Homeostasis refers to the ability of organisms to regulate their internal environment to maintain a stable and constant condition. It involves various control mechanisms that detect changes in factors like temperature, pH levels, and oxygen/carbon dioxide levels and trigger responses to counteract fluctuations and restore balance. A disruption of homeostasis can occur if sensors fail to detect changes, targets don't receive messages, or if injuries or illnesses overwhelm the system, and can potentially lead to disease or death if not addressed.

1. HOMEOSTASIS
WHAT IS HOMEOSTASIS?
DEFINITION: Homeostasis is the ability to maintain a constant internal environment in response to environmental
changes.
EXAMPLES OF HOMEOSTASIS:
 regulation of temperature
 balance between acidity and alkalinity (pH)
: It is a process that maintains the stability of the human body's internal environment in response to changes in
external conditions.
WHAT IS HUMAN HOMEOSTASIS?
DEFINITION: The human body manages a multitude of highly complex interactions to maintain balance or return
systems to functioning within a normal range. These interactions within the body facilitate compensatory changes
supportive of physical and psychological functioning. This process is essential to the survival of the person and to our
species. The liver, the kidneys, and the brain (hypothalamus, the autonomic nervous system and the endocrine system)
help maintain homeostasis.
EXAMPLES OF HOMEOSTASIS:
 Regulation of the pH of the blood at 7.365 (a measure of alkalinity and acidity).
The kidneys are used to remove excess water and ions from the blood. These are then expelled as urine. The kidneys
perform a vital role in homeostatic regulation in mammals, removing excess water, salt, and urea from the blood.
If the oxygen content of the blood falls, or the carbon-dioxide concentration increases, blood flow is increased by more
vigorous heart action and the speed and depth of breathing increases.
If the water content of the blood and lymph fluid falls, it is restored in the first instance by extracting water from the
cells. The throat and mouth become dry, so that the symptoms of thirst motivate the animal to drink.
Sleep timing depends upon a balance between homeostatic sleep propensity, the need for sleep as a function of the
amount of time elapsed since the last adequate sleep episode, and circadian rhythms that determine the ideal timing of
a correctly structured and restorative sleep episode.
WHAT IF THERE IS NO HOMEOSTASIS?
An inability to maintain homeostasis may lead to death or a disease, a condition known as homeostatic imbalance. For
instance, heart failure may occur when negative feedback mechanisms become overwhelmed and destructive positive
feedback mechanisms take over.
Other diseases which result from a homeostatic imbalance include diabetes, dehydration, hypoglycemia, hyperglycemia,
gout and any disease caused by the presence of a toxin in the bloodstream. Medical intervention can help restore
homeostasis and possibly prevent permanent damage to the organs.
WHAT IS GLUCAGON?
DEFINITION: Glucagon is a peptide hormone, produced by alpha cells of the pancreas,that raises the concentration of
glucose in the bloodstream. Its effect is opposite that of insulin, which lowers the glucose concentration. The pancreas
releases glucagon when the concentration of glucose in the bloodstream falls too low. Glucagon causes the liver to
convert stored glycogen into glucose, which is released into the bloodstream.
WHAT IS HYPOGLYCEMIA?
DEFINITION: Hypoglycemia is a medical emergency that involves an abnormally diminished content of glucose in the
blood. The term literally means "low blood sugar“.

2. HOMEOSTASIS
Homeostasis is the maintenance of a stable internal state within an organism. It is also known as steady state.
STIMULUS AND RESPONSE
HOMEOSTATIC CONTROL MECHANISMS
 Devices for maintaining or restoring homeostasis. They
involve virtually all of the body’s organs and systems.
 The body must have appropriate control mechanisms
available that will respond to these changing needs and then
restore and maintain a healthy internal environment.

HOMEOSTATIC CONTROL
Stimulus- produces a change to a variable (the factor being
regulated).
Receptor- detects the change. The receptor monitors the
environment and responds to change (stimuli).
Input- information travels along the (afferent) pathway to the
control center. The control center determines the appropriate
response and course of action.
Output- information sent from the control center travels down
the (efferent) pathway to the effector.
Response- a response from the effector balances out the
original stimulus to maintain homeostasis.
MAIN MECHANISMS
Negative feedback counteracts change and brings the body back to homeostasis.
Positive feedback: increases change away from the set points.
: needed for rapid change in the body.
A disruption of homeostasis can be harmful
Homeostasis can be disrupted for several reasons:
1. sensors fail (don’t detect changes)
2. targets do not receive messages (nerve issues)
3. injury (overwhelm homeostatic controls)
4. illness (viruses or bacteria)
*Disruption of homeostasis can begin in one organ and cause a chain
reaction in the others therefore causing a major body disturbance.
Organ systems must also work together to keep the organism healthy.
Organ systems work together to produce Vitamin D.
Thermoregulation maintains a steady body temperature.

3. Factors That Affect Fluctuations In The Animals Internal Environment
Internal Environment
• The conditions that prevail within the body of an organism, particularly with respect to the composition of the
tissue fluid.
• Maintenance of a constant internal environment was necessary for the survival of an organism in a varying
external environment.
Fluctuation
• An irregular rising and falling in number or amount
Components of the Internal Environment
• Interstitial Fluid -fills the spaces between the cells and constantly bathes the cells and keeps the cell functioning
normally. It is found in the interstitial spaces, also known as the tissue spaces.
• Blood Plasma -is the pale-yellow liquid component of blood that holds the blood cells in suspension.
Factors Affecting the Internal Environment
Physical Factors:
• Temperature
• Osmotic Pressure
• Concentration of oxygen and carbon dioxide in the blood stream
Chemical Factors
• Salt level
• Glucose level
• pH (measure of the acidity or basicity of a solution)
In order for cells of the body to function optimally, the physical factors and the chemical factors within the internal
environment must be maintained at arelatively constant level.
Example: Temperature
 For human, the internal environment need to be maintain around 37 degrees Celsius
 At this temperature, the enzymes give the optimal enzyme activity
 If the temperature too high, enzymes denatured- so lose ability to function
 If the temperature too low, enzymes become inactive.

