Structural Levels of Organization and Major Divisions of the Human 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 in Anatomy and Physiology

Definitions

• 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., visible muscles or veins).

• 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 kidneys and urine production.

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

• Physiology is dynamic and involves understanding processes at the molecular, cellular, tissue, organ, and system levels.

Medical Terminology

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

• Examples:

  • Sublingual: 'Sub-' (under) + 'lingual' (tongue) = under the tongue.

  • Pericarditis: 'Peri-' (around) + 'card' (heart) + '-itis' (inflammation) = inflammation around the heart.

  • Leukocyte: 'Leuko-' (white) + 'cyte' (cell) = white blood cell.

Levels of Structural Organization in the Human Body

The human body is highly organized, from the smallest chemical level to the whole organism level:

• Chemical level: Atoms, molecules, and organelles.

• Cellular level: Single cells, the basic unit of life.

• Tissue level: Groups of similar cells performing a common function.

• Organ level: Contains two or more types of tissues working together (e.g., the heart, liver).

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

• Organismal level: All organ systems combined to make the whole organism.

Body Functions Necessary for Life

To maintain life, the body must perform several essential functions:

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

• Movement: Includes movement of the body, organs, and cells.

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

• Digestion: Breakdown of ingested foodstuffs to simple molecules.

• Metabolism: All chemical reactions that occur within body cells, including catabolism (breaking down) and anabolism (building up).

• Excretion: Removal of wastes from metabolism and digestion (e.g., urea, carbon dioxide, feces).

• Reproduction: Cellular division for growth or repair, and production of offspring.

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

Major Divisions of the Body

• Axial division: Head, neck, and trunk.

• Appendicular division: Limbs (arms and legs).

Directional terms are used to precisely locate one body structure in relation to another.

Body Planes and Sections

Three major planes are used to divide the body:

• 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 Their Subdivisions

The body contains two major sets of internal cavities:

• Dorsal body cavity: Protects the nervous system and is divided into:

  • Cranial cavity: Encases the brain.

  • Vertebral (spinal) cavity: Encases the spinal cord.

• Ventral body cavity: Houses internal organs (viscera) and is divided into:

  • Thoracic cavity: Contains the heart and lungs.

    • Pleural cavities: Each surrounds a lung.

    • Mediastinum: Contains the pericardial cavity, which encloses the heart, and surrounds other thoracic organs (e.g., esophagus, trachea).

  • Abdominopelvic cavity: Contains digestive, urinary, and reproductive organs.

    • Abdominal cavity: Contains stomach, intestines, spleen, liver, and other organs.

    • Pelvic cavity: Contains urinary bladder, reproductive organs, and rectum.

Membranes of the Ventral Body Cavity

• Serosa (serous membrane): Thin, double-layered membrane covering the ventral body cavity and organs.

  • Parietal serosa: Lines internal body cavity walls.

  • Visceral serosa: Covers internal organs (viscera).

Abdominal Regions and Quadrants

The abdominopelvic region is divided for clinical and anatomical reference:

• Four Quadrants (used by clinicians):

  • Right Upper Quadrant (RUQ)

  • Left Upper Quadrant (LUQ)

  • Right Lower Quadrant (RLQ)

  • Left Lower Quadrant (LLQ)

• Nine Regions (used by anatomists):

  • Right hypochondriac region

  • Epigastric region

  • Left hypochondriac region

  • Right lumbar region

  • Umbilical region

  • Left lumbar region

  • Right iliac (inguinal) region

  • Hypogastric (pubic) region

  • Left iliac (inguinal) region

Homeostasis and Its Role in Maintaining Life

Homeostasis is the maintenance of a relatively stable internal environment despite continuous external changes. It is a dynamic state of equilibrium, maintained by contributions of all organ systems.

• Involves feedback systems (mainly negative feedback) to regulate variables such as temperature, blood pressure, and glucose levels.

• Disruption of homeostasis increases risk of disease and contributes to changes associated with aging.

• Positive feedback mechanisms are less common and usually control infrequent events (e.g., blood clotting, labor contractions).

Feedback Mechanisms

• Negative feedback: Reduces or shuts off the original stimulus (e.g., regulation of body temperature, blood glucose by insulin).

• Positive feedback: Enhances or exaggerates the original stimulus (e.g., enhancement of labor contractions by oxytocin).

Summary Table: Levels of Organization

Key Equations

• Homeostatic regulation (generalized):

Clinical Application Example

Understanding anatomical terminology allows healthcare professionals to communicate accurately about patient injuries and conditions. For example, describing a laceration as being in the 'left subscapular region' or a fracture in 'Lumbar 1 and Lumbar 2' provides precise information for diagnosis and treatment.