Structural Levels of Organization and Major Divisions of the Body: Foundations of Anatomy & Physiology

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 in Anatomy & 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., 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 to provide function.

• Physiology is a dynamic and integrative science, often studied at the organ or system level.

Terminology in Anatomy & Physiology

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

• Examples:

  • Sublingual: Under the tongue (sub- = under, -lingual = tongue).

  • Pericarditis: Inflammation around the heart (peri- = around, -card- = heart, -itis = inflammation).

  • Leukocyte: White blood cell (leuko- = white, -cyte = cell).

Levels of Structural Organization in the Human Body

The human body is highly organized, from the smallest chemical building blocks to the entire organism.

• Chemical level: Atoms combine to form molecules, which in turn form organelles (the most basic level of life).

• 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, stomach).

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

• Organismal level: The sum total of all organ systems working together to maintain life in the whole organism (most complex).

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 acts as a barrier).

• Movement: Includes movement of the body as a whole and movement of substances within the body.

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

• Digestion: Breakdown of ingested foodstuffs to simple molecules that can be absorbed.

• Metabolism: All chemical reactions that occur within body cells, including catabolism and anabolism.

• Excretion: Removal of wastes from metabolism and digestion.

• 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 describe the location of one body structure in relation to another, always based on the standard anatomical position.

Body Systems and Their General Functions

The human body contains eleven major organ systems, each with specific functions essential for survival.

System	Main Function
Integumentary	Protects the body, regulates temperature, and provides sensory information.
Skeletal	Supports and protects body organs, provides a framework for muscles, forms blood cells.
Muscular	Allows movement, maintains posture, produces heat.
Nervous	Fast-acting control system, responds to internal and external changes.
Endocrine	Glands secrete hormones that regulate growth, reproduction, and metabolism.
Cardiovascular	Transports blood, oxygen, nutrients, wastes, and hormones.
Lymphatic/Immune	Returns leaked fluid to blood, disposes of debris, houses white blood cells.
Respiratory	Keeps blood supplied with oxygen, removes carbon dioxide.
Digestive	Breaks down food, absorbs nutrients, eliminates waste.
Urinary	Eliminates nitrogenous wastes, regulates water, electrolyte, and acid-base balance.
Reproductive	Produces offspring.

Definition of Organ and Organ System

• Organ: A structure composed of at least two types of tissues that performs a specific function.

• Organ system: A group of organs that work together to perform major functions or meet physiological needs of the body.

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.

• Primarily regulated by the nervous and endocrine systems.

• Involves variables such as body temperature, blood volume, and blood sugar.

• Homeostatic control mechanisms include:

  • Receptor: Detects changes (stimuli).

  • Control center: Determines the set point and appropriate response.

  • Effector: Carries out the response to restore balance.

• Negative feedback: Most common; response reduces or shuts off the original stimulus (e.g., regulation of body temperature, blood glucose).

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

Disturbance of homeostasis increases risk of disease, contributes to aging, and may result in system failure (e.g., heart failure).

Directional Terms and Body Planes

• Directional terms describe the position of one body part relative to another (e.g., superior, inferior, anterior, posterior, medial, lateral, proximal, distal, superficial, deep).

• Always based on the standard anatomical position (body erect, feet slightly apart, palms facing forward).

• Left and right refer to the body being viewed, not the observer.

Body 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 Their Subdivisions

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

• Ventral body cavity: Houses internal organs (viscera); includes the thoracic cavity and abdominopelvic cavity.

  • Thoracic cavity: Contains pleural cavities (each surrounds a lung), mediastinum (contains pericardial cavity, esophagus, trachea), and pericardial cavity (encloses the heart).

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

• Serous membranes (serosa): Thin, double-layered membranes covering surfaces in the ventral body cavity. The parietal serosa lines cavity walls; the visceral serosa covers organs.

Abdominal Regions and Quadrants

To describe locations within the abdominopelvic cavity, two methods are used:

• 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):

Region	Location
Right hypochondriac	Upper right
Epigastric	Upper middle
Left hypochondriac	Upper left
Right lumbar	Middle right
Umbilical	Center
Left lumbar	Middle left
Right iliac (inguinal)	Lower right
Hypogastric (pubic)	Lower middle
Left iliac (inguinal)	Lower left

Clinical Application Example

• Understanding anatomical terminology and body regions is essential for accurate communication in healthcare settings, such as describing the location of injuries or symptoms.

• Example: A patient with a laceration in the left subscapular region and fractures in the lumbar region requires precise anatomical description for diagnosis and treatment.