Organization of the Body
SCIENCE AND SOCIETY
Before we get to the details, we should emphasize that everything you will read in this book is in the context of a broad field of inquiry called science. Science is a style of inquiry that attempts to understand nature in a rational, logical manner. Using detailed observations and vigorous tests, or experiments, scientists winnow out each element of an idea or hypothesis (hye-POTH-eh-sis) until a reasonable conclusion about its validity can be made. Rigorous experiments that eliminate any inﬂuences or biases not being directly tested are called controlled experiments. If the results of observations and experiments are repeatable, they may verify a hypothesis and eventually lead to enough confidence in the concept to call it a theory. Theories in which scientists have an unusually high level of confidence are sometimes called laws. Experiments may disprove a hypothesis, a result that often leads to the formation of new hypotheses (hye-POTH-eh-seez) to be tested.
Figure 1-1 summarizes some of the basic concepts of how new scientific principles are developed. As you can see, science is a dynamic process of getting closer and closer to the truth about nature, including the nature of the human body. Science is definitely not a set of unchanging facts as many people in our culture often assume.
We should also take this opportunity to point out the social and cultural context of the science presented in this book. Scientists drive the process of science, but our culture drives the kinds of questions we ask about nature and how we attempt to answer them. For example, cutting apart human cadavers (dead bodies) for the purpose of studying them has not always been an acceptable activity in all cultures. Today the debate faced by our culture concerns the acceptability of using live animals in scientific experiments. Because our culture does not condone most experiments involving living humans, we have until now often conducted testing on animals that are similar to humans. In fact, most of the theories presented in this book are based on animal experimentation, but cultural influences now are pulling scientists in other experimental directions they otherwise may not have taken.
Similarly, science affects culture. Recent advances in understanding human genes and technological advances in our ability to use so-called “stem cells” and other tissues from human embryos, human cadavers, and living donors to treat devastating diseases have sparked new debates concerning how our culture defines what it means to be a human being.
As you study the concepts presented in this book, keep in mind that they are not set in stone. Science is a rapidly changing set of ideas and processes that not only is influenced by our cultural biases but also affects our cultural awareness of who we are.
ANATOMY AND PHYSIOLOGY
Anatomy is often defined as study of the structure of an organism and the relationships of its parts. The word anatomy is derived from Greek word parts that mean “to cut apart.” Students of anatomy still learn about the structure of the human body by literally cutting it apart. This process, called dissection, remains a principal technique used to isolate and study the structural components or parts of the human body.
Biology is defined as the scientific study of life. Both anatomy and physiology are subdivisions of this very broad area of inquiry. Each of these subdivisions can be further divided into smaller areas of study. For example, the term gross anatomy is used to describe the study of body parts visible to the naked eye. Before invention of the microscope, anatomists had to study human structure relying only on the eye during dissection. These early anatomists could make only a gross, or whole, examination, as you can see in Figure 1-2. With the use of modern microscopes, many anatomists now specialize in microscopic anatomy, including the study of cells, called cytology (sye-TOL-o-jee), and tissues, called histology (hiss-TOL-o-jee).
Other branches of anatomy include the study of human growth and development (developmental anatomy) and the study of diseased body structures (pathological anatomy). In the chapters that follow, you will study the body by systems—a process called systemic anatomy. Systems are groups of organs that have a common function, such as the bones in the skeletal system and the muscles in the muscular system.
Physiology is the science that deals with the functions of the living organism and its parts. The term is a combination of two Greek words (physis, “nature,” and logos, “words or study”). Simply stated, it is the study of physiology that helps us understand how the body works. Physiologists attempt to discover and understand the intricate control systems that permit the body to operate and survive in an often hostile environment.
As a scientific discipline, physiology can be subdivided according to (1) the type of organism involved, such as human physiology or plant physiology; (2) the organizational level studied, such as molecular or cellular physiology; or (3) a specific or systemic function being studied, such as neurophysiology, respiratory physiology, or cardiovascular physiology.
