is the study of how biological systems function.
Therefore, in order to meet the needs of a larger multicellular body composed of millions or billions of cells (~75 ×
or 75 trillion cells in an adult human body)
In order to meet the complex needs of a large body, mammalian systems have 10 distinct
. These are
the nervous system, endocrine system, cardiovascular system, respiratory system, urinary system, musculoskeletal
system, integumentary system, reproductive system, digestive system, and immune system (see Table 1).
exchange of materials between the circulatory system and tissues takes place only across the
In mammals, the circulatory system is such that no cell is more than approximately 10 μm from a capillary.
The endocrine system uses chemical messenger molecules - called
- that travel to distant targets to
convey information from one cell type to another.
The nervous system, in addition to using chemical messenger molecules referred to as
electrical impulses to send and receive information.
, an O
binding protein, serves to dramatically increase the amount of O
that blood can carry.
In an average adult male, the body weight distribution is 60% water, 18% protein, 7% minerals, and 15% fat. In
females the numbers are as follows: 50% water, 18% protein, 7% minerals, and 25% fat.
That is all of the ~10
cells of the human body (with a few exceptions; e.g., cornified cells at the upper layer of the
skin) are surrounded and bathed by a fluid referred to as the
. In contrast, the fluid contained
within the cells in the cytoplasmic compartment is referred to as the
The intracellular fluid in all cells is separated from the extracellular fluid by the cell plasma membrane.
The extracellular fluid compartment is further subdivided into the