SYSTEM INTERACTIONS
In this 8th grade unit the student will describe interactions among systems in the
human organism and be able to identify feedback mechanisms that maintain
equilibrium of systems such as body temperature, turgor pressure, and chemical
reactions. Human body systems were identified and their functions were studied
in the 7th grade. Primary emphasis was on the circulatory, respiratory, digestive,
skeletal and muscular systems. Other systems were covered in less detail and
included the nervous, endocrine, integumentary, and excretory systems. This is a
very short unit in 8th grade, and the concentration should be on interactions
between human systems and an awareness of feedback mechanisms that help the
body to stay in homeostasis (a stable state).
The following table includes examples of how some human body systems interact
with one another. It is not intended to be a complete listing.
Systems
Circulatory and Respiratory
Circulatory and Digestive
Circulatory and Excretory
Circulatory and Integumentary
Circulatory and Integumentary
Circulatory and Skeletal
Nervous, Muscular and Skeletal
Nervous and Integumentary
Muscular and Circulatory
Muscular and Digestive
Muscular and Respiratory
Interaction
Lungs supply oxygen carried by blood to
cells of the body
Nutrients pass into the circulatory
system to be carried to body cells
Kidneys remove wastes from blood
Cooling through dilation of blood vessels
near the surface of the skin
Platelets in blood seal cuts on skin
Production of blood cells in bone marrow
Daily movement and coordination
Sense of touch
Pumping of the heart & blood
Chewing, swallowing and movement of
food through the digestive tract
Movement of the diaphragm in breathing
Homeostasis
Students should be familiar with the term equilibrium and how the body processes
for maintaining equilibrium is referred to as homeostasis. Homeostasis is the
process by which organisms keep the internal environment of their body stable
even if the external environment changes. Positive and negative feedback
mechanisms exist within the human body to aid in the process of homeostasis.
An excellent example of feedback in a system occurs in one’s home is the way an
air conditioner or heater works. If the thermostat senses that it is too warm
(negative feedback), the air conditioner will turn on; when the thermostat senses
that it is cool enough (positive feedback), the air conditioner will stop. Most
human body feedback mechanisms work in exactly the same way.
Below are some interactions that occur in the human body to maintain homeostasis:
1
The body regulates temperature by using nerves that send a message to the
brain telling the brain the temperature of the skin and the internal organs.
If the body is too hot, the person will begin to sweat in an effort to cool
down the body. If the body is too cold, the nerves from the brain will signal
that blood vessels in the extremities (fingers and toes) will close down in an
effort to keep the main part of the body warm. That is why a person’s
hands seem “white” when they get really cold. Another response to being
cold is a signal given to the body from the brain to start to shiver. This is
accomplished by the release of certain chemicals, which are recognized by
the nervous system, and the message is sent to the brain and a reverse
reaction occurs when the brain responds to the message.
2 Human body feedback mechanisms help a person to breathe faster if the
body does not have enough oxygen. For example, a person might seem “short
of breath” after climbing several flights of stairs. The exertion increases the
body’s need for oxygen so the heart begins to pump faster and the pulse rate
increases. Students can be exposed to this concept during lab by using the
probes for heart rate monitoring. When utilizing probes in this unit, the 8th
grade teacher may want to consult with 7th grade teachers to make sure labs
are not repeated.
Feedback mechanisms can also be studied as a system of stimulus and response.
Students studied this concept in 6th and 7th grade, including plant tropisms. An
example of a stimulus-response feedback mechanism is when a plant pot is turned
away from the sun, and the plant responds by growing back towards the sun. The
following illustration shows both the response of a plant stem and roots in a similar
situation:
This unit also includes the concept of turgor pressure. Turgor pressure is usually
a term used in the study of plants. It refers to the plant’s ability to hold enough
water in the plant, and this pressure allows a plant to stay upright. If the plant is
allowed to dry out it will “wilt”. This is when the turgor pressure is so low that the
plant does not have enough water to keep it upright. In some ways, the human
body measures the concentration of water in the circulatory system so that the
body stays healthy. Instead of referring to the body as wilted, one would say the
body is dehydrated. A common demonstration that may be used to study turgor
pressure involves placing celery in different types of water, such as tap, distilled
and salt. Students then observe the response within the celery.