Structural Levels of Organization and Major Divisions of the Human Body

Objective 1: Structural Levels of Organization and Major Divisions of the Body

Learning Objectives

• Define key terms related to anatomy and physiology.

• Discuss the levels of organization of the body.

• Discuss the body functions necessary for life.

• Discuss the eleven body systems and the general function of each.

• Define organ and organ system.

• Discuss homeostasis and its function in maintaining life.

• Discuss the directional terms of the body.

• Identify the three planes of division of the body.

• Identify the two major cavities and the subdivisions of each.

• Identify the nine abdominal regions and the four abdominal quadrants.

Key Terms Related to Anatomy & Physiology

Definitions and Scope

Understanding the language of anatomy and physiology is essential for accurate communication in healthcare and science. The following definitions clarify the foundational concepts:

• Anatomy: The study of the structure of body parts and their relationships to one another.

• Physiology: The study of the function of body parts; how they work to carry out life-sustaining activities.

Subdivisions of Anatomy

• Gross (Macroscopic) Anatomy: Study of large, visible structures.

• Regional Anatomy: Examines all structures in a particular area of the body.

• Systemic Anatomy: Focuses on one system (e.g., cardiovascular, nervous, muscular).

• Surface Anatomy: Studies internal structures as they relate to the overlying skin (e.g., muscles or veins seen on surface).

• Microscopic Anatomy: Deals with structures too small to be seen by the naked eye (e.g., cells, tissues).

• Developmental Anatomy: Studies anatomical and physiological development throughout life.

Subdivisions of Physiology

• Renal Physiology: Concerns the function of the kidney and urine production.

• Neurophysiology: Describes how the nervous system works together to provide function.

• Physiology is a dynamic and fluid process, focusing on how organ systems interact to maintain life.

Medical Terminology

Medical terms often use roots, prefixes, and suffixes from Greek and Latin to describe body sections or regions.

• Sublingual: 'Sub-' means under, 'lingual' refers to the tongue; thus, sublingual means under the tongue.

• Pericarditis: 'Peri-' means around, 'card' refers to the heart, '-itis' means inflammation; pericarditis is inflammation around the heart.

• Leukocyte: 'Leuko-' means white, '-cyte' means cell; leukocyte is a white blood cell.

Levels of Organization in the Human Body

Hierarchical Structure

The human body is highly organized, progressing from the smallest chemical units to the complete organism.

• Chemical Level: Atoms combine to form molecules.

• Cellular Level: Cells are the basic unit of life; each cell has a unique function.

• Tissue Level: Groups of similar cells with a common function (e.g., muscle tissue, nervous tissue).

• Organ Level: Contains two or more types of tissues working together (e.g., heart, kidney).

• Organ System Level: Organs that work closely together to accomplish a common purpose (e.g., digestive system).

• Organismal Level: All organ systems combined to form the whole organism.

Body Functions Necessary for Life

Essential Life Processes

To sustain life, the body must perform several key functions:

• Maintaining Boundaries: Separation between internal and external environments (e.g., skin, cell membranes).

• Movement: Activities promoted by the muscular system, including movement of substances within the body.

• Responsiveness: Ability to sense and respond to stimuli (e.g., withdrawal reflex).

• Digestion: Breakdown of ingested food into simple molecules.

• Metabolism: All chemical reactions that occur within body cells.

• Excretion: Removal of wastes from the body.

• Reproduction: Production of offspring at cellular and organismal levels.

• Growth: Increase in size of a body part or the organism as a whole.

Major Divisions of the Body

Body Regions and Terminology

The body is divided into major regions for anatomical reference:

• Axial Region: Includes the head, neck, and trunk.

• Appendicular Region: Includes the limbs (arms and legs).

Directional terms are used to describe the location of body structures relative to one another (e.g., superior, inferior, anterior, posterior).

Body Planes and Sections

Three Main Planes

• Sagittal Plane: Divides the body into right and left parts.

