Introduction to Human Physiology

page 1

Humans as Mammals

Characteristics of Mammals

Because we are part of a continuum of nature, it is logical to study other animals in order to understand the human animal; it is equally logical to study the human animal in order to gain a deeper understanding of animal life in general.

You should memorize the following points

  1. The human being is a vertebrate which means we have a bony, articulated (jointed) endoskeleton that supports the body and grows as it grows. The spinal cord, which is dorsal in vertebrates (ventral in invertebrates) is surrounded by bony segments, the vertetrae, and the brain is enclosed in a protective casing, the skull.
  2. As in other vertebrates, and most invertebrates as well, the human body contains a cavity, or coelom.

In humans the coelom is divided into two parts, the thoracic cavity and the abdominal cavity. These are separated by a dome-shaped muscle, the diaphragm.

The thoracic cavity contains the heart, the lungs, and the upper portion of the digestive tract. The abdominal cavity contains a large number of organs, including the stomach, intestines, and liver.


  1. One of the most important characteristics of mammals is that they are warm-blooded.

More precisely, they are homeotherms; that is, they maintain a high and relatively constant body termperature. As a consequence, mammals (and birds, which are also homeotherms) are able to achieve and sustain levels of physical activity and mental alertness generally far greater than those of animals whose temperatures rise and fall with that of their external environment.

Were dinosaurs homeotherms or ectotherms? Visit the debate.

  1. They have hair or fur rather than scales or feathers.
  2. They have the most highly developed systems for receiving, processing, and correlating information from the environment
  3. All mammals reproduce sexually and give birth to live young, as distinct from laying eggs (except the primitive mammals called monotremes).
  4. Mammals nurse their offspring, which involves a relatively long period of parental care and makes possible a long learning period.

Contrast this, for example, with most insects and nearly all species of fish, amphibians, and reptiles, which lay eggs and in which the young are independent from the moment they hatch from the egg.

  1. There is a tendency among the large mammals, in particular, toward fewer offspring per litter and prolongation of parental care.

Humans, for instance, rarely have more than two surviving young per birth, only two mammary glands with which to nurse them, and an extraordinarily long period of infancy and childhood, with dependency on parents often lasting well past physical maturity.


  • Finally, for better or worse, Homo sapiens is by far the most intelligent of all mammals, but don't rest on your laurels.


Although they greatly resemble one-celled organisms in their requirements, the cells of multicellular organisms differ from one-celled organisms in that they develop and function as part of an organized whole.

Cells are organized into tissues, groups of cells similar in structure and function. Different kinds of tissues, united structurally and coordinated in their activities, form organs, such as the skin. Organs that function together in an integrated and organized way make up organ systems. The digestive system, for example, is composed of a number of different organs, each of which carries out a specific activity that contributes toward the overall process.


Modified Ap. 17, 2003