The Human Organ Guide: What Each One Does

What Is an Organ?

An organ is a distinct body structure made of two or more tissue types, working together to perform a specific set of functions. The heart, brain, liver, lungs, kidneys, stomach, skin — all are organs. So are smaller structures like the spleen, thyroid and adrenal glands.

The human body contains around 79 recognised organs, depending on how you define the boundaries. Many of these are small and specialised; others are large and central to life. This guide introduces the major organs — where they sit, what they do and why they matter.

The Brain

The brain is the command centre of the nervous system and the most complex structure in the known universe, relative to its size. Weighing about 1.4 kg in an average adult, it contains roughly 86 billion neurons connected by trillions of synapses.

The brain controls voluntary movement, processes sensory information, manages language, generates emotions, stores memories and regulates almost every other organ in the body through hormonal and neural signals. Different regions handle different jobs: the frontal lobes are associated with planning and decision-making; the occipital lobes process vision; the cerebellum coordinates balance and fine motor control; the brainstem governs breathing, heart rate and consciousness.

The Heart

The heart is a hollow, muscular pump about the size of a fist, located slightly left of the centre of the chest. Its job is to circulate blood continuously around the body — delivering oxygen and nutrients to every cell and collecting carbon dioxide and waste products for removal.

The heart has four chambers: two upper chambers (atria) that receive blood, and two lower chambers (ventricles) that pump it out. The right side receives oxygen-depleted blood from the body and sends it to the lungs; the left side receives freshly oxygenated blood from the lungs and pumps it out to the body. The heart beats roughly 60–100 times per minute at rest, adding up to around 100,000 beats per day.

To explore cardiovascular function further, try our heart rate calculator.

The Lungs

The two lungs fill most of the chest cavity on either side of the heart. Their primary role is gas exchange: drawing oxygen from inhaled air into the bloodstream and releasing carbon dioxide (a metabolic waste product) from the blood into exhaled air.

Inside the lungs, the airways branch into progressively smaller tubes, ending in tiny air sacs called alveoli. An adult has roughly 480 million alveoli, providing a total gas-exchange surface area of about 70 square metres — roughly the floor area of a small flat. This enormous surface area allows rapid, efficient gas exchange with every breath.

The Liver

The liver sits in the upper right of the abdomen, beneath the ribcage, and is the body's largest internal organ. It is also one of the most functionally diverse — performing over 500 distinct chemical functions, including:

• Processing nutrients absorbed from the digestive system.
• Producing bile, which is stored in the gallbladder and used to digest fats.
• Breaking down alcohol, medications and other toxins.
• Synthesising blood clotting proteins and other plasma proteins.
• Storing glucose as glycogen and releasing it to maintain blood sugar levels.
• Regulating cholesterol levels in the blood.

The liver is also one of the few organs with significant regenerative capacity — it can regrow substantially after surgery or injury, provided the underlying cells are healthy. You can read more in our liver function guide.

The Kidneys

The two kidneys sit at the back of the abdominal cavity, one on each side of the spine. Each is about the size of a fist. Their central role is to filter blood — removing waste products, excess salts and water in the form of urine.

The kidneys filter the entire blood supply around 40 times per day. Within each kidney are roughly one million tiny filtering units called nephrons, which perform the precise work of separating useful substances (returned to the blood) from waste (passed to the bladder). The kidneys also regulate blood pressure, maintain fluid and electrolyte balance, and produce a hormone (erythropoietin) that stimulates red blood cell production. Our kidney function guide covers all of this in depth.

The Digestive Organs

The digestive system is a team of organs working in sequence. Food enters through the mouth, is processed in the stomach and small intestine, and residue exits as waste through the large intestine.

The stomach mixes food with acid and enzymes, breaking it down into a semi-liquid called chyme. The small intestine (about 6–7 metres long) is where most nutrient absorption happens — the inner wall is covered in finger-like projections called villi that maximise surface area. The large intestine absorbs water from the remaining material and compacts it into faeces. The pancreas contributes digestive enzymes, and the liver and gallbladder supply bile for fat digestion.

Where to Go Next

This guide gives a starting-point overview. For each organ, there is much more to explore. The organ learning quiz helps you test and retain what you have read here. For a structured learning path from cells to full systems, follow the beginner anatomy roadmap.

If you are interested in how these organs are studied and explored in anatomy education, the anatomy quiz and body systems explorer are good next steps. For a deeper look at how individual organs connect into functioning systems, explore our digestion and organs category.

Glands and Hormonal Organs

Beyond the major structural organs, the body contains a collection of specialised glands that produce and release hormones — chemical messengers that travel through the bloodstream to regulate distant organs and processes.

The thyroid gland in the neck produces hormones that regulate metabolic rate — how quickly the body converts food to energy. The adrenal glands, sitting on top of the kidneys, produce adrenaline (for the "fight or flight" response) and cortisol (involved in stress response and metabolism). The pancreas has both digestive and hormonal functions: it releases enzymes into the digestive tract and also produces insulin and glucagon, which regulate blood sugar levels.

The pituitary gland, a pea-sized structure at the base of the brain, is sometimes called the "master gland" because it releases hormones that regulate many other glands — including the thyroid, adrenal glands, and the gonads (ovaries and testes). It is itself regulated by the hypothalamus, a region of the brain that links the nervous system to the hormonal system.

Protective and Immune Organs

Several organs contribute primarily to the body's defence. The spleen, located in the upper left abdomen, filters blood, removes aged or damaged red blood cells, and stores white blood cells and platelets. It also plays a role in immune responses, though its functions can mostly be compensated for if it is removed.

The lymph nodes — small, bean-shaped structures distributed throughout the body — filter lymph fluid and house immune cells that respond to infection. The thymus, most active in childhood, is where a class of immune cells called T lymphocytes mature before entering circulation.

Bone marrow — the soft tissue inside bones — is not typically listed as a separate organ but functions as one: it is the site of blood cell production, generating millions of red blood cells, white blood cells and platelets every second.

The Skin: The Body's Largest Organ

Skin is often overlooked in organ discussions, but it is the body's largest organ by surface area. An adult's skin covers roughly 1.5–2 square metres and performs a remarkable range of functions:

• Protection — a physical barrier against pathogens, UV radiation, mechanical damage and chemical exposure.
• Temperature regulation — sweat glands cool the body through evaporation; blood vessels near the surface dilate or constrict to control heat loss.
• Sensation — the skin is packed with sensory receptors that detect pressure, pain, temperature and texture.
• Vitamin D synthesis — UV light hitting the skin triggers production of vitamin D, essential for bone health and immune function.
• Water retention — the outer skin layer is nearly waterproof, preventing excessive water loss from the body.

Skin is organised in three main layers: the outer epidermis (which is constantly renewed as cells move up from below and are shed from the surface), the dermis (containing hair follicles, glands and blood vessels), and the subcutaneous fat layer beneath.