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Organization of the Human Body

Introduces cells, tissues, organs, and organ systems in humans.

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Unit 1: Organization of the Human Body Plan

Lesson 1- Learning Objectives

  1. 1.  Contrast the sciences of anatomy and physiology.
  2. 2.  Describe the six levels of structural organization of the human body (chemical, cellular, tissue, organ, system, organism)
  3. 3.  Describe metabolism and its anabolic and catabolic processes.
  4. 4. Apply commonly used planes to divide the body (sagittal, midsagittal, transverse [horizontal], frontal [coronal])
  5. 5.   Apply directional terms used in human anatomy (posterior/anterior, medial/lateral, proximal/distal, superficial/deep, superior/inferior, cranial/caudal, peripheral/palmar/plantar) 
  6. 6.  Identify the body cavities and locate the following organs within each cavity. 
    7.  Contrast homeostasis, stressors and positive and negative feedback loops.   


How is the human body similar to a well-tuned machine?

Many people have compared the human body to a machine. Think about some common machines, such as drills and washing machines. Each machine consists of many parts, and each part does a specific job, yet all the parts work together to perform an overall function. The human body is like a machine in all these ways. In fact, it may be the most fantastic machine on Earth.

As a preview of the human machine, the Emmy award-winning video, Inside The Living Body, at this link is highly recommended: http://www.youtube.com/watch?v=chqwSh4ii84&feature=related.

Anatomy: Anatomy is the scientific study of structures and the relationship of structures to each other. (FORM) Physiology: Physiology is the scientific study of how body structures and systems function to perform life processes. (FUNCTION)

How does anatomy determine function in the body?   

Levels of Organization

The human machine is organized at different levels, starting with the cell and ending with the entire organism (see Figure below). At each higher level of organization, there is a greater degree of complexity.

The human organism has several levels of organization.


The chemical level includes all elements (atoms) and molecules essential for maintaining life. Examples of the four most common elements in the body include: (C) carbon, (H) hydrogen, (O) oxygen and (N) nitrogen. Examples of molecules include glucose and water.


The most basic parts of the human machine are cells—an amazing 100 trillion of them by the time the average person reaches adulthood! Cells are the basic units of structure and function in the human body, as they are in all living things. Each cell carries out basic life processes that allow the body to survive. Many human cells are specialized in form and function, as shown in Figure below. Each type of cell in the figure plays a specific role. For example, nerve cells have long projections that help them carry electrical messages to other cells. Muscle cells have many mitochondria that provide the energy they need to move the body.

You can watch a video about some of the specialized cells of the human body and how they function at this link: http://www.youtube.com/watch?v=I8uXewS9dJU&feature=related

Different types of cells in the human body are specialized for specific jobs. Do you know the functions of any of the cell types shown here?


After the cell, the tissue is the next level of organization in the human body. A tissue is a group of connected cells that have a similar function. There are four basic types of human tissues: epithelial, muscle, nervous, and connective tissues. These four tissue types, which are shown in Figure below, make up all the organs of the human body.

The human body consists of these four tissue types.

  • Connective tissue is made up of cells that form the body’s structure. Examples include bone and cartilage.
  • Epithelial tissue is made up of cells that line inner and outer body surfaces, such as the skin and the lining of the digestive tract. Epithelial tissue protects the body and its internal organs, secretes substances such as hormones, and absorbs substances such as nutrients.
  • Muscle tissue is made up of cells that have the unique ability to contract, or become shorter. Muscles attached to bones enable the body to move.
  • Nervous tissue is made up of neurons, or nerve cells, that carry electrical messages. Nervous tissue makes up the brain and the nerves that connect the brain to all parts of the body.

Organs and Organ Systems

After tissues, organs are the next level of organization of the human body. An organ is a structure that consists of two or more types of tissues that work together to do the same job. Examples of human organs include the brain, heart, lungs, skin, and kidneys. Human organs are organized into organ systems, many of which are shown in Figure below. An organ system is a group of organs that work together to carry out a complex overall function. Each organ of the system does part of the larger job.

You can watch overviews of the human organ systems and their functions at the links below.

Many of the organ systems that make up the human body are represented here. What is the overall function of each organ system?

Organism Level

An organism is a group of organ systems which function together to meet the needs of the individual.


Definition - Metabolism is the total of all chemical processes that occur in the body. Metabolism refers specifically to the two processes associated with converting nutrients into energy.

Anabolism - Anabolism uses energy to synthesize or manufacture new cells, tissues or molecules.

Catabolism - Catabolism is the breakdown of tissues or chemicals to produce energy.

Watch this short video created by Jenny Walsh on You Tube showing the difference between anabolism and catabolism found at this link http://www.youtube.com/watch?v=v0OM-Qjdj88.


Body planes refer to any slice or cut through a three-dimensional structure allowing us to visualize relationships between those parts. CT (Computed Tomography Imaging) and MRI (Magnetic Resonance Imaging) technology use these principles.

