HOMEOSTASIS – REGULATION
OF INTERNAL CONDITIONS
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Patterns of internal regulation in animals
Principles of regulatory systems
Signaling in internal regulation
Animal example: mineral-balance
regulation in animals
• Plant example: plant responses to
drought
-50 F, Body = 98.6 F
+ 119 F, Body =
98.6 F
Homeostasis
• = ability of animals to regulate their internal
environment
• Regulator = uses mechanisms of homeostasis to
moderate internal change in face of external
fluctuation
Fresh
Salt
Water
Water
Constant solute, water concentration in blood, body fluids
Conformers – allow some conditions within their
bodies to vary with certain external changes
Homeostasis
• Some constancy
• But also includes regulated change
essential for normal function, survival
– Hormonal changes in reproductive cycles
– Responses to challenges
Homeostasis depends on feedback circuits
• Three components
Receptor – detects a change in some variable
of the animal’s internal environment
Control Center – process information from
receptor, directs signal to the effector
Effector – brings about the change to return
conditions toward normal
NEGATIVE
FEEDBACK
SYSTEM
FEEDBACK SYSTEMS
Negative – a change in one direction fuels
response in a control system and effector
in the opposite direction
- inherently regulatory
Positive – a change in one direction fuels
response in a control system and effector
in the same direction
- non-regulatory
Positive Feedback Systems
• Non-regulatory
• Unstable
• Short-lived, produce radical change
– Mammalian birth
– Generation of nerve impulse
– Swallowing or vomiting
Negative feedback
in regulation of
mammalian body
temperature
Homeostatic Mechanisms
• Communication and signaling between a
receptor and a control center
• AND between a control center and an
effector
Homeostatic Mechanisms
• Communication and signaling between a
receptor and a control center
• AND between a control center and an
effector
• Signaling and
communication are dominant
themes in biology
Signaling and Communication in Homeostasis
• Nervous system – high-speed, electrical signals
along specialized cells (neurons)
• Endocrine system – slower communication, via
hormones
= chemical messengers secreted directly into
body fluids by endocrine glands (organs)
Cell signaling in nervous and endocrine systems
Produce protein, change in membrane
permeability, release of material
Nervous and endocrine systems are closely
linked
• Epinephrine (adrenalin)
– Produced in adrenal gland (an endocrine
organ)
– Hormone: “flight or fight” response
– Neurotransmitter – conveys signals between
neurons in the nervous system
• Neurosecretory cells – specialized nerve cells
that secrete hormones in endocrine organs and
tissues
INSECT
DEVELOPMENT
Mineral balance in herbivorous
mammals
Sodium - predominant
cation in extracellular
fluids, needed for
many metabolic
purposes
Most plants – do not
require sodium, do not
accumulate it
-very high potassium
levels when growing
High sodium
intake from
animal flesh
Herbivores face physiological challenges in mineral
balance
- how to take in enough sodium?
- how to reduce sodium loss?
- how to get rid of enough potassium
Very little sodium in urine
and feces (sodium retention)
Salt blocks, mineral licks,
geophagy (behavioral
solution)
High excretion of potassium
Mammalian kidney
Blood
vessel
Urine
(glycoprotein)
Steroid
hormone
(Enzyme)
High K+
in blood
K+ excretion
High K+
in blood
ACTH
Hypothalamus
K+ excretion
STRESS
Plant responses to external changes
(drought stress)
Water is lost through
leaves via transpiration
(stomates)
Drought: transpiration >
water uptake
Processes to control
Plant response to water deficit
• Stomates close due to reduced turgor
Plant response to water deficit
Water deficit increases synthesis of abscissic
acid, hormone that keeps stomates closed
(changed permeability)
Reduced leaf growth = lower rate of increase
in leaf surface = lower transpiration
Leaves wilt, roll, expose less surface area to
air
Root growth in deeper, moist soil, inhibited
shallow root growth
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