Organ (biology)

• 1. Counting Organs

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  Ask an anatomist how many organs the human body contains, and the answer is approximately 79, followed by a caveat. The exact figure remains debated. The disagreement is not about anatomy so much as vocabulary, because the total shifts depending on which definition of an organ a person adopts.

  That gap raises the questions this documentary sets out to answer. What does a part of the body have to do to earn the name organ? How do organs team up, and what do plants have in place of a heart or a liver? When in the history of life did organs first appear? And why would some of the earliest physicians have rejected the idea altogether?

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• 2. Tissues Into Units

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  The wall of the intestine shows how an organ is put together. It is built from epithelial tissue and smooth muscle tissue. A tissue is a group of cells of the same type that act together on one task. When tissues of different types join into a structural unit with a shared purpose, that unit is an organ.

  Within any organ, anatomists separate the tissues into two broad groups. Parenchyma is the functional tissue, and stroma is the supporting tissue. A gland that makes hormones shows the split clearly. The hormone-producing gland tissue is the parenchyma. The stroma includes the nerves that reach it, the blood vessels that bring oxygen and nourishment and carry away metabolic wastes, and the connective tissue that gives structure, position, and anchoring.

  The primary tissues of an organ usually share an embryological origin, and they often arise from the same germ layer. Organs appear in most multicellular organisms. A single-celled eukaryote has none, but it does have a functional analogue, which is the organelle.

  Once organs exist, they rarely work alone. Their teamwork produces the body systems that medicine studies one at a time.

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• 3. Body Systems

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  Two or more organs cooperating on one specific bodily function form an organ system, which is also called a biological system or body system. Multicellular animals, including humans, have a range of these systems, with the exception of placozoans. Eleven are commonly described.

  The cardiovascular system pumps blood to and from the body and the lungs, using the heart, the blood, and the blood vessels. The digestive system processes food with the salivary glands, esophagus, stomach, liver, gallbladder, pancreas, intestines, colon, mesentery, rectum, and anus. The endocrine system communicates through hormones made by glands such as the hypothalamus, the pituitary, the pineal gland, the thyroid, the parathyroids, and the adrenals. The excretory system relies on the kidneys, ureters, bladder, and urethra to balance fluids and electrolytes and to remove urine.

  The lymphatic system carries lymph between the tissues and the bloodstream through its nodes and vessels. It includes the immune system, which defends against disease-causing agents using leukocytes, tonsils, adenoids, thymus, and spleen. The integumentary system covers mammals in skin, hair, and nails. It also includes the scales of fish and reptiles and birds, and the feathers of birds.

  The muscular system produces movement. The nervous system collects, transfers, and processes information through the brain, spinal cord, and nerves. The reproductive system comprises the sex organs, among them the ovaries, oviducts, uterus, vulva, vagina, testicles, vasa deferentia, seminal vesicles, prostate, and penis. The respiratory system handles breathing with the pharynx, larynx, trachea, bronchi, lungs, and diaphragm. The skeletal system supplies support and protection through bones, cartilage, ligaments, and tendons.

  The boundaries between these systems blur. The hypothalamus serves both the nervous system and the endocrine system, so the two are often studied together as the neuroendocrine system. The muscular and skeletal systems are merged for a similar reason, as the musculoskeletal system.

  Many of the organs named above sit inside the body's cavities, and anatomists have a separate word for them.

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• 4. Inside the Cavities

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  Viscera is the anatomical term for the internal organs of three cavities: the abdominal, the thoracic, and the pelvic. The study of the viscera is called splanchnology, and the adjective for anything relating to them is visceral, or splanchnic.

  The abdominal organs fall into two kinds. The solid organs are the liver, pancreas, spleen, kidneys, and adrenal glands. The hollow organs are the stomach, intestines, gallbladder, bladder, and rectum. In the thoracic cavity, the heart counts as a hollow, muscular organ.

  The word visceral has an opposite, a term meaning of or relating to the wall of a body part, organ, or cavity. Anatomists use the pair to describe the two faces of a membrane or a piece of connective tissue.

  The Roman pagan priests known as haruspices, along with the augurs, once examined the viscera of animals in order to foretell the future. They read the shape, the dimensions, and other features of the organs. The practice remains an important ritual in some remote, tribal societies.

