As can be seen, anatomy can be studies in multiple ways, and at various levels. Understanding the hierarchy of these levels, provides context to the complexity of the human organism. The simplest level of organization (although first year chemistry students would disagree) is the chemical level of organization. At this level, simple atoms combine to form relatively simple molecules. For example, carbon dioxide (CO2) is made up of one carbon atom, and two oxygen atoms, and water (H2O) is made up of two hydrogen atoms and one oxygen atom. Macromolecules (macro: big) are larger and more complex, and include four key types in the human body; carbohydrates (sugars), lipids (fats), proteins and nucleic acids (DNA). These four macromolecules form the building blocks of the next level of organization: the cellular level. Cells are the smallest units of life and are responsible for regulating their own environment. The four macromolecules, interact to complete complex tasks for the cell, like generating energy (ATP) or producing muscle contractions (through the interactions of two protein complexes: actin and myosin).
The tissue level of organization consists of a group of cells that work together to accomplish one or more specific functions. There are only four distinct types of tissue in an adult human. Muscle tissue is specialized for contraction to generate movement; neural tissue is specialized for generating action potentials for rapid communication within the body; epithelial tissue provides a physical barrier for entry into the body and produces specialized secretions via glands; and connective tissue shows the greatest variability of all of the tissues, and forms much of the structure of the body (among many other things).
The organ level of organization is when two or more tissues work together for a specific function. For example, the bladder consists of an inner lining of epithelial tissue, bound by various connective tissues to (smooth) muscle. Throughout the bladder, there are also neurons that control the muscle tissue directing it to contract or relax during the urination reflex.
The organ system level of organization is when two or more organs work together for a specific function. The bladder mentioned, when combined with the kidneys (another organ), and the ureters (“tubes” connecting the kidneys to the bladder, form the urinary system (or urinary tract). The kidneys filter the blood, and the waist products drain through the ureters to be stored in the bladder. The waist is eliminated from the body when we urinate.The urinary system is one of the eleven body systems that can be examined using systemic anatomy.
The most complex level of organization is the organismal level, where all eleven organ systems function in the human organism, the whole living person.
Before you begin to study the different structures and functions of the human body, it is helpful to consider its basic architecture; that is, how its smallest parts are assembled into larger structures. It is convenient to consider the structures of the body in terms of fundamental levels of organization that increase in complexity: subatomic particles, atoms, molecules, organelles, cells, tissues, organs, organ systems, organisms and biosphere (Figure 1).
Figure 1. Levels of Structural Organization of the Human Body. The organization of the body often is discussed in terms of six distinct levels of increasing complexity, from the smallest chemical building blocks to a unique human organism.To study the chemical level of organization, scientists consider the simplest building blocks of matter: subatomic particles, atoms and molecules. All matter in the universe is composed of one or more unique pure substances called elements, familiar examples of which are hydrogen, oxygen, carbon, nitrogen, calcium, and iron. The smallest unit of any of these pure substances (elements) is an atom. Atoms are made up of subatomic particles such as the proton, electron and neutron. Two or more atoms combine to form a molecule, such as the water molecules, proteins, and sugars found in living things. Molecules are the chemical building blocks of all body structures.
A cell is the smallest independently functioning unit of a living organism. Even bacteria, which are extremely small, independently-living organisms, have a cellular structure. Each bacterium is a single cell. All living structures of human anatomy contain cells, and almost all functions of human physiology are performed in cells or are initiated by cells.
A human cell typically consists of flexible membranes that enclose cytoplasm, a water-based cellular fluid together with a variety of tiny functioning units called organelles. In humans, as in all organisms, cells perform all functions of life. A tissue is a group of many similar cells (though sometimes composed of a few related types) that work together to perform a specific function. An organ is an anatomically distinct structure of the body composed of two or more tissue types. Each organ performs one or more specific physiological functions. An organ system is a group of organs that work together to perform major functions or meet physiological needs of the body.
This book covers eleven distinct organ systems in the human body (Figure 2 and Figure 3). Assigning organs to organ systems can be imprecise since organs that “belong” to one system can also have functions integral to another system. In fact, most organs contribute to more than one system.
Figure 2. Organ Systems of the Human Body. Organs that work together are grouped into organ systems.Figure 3. Organ Systems of the Human Body (continued). Organs that work together are grouped into organ systems.The organism level is the highest level of organization. An organism is a living being that has a cellular structure and that can independently perform all physiologic functions necessary for life. In multicellular organisms, including humans, all cells, tissues, organs, and organ systems of the body work together to maintain the life and health of the organism.
Life processes of the human body are maintained at several levels of structural organization. These include the chemical, cellular, tissue, organ, organ system, and the organism level. Higher levels of organization are built from lower levels. Therefore, molecules combine to form cells, cells combine to form tissues, tissues combine to form organs, organs combine to form organ systems, and organ systems combine to form organisms.
Review Questions
1. The smallest independently functioning unit of an organism is a(n) ________.
2. A collection of similar tissues that performs a specific function is an ________.
- organ
- organelle
- organism
- organ system
3. The body system responsible for structural support and movement is the ________.
- cardiovascular system
- endocrine system
- muscular system
- skeletal system
Critical Thinking Questions
1. Name the six levels of organization of the human body.
2. The female ovaries and the male testes are a part of which body system? Can these organs be members of more than one organ system? Why or why not?