A. Sagittal Plane - The sagittal plane is a vertical plane (lengthwise) dividing the body or an organ into right and left sections.

B. Midsagittal Plane - The midsagittal plane is a vertical plane (lengthwise) dividing the body or an organ into equal right and left halves.

C. Transverse (Cross-Section, Horizontal) - The transverse plane is a horizontal plane dividing the body or an organ into superior (upper) and inferior (lower) sections.

D. Frontal (Coronal) - The frontal plane is a vertical plane dividing the body or an organ into anterior (front) and posterior (back) sections.


These terms will help to standardize discussion about locations and directions of the body.  

  1. Posterior – to the back
  2. Anterior – to the front
  3. Medial – towards the middle
  4. Lateral – towards the side
  5. Proximal – closest to the trunk or main part of the body
  6. Distal – away from the trunk or the main part of the body
  7. Superficial – towards the surface
  8. Deep – away from the surface
  9. Superior – above
  10. Inferior -- below
  11. Cranial- towards the head
  12. Caudal- directed toward or situated in or near the tail or posterior part of the body 
  13. Peripheral - near the surface or outside of; external
  14. Palmar- pertaining to, or located in or on the palm of the hand 
  15. Plantar-pertaining to the sole of the foot

Directional terms are used to describe joint movements that occur in different directions and planes. The body is assumed to be in anatomical position; standing erect, the face forward, and the arms at the sides with the palms forward. A person may be lying down in anatomical position. The person is supine when lying on the back, face up or prone when lying face down on the abdomen.  Provided is a video clip from YouTube called Joine Movement Anatomy Project  that does a great job of showing the joint movements-

A. Abduction - Moving a body part away from the midline.
B. Adduction - Moving a body part toward the midline.
C. Circumduction - Moving a body part in a circular motion.
D. Depression - Lowering a body part.
E. Dorsiflexion - Bending the foot upward by flexing the foot at the ankle.
F. Elevation - Raising a body part.
G. Eversion - Turning the foot so the sole is outward.
H. Extension - Increasing the joint angle to straighten body parts.
I. Flexion - Decreasing the joint angle to bring two body parts closer together.
J. Hyperextension - Excessive extension of body parts at a joint; moving a part beyond normal anatomical position.
K. Inversion - Turning the foot so the sole of the foot is inward.
L. Plantar flexion - Bending the foot downward by extending the foot at the ankle.
M. Pronation - Turning the hand with the palm down or turning the foot so the medial margin is lowered.
N. Protraction - Moving a body part forward.
O. Retraction - Moving a body part backward.
P. Supination - Turning the hand with the palm upward or turning the foot so the medial margin is raised


Body cavities are openings within the torso which contain organs, protect delicate organs from accidental shocks and bumps, and permit the expansion and contraction of organs without disrupting the activities of other organs.

A. Dorsal Cavity - The dorsal cavity is located on the posterior/dorsal surface of the body and surrounds the brain and the spinal cord.
     1. Spinal (Vertebral) Cavity - The spinal cavity is formed by the vertebrae of the spine and surrounds spinal cord..

     2. Cranial Cavity - The bones of the skull create the cranial cavity to protect the brain.

B. Ventral Cavity - The ventral cavity is located on the anterior/ventral surface of the body and contains the thoracic cavity and the abdominopelvic cavity. The walls of the cavities are composed of skin, muscle, connective tissue, bone (for two cavities), and the serous membrane.

     1. Thoracic Cavity - The thoracic cavity is the portion of ventral cavity superior to the diaphragm.

          a. Pleural Cavities - The pleural cavities are the spaces surrounding each lung.

          b. Mediastinum - The mediastinum is the broad, middle tissue mass of the thoracic cavity dividing the lungs  
              into two cavities. It includes the aorta, other great blood vessels, esophagus, trachea, thymus, pericardial
              cavity, and heart.

          c. Pericardial Cavity - The pericardial cavity is the space in which the heart is located.

     2.Abdominopelvic Cavity - The abdominopelvic cavity is portion of the ventral cavity inferior to the diaphragm.

          a. Abdominal Cavity

               (1) The abdominal cavity is the superior portion of the abdominopelvic cavity. It extends from the diaphragm    
               to the superior margin of the pelvic girdle.

               (2) The abdominal cavity contains the organs known as the viscera. The organs include the stomach,
               spleen, liver, gallbladder, pancreas, small intestines, and most of the large intestine.

          b. Pelvic Cavity

               (1) The pelvic cavity is surrounded by the pelvic bones.

               (2) The pelvic cavity contains the urinary bladder, cecum, appendix, sigmoid colon, rectum, intestines, and
               the male or female internal reproductive organs.


