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External Environment
External Environment
External Environment
External Environment
Internal Environment
Homeostasis is about staying the same
Conditions at external environment change constantly
The Internal Environment should not change
THE MAINTENANCE OF STATIC OR CONSTANT CONDITIONS IN THE
INTERNAL ENVIRONMENT
Homeostasis
It is similar to the idea of equilibrium.
All of our body's systems work together to maintain homeostasis inside our body. Homeostasis is achieved by making sure the temperature, pH (acidity), and oxygen levels (and many other factors) are set just right for your cells to survive.
Homeostasis levels are different for each species.
Homeostasis: Why? Our body and its individual cells need just the right
conditions to perform at their best.
A cell’s delicately balanced chemical reactions workbest within narrow limits of temperature, pH, soluteconcentration etc.
Homeostasis: Why?
• Homeostasis is continually being disrupted by:
– External stimuli • heat, cold, lack of oxygen,
pathogens, toxins
– Internal stimuli• Body temperature• Blood pressure• Concentration of water, glucose,
salts, oxygen, etc.• Physical and psychological
distresses
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.
Activities such as exercise change the rate at which we breathe ...
Which changes the pH of the blood...
•Which is dangerous•Potentially fatal, unless ...
Which increases the amount of CO2 in our blood
The body responds homeostatically by changing the volumes of air we breathe and adjusting blood pH with buffers (HCO3 , Hb and others)
Homeostasis: Example
Maintaining HomeostasisThe various organ systems of the body act to maintain homeostasis through a combination of hormonal and nervous mechanisms.
In everyday life, the body must regulate respiratory gases, protect itself against agents of disease (pathogens), maintain fluid and salt balance, regulate energy and nutrient supply, and maintain a constant body temperature.
All these must be coordinated and appropriate responses made to incoming stimuli.
In addition, the body must be able to repair itself when injured and be capable of reproducing (leaving offspring).
Some Homeostasis Examples in Humans
1. Maintaining steady body temperature (~37ºC)
2. Maintaining steady water balance
3. Steady state of blood alkalinity (pH 7.34 – 7.43)
4. Steady sugar level in blood
5. Number of red blood cells
Feedback Control System: Level Control
Flow in
Flow out
The inlet flow comes from an
upstream process, and may change
with time.The level in the
tank must be kept constant in spite of
these changes.
Feedback Control System: Level Control
LTLC
SP
Flow in
Flow out
The level controller (LC) looks at the
level (monitoring)
If the level starts to increase, the LC
sends a signal to the output valve to vary
the output flow (change)
This is the essence of feedback control
Feedback Control System
Feedback control is the most important and widely usedcontrol strategy. It is a closed-loop control strategy
process
transmitter
controller
disturbance
comparator manipulatedvariable
controlledvariable
+– errorset-point
ysp y
Block diagram:
LTLC
SP
Flow in
Flow out
desired value(set-point)
transmitter
controllercontrolledvariable(measurement)
manipulatedvariable
disturbance
process
Feedback Control System: Level Control
Process models
Laplace domain model
)(
);;;(d
)(d
xy
xx
h
tduftt
=
=
State-space model Input-output model
)()()( sUsGsY =
)(sG)(sU )(sYstates
output G (s) is called transfer functionof the process
State-space models can be derived directly from the general conservation equation:
Accumulation = (Inlet – Outlet) + (Generation – Consumption)
Time-domain model
First-order systems
• KP is the process steady state gain• τP is the process time constant
)(dd tuKy
ty
PP =+τ )(1
)( sUs
KsYP
P
+τ
=
1)(
+τ=
sKsG
P
P
Time-domain model Laplace-domain model
Transfer function of a first-order system:
Response of a First-order System• We only consider the response to a step
forcing function of amplitude A
−= τ
−P
t
P eAKty 1)(
The time-domain response is:
It takes ∼ 4 to 5 time constants for the process to reach the new steady state
0
0
A
inpu
t, u
time
0.632 AKP
τP
AKP
outp
ut, y
Negative/Positive Feedback
Negative FeedbackNegative feedback is a process that happens when your systems need to slow down or completely stop a process that is happening.
Positive FeedbackPositive feedback is the opposite of negative feedback in that encourages a physiological process or amplifies the action of a system. Positive feedback is a cyclic process that can continue to amplify your body's response to a stimulus until a negative feedback response takes over.
During positive feedback, the system responds to the perturbation in the same direction as the perturbation. This feedback mechanism results in the amplification or growth of the output signal.