In the chapters that follow, both anatomy and physiology are studied by dividing the human body into specific organ systems. This unit begins with an overview of the body as a whole. In subsequent chapters the body is dissected and studied, both structurally (anatomy) and functionally (physiology), into “levels of organization” so that its component parts can be more easily understood and then “fit together” into a living and integrated whole. It is knowledge of anatomy and physiology that allows us to understand how nerve impulses travel from one part of the body to another; how muscles contract; how light energy can be transformed into visual images; how we breathe, digest food, reproduce, excrete wastes, and sense changes in our environment; and even how we think and reason.
LANGUAGE OF SCIENCE AND MEDICINE
You may have noticed by now that many scientific terms, such as anatomy and physiology, are made up of non-English word parts. Many such terms make up the core of the language used to communicate ideas in science and medicine. Learning in science thus begins with learning a new vocabulary, just as when you learn a new language to help you understand and communicate in a region of the world other than the one you call home.
To help you learn the vocabulary of anatomy and physiology, we have provided several helpful tools for you. Within each chapter, lists of new terms titled Language of Science and Language of Medicine give you each new key (boldface) term that you will be learning in that chapter. Each term in the list has a pronunciation guide and an explanation (or meaning) of each of the word parts that make up the term.
We have also included a separate compact reference called Language of Science and Medicine with this textbook. Take a moment now to locate it. After you have finished reading this chapter, quickly review the tips for learning scientific language. Then keep it nearby so that you will have a handy list of commonly used word parts at your fingertips.
You will see that most scientific terms are made up of word parts from Latin or Greek. Most Western scientists first began corresponding with each other in these languages, because it was commonly the first written language learned by educated people. Other languages such as German, French, and Japanese are also sources of some scientific word parts.
As with any language, scientific language changes constantly. This is useful because we often need to fine-tune our terminology to reflect changes in our understanding of science and to accommodate new discoveries. But it also sometimes leads to confusion. In an attempt to clear up some of the confusion, the International Federation of Associations of Anatomists (IFAA) formed a worldwide committee to publish a list of “universal” or standard anatomical terminology. The list for gross anatomy, the structure we can see without magnification, was published in 1998 as Terminologia Anatomica (TA). In 2008 the Terminologia Histologica (TH) was published for microscopic anatomy—the study of body structure requiring significant magnification for the purpose of visualization.
Although there remain some alternate (and newer) terms used in anatomy, the lists are useful standard references. The lists show each term in Latin and English (based on the Latin form), along with a reference number. In this textbook we use the English terms from the published lists as our standard reference, but we do occasionally refer to the pure Latin form or an alternate term when appropriate for beginning students.
One of the basic principles of the standardizing terminology is the avoidance of eponyms (EP-o-nimz), or terms that are based on a person’s name. Instead, a more descriptive Latin-based term is always preferred. Thus the term eustachian tube (tube connected to the middle ear, named after the famed Italian anatomist Eustachius) is now replaced with the more descriptive auditory tube. Likewise, the islets of Langerhans (in the pancreas) are now simply pancreatic islets. In the rare cases where eponyms do appear in a standard list, we now avoid the possessive form. Thus Bowman’s capsule (in kidney tissue) is now either glomerular capsule or Bowman capsule.
There are no such standard lists of physiological terms. However, many principles used in anatomical terminology are used in physiology. For example, most terms have an English spelling but are based on Latin or Greek word parts. And, as in anatomy, eponyms are less favored than descriptive terms.
This may all seem like a lot more than you want to know right now. However, if you focus on learning the new words as you begin each new topic, as though you are in a foreign land and need to pick up a few phrases to get by, you will find your study of anatomy and physiology easy and enjoyable.
CHARACTERISTICS OF LIFE
Anatomy and physiology are important disciplines in biology—the study of life. But what is life? What is the quality that distinguishes a vital and functional being from a dead body? We know that a living organism is endowed with certain characteristics not associated with inorganic matter. However, it is sometimes hard to find a single criterion to define life. One could say that living organisms are self-organizing or self-maintaining and nonliving structures are not. This concept is called autopoiesis (aw-toe-poy-EE-sis), which literally means “self making.” Another idea, called the cell theory, states that any independent structure made up of one or more microscopic units called cells is a living organism.
