An organ is a structurally organised part of a multicellular organism composed of multiple tissues that work together to perform one or more specialised functions. Organs represent a level of biological organisation above tissues and below organ systems in organisms that possess organ systems. An organ is not simply a collection of cells or a single tissue. Its function depends on the coordinated arrangement of different tissues, blood vessels, nerves, extracellular structures and, where relevant, ducts or other specialised pathways.
Examples in humans include the heart, lungs, kidneys, liver, stomach, brain and skin. Each organ has a characteristic architecture that reflects its physiological role. The heart is organised to generate and direct blood flow, the lungs provide surfaces for gas exchange, the kidneys regulate the composition of body fluids and the liver performs extensive metabolic and biochemical processing.
Organs contain multiple tissue types that contribute different components of the overall function. The heart contains cardiac muscle tissue responsible for contraction, connective tissues that provide structural support, endothelial tissue lining the chambers and blood vessels, and nervous and signalling components involved in regulation. The stomach contains epithelial tissue for secretion and protection, connective tissue, smooth muscle for mechanical mixing and nervous components that coordinate activity.
The kidney contains specialised epithelial structures, connective tissue, blood vessels and extensive neural and hormonal interactions. These combinations allow organs to carry out functions that individual tissues could not perform alone. Organ architecture is therefore an example of biological division of labour, in which different cell types and tissues perform complementary tasks within a coordinated structure.
Organs often have specialised regions and internal compartments. The kidney contains structures including nephrons, collecting ducts, blood vessels and supporting tissues arranged to enable filtration, selective reabsorption, secretion and concentration of urine. The lungs contain branching airways that lead to vast numbers of microscopic alveoli surrounded by capillaries. The liver contains organised arrangements of hepatocytes, blood vessels and bile channels that allow it to process nutrients, detoxify substances and produce or modify many biochemical compounds. The brain contains specialised regions with distinct cellular organisation and connections that support sensory processing, movement, memory, language and other functions. Internal architecture is therefore critical because the spatial arrangement of tissues determines how substances, signals and forces move through an organ.
Organs require transport and communication systems to remain functional. Blood vessels supply oxygen and nutrients and remove carbon dioxide and metabolic waste. Nerves can regulate organ activity and transmit sensory information. Hormones can alter organ function over longer distances through the circulation. Lymphatic vessels contribute to fluid balance and immune surveillance in many organs. Local signalling systems also influence cells within an organ. The density and arrangement of these supporting systems vary according to the organ's requirements.
The brain has an extensive network of blood vessels because neurons require continuous oxygen and glucose delivery, while skeletal muscle has vascular and nervous arrangements adapted to force generation and rapid changes in metabolic demand. Organ function therefore depends on integration between specialised tissues and broader physiological networks.
Organ development occurs through coordinated processes of cell differentiation, tissue formation, growth and morphogenesis. During embryonic development, cells receive molecular signals that influence their identities and spatial organisation. Developing tissues interact with one another, and these interactions can determine the shape and function of emerging organs. Organs may continue to grow, remodel and mature after birth. Some tissues have substantial regenerative capacity, while others have limited ability to replace damaged cells.
The liver, for example, has notable regenerative capacity under appropriate conditions, whereas extensive damage to certain regions of the nervous system may be much more difficult to repair. Organ development and regeneration are therefore influenced by cell type, extracellular environment, signalling pathways, blood supply and the extent of injury.
Organ function is usually regulated dynamically. The heart changes its rate and force of contraction in response to nervous, hormonal and local signals. The kidneys alter filtration and reabsorption according to hydration, blood pressure, hormone levels and electrolyte status. The pancreas adjusts endocrine and exocrine activity according to nutritional and physiological conditions. The skin changes blood flow and sweat production in response to temperature. These examples show that an organ does not operate independently of the organism. Its activity is continuously adjusted according to information from other organs and the internal environment. Organ function is therefore integrated into homeostasis and depends on communication through nervous, endocrine, circulatory and local signalling mechanisms.
Organ failure occurs when an organ can no longer perform sufficient functional activity to maintain physiological requirements. Causes include genetic disorders, infection, trauma, cancer, chronic inflammation, vascular damage, toxic exposure and ageing related changes. Failure of one organ can affect many others because organ systems are interconnected. Kidney failure, for example, can disrupt fluid, electrolyte and acid base balance and cause the accumulation of metabolic waste products.
Heart failure can impair blood delivery to tissues and affect kidney and lung function. Liver dysfunction can alter metabolism, detoxification and protein production. Treatment may involve medicines, supportive therapies, surgery, transplantation or mechanical replacement depending on the organ and condition. Understanding organ structure and function is therefore central to medicine and pathology.
The word organ derives from a Greek term associated with a tool or instrument, reflecting the idea that an organ is a specialised structure serving a particular function. In biology, the concept helps connect microscopic organisation with whole organism physiology. Organs are found in many complex multicellular organisms, although their arrangements differ substantially among animal and plant lineages.
Plants contain structures such as roots, stems and leaves that can be regarded as organs because they consist of multiple tissues organised for specialised functions. A plant root, for example, contains protective, conductive and absorptive tissues arranged to anchor the plant and acquire water and minerals. The organ concept therefore applies broadly to multicellular biology, while the exact structures and functions differ according to evolutionary history, anatomy and ecological requirements.
