The human body contains roughly 80 organs, each built from multiple tissue types working together to perform a specific job. Some are essential for survival, others support digestion or movement, and a few have only recently been recognized as organs at all. Here’s a clear breakdown of what counts as an organ, which ones matter most, and how they’re organized.
What Makes Something an Organ
An organ is a collection of tissues that form a structural unit specialized to perform a particular function. Your kidneys, liver, and heart all qualify because each has a defined shape, a fixed location in the body, and a clear job. This distinguishes organs from simpler structures like individual muscles or blood vessels, which are made of fewer tissue types. Cells group into tissues, tissues combine into organs, and organs coordinate within larger organ systems.
The total count depends on how you draw the lines. If the interstitium, a network of fluid-filled compartments beneath the skin proposed as an organ in 2018, gains full acceptance, the count reaches 80. As pathologist Neil Theise of NYU put it, “There’s no governing body that says, ‘Yes, this meets the definition of an organ.’ It’s how the collective community treats it over time.”
The Five Vital Organs
Five organs are considered essential for life. You cannot survive if any of them fails completely.
- Brain: Controls thought, movement, sensation, and every automatic process from breathing to temperature regulation. It consumes about 20% of the body’s energy despite making up roughly 2% of body weight.
- Heart: Pumps blood through roughly 60,000 miles of blood vessels, delivering oxygen and nutrients to every cell. It beats about 100,000 times per day without rest.
- Lungs: Exchange oxygen and carbon dioxide with each breath. You take around 20,000 breaths daily, and the total internal surface area of the lungs is roughly the size of a tennis court.
- Liver: Filters toxins from the blood, produces bile for digestion, stores energy, and manufactures proteins the body needs for clotting and immune function. It performs over 500 distinct tasks.
- Kidneys: Filter about 50 gallons of blood per day, removing waste and balancing fluid levels. They also regulate blood pressure and produce hormones that stimulate red blood cell production.
Solid Organs vs. Hollow Organs
Anatomists split abdominal organs into two structural categories. Solid organs are dense tissue throughout: the liver, kidneys, spleen, pancreas, and gallbladder all fall into this group. Hollow organs are tubes or pouches with open interior spaces. The stomach, small intestine, large intestine, and bladder are the primary hollow organs.
This distinction matters practically because the two types behave differently when injured. A damaged solid organ bleeds internally. A ruptured hollow organ spills its contents (stomach acid, bile, or waste) into the abdominal cavity, which can trigger serious infection.
The 11 Major Organ Systems
Organs don’t work in isolation. They’re grouped into 11 systems, each handling a broad category of body function. Many organs belong to more than one system.
- Circulatory system: Heart, blood vessels, and blood. Delivers oxygen and removes waste.
- Respiratory system: Lungs, trachea, bronchi, and diaphragm. Handles gas exchange.
- Digestive system: Mouth, esophagus, stomach, intestines, liver, pancreas, and gallbladder. Breaks down food and absorbs nutrients.
- Nervous system: Brain, spinal cord, and nerves. Processes information and coordinates responses.
- Endocrine system: Thyroid, adrenal glands, pancreas, pituitary gland, and others. Produces hormones that regulate metabolism, growth, and mood.
- Musculoskeletal system: Bones, muscles, tendons, and ligaments. Provides structure and movement.
- Integumentary system: Skin, hair, and nails. Protects against infection, regulates temperature, and senses the environment.
- Urinary system: Kidneys, ureters, bladder, and urethra. Filters blood and excretes waste.
- Reproductive system: Ovaries and uterus (female) or testes (male). Produces hormones and enables reproduction.
- Lymphatic/immune system: Spleen, thymus, lymph nodes, and tonsils. Fights infection and drains excess fluid.
- Respiratory system often overlaps with the circulatory system since gas exchange depends on blood flow through the lungs.
The Largest and Smallest Organs
Skin is the body’s largest organ. In an average adult, it weighs about 3.9 kilograms (roughly 8.5 pounds) and covers approximately 1.7 square meters of surface area. That’s about 5.5% of total body weight. Skin serves as the first barrier against bacteria, ultraviolet radiation, and water loss, while also housing nerve endings for touch, pain, and temperature.
The pineal gland is one of the smallest. Tucked deep in the brain, it measures just 5 to 10 millimeters long and weighs between 80 and 150 milligrams, roughly the weight of a single grain of rice. Despite its size, it plays an outsized role in regulating your sleep-wake cycle by producing melatonin.
Organs You Can Live Without
Not every organ is essential. You can live a normal life after losing your spleen, gallbladder, appendix, one kidney, one lung, or portions of your stomach and intestines. The body compensates remarkably well. A remaining kidney enlarges slightly and takes over the full filtering workload. After gallbladder removal, bile flows directly from the liver into the small intestine instead of being stored.
The liver stands out for its regenerative ability. In surgical models, up to two-thirds of the liver can be removed, and the remaining tissue regrows to near its original mass in under two weeks. Even when 80% to 90% is removed, cells from the bile ducts can transform into liver cells and repopulate the organ. No other internal organ regenerates at this scale.
Recently Recognized Organs
The list of organs is not fixed. The mesentery, a fan-shaped fold of tissue that anchors the intestines to the abdominal wall, was long considered a collection of separate fragments. Closer study revealed it is one continuous structure with a clear purpose: it supports, nourishes, and physically connects the digestive organs. Sections once given separate names, like the mesocolon and mesorectum, are now understood as branches of a single organ.
The interstitium is another candidate. Described in 2018, it refers to a body-wide network of fluid-filled spaces found beneath the skin, lining the digestive tract, and surrounding muscles and blood vessels. If widely accepted as the body’s 80th organ, it would also be one of the largest, since these compartments exist nearly everywhere. Researchers believe it may play a role in how cancer cells spread through the body, though its exact functions are still being mapped.