Structural Levels of Organization and Major Divisions of the Human Body

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.

• Describe 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.

• Describe the directional terms of the body.

• Identify the three planes of division of the body.

• Identify the two major body 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: Examines 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 dynamic and fluid, focusing on processes rather than static images.

Terminology in Anatomy & Physiology

• 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.

• 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 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 the organism as a whole.

Body Systems and Their General Functions

The human body consists of eleven organ systems, each with specific functions essential for survival.

System	Main Function
Integumentary	Protects body, regulates temperature, prevents water loss
Skeletal	Supports and protects organs, provides framework for muscles
Muscular	Allows movement, maintains posture, produces heat
Nervous	Fast-acting control system, responds to internal and external changes
Endocrine	Secretes hormones for regulation of growth, reproduction, metabolism
Cardiovascular	Transports blood, nutrients, gases, and wastes
Lymphatic/Immune	Returns leaked fluids to blood, defends against pathogens
Respiratory	Supplies blood with oxygen, removes carbon dioxide
Digestive	Breaks down food, absorbs nutrients, eliminates waste
Urinary	Eliminates nitrogenous wastes, regulates water and electrolytes
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 the contributions of all organ systems.

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

• Maintained through feedback systems:

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

  • Positive feedback: Enhances or exaggerates the original stimulus (e.g., labor contractions, blood clotting).

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

Directional Terms and Body Planes

Directional Terms

• Used to describe the location of one body structure relative to another.

• Based on the standard anatomical position: body erect, feet slightly apart, palms facing forward, thumbs pointing away from the body.

• Right and left refer to the body being viewed, not the observer.

Major Body Divisions

• Axial: Head, neck, and trunk.

• Appendicular: Limbs (arms and legs).

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

The body contains two major cavities, each with subdivisions that house internal organs.

Major Cavity	Subdivisions	Contents
Cranial	Vertebral	Brain, spinal cord
Thoracic	Pleural, Pericardial, Mediastinum	Lungs, heart, esophagus, trachea
Abdominopelvic	Abdominal, Pelvic	Digestive organs, urinary bladder, reproductive organs, rectum

• Serous membranes (serosa): Thin, double-layered membranes covering surfaces in the ventral body cavity.

• Pleurae: Surround the lungs.

• Pericardium: Surrounds the heart.

• Peritoneum: Surrounds abdominal organs.

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

Summary Table: Levels of Organization

Level	Description
Chemical	Atoms combine to form molecules
Cellular	Cells are made up of molecules
Tissue	Consists of similar types of cells
Organ	Made up of different types of tissues
Organ System	Consists of different organs that work together
Organismal	The human organism is made up of many organ systems

Additional info: Some content was expanded for clarity and completeness, including the full list of body systems and more detailed definitions of anatomical terms and planes.