(08 Febr 2010)
What is physiology? The science of normal function of living
things (esp. animal). How do living organisms function? (how animal
function) Can be investigated at several level:Molecular level
Cellular level Tissue level Organ level
Investigation Core of Animal Physiology
HOMEOSTATIC MECHANISMS
HomeostasisA condition in which the internal environment of the
body remains relatively constant despite changes in the external
environment. Examples would be the maintenance of body temperature
and levels of glucose in the blood
Homeostasis refers to. Relative stability of internal milieu.as
opposed to external environment
Dynamic process, not a static onePhysiological variable
fluctuates Does not remain constant Balance does not mean
unchanging
Glucose, water
HOMEOSTASIS Related to relative stability of internal milieu.
What is regulated? Water volume, concentration [Na+], [K+], [Cl-],
[HCO3-], [Ca++], [H+] T Blood pressure [Glucose] Oxygen Waste
products (urea)
Water balance in an organism
Homeostatic control systems Steady state (fluctuates within a
predictable range) Set point Input = output
Feedback Negative Positive
Feed forward regulation
Homeostatic reflex arc
A sequence of events that link a stimulus to the response.
A reflex that regulates body T
o
Temperature homeostasis Humans, other mammals and birds, are
homoeothermic warm-blooded - and so maintain their body temperature
within narrow limits despite variations in the external
temperature. The advantage of the homoeothermic state may be that
critical enzymes in the body's metabolism work optimally at around
normal average body temperature: 37 degrees centigrade. The
maintenance of a small range of temperature is a good example of a
homeostatic physiological control system. At its most basic level,
a cerebral integrative centre samples the body temperature at
several sites, compares this value with a desired set point, and
modulates heat production and heat loss accordingly.
Temp Regulation cont. In vertebrates temperature homeostasis is
controlled by the thermoregulatory centre in the hypothalamus. It
receives input from two sets of thermoreceptors: receptors in the
hypothalamus itself monitor the temperature of the blood as it
passes through the brain (the core temperature), and receptors in
the skin monitor the external temperature. Both pieces of
information are needed so that the body can make appropriate
adjustments. The thermoregulatory centre sends impulses to several
different effectors to adjust body temperature:
Homeostatic control
Glucose homeostasis
Regulation of Ca
++
Circadian rhythm of Ca8
++
6
Ca++ Mm4
2
0 0
6
12
18
24
30
36
42
48
Hours
Scheme for negative feedback
Regulating enzymatic processes
Feed forward controls Digestion of food Stomach produces HCl in
anticipation of food Stimulated by odor
Regulation of body temperature Anticipation of cooling of body
in response to dropping external To Thermal receptors in skin
Signaling to neurons in brain
Chemicals that regulateHormones, Neurotransmitters
Local chemical messengers
Positive Feedback Mechanisms:Homeostatic systems utilizing
positive feedback exhibit two primary characteristics: 1. Time
limitation Processes in the body that must be completed within a
constrained time frame are usually modified by positive feedback.
2. Intensification of stress During a positive feedback process,
the initial imbalance or stress is intensified rather than reduced
as it is in negative feedback.
Typical Positive Feedback ProcessStress Sensor Control
Center
Intensifies
Effector
Homeostatic Regulation of Child Birth through Positive
FeedbackPressure of Fetus on the Uterine Wall Nerve endings in the
uterine wall carry afferent messages to the Hypothalamus
Intensifies
Increasing strength of uterine contractions
Production and Release of Oxytocin into the Blood
The birth of the child will bring this process to a close. Other
examples of positive feedback regulation occur during milk letdown
and blood clotting.
Feedback in Coagulation
Positive feedback mini-loops are built into pathway to speed up
production of chemicals needed to form the clot. Entire sequence of
clotting is a negative feedback pathway:
Harmful Effects of Positive FeedbackPositive feedback can be
harmful. Two specific examples of these harmful outcomes would be:
1. Fever can cause a positive feedback within homeostasis that
pushes the body temperature continually higher. If the temperature
reaches 45 degrees centigrade (113 degrees Fahrenheit) cellular
proteins denature bringing metabolism to a stop and death. 2.
Chronic hypertension can favor the process of atherosclerosis which
causes the openings of blood vessels to narrow. This, in turn, will
intensify the hypertension bring on more damage to the walls of
blood vessels.
More mechanisms of homeostasis Adaptation Allen's rule: a rule
which states that among endotherms, populations of the same species
living near the equator tend to have more protruding body parts and
longer limbs than do populations farther away from the equator.
Acclimatization Continuous exposure to stressors lead to
improved adaptation
Biological rhythms Variations that occur on a daily or annual
basis
Allens Rule in one genus of hares
From the arctic to desert jack rabbits.
Adaptation and Acclimatization Adaptation Inherited
characteristic that promotes survival in a specific habitat
Includes homeostatic control mechanisms
Acclimatization Lengthy exposure to a condition that improves
ability to survive No genetic change involved
PO2 at different altitudes
Acclimatization to different altitudes Mountain sickness due to
low barometric pressure. Inc in tidal vol, frequency of breathing,
pH, loss of CO2 from blood. At high altitude, larger lung capacity,
more red cells, myoglobin, capillaries in tissue, change in
bicarbonate conc. in blood that permits acidbase balance.
EXERCISE
1. What is homeostasis? Why is it called a dynamic equilibrium?
2. Describe these components of a homeostasis loop: stress,
receptor, control center, effector, response. Using an example, put
them in order. 3. What are the benefits of a negative feedback
response? In what direction does a variable change as a result of a
negative feedback response to a stress? 4. What are the benefits of
a positive feedback response? In what direction does a variable
change as a result of a positive feedback response to a stress? 5.
What are the risks associated with positive feedback responses? 6.
Draw 2 graphs to show negative & positive feedback responses.
7. Explain how positive feedback events can be built into a
negative feedback loop. 8. What is homeostasis failure? What has
happened with this
