V. Organization of the Body
The levels of organization that have been examined so far are, for the most part, microscopic. The next levels of organization, organ and organ system are large (gross) and constitute the visible body. Before beginning a detailed examination of organs and organ systems it is necessary to get an overview of gross anatomy and the major organ systems.
A. Anatomical orientation - The study of anatomy is a systematic process with specific terminology and procedures. Three items of particular importance are the anatomical position, terms of direction, and planes of section.
1. Anatomical position - All anatomical descriptions are always in reference to the anatomical position. In humans, this position is the body standing erect with the arms to the side and the palms of the hand facing forward.
2. Terms of direction - All structures are located with reference to some point and with respect to the anatomical position. The major terms of direction are listed below.
a. Anterior - front (ventral - sometimes used)
b. Posterior - back (dorsal - sometimes used)
c. Medial - towards the midline
d. Lateral - towards the side - away from the midline
e. Superior - towards the head (cranial - sometimes used)
f. Inferior - towards the feet (caudal - sometimes used)
g. Proximal - towards the body
h. Distal - away from the body
i. Superficial - near the surface
j. Deep - away from the surface
(l) The chin is inferior with respect to the nose.
(2) The nose is superior with respect to the chin.
(3) Fingers are distal with respect to the elbow.
(4) The sternum is medial to the ribs, inferior to the neck, superior to the umbilicus, and anterior to the vertebral column.
3. Regional terms - These terms refer to special regions of the body. The major ones are listed below.
a. cervical - neck region
b. thoracic - chest region
c. lumbar - small of the back
d. sacral - lowest portion of the back
e. plantar - sole of the foot
f. leg - region between the knee and the ankle
g. thigh - region between the hip and the knee
h. groin - the region formed by the junction of the thigh and the abdomen
i. abdomen - The region between the thighs and the thorax. To facilitate location of the abdominal organs, the abdomen is subdivided into several different regions.
(l) umbilical - area surrounding the umbilicus
(2) lumbar - located to the left and right of the umbilical area
(3) epigastric - immediately superior to the umbilical region and along the body midline
(4) hypochondriac - located to the left and right of the epigastric region
(5) hypogastric - immediately inferior to the umbilical region and along the midline of the body
(6) iliac - located to the left and right of the hypogastric region
j. axilla - armpit
k. arm - the region between the shoulder and the elbow
l. forearm - region between the elbow and the hand
m. palmar - the anterior (palm) of the hand
4. Planes of section - It is frequently convenient to slice the body into sections. These sections cut through all of the organs in the plane of the section and permits the relationships between the various organs to be visualized. There are three major planes of section.
a. Transverse - cross section - This divides the body into superior and inferior sections.
b. Frontal - coronal - This plane divides the body into anterior and posterior sections.
c. Sagittal - This divides the body into left and right portions.
B. Body cavities - The body has two major cavities, lined by membranes, and filled with organs.
l. Dorsal (posterior) cavity - This is subdivided into two regions.
a. Cranial cavity - This is located inside of the skull and contains the brain.
b. Spinal cavity - Located inside of the vertebral column, this cavity contains the spinal cord.
2. Ventral (anterior) cavity - This large cavity is subdivided into two major regions and several subregions. Each subdivision is lined by a double serous membrane. The outer membrane is termed the parietal layer and lines the cavity. The inner membrane is the visceral membrane and covers the organ(s) that lie in the cavity. The major subdivisions and their membranes are as follows.
a. Thoracic cavity
(l) Pericardial cavity - This surrounds the heart and divided off from the remainder of the thoracic cavity by the two pericardial membranes.
(2) Pleural cavities - There is a left and a right pleural cavity, each containing a lung. They are lined by the pleural membranes.
b. Abdominopelvic cavity (Peritoneal cavity)- This cavity is separated from the thoracic cavity by the diaphragm a broad, flat muscle, used in breathing. It is lined by a double membrane known as the peritoneum. The bulk of the organs found in this cavity are supported by extensions of the peritoneal membrane from the posterior wall of the cavity. These peritoneal extensions are termed mesenteries.
(l) Abdominal cavity - This is the superior subdivision and lies above the pelvis.
(2) Pelvic cavity - This inferior portion lies completely within the confines of the circle made by the two pelvic (hip) bones.
C. Organ systems - The body is composed of ten different organ systems. Below is a listing of the organ systems along with a brief description of each.
l. Integumentary system - This consists of the skin and associated structures such as hair and nails. The skin is a complex organ which functions in prevention of water loss, temperature regulation, and protection against microorganisms.
2. Skeletal system - This is made up of bones and cartilage. It provides the framework for the body and protection for other organs. It contains joints that permit movement.
3. Muscular system - This consists of the skeletal muscles which are attached to the skeleton and cause movement.
4. Nervous system - This consists of the brain, spinal cord, nerves, and receptor organs. Along with the endocrine system it functions to provide control, communication, and integration of the body.
5. Endocrine system - A series of ductless glands which secrete hormones. It works with the nervous system and carries out the same basic functions as the nervous system.
6. Cardiovascular system - The circulatory system is another name for this system. It is composed of the blood, vessels, and heart. It functions in the transport and delivery of materials to and from the cells.
7. Respiratory system - The lungs and associated structures make up this system. It functions in the exchange of gases between the blood and atmosphere.
8. Digestive system - This is essentially a tube that runs throughout the body with a series of accessory glands. It functions in the breakdown and absorption of nutrients.