Negative/Positive Feedback
• Negative feedback loop– Original stimulus reversed (shut off)– Most feedback systems in the body are negative
• Positive feedback loop– Original stimulus intensified– Not very common in nature– Some positive feedback systems are used by our
body to its advantage
Negative/Positive Feedback: Examples
This is a positive feedback loop. The input is increased carbon dioxide, which begins this positive feedback loop.
• Increases the changes away from set points• Important when rapid changes needed• Ex: Skin Cut
– Clotting proteins increased to seal the wound
Positive Feedback
Positive Feedback
An example of positive feedback is the processof blood clotting.Hemostasis:
1. Vascular constriction limits blood flow2. The loop is initiated when injured tissuereleases signal chemicals (Thrombin) thatactivate platelets in the blood.3. An activated platelet releases chemicals toactivate more platelets, causing a rapid cascadeand the formation of a blood clot.4. To insure stability of the initially loose platelet plug, a fibrin mesh forms and entraps the plug.
Excess thrombin splitting of more prothrombin to thrombin more thrombin more clotting!
Fibrinolysis, antithrombin, fibrin adsorbs excess thrombin and makes it inactive
Positive Feedback of FeverFever may be provoked by many stimuli. Most often, they are bacteria and their endotoxins, viruses, yeasts, spirochets, protozoa, immune reactions, several hormones, medications and synthetic polynucleotides. These are called exogenic pyrogens.
Cells stimulated by exogenic pyrogens form and produce cytokines called endogenic pyrogens.
These cytokines bind to their own specific receptors located in close proximity to the hypothalamus. This process leads to production of prostaglandin E2 (PGE2).
PGE2 diffuses across the blood brain barrier, where it causes the set-point of the hypothalamic thermostat to rise.
Positive Feedback of Fever
Aspirin and the non-steroidal anti-inflammatory drugs display antipyretic activity by inhibiting the cyclo-oxygenase, an enzyme responsible for the synthesis of PGE2.
There are indications that the development of fever is of benefit as a normal body defense in combating some infections. Temperature elevation has been shown to enhance several parameters of immune function, including antibody production, T-cell activation, production of cytokines, and enhanced neutrophil and macrophage function.
Fever increases the chemical reactions of the body by an average of about 12 per cent for every 1°C rise in temperature. It increases the metabolic rate, which increases heat production, which in turn raises body temperature even more. This is a positive feedback mechanism that will continue until an external event (such as antipyretic or death of the pathogens) acts as a brake.
Positive Feedback of InflammationInflammation is characterized by increased blood flow to the tissue causing increased temperature, redness, swelling, and pain.
Inflammation is a beneficial process up to a point. Increased blood flow accelerates delivery of the white blood cells that combat invading foreign substances or organisms and clean up the debris of injured and dead cells.
In addition, increased blood flow provides more oxygen and nutrients to cells at the site of damage and facilitates removal of toxins and wastes.
It may, however, become a vicious cycle of damage, inflammation, more damage, more inflammation, and so on—a positive feedback mechanism.
Normal cortisol secretion seems to be the brake, to limit the inflammation process to what is useful for tissue repair, and to prevent excessive tissue destruction.
Homeostasis Needs
• Sensors to detect changes in the internal environment
• A comparator which fixes the set point of the system (e.g. body temperature).
• The set point will be the optimum condition under which the system operates
• Effectors which bring the system back to the set point
• Feedback control. Negative feedback stops the system over compensating (going too far)
• A communication system to link the different parts together
SensorPerturbation in
the internal environment
Return to normal internal
environment
EffectorComparator
Sensor
Negative feedback
Homeostasis needs
Homeostasis Components
• There are three main components involved when homeostasis is disrupted by a stimuli.
• The Receptor• The Control Center• The Effector
Homeostasis Components: Receptor
• The receptor is an organ or sensor that receives the chemical signal and communicates to the next Component ( the control center).
• In the case of blood sugar the liver is the main receptor.
Homeostasis Components: Control System
• The control system must be able to:– Receive signal from the receptor. It also can sense
deviations from the norm itself.– Integrate this information with other relevant
information. – Send a signal to the appropriate organ or gland to
make the necessary adjustment.
Generally the Brain (hypothalamus) is the control center. However, the pancreas is its own control center for blood sugar.
Homeostasis Components: Effector
• The effector is the component that causes the change. It sends out the chemical to deal with the stimulus.
• In the case of blood sugar the pancreas would be the effector because it sends out the insulin.
