Structural Levels of Organization and Major Divisions of the Human Body

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

• Anatomy: The study of the structure of body parts and their relationship 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 (e.g., heart, lungs).

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

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

• Surface Anatomy: Studies internal structures as they relate to the overlying skin (e.g., visible 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 kidney and urine production.

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

Additional info: Physiology is a dynamic and fluid process, focusing on how body systems interact to maintain life.

Medical Terminology

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

• Sublingual: Underneath the tongue.

• Pericarditis: Inflammation of the pericardium (the membrane surrounding the heart).

• Leukocyte: White blood cell.

Levels of Organization in the Human Body

The human body is hierarchically organized, from the smallest chemical units to the whole organism.

• Chemical Level: Atoms, molecules, and organelles.

• Cellular Level: Single cells; basic unit of life.

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

• Organ Level: Contains two or more types of tissues; organs perform specific functions.

• Organ System Level: Organs that work closely together to accomplish a common purpose.

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

Example: The heart (organ) is made of muscle tissue, connective tissue, and nervous tissue, and is part of the cardiovascular system (organ system).

Body Functions Necessary for Life

Life involves several essential functions that maintain homeostasis and survival.

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

• 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 foodstuffs to simple molecules.

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

• Excretion: Removal of wastes from metabolism and digestion.

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

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

Major Divisions of the Body

The body is divided into major regions for anatomical reference.

• Axial Region: Head, neck, and trunk.

• Appendicular Region: Limbs (arms and legs).

Additional info: Directional terms designate specific areas within these regions for precise communication.

Directional Terms

Directional terms are used to describe the locations of structures relative to other structures or locations in the body.

• 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.

Additional info: Anatomical position is the standard reference: body erect, feet slightly apart, palms facing forward.

Planes of Division of the Body

Three main planes are used to section the body:

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

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

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

Body Cavities and Their Subdivisions

The body contains two major cavities, each with subdivisions:

Major Cavity	Subdivisions	Main Contents
Cranial (Dorsal)	Cranial cavity, Vertebral (spinal) cavity	Brain, Spinal cord
Ventral	Thoracic cavity, Abdominopelvic cavity	Heart, Lungs, Digestive organs, Urinary and reproductive organs

• Thoracic cavity: Contains pleural cavities (lungs), pericardial cavity (heart), and mediastinum.

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

Additional info: Cavities are lined by membranes (e.g., serous membranes such as pleura, pericardium, peritoneum).

Abdominal Regions and Quadrants

For clinical and anatomical reference, the abdomen is divided into regions and quadrants.

Quadrant	Location
Right Upper Quadrant (RUQ)	Upper right section
Left Upper Quadrant (LUQ)	Upper left section
Right Lower Quadrant (RLQ)	Lower right section
Left Lower Quadrant (LLQ)	Lower left section

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

Homeostasis and Its Function in Maintaining Life

Homeostasis is the maintenance of a relatively stable internal environment despite continuous changes in the external environment.

• Dynamic state of equilibrium, always readjusting as needed.

• Maintained by contributions of all organ systems.

• Involves monitoring and regulation of variables (e.g., temperature, blood volume).

Components of Homeostatic Control

• Receptor: Senses changes in the environment.

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

• Effector: Carries out the response to restore balance.

Feedback Mechanisms

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

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

Example: Regulation of blood glucose by insulin is a negative feedback mechanism.

Homeostatic Imbalance

• Increases risk of disease.

• Contributes to changes associated with aging.

• Negative feedback mechanisms become less efficient; positive feedback may take over (e.g., heart failure).