Introduction to the Human Body Help (page 2)

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By — McGraw-Hill Professional
Updated on Aug 18, 2011

Anatomical Position and Terminology

All terms of direction that describe the relationship of one body part to another are made in reference to a standard anatomical position. In anatomical position, the body is erect, feet are parallel and flat on the floor, eyes are directed forward, and arms are at the sides of the body with the palms of the hands turned forward and the fingers pointing downward.

Descriptive and directional terms are used to communicate the position of structures, surfaces, and regions of the body with respect to anatomical position. Commonly used terms are listed and defined in Table 1.2.

In addition to the terms listed in Table 1.2, three planes of reference are used to locate and describe structures within the body. The midsagittal plane is the plane of symmetry, dividing the body into right and left halves. A coronal plane divides the body into front and back portions, and a transverse (horizontal or cross-sectional) plane divides the body into superior and inferior portions.

Body Regions and Body Cavities

The principal body regions are the head, neck, trunk (divided into the thorax and the abdomen), upper extremity (two), and lower extremity (two).

The body cavities are confined spaces in which organs are protected, separated, and supported by associated membranes. The posterior (dorsal) cavity includes the cranial and vertebral cavities and contains the brain and spinal cord.

The anterior (ventral) cavity includes the thoracic, abdominal, and pelvic cavities and contains the visceral organs. The abdominal cavity and the pelvic cavity are frequently referred to collectively as the abdominopelvic cavity because there is no physical division between these two regions. The visceral organs located in the thoracic cavity are the heart and lungs. The thoracic cavity is partitioned into two pleural cavities, one surrounding each lung and the pericardial cavity surrounding the heart. The area between the two lungs is known as the mediastinum. The viscera of the abdominal cavity include the stomach, small intestine, large intestine, spleen, liver, and gallbladder.

The body cavities serve to segregate organs and systems by function: the major portion of the nervous system occupies the posterior cavity; the principal organs of the respiratory and circulatory systems are in the thoracic cavity; the primary organs of digestion are in the abdominal cavity; and the reproductive organs are in the pelvic cavity.

Body membranes, composed of thin layers of connective and epithelial tissue, serve to cover, protect, lubricate, separate, or support visceral organs or to line body cavities. The two principal types are mucous membranes and serous membranes.

Mucous membranes secrete a thick, viscous substance called mucous that lubricates and protects the body organs where it is secreted. Examples of mucous membranes are the epithelial membranes lining the nasal cavity, the trachea, and the oral cavity. Mucous membranes are found lining the inside walls of many other body organs.

Serous membranes line the thoracic and abdominopelvic cavities and cover the visceral organs (described above). They are composed of thin sheets of epithelial tissue that lubricate, support, and compartmentalize visceral organs. Serous fluid is the watery lubricant they secrete. The serous membranes of the thoracic cavity are the parietal and visceral pleura, lining the thoracic walls and diaphragm and the outer surface of the lungs respectively, and the parietal and visceral pericardium surrounding the heart. The serous membranes of the abdominopelvic cavity are the parietal and visceral peritoneum, lining the abdominal wall and covering the abdominal viscera respectively; and the mesentery, a double fold of which supports the viscera and loosely anchors it to the abdominal body wall.

Practice problems for these concepts can be found at:

Introduction To the Human Body Practice Problems

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