In animals there are two communication systems:
• The endocrine system based upon hormones
• The nervous system based upon nerve impulses
Communication Systems in Homeostasis
Hormones
• Organic substances • Produced in small quantities • Produced in one part of an organism (an
endocrine gland)• Transported by the blood system to a
target organ or tissue where it has a profound effect
The Endocrine System • The endocrine system produces chemical
signals
• Each hormone is different and they travel relatively quickly through the blood stream all over the body
• Their effects may be very slow or very fast: - growth hormone acts over years- adrenaline acts in seconds
Nerve Impulses
• The nervous system sends signals along nerves to specific parts of the body
• The nerve impulses travel very quickly and affect their target tissues in milliseconds
The nervous system• The nervous system is composed of excitable cells
called neurons• Neurons have long thin extensions which carry
electrical nerve impulses• This electrical signal of the nerve impulse needs to be
converted into a chemical signal (a neurotransmitter) so that it can pass from one nerve cell to another nerve cell
Thermoregulation
How much energy can be saved by staying in bed all day?
Surprisingly, the answer is only about 30%.
The other 70% keeps his body temperature at 37 C (98.6 °F), and the solutions around his cells at just the right concentration.
Homeostasis – Temperature Regulation
– Core body temperature• Humans: 37º C (98.6º F)• Hypothermia = decrease in body temperature• Hyperthermia = increase in body temperature
– Above 41º C is dangerous– Above 43º C is deadly
Homeostasis – Temperature Regulation
– Mechanisms of heat transfer between body and external environment
– Radiation—thermal energy as electromagnetic waves– Conduction—thermal energy through contact– Evaporation—heat loss through evaporation of water
• Insensible water loss• Sweating
– Convection—heat transfer by movement of fluid or air
Thermoregulation: Household Thermostat
Negative feedback control system
Let us consider that we want to maintain the room temperature is set to 25°C (“normal temperature”).
When the temperature falls below 25°C, the thermostat recognizes change in “normal” temperature and switches on the furnace.
When the thermometer detects a temperature above 25°C, the thermostat switches off the furnace.
Homeostasis Example: Body Temperature
(Internal temperature)
Effect of environmental temperature on human
Effect of environmental temperature on different animals
Homeostasis – Temperature Regulation: Components
– Receptors = thermoreceptors• Central: found in CNS (hypothalamus)• Peripheral: found in PNS (mainly skin)
– Effectors• Glands: sweat glands• Muscles: skeletal muscles, and smooth muscle of cutaneous
blood vessels
– Integrating center• Thermoregulatory center in hypothalamus
– Signals• Nerve impulses via neurons• Chemicals via hormones
Mechanism of Homeostasis: Body Temperature: Fever
– Rise in core body temperature– Accompanies infection– White blood cells secrete pyrogens– Body temperature set point increases– Fever enhances immune response
Long Term: Diabetes
• Normal Cells– Glucose circulates in blood;
pancreas releases insulin– If high glucose levels: insulin
tells cells to intake glucose– If low glucose levels:
pancreas creates glucose• Type 1 Diabetes
– Immune system destroys cells to produce insulin
– Pancreas fails– Blood pH decreases (more
acidic)• Type 2 Diabetes
– Insulin production decreases– Glucose level in blood rises– Cells starve
β-cells produce Insulin α-cells produce Glucagon
INSULIN• Lowers blood glucose levels!!!• Is released when blood glucose rises above
110 mg/dl• Forces liver and muscles to take up glucose
from blood stream• Forces liver to make glycogen (animal
starch) by linking glucose molecule together
Glucose Control by Pancreas
β-cells produce Insulin α-cells produce Glucagon
Glucose Control by Pancreas
GLUCAGON• Raises blood glucose• Is released when blood glucose falls
below 70 mg/dl• forces liver break down glycogen into
glucose and release it into the blood stream
Negative Feedback Control of Blood Pressure
The body has mechanisms to alter or maintain blood pressure and bloodflow. There are sensors that sense blood pressure in the walls of the arteries and send signals to the heart, the arterioles, the veins, and the kidneys that cause them to make changes that lower or increase blood pressure.
Negative Feedback Control of Blood Pressure
Blood volume is regulated by the hormone aldosterone
Aldosterone affects the rate of sodium ion reabsorption, which in turn affects the rate of water reabsorption
Increased aldosterone ➔ increased water reabsorption ➔ higher blood pressure
Decreased aldosterone ➔decreased water reabsorption ➔lower blood pressure
• Hypothalamus directs the pituitary gland of the endocrine system to control levels of the hormone vasopressin or antidiuretichormone (ADH) in the blood
• This hormone travels through the blood to the kidneys where it directs the rate of water reabsorption
• Increased vasopressin ➔increased water reabsorption
• Decreased vasopressin ➔decreased water reabsorption
Negative Feedback Control of Water Balance