40.1 Why Must Animals Regulate Their Internal
All animals need nutrients and oxygen and must eliminate carbon dioxide and other
waste products of metabolism. Single-celled organisms meet all these needs by direct
exchanges with the external environment. Even some simple multicellular animals meet
the needs of their cells in this way. Such animals are common in the sea; they tend to be
small and flat or, as in sponges, perforated with channels through which seawater can
flow. In such an animal, no cell is far from direct contact with seawater, which contains
nutrients, absorbs waste, and provides a relatively unchanging physical environment. In
larger animals, however, most cells do not have direct contact with the external
An internal environment makes complex multicellular animals
The cells of multicellular animals exist within an internal environment of
extracellular fluid that bathes every cell of the body. Individual cells get their nutrients
from this extracellular fluid and dump their waste products into it. As long as the
conditions in the internal environment are held within certain limits, the cells are
protected from changes or harsh conditions in the external environment. Thus, a stable
internal environment makes it possible for an animal to occupy habitats that would kill
its cells if they were exposed to it directly. How is the internal environment kept
As multicellular organisms evolved, cells became specialized for maintaining specific
aspects of the internal environment. In turn, the development of an internal
environment enabled these specializations, since each cell did not have to be a generalist
and provide for all of its own needs. Some cells evolved to be the interface between the
internal and the external environments and to provide the necessary transport functions
to get nutrients in, move wastes out, and maintain appropriate ion concentrations in the
internal environment. Other cells became specialized to provide internal functions such
as circulation of the extracellular fluids, energy storage, movement, and information
processing. The evolution of physiological systems to maintain different aspects of the
internal environment made it possible for multicellular animals to become larger,
thicker, more complex, and more adaptable to external environments that are very
different from the internal environment.
The composition of the internal environment is constantly being challenged by the
external environment and by the metabolic activity of the cells of the body. The
maintenance of stable conditions (within a narrow range) in the internal environment is
called homeostasis. Homeostasis is an essential feature of complex animals. If a
physiological system fails to function properly, homeostasis is compromised, and as a
result cells are damaged and can die. To avoid loss of homeostasis, physiological systems
must be controlled and regulated in response to changes in both the external and
www.notesolution.com Figure 40.1 Maintaining Internal Stability Organ systems maintain a constant internal environment
that provides for the needs of all the cells of the body, making it possible for animals to travel between
different, often highly variable, external environments. Each organ system controls different aspects of the
internal environment. Arrows indicate the movement of nutrients and water to areas of need and the
removal of metabolic waste products from those areas.
Homeostasis requires physiological regulation
The activities of all physiological systems are controlledspeeded up or slowed down
by actions of the nervous and endocrine systems. But to regulate the internal
environment, information is required.
Think of it this way. You control the speed of your car with the accelerator and the
brakes, but when you use the accelerator and brakes to regulate the speed of your car,
you have to know both how fast you are going and how fast you want to go. The desired
speed is a set point, or reference point, and the reading on your speedometer is
feedback information. When the set point and the feedback information are
compared, any difference between them is an error signal. Error signals suggest
corrective actions, which you make by using the accelerator or brake.
Some components of physiological systems are called effectors because they effect
changes in the internal environment. Effectors are controlled systems because their
activities are controlled by commands from regulatory systems. Regulatory systems,
in contrast, obtain, process, and integrate information, then issue commands to
controlled systems. An important component of any regulatory system is a sensor,
which provides the feedback information that is compared to the internal set point.
A fundamental way to study a regulatory system is to identify the information it uses.
What are its sensors? How is the information from the sensors used? Negative
www.notesolution.com feedback is the most common use of sensory information in regulatory systems. The
word negative indicates that this feedback information causes the effectors to reduce
or reverse the process or counteract the influence that created an errorsignal. In our car
analogy, the recognition that you are going too fast is negative feedback if it causes you
to slow down. Negative feedback is a stabilizing influence in physiological systems; it
tends to return a variable of the internal environment to the set point from which it
Although not as common as negative feedback, positive feedback is also seen in some
physiological systems. Rather than returning a system to a set point, positive feedback
amplifies a response (i.e., increases the deviation from the set point). Examples of
regulatory systems that use positive feedback are the responses that empty body
cavities, such as urination, defecation, sneezing, and vomiting. Another example is
sexual behavior, in which a little stimulation causes more behavior, which causes more
stimulation, and so on.
Feedforward information is another feature of regulatory systems. The function of
feedforward information is to change the set point. Seeing a deer ahead on the road
when you are driving is an example of feedforward information; this information takes
precedence over the posted speed limit, and you change your set point to a slower speed.
Before the start of a race, the on your marks, get set commands are feedforward
information that raises your heart rate before you begin to run. Feedforward
information anticipates change in the internal environment before that change occurs.
These principles of control and regulation help organize our thinking about physiological
systems. Once we understand how a system works, we can then ask how it is regulated.
Physiological systems are made up of cells, tissues, and organs
Each physiological system is composed of discrete organs, such as the liver, heart, lungs,
and kidneys, that serve specific functions in the body. These organs are made up of
tissues. A tissue is an assemblage of cells and, although there are many, many types of
cells, there are only four kinds of tissue: epithelial, connective, muscle, and nervous. The
word tissue is often used in a general way to refer to a piece of an organ, such as lung
tissue or kidney tissue; however, as we will see, a piece of an organ will almost always
consist of more than one of the four kinds of tissues.
EPITHELIAL TISSUES Epithelial tissues are sheets of densely packed, tightly
connected epithelial cells that cover inner and outer body surfaces. They act as
barriers and provide transport across those barriers. Epithelial tissues form the
skin and line the hollow organs of the body, such as the gut.
Epithelial cells have many roles in the body. Some secrete hormones, milk, mucus,
digestive enzymes, or sweat. Others have cilia that move substances over surfaces or
through tubes. Epithelial cells can also provide information to the nervous system.
Smell and taste receptors, for example, are epithelial cells that detect specific chemicals.
Epithelial cells create boundaries between the inside and the outside of the body and
between body compartments; they line the blood vessels and make up various ducts and
tubules. Filtration and transport are important functions of epithelial cells. They
control what molecules and ions can leave the blood to enter the internal
environment or the urine. They can selectively transport ions and molecules from one
www.notesolution.com side of an epithelial membrane to the other. Examples are the absorption of nutrient
molecules from your gut and the secretion of acid into your stomach.
The skin and the lining of the gut are examples of epithelial tissues that receive much
wear and tear. Accordingly, cells in these tissues have a high rate of cell division to
replace cells that die and are shed. Dandruff is discarded skin cells.
MUSCLE TISSUES Muscle tissues consist of elongated cells that can contract to
generate forces and cause movement. Muscle tissues are the most abundant tissues
in the body, and when animals are active, muscles use most of the energy produced in
the body. There are three types of muscle tissuesskeletal, cardiac, and smooth.
Skeletal muscles (so named because they mostly attach to bones) are responsible for
locomotion and other body movements such as facial expressions, shivering, and
breathing. These muscles are under both conscious and unconscious control.
Cardiac muscle makes up the heart and is responsible for the heart beat and the
pumping of blood. Smooth muscle is responsible for movement and generation of
forces in many hollow internal organs such as the gut, bladder, and blood vessels.
Cardiac and smooth muscle are n