Human Physiology ❮ 185Thermoregulation
A fairly constant body temperature is important for many living organisms. The process by
which this temperature is maintained is known as thermoregulation.A major organ
involved in thermoregulation is the skin, which also plays a role in excretion through sweat-
ing. Four major thermoregulatory processes are conduction, convection, evaporation, and
radiation.Conductionis the process by which heat moves from a place of higher temper-
ature to a place of lower temperature. For example, let’s say that two people are sleeping in
the same bed, and that person A is cold all the time. Person A would not make it through
the night if it were not for this process. Since person B tends to be warmer than person A,
person A takes advantage of conduction by pulling the heat from person B’s body to hers.
Convectionis heat transfer caused by airflow. Thinking about my baseline warmth (simi-
lar to that of person B), if it were not for our air conditioners in the summer, we would
probably not be here today to write this book. But we curse convection in the winter as the
cold wind removes heat from our bodies, making it feel that much colder outside.
Evaporationis the process by which water leaves our bodies in the form of water vapor:
sweat. Why do humid days feel so much warmer than nonhumid days? Because humidity
increases the amount of water in the air, decreasing the driving force for water to leave our
bodies.Radiationis the loss of heat through ejection of electromagnetic waves.
Before moving on to the nervous system, we must mention two more terms: endotherm
and ectotherm. An endothermis an organism whose body temperature is not dramatically
affected by the surrounding temperature. We humans are endothermic creatures. Sure, a
cold day can feel really cold, but at least it does not dramatically lower the human body
temperature.Ectothermic animalsare organisms whose body temperatures areaffected by the
surrounding temperature. Fish, reptiles, and amphibians are good examples of ectothermic
organisms.Nervous System
The nervous system is divided into two systems: the central nervous system(CNS) and
theperipheral nervous system(PNS). The CNS contains the brain and the spinal
cord. The PNS can be broken down into a sensory and a motor division. The sensory
division carries information tothe CNS while the motor division carries information
awayfrom the CNS. The motor division can be further broken down in the somatic
nervous system(SNS), also known as the voluntary nervous system, and the auto-
nomic nervous system(ANS). As indicated by its name, the SNS controls the volun-
tary contraction of muscles, while the ANS controls the involuntary activities of the
body: smooth muscle, cardiac muscle, and glands. The ANS is divided into the sympa-
theticandparasympatheticdivisions.
Before delving into the various divisions of the nervous system, it is important to look
at the mechanics of nerve cell transmissions (Figure 15.4). The functional unit of the
nervous system is the neuron(nerve cell).Outside to the left of the bus is a nerve cell from
the CNS. There are three main parts to a nerve cell: the cell body, the dendrite, and the
axon. The cell bodyis the main body of the neuron. The dendriteis one of many short,
branched processes of a neuron that help bring the nerve impulses toward the cell body.
Theaxonis the longer extension that leaves from the neuron and carries the impulse away
from the cell body toward the target cell. Some CNS nerve cells, as well as most PNS neurons,
aremyelinated neurons,which means that they have a layer of insulation around the axon,
allowing for faster transmission. It is the cable Internet of the body.BIG IDEA 3.E.2
Animals have a
nervous system that
detects signals,
transmits informa-
tion, and produces
responses.
http://www.ebook3000.com