HOMEOSTASIS &

CONTROL SYSTEMS

Objectives

• H - Explain the importance of maintaining a constant

internal environment

• How do the different systems interact together to maintain

homeostasis?

Guiding Question

Important variables are regulated within

the body:

• Blood sugar

• Fluid balance

• Body temperature

• Oxygen levels

• Blood pressure

• pH

• These variables must stay within certain ranges

• Changes in the external environment can cause shifting within

these ranges

Homeostasis

• The process by which a constant internal environment is

maintained despite changes in the external environment.

• Body maintains a constant balance, or steady state,

through a series of monitored adjustments.

• This balance requires constant monitoring and feedback

about body conditions

Components of a Control System:

Component Function

Monitor Detects a change

Coordinating Centre Receives a message from the

monitor

Directs a response via a regulator

Regulator Carries out the response initiated by

the coordinating centre

Hypothalamus & Pituitary Glands

• Hypothalamus:

• Often serves as the coordinating centre to either tell the pituitary to

work more or work less depending on the message

• Receives messages from monitors

• Initiates a hormonal/nervous response

• Attempts to keep the body within accepted ranges

• Pituitary:

• Master gland

• Receives communications from hypothalamus and tells other

glands what to produce

How to Maintain Homeostasis

• Via use of feedback systems

• Two types of feedback systems:

• Negative feedback

• Positive feedback

The clapping game

• Player 1: Clap half the number of player 3’s claps.

• Player 2: Clap twice the number of player 1’s claps.

• Player 3: Clap twice the number of player 2’s claps.

The clapping game

• Player 1: Clap half the number of player 3’s claps, unless

you hear “Inhibit”, then clap a quarter the number of

player 3’s claps.

• Player 2: Clap twice the number of player 1’s claps.

• Player 3: Clap twice the number of player 2’s claps.

• Player 4: When player 3 claps 8 times or more, say

“Inhibit”

Negative Feedback

• Change in condition triggers a response to reverse the

change

• e.g. Body’s response to increase in temperature

• Most responses are under this category

Positive Feedback

• Change in conditions triggers to response, and reinforces

the change

• Less common in biological systems

• e.g. uterine contractions during birth of baby, mosquito bites, drug

addictions

Ben’s Story

• Work on the following module about how the body

maintains homeostasis and how all systems interact

• http://ats.doit.wisc.edu/biology/ap/ho/ho.htm

Internal Factors to Monitor

1. Temperature:

• Liver and muscle contractions are primarily responsible for generating heat within the body.

• When the temperature of the body is greater than the surroundings, the skin loses heat.

• Evaporation is also a means of cooling down the body temperature and getting rid of excess heat.

• The brain also produces a lot of heat.

• The system of blood vessels comprising the head, allow the excess heat to escape and cool the head off.

2. Osmoregulation:

• The body makes sure that the water content within the

body does not become too diluted or too concentrated.

• Kidneys help by removing excess ions from the blood.

This is then excreted as urine and affects blood pressure

3. Sugar:

• The pancreas secretes two hormones essential to

regulating blood sugar levels (glucagon and insulin).

• ↑ sugar levels = pancreas ↑ insulin = glucose is stored as

glycogen = ↓ blood sugar levels

• Reverse is true when blood sugar is low

4. Calcium:

• ↓ calcium regulation = detection bh receptors in

parathyroid gland.

• This releases PTH which ↑ blood calcium levels by

release of calcium from bones.

• Thyroid gland releases calcitonin, which helps lower

calcium levels by absorption of calcium into the bones.

• 5. Balance of Fluids:

• adequate balance of fluids within the body includes both

the gain as well as loss of fluids.

• Antidiuretic hormone (ADH) and aldosterone are two

major hormones that help maintain a fluid balance.

ORGAN SYSTEMS:Their Role in Maintaining Homeostasis

• The proper functioning of the body requires all systems to

work together and in proper condition.

• Many diseases can affect the various organs and organ

systems of the body.

• When homeostasis within the body cannot be maintained

it can lead to death.

A person is stuck in icy water and

loses heat faster than they can

generate it. Their body temperature

drops below 35°C

Enzymes become less active.

Exergonic reactions that release

heat become less active.Less heat is released.

The body cools down.

A person loses 2+ litres of blood in an accident

Arterial blood pressure falls

Flow of blood to

the heart

vessels

decreasesThe heart weakens

The heart pumps less blood

Baroreceptors signal

the medulla that the

blood pressure is too

high

The medulla signals

the heart to

decrease rate,

which decreases

cardiac output.

The medulla

signals blood

vessels to dilate

(vasodilation)

This also decreases

venous return, which

decreases cardiac

output.

This lowers

blood pressure.

Baroreceptors signal

the medulla that

blood pressure is

too low.

The medulla signals

the heart to increase

rate, which

increases cardiac

output.

The medulla signals

blood vessel to

constrict

(vasoconstriction).

This also increases

venous return which

increases cardiac

output.

This raises

blood pressure.

Head of fetus pushes against cervix

Nerve impulse from

cervix transmitted to

brain.

Brain stimulates

pituitary gland to

secrete oxytocin

Oxytocin carried in

bloodstream to

uterus

Oxytocin

stimulates

uterine

contractions and

pushes fetus

towards cervix.

Kidneys detect reduced

O2 carrying capacity of

blood

When less O2 is delivered

to the kidneys they

secrete erythropoietin

into the blood

Erythropoietin

stimulates red blood

cell production

(erythropoiesis) by

bone marrow

Additional red

blood cells

(erythrocytes)

increase O2

carrying capacity

of blood

Increased O2 carrying

capacity of blood stops

erythropoietin secretion

Fever!

• What sets body temperature? what can change the set

point for body temperature?

• How does stress affect body temperature?

• How is fever different from a simple rise in body

temperature?

• What role might fever play in fighting infection?

• Why does the body sweat when a fever breaks?

Hypothermia