled, with nerves helping regulate blood flow and blood vessels supplying the nerves themselves.
Mostly Water, Then Protein
At the chemical level, organs are overwhelmingly water. The brain and heart are about 73% water. Lungs are roughly 83%. Muscles and kidneys sit around 79%, and skin is about 64%. Even bones, the hardest structures in the body, are 31% water. This water isn’t just filler. It’s the medium in which every chemical reaction takes place, the solvent that carries nutrients and waste, and a major component of the gel-like matrix between cells.
After water, the next largest component by mass is protein. Collagen alone accounts for a significant share, but thousands of other proteins handle everything from catalyzing chemical reactions to transporting molecules across cell membranes. Fats make up cell membranes and insulating layers, while minerals like calcium and phosphorus harden bone tissue. A healthy kidney, for reference, has an interstitial fluid volume of about 7 to 8% of its total volume, meaning the spaces between cells are filled with a thin layer of fluid that bathes every cell surface.
Skin as an Example
Skin is a good case study because it’s the organ you can see and touch. It has three distinct layers, each built from different tissues. The epidermis, the outermost layer, is epithelial tissue. Its cells produce keratin, a tough protein that makes the surface water-resistant and protective. Below that, the dermis is dense connective tissue rich in collagen for strength and elastin for flexibility. Blood vessels, nerve endings, hair follicles, and sweat glands all sit in this middle layer. The deepest layer, the hypodermis, is mostly fat cells cushioning the body and connecting the skin to the muscles and bones underneath.
This layered architecture, with epithelial tissue on the outside, connective tissue providing structure, nerves carrying sensation, and blood vessels delivering nutrients, is a pattern repeated in nearly every organ. The specifics change (the liver doesn’t need keratin, and the heart doesn’t need hair follicles), but the organizational logic is the same.
Scale of Construction
The sheer number of cells involved is staggering. A single gram of liver tissue contains roughly 100 million cells. An adult liver weighs about 1.4 kilograms, which puts the total cell count for just that one organ in the hundreds of billions. Each of those cells is individually wrapped in a membrane, connected to its neighbors through the extracellular matrix, and supplied by a capillary never more than a fraction of a millimeter away.
This is what makes an organ fundamentally different from a simple tissue: it’s not just a collection of similar cells, but a precisely organized structure where multiple tissue types, a protein scaffold, a vascular network, and a nervous system all integrate into a single functional unit.