CHAPTER 1 The Human Organism

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4. Growth refers to an increase in the size or number of cells,which produces an overall enlargement of all or part of anorganism. For example, a muscle enlarged by exercise iscomposed of larger muscle cells than those of an untrainedmuscle, and the skin of an adult has more cells than the skinof an infant. An increase in the materials surrounding cells canalso contribute to growth. For instance, bone grows because ofan increase in cell number and the deposition of mineralizedmaterials around the cells.5. Development includes the changes an organism undergoesthrough time, beginning with fertilization and ending at death.The greatest developmental changes occur before birth, butmany changes continue after birth, and some go on through-out life. Development usually involves growth, but it alsoinvolves differentiation and morphogenesis. Differentiation involves changes in a cell’s structure and function from animmature, generalized state to a mature, specialized state. Forexample, following fertilization, immature cells differentiateto become specific cell types, such as skin, bone, muscle, ornerve cells. These differentiated cells form tissues and organs. Morphogenesis (m ō r-f ō -jen′ ĕ -sis) is the change in shape oftissues, organs, and the entire organism.6. Reproduction is the formation of new cells or new organ-isms. Without reproduction of cells, growth and developmentare not possible. Without reproduction of organisms, speciesbecome extinct.

ASSESS YOUR PROGRESS

8. What are the six characteristics of living things? Brieflyexplain each.

9. How does differentiation differ from morphogenesis?

obtained only from humans because other organisms differ fromhumans in significant ways. A failure to appreciate the differencesbetween humans and other animals led to many misconceptionsby early scientists. One of the first great anatomists was a Greekphysician, Claudius Galen (ca. 130–201). Galen described a largenumber of anatomical structures supposedly present in humansbut observed only in other animals. For example, he describedthe liver as having five lobes. This is true for rats, but not forhumans, who have four-lobed livers. The errors introduced byGalen persisted for more than 1300 years until a Flemish anato-mist, Andreas Vesalius (1514–1564), who is considered the firstmodern anatomist, carefully examined human cadavers andbegan to correct the textbooks. This example should serve as aword of caution: Some current knowledge in molecular biologyand physiology has not been confirmed in humans.

ASSESS YOUR PROGRESS

10. Why is it important to recognize that humans share many,but not all, characteristics with other animals?

1.5 Homeostasis

LEARNING OUTCOMES

After reading this section, you should be able to

A. Define homeostasis and explain why it is importantfor proper body function.

B. Describe a negative-feedback mechanism and givean example.

C. Describe a positive-feedback mechanism and givean example.

1.4 Biomedical Research

LEARNING OUTCOME

After reading this section, you should be able to

A. Explain why it is important to study other organismsalong with humans.

Studying other organisms has increased our knowledge abouthumans because humans share many characteristics with otherorganisms. For example, studying single-celled bacteria providesmuch information about human cells. However, some biomedicalresearch cannot be accomplished using single-celled organismsor isolated cells. Sometimes other mammals must be studied, asevidenced by the great progress in open heart surgery and kidneytransplantation made possible by perfecting surgical techniqueson other mammals before attempting them on humans. Strict lawsgovern the use of animals in biomedical research; these laws aredesigned to ensure minimal suffering on the part of the animal andto discourage unnecessary experimentation.Although much can be learned from studying other organ-isms, the ultimate answers to questions about humans can be

Homeostasis (h ō ′m ē - ō -st ā ′sis) is the existence and maintenance of arelatively constant environment within the body. To achieve homeo-stasis, the body must actively regulate conditions that are constantlychanging. As our bodies undergo their everyday processes, we arecontinuously exposed to new conditions. These conditions are called variables because their values can change. For example, a smallamount of fluid surrounds each body cell; for cells to function nor-mally, the volume, temperature, and chemical content of this fluidmust be maintained within a narrow range.One variable familiar to all of us is body temperature. Bodytemperature is a variable that can increase in a hot environmentor decrease in a cold one. Homeostatic mechanisms, such assweating or shivering, normally maintain body temperature nearan ideal normal value, or set point (figure 1.4). Note that thesemechanisms are not able to maintain body temperature precisely at the set point. Instead, body temperature increases and decreasesslightly around the set point to produce a normal range of values.As long as body temperature remains within this normal range,homeostasis is maintained. Keep in mind that the fluctuations areminimal, however. Note in figure 1.4 that the normal body tem-perature range is no more than 1 degree Fahrenheit above or belownormal. Our average body temperature is 98.6 degrees Fahrenheit.