Homeostasis Intro
The Main Point

The organization and function of biological systems at many levels are
maintained by feedback control-mechanisms that maintain the constancy
and control the activity of the system.

The classic example of feedback control is the thermostat: it keeps the room
temperature constant by detecting when the temperature goes below or
above its setting, and turning the furnace on or off accordingly. Large arrays
of analogous mechanisms enable cells and organisms to maintain constant
internal conditions.

What are some variables that the human body needs to regulate?

What are some variables that a bacterium needs to regulate?

What is similar and what is different about what you listed for the two
organisms?
Hierarchy of Multicelluar Life

The smallest level of organization for all life is a cell.

In multicellular organisms, cells can be grouped into tissues, tissues
into organs, and organs into organ systems.
Humans have ~1014 human cells
that make up our bodies,
categorized into a few hundred
different types
Homeostasis

From greek “Homoios” (similar) and “Stasis” (standing still), it is a property
of a system that regulates its internal environment around a stable, fairly
constant condition.

The systems on the previous page work together to monitor and maintain a
balanced environment. If one system fails, others will be affected, and
homeostasis may not be maintained.

Homeostatic mechanisms occur at the level of the cell, tissue, organ, and
organism as a whole.
Variables in Homeostasis

Some examples of variables in humans that are maintained within a stable
range are:

blood sugar

osmotic pressure (fluid balance)

body temperature

oxygen levels

blood pressure

blood pH

Changes in the external environment can cause these variables to change,
and internal systems must respond to maintain homeostasis.
A Recent Example

Survivors from the Nepal Earthquake

In Nepal, people were pulled out alive up to a week after the
earthquake. What factors do you think were at play in keeping them
alive? What organ systems were struggling to maintain homeostasis?
Feedback Systems

If variables are continually disturbed by
stimuli, how is it that the body is able to
maintain homeostasis?

Organs monitor changes and maintain
homeostasis primarily through negative
feedback systems.
Negative Feedback Loop

There are many examples of
negative feedback loops in the
human body, and all work to
keep a variable within a set
range of values.
Components of Feedback System
The Hympothalamus

One of the most important “control
centres” of the human body is a part of
the brain known as the hypothalamus.

It receives messages from sensors that
are located both nearby and further
away and initiates a response to
maintain homeostasis.
The hypothalamus controls body
temperature, hunger, important aspects of
parenting and attachment behaviours,
thirst, fatigue, sleep and circadian rhythms.
Temperature Control

Sensors on the
hypothalamus can detect
changes in blood
temperature, and can send
signals to effectors that
will counteract any
increase or decrease from
37oC
Blood Glucose Regulation
Positive Feedback Loops

A positive feedback mechanism is the exact
opposite of a negative feedback mechanism.
With negative feedback, the output reduces
the original effect of the stimulus. In a
positive feedback system, the output
enhances the original stimulus.

Blood clotting is an example of a variable
controlled by a positive feedback loop.
Milk Secretion

Milk secretion from a
mother is an example
of a positive feedback
loop controlled by the
hypothalamus.
Review Questions

List the four biological levels of organization in a human in order from
most simple to most complex.

▪Define the term “homeostasis.”

▪What does the term “feedback system” refer to with respect to
homeostasis?

▪List the three components of a feedback system and describe each
briefly.

▪What are some relationships between the integumentary system and
the nervous system?

▪Using the pancreas as an example, illustrate the four biological levels
of organization in a human.