Human Anatomy, Part 1 - K. A. Dyubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2002

General Part
Organs, organ systems, and apparatuses

Organs are structurally distinct PARTS OF THE Organism that perform specialized Functions. They are composed of various interacting Tissues, which accounts for their higher level of Organization compared to tissues alone. Based on their General Structural Features, the organs of vertebrate and human bodies can be divided into two main categories: hollow (tubular) and parenchymal. Hollow organs include those of The Cardiovascular system, digestive tract, respiratory pathways, as well as the male and female reproductive and urinary tracts. Parenchymal organs include endocrine and exocrine glands, the Liver, Kidneys, Lungs, Hematopoietic organs, and others.

The wall of most hollow organs consists of three layers, with the inner layer being the primary one. If these organs open to the external environment (such as the digestive tract, respiratory pathways, Urinary Organs, and reproductive tracts), their inner lining is a mucous membrane (mucosa). Conversely, if the hollow organs are closed and lack direct connection to the external environment (such as those in the cardiovascular system), their inner lining in Arteries and Veins is termed the tunica intima, while in The Heart it is known as the endocardium. The Middle layer of hollow organs is muscular, whereas the outer layer is composed of connective or serous tissue.

Parenchymal organs consist of two main structural components: a supportive-trophic framework called the stroma, and the parenchyma, which performs the organ's specific function. In most parenchymal organs, the stroma is formed by Connective Tissue. However, in the Organs of the Central Nervous system, the primary stromal element is a specialized type of neural tissue known as neuroglia.

Organs are integrated into morphological systems. These systems group together organs that share a common origin, topography, and function. For instance, the Digestive System comprises the hollow organs of the digestive tract along with The Liver and Pancreas, which are parenchymal organs. Similarly, the Respiratory system includes the respiratory pathways and the respiratory Divisions of the lungs responsible for gas exchange.

Groups of organs that originate from different sources yet work together to perform a common function are referred to as apparatuses. For example, the Locomotor Apparatus includes the Skeletal System, skeletal joints, and skeletal Muscles. The Endocrine System (or apparatus) encompasses Endocrine glands alongside isolated hormone-producing Cells of diverse origin that collectively participate in the Hormonal Regulation of other organs' activities.

Influenced by the research of Academician P. K. Anokhin, functional systems are now also distinguished; these comprise a complex of neural structures and corresponding effector organs that ensure the execution of a specific, adaptive function beneficial to the system or the organism as a whole. Organs belonging to different anatomical systems can participate in The formation of functional systems. For example, the functional respiratory system includes not only the organs responsible for air conduction and gas exchange, but also afferent and efferent nerve fibers and their endings, Respiratory Muscles, Ribs, the respiratory center, and relevant Components of the cardiovascular system.

The microscopic and ultramicroscopic Structure of organs, including their specific vascularization and innervation patterns, is closely tailored to the functions they perform.