Homeostasis - ME14mech14.weebly.com/uploads/6/1/0/6/61069591/homeostasis.pdf · It is similar to the idea of . equilibrium. All of our body's systems work together to maintain homeostasis

Homeostasis

Homeostasis

homeo: same/steadystasis: state

Homeostasis

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.

Hypothalamus

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





Human body temperature

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.


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


Closed-loop Feedback Control System

Open-loop System

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

Why Feedback?

Open-loop system:

Closed-loop system:

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

Positive Feedback

Negative Feedback

Negative/Positive Feedback: Examples

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

https://en.wikipedia.org/wiki/Coagulation

Positive Feedback

Another example of positive feedback: Child birth

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:

Homeostasis of Body Temperature

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


– 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

Thermoregulation: Penguins huddling to keep warm

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

Negative Feedback Control of Body Temperature

Thermoregulation: Negative Feedback

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

Homeostasis: Body Temperature

The Skin

Mechanism of Homeostasis: Body Temperature

Mechanism of Homeostasis: Body Temperature

Mechanism of Thermoregulation: Body Temperature

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

Glucose Homeostasis

Glucose Homeostasis

Glucose Homeostasis

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

Glucose and Insulin Level in a Healthy Person

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

Glucose Control

Glucose Control

Artificial Pancreas: Glucose Control

Open-loop control

Closed-loop control

Artificial Pancreas: Glucose Control

Glucose Control: Modeling

Glucose Control: Modeling

Minimal Model (Bergman, 1981):

Glucose Control: Model Validation

Glucose Control: Closedloop Control

Glucose Control: Fuzzy Logic Control

Glucose Control: Fuzzy Logic Control

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.






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

Homeostasis - ME14mech14.weebly.com/uploads/6/1/0/6/61069591/homeostasis.pdf · It is similar to the idea of . equilibrium. All of our body's systems work together to maintain homeostasis

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