Human Anatomy and Physiology - I. V. Gayvoronsky 2011

The human body as a single entity. Fundamentals of cytology and histology
Organs

Tissues do not exist in isolation; they participate in The formation of organs. An organ is a part of The Human Body and a component of a specific system, constructed from various tissues, one of which performs The primary function. For example, the Liver consists of all types of tissue, but the Epithelial Tissue is paramount (responsible for Bile production and the detoxification of substances delivered to the liver from the gastrointestinal tract).

Providing an exhaustive Classification of organs is difficult, as they differ significantly in their Location, shape, as well as external and internal Structure.

Organs can be categorized into Internal Organs, organs of The Musculoskeletal System, and somatosensory organs (Sense Organs and the Skin). In turn, internal organs are divided into hollow and parenchymal organs. The musculoskeletal system includes such organs as bones, ligaments, and Muscles.

All hollow organs share a common structural plan and consist of three layers: an inner mucous membrane, a middle muscular layer, and an outer layer. The outer layer may be represented by loose Connective Tissue, known as the adventitia, or by a serous membrane (Peritoneum, Pleura, or Pericardium). Parenchymal organs consist of a stroma—connective tissue that forms the framework—and parenchyma, which is the functional substance of the organ.

Thus, an organ is a relatively distinct anatomical entity and a structural element that builds the next higher level of Organization—the organ system.