Instead of trying to find a single difference that separates living and nonliving things, scientists sometimes define life by listing what are often called characteristics of life. Lists of characteristics of life may differ from one physiologist to the next, depending on the type of organism being studied and the way in which life functions are grouped and defined. Attributes that characterize life in bacteria, plants, or animals may vary. Characteristics of life that are considered most important in humans are described in Table 1-1.
|Responsiveness||Ability of an organism to sense, monitor, and respond to changes in both its external and internal environments|
|Conductivity||Capacity of living cells to transmit a wave of electrical disturbance from one point to another within the body|
|Growth||Organized increase in the size and number of cells and therefore an increase in size of the individual or a particular organ or part|
|Respiration||Exchange of respiratory gases (oxygen and carbon dioxide) between an organism and its environment|
|Digestion||Process by which complex food products are broken down into simpler substances that can be absorbed and used by individual body cells|
|Absorption||Movement of molecules, such as respiratory gases or digested nutrients through a membrane and into the body fluids for transport to cells for use|
|Secretion||Production and release of important substances, such as digestive juices and hormones, for diverse body functions|
|Excretion||Removal of waste products from the body|
|Circulation||Movement of body fluids containing many substances from one body area to another in a continuous, circular route through hollow vessels|
|Reproduction||Formation of new individual offspring|
Each characteristic of life is related to the sum total of all the physical and chemical reactions occurring in the body. The term metabolism is used to describe these various processes. They include the steps involved in the breakdown of nutrient materials to produce energy and the transformation of one material into another. For example, if we eat and absorb more sugar than needed for the body’s immediate energy requirements, it is converted into an alternate form, such as fat, that can be stored in the body. Metabolic reactions are also required for making complex compounds out of simpler ones, as in tissue growth, wound repair, or manufacture of body secretions.
Each characteristic of life—its functional manifestation in the body, its integration with other body functions and structures, and its mechanism of control—is the subject of study in subsequent chapters of the text.
LEVELS OF ORGANIZATION
Before you begin the study of the structure and function of the human body and its many parts, it is important to think about how the parts are organized and how they might logically fit together and function effectively. The differing levels of organization that influence body structure and function are illustrated in Figure 1-3.
Chemical Level—Basis for Life
Note that organization of the body begins at the chemical level (see Figure 1-3). There are more than 100 different chemical building blocks of nature called atoms—tiny spheres of matter so small they are invisible. Every material thing in our universe, including the human body, is composed of atoms.
Combinations of atoms form larger chemical groupings, called molecules. Molecules, in turn, often combine with other atoms and molecules to form larger and more complex chemicals, called macromolecules.
The unique and complex relationships that exist between atoms, molecules, and macromolecules in living material form a gel-like material made of fluids, particles, and membranes called cytoplasm (SYE-toe-plaz-em)—the essential material of human life. Unless proper relationships between chemical elements are maintained, death results. Maintaining the type of chemical organization in cytoplasm required for life requires the expenditure of energy. In Chapter 2 important information related to the chemistry of life is discussed in more detail.
Chemical structures may be organized within larger units called cells to form various structures called organelles (or-gah-NELLZ), the next level of organization (see Figure 1-3). An organelle may be defined as a structure made of molecules organized in such a way that it can perform a specific function. Organelles are the “tiny organs” that allow each cell to live. Organelles cannot survive outside the cell, but without organelles the cell itself could not survive either.
Dozens of different kinds of organelles have been identified. A few examples include mitochondria (my-toe-KON-dree-ah), the “power houses” of cells that provide the energy needed by the cell to carry on day-to-day functioning, growth, and repair; Golgi (GOL-jee) apparatus, which provides a “packaging” service to the cell by storing material for future internal use or for export from the cell; and endoplasmic reticulum, the network of transport channels within the cell that act as “highways” for the movement of chemicals. Chapter 3 contains a complete discussion of organelles and their functions.
The characteristics of life ultimately result from a hierarchy of structure and function that begins with the organization of atoms, molecules, and macromolecules. Further organization that results in organelles is the next step. However, in the view of the anatomist, the most important function of the chemical and organelle levels of organization is that of furnishing the basic building blocks required for the next higher level of body structure—the cellular level.
Cells are the smallest and most numerous structural units that possess and exhibit the basic characteristics of living matter. How many cells are there in the body? One estimate places the number of cells in a 150-pound adult human body at 100,000,000,000,000.
In case you are having trouble translating this number—1 with 14 zeroes after it—it is 100 trillion! or 100,000 billion! or 100 million million!