4. Biological Rhythms in Animals
Biological Rhythms
Our surrounding undergoes predictable, cyclical fluctuations as a result of changes in seasons, or the time of day. When
events and conditions repeat in a rhythmic pattern, the ability to predict, anticipate, and prepare for them is a highly
beneficial trait. Not surprisingly, most animals are able to adjust their physiology, behaviour, and life cycles to the
upcoming conditions.
Definitions
 Biological clock is an internal timing system which continues without external time clues, and controls the time of
activities of plants and animals
 Period of the rhythm the time it takes to complete one cycle of activity
 Phase shift when the onset of the period of the rhythm is changed either earlier or later. This occurs when you travel
around the earth into different time zones. It can be artificially induced by controlling the light and dark periods
 Free running period this is the time when the clock is running without any clues from the environment, so it ‘runs free’
 Entrainment this is the resetting of the clock on a regular basis, forcing it to take up the period of the environment
 Zeitgeber – the environmental agent that resets the biological clock eg light or temp
 Circa – because each of the rhythms is not exactly the time length stated, eg daily is not 24 hours, their names start
with circa (which means ‘about’)
Biological Timing responses to the abiotic world
All organisms respond to various cues. The responses can be:
Annual cycles – yearly changes of the season
Daily – night and day
Lunar – monthly, often related to the moon
Tidal – related to the ebb and flow of the tides
It is to an individual’s advantage to synchronize its activities to these rhythms
Annual rhythms
-Mating and rearing of young is timed to exploit annual periods of abundant food and suitable environmental
conditions.
Compound Rhythms
-The shore environment changes in a more complex way than elsewhere because of the combined effect of
several cycles such as day-night, tidal, lunar, semi-lunar and annual each with a different period.
For example:
To avoid desiccation, sandhoppers feed only when low tide occurs at night. Therefore they are responsive to
daily and tidal cycles.
Synchronising to rhythms
Exogenous - A rhythm that is control by the external, environmental stimulai detected by the organisms
- Externally driven rhythms based on environmental changes. The normal response does not happen in the
absence of the environmental cue.
- - Environment dependent. The behaviour is cyclic because neural mechanisms are responding to cyclic
variations in the environmental cues. If kept under constant conditions the rhythm should disappear.
Endogenous – A rhythm that is controlled by an internal biological clock
- Environment independent. It can be entrained by exposure to an external cue that synchronises the rhythm to
a natural cycle. The behaviour is cyclic because an internal clock acts independently of any cues from the animals
external environment. If kept under constant conditions the animal should exhibit the behaviour at the same period
each cycle.
Internally driven responses controlled by an internal ‘clock’ usually ‘reset’ by an environmental change.
Combination – of both endogenous and exogenous.
Biological Clocks
The existence of circadian and circannual rhythms means that animals must have a way of keeping track of time. They
have an internal clock which lets them predict and prepare for changes to come. The biological clock in animals is found
in the hypothalamus of the brain.
It is
Sensitive to environmental cues, Can be stopped and reset, Is very accurate, Is inherited.

5. Biological clocks are used for:
Control of the daily rhythms of the body, Reproduction timing, Preparing for migration by eating of plenty of food,
Preparing for winter by storing of food, increasing thickness of coat and hibernating, Navigating by the sun or stars.
Circadian Rhythms
Animals are active at different times of the day.
Diurnal – active during the day, inactive at night
Nocturnal – active at night, inactive
during the day
Crepuscular – active at dawn and dusk
Arrhythmic – no regular pattern – tend to be found were changes in the microclimate are negligible
Examples of circadian rhythms
Periods of activity
Periods of sleep
Physiological processes
Endocrine system rhythms
Temperature changes
Heart rate rhythms
Pain rhythms
Alcohol metabolism rhythms
Times of births and deaths, etc
Circamonthly Rhythms
Some animals synchronize their behavior with the phases of the moon and changes associated with tidal patterns are
also considered circa monthly (lunar). The spawning behavior of some marine worms is synchronized by the moon so
that the egg and sperm are released at the same time.
Circannual Rhythms
These result from the rotation of the earth around the sun and also a factor is seasonal changes caused by the tilt of the
earth and the seasons produced as it rotates around the sun.
Examples:
Migration to and from breeding sites
Hibernation
Reproduction cycles
Hibernation
Aestivation
Hibernation - This is the way some animals survive over winter by slowing their metabolic rate
Aestivation – this is a form of hibernation over summer when the weather gets too dry or temperature gets too hot
Reproduction – the method by which most animals reproduce when conditions are most favourable, ie spring
EXAMPLES
Owls begin to stir in the late afternoon and evening, awaiting the emergence of small mammals at dusk. Ground
squirrels gather rations and pack on fat reserves in the fall in preparation for cold winters spent underground. Moose
reproductive cycles match the birth of fawns in the spring to the rich emergence of forage at that time.
SLEEP
Sleep, an essential state of natural rest, is observed in most animals. It is characterized by reduced voluntary body
movement, decreased reaction to external stimuli, and a loss of consciousness. Humans sleep an average of 7.5 hours,
during which they proceed in around 100 minute cycles of two broad types.
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