Human Anatomy - Kotsan I. Ya. 2009

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General Characteristics of Internal Organs

Internal Organs are those located primarily within the Body Cavities (the thoracic, abdominal, and pelvic cavities). These organs form the digestive, respiratory, urinary, and reproductive systems. The Digestive System supplies the body with nutrients and eliminates undigested food residues. The Respiratory system delivers oxygen to Cells and Tissues while expelling carbon dioxide. The Urinary Organs eliminate harmful Metabolic waste products dissolved in Water. Thus, the digestive, respiratory, and urinary systems participate in the METABOLISM between the Organism and its environment. They belong to the systems that sustain the body's vital activities.

The Reproductive System performs the function of propagation, that is, the reproduction of its own kind. Given that the URINARY AND REPRODUCTIVE organs share common developmental origins, maintain a close TOPOGRAPHIC AND ANATOMICAL relationship, and that some of their parts serve as both urinary and genital passages, they are collectively referred to as the Urogenital apparatus of the organism for convenience of study.

By their Structure, internal organs are divided into parenchymatous and tubular, or hollow, organs.

Parenchymatous organs are built of working tissue—the parenchyma—which comprises specialized cellular elements. Externally, the parenchyma is covered by a Connective Tissue capsule from which septa extend into the substance of the organ, dividing it into lobes, segments, and lobules. The capsule and septa constitute the framework of the parenchymatous organ, known as the stroma. The stroma performs supporting (acting as a soft Skeleton) and trophic Functions; it houses Blood and Lymphatic vessels, as well as nerves. Examples of parenchymatous organs include the Liver, Pancreas, Lungs, and Kidneys.

Tubular (hollow) organs resemble a tube whose walls enclose a lumen (for example, the Esophagus, Stomach, intestine, and Ureters). Despite differences in shape and function, the walls of tubular organs share a similar General structural plan. The wall of a tubular organ consists of four layers: an inner mucosal layer (mucosa), a submucosal base (submucosa), a muscular layer (muscularis), and an outer connective tissue layer, which is either adventitia or serosa.

The mucous membrane (tunica mucosa) faces the lumen of the tube. It performs protective, secretory, absorptive, and other functions. The mucous membrane consists of three layers: the epithelium, the lamina propria of the mucosa, and the muscularis mucosae.

The epithelium covers the mucous membrane from the side of the organ's lumen. It can be of various types: stratified squamous non-keratinized (lining the Oral Cavity, Pharynx, esophagus, and terminal part of the rectum); simple columnar (lining The Stomach, small and large intestines, Trachea, and Bronchi); and transitional (lining the urinary tracts). The epithelium lacks Blood Vessels.

The lamina propria of the mucosa, upon which the epithelium rests, is formed of loose fibrous unformed connective tissue. It contains clusters of lymphoid tissue, glands, vessels (arterial, venous, and lymphatic), and nerves.

The muscular layer of the mucosa lies at the border between the mucosa and the submucosa. It consists of Cytology/cytology/32.html">Smooth Muscle tissue, the contraction of which causes The formation of mucosal folds.

The submucosa (tela submucosa) is formed of loose fibrous unformed connective tissue, which houses clusters of lymphoid tissue, glands, submucosal nerve plexuses, and vascular plexuses (arterial, venous, and lymphatic). Thanks to this layer, the mucous membrane can shift and form folds. In the absence of a submucosa, the mucous membrane fuses with the underlying layer and does not form folds.

The Glands of the mucous membrane are derivatives of the epithelium. They produce mucus, which covers and moisturizes the mucous membrane (protecting it from mechanical and chemical damage), or digestive juices that break down complex nutrients into simpler ones. Depending on the number of cells forming them, mucosal glands are divided into unicellular and multicellular. Unicellular glands, such as the goblet Cells of the intestine that produce mucus, are located among the epithelial cells of the mucous membrane. Multicellular glands are located in the lamina propria and submucosa, and also form independent organs situated adjacent to the hollow organ (for example, the major Salivary Glands—parotid, submandibular, and sublingual). By shape, multicellular glands are divided into tubular, alveolar, and tubulo-alveolar; by structure, they are divided into simple, branched, and compound. In large, more complexly structured glands, the secretory epithelium is located only in the blind-ended region, and the release of secretion into the organ's lumen occurs through excretory ducts whose walls contain a layer of smooth muscle tissue. The glands of the digestive, respiratory, and urogenital systems belong to exocrine organs that secrete their product into the lumen of the organ (with the exception of the pancreas and Gonads, which have a mixed secretory function).

Clusters of lymphoid tissue present in the mucosa and submucosa perform a protective function. They have a diffuse or concentrated character (solitary and Aggregated lymphoid follicles) (see "Organs of Hematopoiesis and The Immune System").

The muscular layer (tunica muscularis) is located outward from the submucosa. In most organs, it consists of two layers: an inner circular and an outer longitudinal layer, separated by a layer of loose fibrous unformed connective tissue that contains intermuscular nerve plexuses, blood vessels, and lymphatics. In some organs (such as the stomach), the muscular layer is formed of three layers, with the additional layer comprising oblique fibers. As a result of the simultaneous contractions of the longitudinal and circular fiber layers, so-called peristaltic movement occurs. This is particularly distinct in the stomach, intestines, and the excretory ducts of various glands. Its essence lies in the fact that when the circular layer contracts in one area, it simultaneously relaxes in an adjacent area. This contraction moves along the digestive tube, gland ducts, etc., forming a peristaltic wave. Peristaltic waves facilitate the movement of the given organ's contents.

In the walls of most of the digestive tube, the muscular layer is built of smooth muscle tissue; only in the upper section (pharynx, upper third of the esophagus) and the lower section (external anal sphincter) does it consist of Striated muscle tissue. The Muscles of the Larynx are also built of striated muscle tissue, while those of the trachea, bronchi, and urinary and reproductive organs are composed of smooth muscle.

The serous membrane (tunica serosa) is formed of Fibrous connective tissue covered on its outer free surface by a simple squamous epithelium (mesothelium). The cells of this epithelium produce serous fluid, which helps reduce friction between organs and cavity walls. The serous membrane covers most internal organs, notably those of the abdominal and thoracic cavities. In the Abdominal cavity, it is called the Peritoneum, and in the thoracic cavity, the Pleura and Pericardium. Each serous membrane has two layers. One of these—the parietal layer—lines the walls of the cavities (e.g., abdominal or thoracic) in which certain organs are located, while the other—the visceral layer—covers the organs themselves. Between these two layers and between individual organs covered by the serous membrane lies a slit-like space called the serous cavity (peritoneal cavity, pleural cavity).

Between the serous and muscular layers lies a layer of loose connective tissue called the subserous base (tela subserosa). Like the submucosa, it houses blood and Lymphatic vessels and nerves.

There are places where the serous membrane is absent (for example, the pharynx, the cervical and thoracic PARTS OF THE esophagus, etc.); in such cases, its function is performed by the outer or adventitial layer (tunica adventitia) (from Latin adventitia—external). It is built of loose fibrous connective tissue that houses Vessels and nerves. Unlike the serous membrane, the adventitial layer lacks a mesothelial covering.

Internal organs are located in body cavities alongside skeletal bones, muscles, vessels, and nerves. When describing THE POSITION OF organs relative to these structures, special anatomical terms are used: skeletotopy (from Greek topos—place), which explains the Location of an organ relative to the skeletal bones; holotopy (from Greek holos—whole), which defines the position of an organ or a group of organs within The Human Body or its cavities; and syntopy (from Greek syn—together), which determines the location of an organ in relation to neighboring organs.