Organ (biology)

In a multicellular organism, an organ is a collection of tissues joined together to serve a common function. Think of an organ as a team of different workers all doing their part to help something important get done. In the grand hierarchy of life, an organ sits between smaller groups called tissues and larger groups called organ systems. Tissues are made from the same type of cells working together, and when different types of tissues combine, they form an organ with a special job to do. For example, the intestinal wall is made from epithelial tissue that lines the inside and smooth muscle tissue that helps it move things along.

Organs are made up of two main kinds of tissues: parenchyma, which does the actual work, and stroma, which supports and connects everything. For instance, in a gland, the tissue that makes hormones is the parenchyma, while the stroma includes nerves, blood vessels, and connective tissues that help it function. These tissues often come from the same place early in development, known as the same germ layer.

Organs are found in most living things that have more than one cell. In simpler, single-celled organisms like eukaryotes, something called an organelle acts kind of like an organ. Plants have three main organs, and the number of organs in any creature depends on how we define them. In the human body, there are about 79 organs, though scientists still debate the exact number.

Animals

See also: List of organs of the human body and Biological system

Except for placozoans, multicellular animals including humans have many organ systems. These systems are studied in human anatomy. Some systems share functions. For example, the nervous and endocrine systems both use the hypothalamus, so they are often studied together as the neuroendocrine system. The musculoskeletal system is also a combination of the muscular and skeletal systems.

Some important animal organ systems include:

• Cardiovascular system: pumps and moves blood with the heart, blood, and blood vessels.
• Digestive system: breaks down food using salivary glands, esophagus, stomach, liver, gallbladder, pancreas, intestines, colon, mesentery, rectum and anus.
• Endocrine system: sends messages using hormones from glands like the hypothalamus, pituitary gland, and adrenals.
• Excretory system: removes waste with kidneys, ureters, bladder and urethra.
• Lymphatic system: helps defend the body and moves lymph with nodes and immune system.
• Integumentary system: includes skin, hair and nails of mammals, plus scales of fish, reptiles, birds, and feathers of birds.
• Muscular system: allows movement with muscles.
• Nervous system: collects and processes information with brain, spinal cord and nerves.
• Reproductive system: includes sex organs such as ovaries, uterus, testicles, prostate and penis.
• Respiratory system: helps with breathing using the pharynx, larynx, trachea, bronchi, lungs and diaphragm.
• Skeletal system: gives shape and support with bones, cartilage, ligaments and tendons.

Viscera

In anatomy, viscera are the internal organs found in the abdominal, thoracic, and pelvic cavities. Abdominal organs can be solid, like the liver, pancreas, spleen, kidneys, and adrenal glands, or hollow, like the stomach, intestines, gallbladder, bladder, and rectum. The heart in the thoracic cavity is also a hollow organ. The study of these organs is called Splanchnology.

Origin and evolution

The organ level of life can be seen in simple animals like flatworms and becomes more complex in advanced animals. For example, the liver and heart evolved in certain animals around 550–500 million years ago, while the gut and brain are even older, developing about 700–650 million years ago. Scientists study how organs develop by looking at newer organs like the placenta, which has evolved many times in different animals.

Plants

See also: Plant morphology, Plant anatomy, and Plant physiology

The study of plant organs is covered in plant morphology. Plant organs are divided into two main types: vegetative and reproductive. Vegetative organs include roots, stems, and leaves. These help the plant grow, take in water, and make food. Reproductive organs vary between plant types. In flowering plants, they include the flower, seed, and fruit. In conifers, reproductive structures are called cones.

Vegetative organs are essential for a plant's life, helping it stay alive and grow. While animals have many organ systems, plants have fewer, with some organs handling important tasks like photosynthesis. Reproductive organs are crucial for creating new plants, either through flowers or other structures depending on the plant type.

Society and culture

Many societies have systems for organ donation, where a healthy organ from a living or deceased person is transplanted into someone who needs a new one. These transplants sometimes need special medicines to help the body accept the new organ.

There is growing interest around the world in making laboratory-grown or artificial organs. Starting in the 20th century, organ transplants became possible as scientists learned more about how organs work. Today, scientists are also studying ways to use organs from animals, called xenotransplant, which could help meet the need for more transplant organs.

History

The word "organ" has been used since the twelfth century, originally to describe musical instruments and later to mean a special body part that performs a certain function. In ancient times, people studied internal body parts to understand more about nature and the future.

Ancient thinkers like Aristotle thought of body organs as tools that help us do various activities. Different cultures, such as Chinese traditional medicine, have their own ways of understanding and naming organs based on their beliefs and elements.