Each cell is surrounded by a membrane and is characterized by a single nucleus surrounded by cytoplasm that includes the numerous organelles required for the normal processes of living. Although all cells have certain features in common, they specialize or differentiate to perform unique functions. Fat cells, for example, are structurally modified to permit the storage of fats, whereas other cells, such as cardiac muscle cells, are able to contract with great force (see Figure 1-3). Muscle, bone, nerve, and blood cells are other examples of structurally and functionally unique cells.
The next higher level of organization beyond the cell is the tissue level (see Figure 1-3). Tissues represent another step in the progressive organization of living matter. By definition, a tissue is a group of a great many similar cells that all developed together from the same part of the embryo and all perform a certain function. Tissue cells are surrounded by varying amounts and kinds of nonliving, intercellular substances, or the matrix. Tissues are the “fabric” of the body.
There are four major or principal tissue types: epithelial, connective, muscle, and nervous. Considering the complex nature of the human body, this is a surprisingly short list of major tissues. Each of the four major tissues, however, can be subdivided into several distinct subtypes. Together the body tissues are able to meet all the structural and functional needs of the body.
The tissue used as an example in Figure 1-3 is a type of muscle called cardiac muscle. Note how the cells are branching and interconnected. The details of tissue structure and function are covered in Chapter 6.
Organ units are more complex than tissues. An organ is defined as a structure made up of several different kinds of tissues arranged so that, together, they can perform a special function.
If tissues are the “fabric” of the body, an organ is like an item of clothing with a specific function made up of different fabrics. The heart is an example of the organ level: muscle and connective tissues give it shape and pump blood; epithelial tissues line the cavities, or chambers; and nervous tissues permit control of the pumping contractions of the heart.
Tissues seldom exist in isolation. Instead, joined together, they form organs that represent discrete, but functionally complex, operational units. Each organ has a unique shape, size, appearance, and placement in the body, and each can be identified by the pattern of tissues that form it. The lungs, heart, brain, kidneys, liver, and spleen are all examples of organs.
Systems are the most complex of the organizational units of the body. The system level of organization involves varying numbers and kinds of organs arranged so that, together, they can perform complex functions for the body.
Eleven major systems compose the human body: integumentary, skeletal, muscular, nervous, endocrine, circulatory, lymphatic/immune, respiratory, digestive, urinary, and reproductive. Systems that work together to accomplish the general needs of the body are summarized in Table 1-2.
|FUNCTIONAL CATEGORY||SYSTEM||PRINCIPAL ORGANS||PRIMARY FUNCTIONS|
|Support and movement (Unit One)||Integumentary (Chapter 7)||Skin||Protection, temperature regulation, sensation|
|Skeletal (Chapters 8–10)||Bones, ligaments||Support, protection, movement, mineral and fat storage, blood production|
|Muscular (Chapters 11–12)||Skeletal muscles, tendons||Movement, posture, heat production|
|Communication, control, and integration (Unit Two)||Nervous (Chapters 13–17)||Brain, spinal cord, nerves, sensory organs||Control, regulation, and coordination of other systems, sensation, memory|
|Endocrine (Chapters 18–19)||Pituitary gland, adrenals, pancreas, thyroid, parathyroids, and other glands||Control and regulation of other systems|
|Transportation and defense (Unit Three)||Cardiovascular (Chapters 20–22)||Heart, arteries, veins, capillaries||Exchange and transport of materials|
|Lymphatic (Chapters 23–25)||Lymph nodes, lymphatic vessels, spleen, thymus, tonsils||Immunity, fluid balance|
|Respiration, nutrition, and excretion (Unit Four)||Respiratory (Chapters 26–27)||Lungs, bronchial tree, trachea, larynx, nasal cavity||Gas exchange, acid-base balance|
|Digestive (Chapters 28–30)||Stomach, small and large intestines, esophagus, liver, mouth, pancreas||Breakdown and absorption of nutrients, elimination of waste|
|Urinary (Chapters 31–33)||Kidneys, ureters, bladder, urethra||Excretion of waste, fluid and electrolyte balance, acid-base balance|
|Reproduction and development (Unit Five)||Reproductive (Chapters 34–37)||Reproduction, continuity of genetic information, nurturing of offspring|
Take a few minutes to read through Table 1-2