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• 5. Deep Time Origins

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  Flatworms are the earliest animals in which organs can be detected, and the same holds for the more derived bilaterians. The simpler groups, namely placozoans, sponges, comb jellies, and cnidarians, do not gather their tissues into organs.

  The more complex animals carry organs that evolved over long stretches of time. The gut and the brain are the older pair. They arose in the shared ancestor of vertebrates, insects, molluscs, and worms about 700-650 million years ago. The liver and the heart came later, evolving in the chordates about 550-500 million years ago.

  Because most vertebrate organs are so ancient, researchers have searched for organs that evolved more recently, and ideally more than once. The placenta fits that description. It has evolved more than 100 times independently in vertebrates, it arose fairly recently in some lineages, and intermediate forms still exist among living species.

  Studies of the placenta have pointed to several routes by which organs come into being. Existing tissues can be re-purposed, those tissues can acquire new functional properties, and distinct tissue types can begin to interact in new ways.

  Animals are only half of the picture, since plants also build organs, though with far fewer systems.

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• 6. Plant Organs

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  Plant biologists describe three main organs in plants. The vegetative group consists of roots, stems, and leaves, and these organs keep the plant alive. Some of them carry out vital functions such as photosynthesis.

  Reproductive organs vary much more from group to group. A flowering plant reproduces through its flower, seed, and fruit. In conifers, the reproductive structures are borne on a cone. In Lycopodiophyta they are called strobili, and in mosses they are simply gametophores.

  Plants have a simpler arrangement of systems than animals, which can have 11. The usual division is between shoot and root. The shoot organ system gathers all the above-ground parts of a plant that is not an epiphyte, including the distinct leaf and flower organs.

  Some plants reproduce asexually. In that case, the vegetative organs themselves create the next generation, a process that can give rise to a clonal colony.

  Beneath all of this, plant organs are built from three tissue types: ground, vascular, and dermal.

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• 7. Ancient Organ Theories

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  In the twelfth century, the English word organ meant any musical instrument. By the late 14th century, the musical sense had narrowed to the keyboard instrument. A second meaning arrived at the same moment, describing a body part adapted to a certain function.

  Aristotle used the word often. In ancient Greek, organon means tool, and he believed the organs of the body were tools with which we do things, whether the roots of a tree or the heart and liver of an animal. His logical works are collectively called the Organon for a similar reason, because logic is a tool for philosophical thinking. Earlier thinkers, including those who wrote the texts of the Hippocratic corpus, generally did not recognize organs at all and spoke only of different parts of the body.

  Some alchemists, Paracelsus among them, took up a Hermetic Qabalah scheme that matched seven vital organs to the seven classical planets. The Sun was paired with the heart and the Moon with the brain. Mercury went with the lungs, Venus with the kidneys, Mars with the gall bladder, Jupiter with the liver, and Saturn with the spleen.

  Traditional Chinese medicine recognizes eleven organs, tied to the five traditional elements and to yin and yang. Wood pairs the yin liver with the yang gall bladder. Fire pairs the yin heart with the yang small intestine, or san jiao. Earth pairs the spleen with the stomach, and metal pairs the lungs with the large intestine. Water pairs the kidneys with the bladder. The yin and yang split approximates the modern notion of solid and hollow organs.

  The Chinese also matched the five elements to five planets: Jupiter, Mars, Venus, Saturn, and Mercury. This mirrored the way the classical planets had been linked with different metals.

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• 8. Transplants and Replacements

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  Organ transplants began in the 20th century, once scientists understood organ anatomy well enough to attempt them. The procedures were dangerous and difficult, which is why they came late. Many societies now run a donation system, in which an organ from a living or deceased donor is transplanted into a person whose own organ is failing. Larger solid organs usually require immunosuppression, to prevent organ rejection or graft-versus-host disease.

  Ethics shadows every part of the process. Both the source of an organ and the method of obtaining it raise major questions. Organs available for transplant are always scarcer than the demand, so ethicists draw on various notions of justice, including distributive justice. That tension persists as long as transplantation depends on donors rather than on technological innovation, testing, and industrial manufacturing.

  Since the 1960s, researchers have studied organs and tissue from animal donors. Some xenotransplant tissues, heart valves in particular, are already in common use. Xenotransplant could ease the critical shortage of organ grafts. Interest in laboratory-grown and artificial organs is considerable throughout the world.

  The science behind xenotransplant trials has advanced considerably, and human clinical trials using porcine xenografts are quickly approaching.

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