• Frontal (Coronal) Plane: Divides the body into anterior (front) and posterior (back) parts.

• Transverse (Horizontal) Plane: Divides the body into superior (upper) and inferior (lower) parts.

Body Cavities and Subdivisions

Major Body Cavities

• Dorsal Body Cavity: Protects the nervous system; includes the cranial cavity (brain) and vertebral cavity (spinal cord).

• Ventral Body Cavity: Houses internal organs (viscera); subdivided into thoracic and abdominopelvic cavities.

Ventral Cavity Subdivisions

• Thoracic Cavity: Contains pleural cavities (each surrounds a lung), the mediastinum (contains the pericardial cavity, which encloses the heart, and other thoracic organs).

• Abdominopelvic Cavity: Subdivided into the abdominal cavity (contains stomach, intestines, spleen, liver) and pelvic cavity (contains urinary bladder, reproductive organs, rectum).

Membranes

• Serous Membranes: Thin, double-layered membranes that cover surfaces in the ventral body cavity. The parietal serosa lines cavity walls; the visceral serosa covers internal organs.

Abdominal Regions and Quadrants

Four Quadrants

• Right Upper Quadrant (RUQ)

• Left Upper Quadrant (LUQ)

• Right Lower Quadrant (RLQ)

• Left Lower Quadrant (LLQ)

Nine Regions

Region	Location
Right Hypochondriac	Upper right, beneath the ribs
Epigastric	Upper middle, above the stomach
Left Hypochondriac	Upper left, beneath the ribs
Right Lumbar	Middle right, near the waist
Umbilical	Center, around the navel
Left Lumbar	Middle left, near the waist
Right Iliac (Inguinal)	Lower right, near the groin
Hypogastric (Pubic)	Lower middle, below the stomach
Left Iliac (Inguinal)	Lower left, near the groin

Homeostasis and Its Function in Maintaining Life

Definition and Mechanisms

Homeostasis is the maintenance of a relatively stable internal environment despite continuous changes in the external environment. It is a dynamic state of equilibrium, constantly readjusting as needed.

• Maintained by contributions of all organ systems, especially the nervous and endocrine systems.

• Involves feedback mechanisms: negative feedback (reduces the effect of the stimulus) and positive feedback (enhances the effect of the stimulus).

Examples of Feedback Mechanisms

• Negative Feedback: Regulation of body temperature, blood glucose levels.

• Positive Feedback: Enhancement of labor contractions by oxytocin, blood clotting.

Consequences of Homeostatic Imbalance

• Increases risk of disease.

• Contributes to changes associated with aging.

• Negative feedback mechanisms may become overwhelmed, and positive feedback may take over (e.g., heart failure).

Directional Terms

Standard Anatomical Position

Directional terms are always based on the standard anatomical position: body erect, feet slightly apart, palms facing forward.

• Superior (cranial): Toward the head or upper part of a structure.

• Inferior (caudal): Away from the head or toward the lower part of a structure.

• Anterior (ventral): Toward the front of the body.

• Posterior (dorsal): Toward the back of the body.

• Medial: Toward the midline of the body.

• Lateral: Away from the midline of the body.

• Proximal: Closer to the origin of the body part or point of attachment.

• Distal: Farther from the origin of the body part or point of attachment.

• Superficial: Toward or at the body surface.

• Deep: Away from the body surface; more internal.

Summary Table: Levels of Organization

Level	Description	Example
Chemical	Atoms and molecules	Water, proteins
Cellular	Basic unit of life	Muscle cell, neuron
Tissue	Group of similar cells	Muscle tissue
Organ	Two or more tissue types	Heart, kidney
Organ System	Organs working together	Digestive system
Organismal	All systems combined	Human body

Example Application

Understanding anatomical terminology and body organization is crucial for clinical practice. For example, when describing injuries or symptoms, precise terms such as 'lumbar region', 'subscapular', or 'posterior' help healthcare professionals communicate effectively and provide accurate care.