Human Body - Levels of Organization & Organ Systems

Learning Objectives

  • Describe the structure of the human body in terms of six levels of organization.
  • List the eleven organ systems of the human body and identify at least one organ and one major function of each.

Levels of Organization

  • The structures of the body can be considered in terms of fundamental levels of organization that increase in complexity:
    • Subatomic particles
    • Atoms
    • Molecules
    • Organelles
    • Cells
    • Tissues
    • Organs
    • Organ systems
    • Organisms
    • Biosphere

Chemical Level

  • Scientists consider the simplest building blocks of matter: subatomic particles, atoms, and molecules.
  • All matter is composed of one or more unique pure substances called elements (e.g., hydrogen, oxygen, carbon, nitrogen, calcium, and iron).
  • The smallest unit of an element is an atom.
  • Atoms are made up of subatomic particles such as protons, electrons, and neutrons.
  • Two or more atoms combine to form a molecule (e.g., water molecules, proteins, and sugars).
  • Molecules are the chemical building blocks of all body structures.

Cellular Level

  • A cell is the smallest independently functioning unit of a living organism.
  • Examples:
    • Bacteria (single-celled organisms)
  • All living structures of human anatomy contain cells.
  • Almost all functions of human physiology are performed in cells or initiated by cells.
  • A human cell typically consists of:
    • Flexible membranes
    • Cytoplasm (water-based cellular fluid)
    • Organelles (tiny functioning units)
  • Cells perform all functions of life.

Tissue Level

  • A tissue is a group of many similar cells that work together to perform a specific function.
  • Sometimes tissues are composed of a few related types of cells.

Organ Level

  • An organ is an anatomically distinct structure composed of two or more tissue types.
  • Each organ performs one or more specific physiological functions.

Organ System Level

  • An organ system is a group of organs that work together to perform major functions or meet physiological needs.
  • There are eleven distinct organ systems in the human body.
  • Assigning organs to organ systems can be imprecise because some organs contribute to multiple systems.

Organ Systems of the Human Body

  • Integumentary System
    • Organs: Skin, hair, nails
    • Functions: Encloses internal body structures, site of many sensory receptors
  • Skeletal System
    • Organs: Cartilage, bones, joints
    • Functions: Supports the body, enables movement (with the muscular system)
  • Muscular System
    • Organs: Skeletal muscles, tendons
    • Functions: Enables movement (with the skeletal system), helps maintain body temperature
  • Nervous System
    • Organs: Brain, spinal cord, peripheral nerves
    • Functions: Detects and processes sensory information, activates bodily responses
  • Endocrine System
    • Organs: Pituitary gland, thyroid gland, pancreas, adrenal glands, testes, ovaries
    • Functions: Secretes hormones, regulates bodily processes
  • Cardiovascular System
    • Organs: Heart, blood vessels
    • Functions: Delivers oxygen and nutrients to tissues, equalizes temperature in the body
  • Lymphatic System
    • Organs: Thymus, lymph nodes, spleen, lymphatic vessels
    • Functions: Returns fluid to blood, defends against pathogens
  • Respiratory System
    • Organs: Nasal passage, trachea, lungs
    • Functions: Removes carbon dioxide from the body, delivers oxygen to blood
  • Digestive System
    • Organs: Stomach, liver, gall bladder, large intestine, small intestine
    • Functions: Processes food for use by the body, removes wastes from undigested food
  • Urinary System
    • Organs: Kidneys, urinary bladder
    • Functions: Controls water balance in the body, removes wastes from blood and excretes them
  • Male Reproductive System
    • Organs: Epididymis, testes
    • Functions: Produces sex hormones and gametes, delivers gametes to female
  • Female Reproductive System
    • Organs: Mammary glands, ovaries, uterus
    • Functions: Produces sex hormones and gametes, supports embryo/fetus until birth, produces milk for infant

Organism Level

  • The organism level is the highest level of organization.
  • An organism is a living being that:
    • Has a cellular structure
    • Can independently perform all physiologic functions necessary for life
  • In multicellular organisms, all cells, tissues, organs, and organ systems work together to maintain life and health.