A. Abdominopelvic Quadrants - The abdominopelvic quadrants are imaginary lines intersecting through the umbilicus to divide the abdominopelvic cavity into four areas. The quadrants are used by clinical personnel to describe the location of abdominopelvic pain, tumors, or other abnormalities.

          1. Right Upper Quadrant (RUQ)

               a. Liver

               b. Right Kidney

               c. Gallbladder

          2. Left Upper Quadrant (LUQ)

               a. Spleen

               b. Stomach

               c. Left Kidney

          3. Right Lower Quadrant (RLQ)

               a. Cecum 

               b. Appendix

               c. Right Ovary

          4. Left Lower Quadrant (LLQ)

               a. Left ovary


A. Homeostasis -Homeostasis is the body’s ability to maintain a stable internal environment despite changes that occur internally or externally.

Types of Homeostatic Regulation in the Body


Homeostatic Processes

Hormones and Other Messengers

Tissues, Organs and Organ Systems Involved

Osmoregulation (also called excretion)

Excess water, salts, and urea expelled from body

Antidiuretic hormone (ADH), aldosterone, angiotensin II, carbon dioxide

Kidneys, urinary bladder, ureters, urethra (urinary system), pituitary gland (endocrine system), lungs (respiratory system)


Sweating, shivering, dilation/constriction of blood vessels at skin surface, insulation by adipose tissue, breakdown of adipose tissue to produce heat

Nerve impulses

Skeletal muscle (muscular system), nerves (nervous system), blood vessels (cardiovascular system), skin and adipose tissue (integumentary system), hypothalamus (endocrine system)

Chemical Regulation (including glucoregulation)

Release of insulin and glucagon into the blood in response to rising and falling blood glucose levels, respectively; increase in breathing rate in response to increases carbon dioxide levels in the blood, and release of carbon dioxide into exhaled air from lungs, secretion of erythropoietin by kidneys to stimulate formation of red blood cells

Insulin, glucagon, cortisol, carbon dioxide, nerve impulses, erythropoietin (EPO)

Pancreas (endocrine system), liver (digestive system); adrenal glands (endocrine system) lungs (respiratory system), brain (nervous system), kidneys (urinary system)

B. Stress - Stress is an imbalance in the body’s internal environment. A stressor is something that causes stress and may be physical (illness or injury), emotional (such as bipolar disorder or obsessive compulsive disorder), metabolic (starvation), or environmental (heat, cold). Stressors may be further classified as external or internal.

     1. External stressors: heat, cold, noise. light, exercise

     2. Internal stressors: pain, tumors, hypertension, chemicals


A. Components of Feedback Mechanisms - Feedback mechanisms function continuously to monitor the level of chemicals, molecules, gases, pressure, pH, nutrients, glucose, water, temperature, and other vital parameters. The feedback model contains the following components:

          1. Stimulus - Any stress that changes a controlled condition.

          2. Receptor - Monitors changes in the controlled condition and sends information (Input) to the control center.

          3. Control Center - An area in the body that receives information about the status of a controlled condition from
          a receptor and determines an appropriate course of action.

          4. Effector - Receives information from the control center and produces a response.

          5. Response - The action of the effector.

B. Types of Feedback Mechanisms

     1. Negative Feedback Mechanisms (Inhibitory)

The reaction of the body (output) counteracts the stress (input) in order to restore homeostasis. Simply stated, negative feedback mechanisms reverse the effects of the change. EXAMPLE: Blood Glucose. Blood glucose normally ranges between 80 milligrams and 120 milligrams in a blood sample. When we eat, our blood sugar rises above normal levels. This causes insulin to be released from the pancreas to facilitate the movement of glucose from the blood and into the body cells. The blood sugar level drops back to the normal. Examples of other negative feedback mechanisms include regulation of blood pressure, body temperature, and water balance. Watch this video clip from YouTube called Homeostasis: Negative Feedback  found at the link http://www.youtube.com/watch?v=YQMgV9pkwwA. 

License: CC BY-NC 3.0


     2. Positive Feedback Mechanisms (Stimulatory)

The reaction of the body (output) is stimulated or intensified by the input. In other words, the response enhances the stimulus. EXAMPLE: Breast feeding shows the hormonal control of milk let-down by a suckling infant. Other non-lethal examples of positive feedback mechanisms include labor contractions and blood clotting.

Watch a video clip found on You Tube called Postive Feedback found at http://www.youtube.com/watch?v=fANoQHFsj-g to help you understand this concept.


What happens when homeostasis is disrupted?  The state of homeostasis is very important for the body. All the chemical reactions and the metabolic processes are carried it perfectly if homeostasis is maintained. The disturbance in the homeostasis invites diseases, fatigue and can endure longterm consequences if not treated properly. 

Read the following article about concussions found at http://discoverer.prod.sirs.com/discoweb/disco/do/article?urn=urn%3Asirs%3AUS%3BARTICLE%3BART%3B0000352157 to see how disruptions in the nervous system affect the body. 

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