Physiology: Homeostasis

Question 1

What is homeostasis?

Answer 1

Maintenance of a relatively constant internal environment despite changes in the external environment.

Question 2

Internal environment

Answer 2

Cell cytosol

Question 3

External environment

Answer 3

Ambient conditions

Presence or absence of food and drink

Level of exercise (causing excesses and demands)

Question 4

Elements of the internal environment

Answer 4

Core temp
Blood pressure
Blood osmolarity 
Blood pH
Blood glucose level

Question 5

What is osmolarity?

Answer 5

Osmolality of blood increases with dehydration and decreases with overhydratio.

Question 6

Elements of the external environment

Answer 6

Heart rate
Body fat percentage

etc.

Question 7

Normal ranges for blood valves

Arterial pH

Answer 7

7.35 - 7.45

Question 8

Normal ranges for blood valves

Glucose pH

Answer 8

75-110mg / 100ml

Question 9

Why is homeostasis so important?

Answer 9

Changes in the conditions with the cell (such as pH and temperature) can or irreversible damage to proteins

Important proteins within cells include enzymes, receptors and transport proteins.

Changes to these proteins can threaten the life of the cell.

Question 10

Summary of homeostasis `

Chain

Answer 10

Normal homeostatic state.

Change causes loss of homeostasis,

Attempts made to compensate for change

If compensation inadequate = health risk
If compensation adequate = health maintained

Question 11

What are homeostatic mechanisms?

Answer 11

Regulatory mechanisms that compensate for changes away from a stable condition

Question 12

Homeostatic Mechanisms

Local / Intrinsic mechanisms

Answer 12

Regulation within a tissue involving no external systems

May be direct effect or local nerve reflexes

Question 13

Homeostatic Mechanisms

Reflex / extrinsic mechanisms (usually feedback loops)

Answer 13

Control system outside organ or tissue being controlled.

Involves nervous or endocrine systems and CNS

Question 14

Homeostatic Mechanisms

Intracellular mechanisms

Answer 14

Action taken within the cells

Allosteric modification of enzymes (substances bind to sites on enzymes (not active sites) that change their function).

Question 15

Examples of local mechanisms

Autoregulation

e.g. kidney

Answer 15

Stretching the renal afferent arteriole walls causes them to constrict, reducing blood flow, and vice versa.

Question 16

Process of homeostatis requires:

Receptors

Answer 16

Some kind of receptor to detect any changes in the internal environment

Question 17

Process of homeostatis requires:

Integrating system

Answer 17

A control centre that monitors inputs from the receptors and makes decisions on what to do.

Question 18

Process of homeostatis requires:

Effectors

Answer 18

Effector cells or organs that respond to the hormones or neural signals

Question 19

Process of homeostatis requires:

Effects

Answer 19

Effects or responses that act to reduce the change / stimulus

Question 20

Process of homeostatis requires:

Internal Environment compared

Answer 20

A pre-programmed reference point or set point

Question 21

Process of homeostatis requires:

Hormones / Neurons

Answer 21

Carry the messages

Question 22

Negative feedback (reflex)

Answer 22

Receptors

Intergrating centre**

Effectors

Effects*

Chnage to external environment

• The effects result in a change back towards the norm.
• *Internal environment compared to a set point

Question 23

Negative Feedback

What happens…

Answer 23

The value (e.g pH) will increase until it reaches a threshold above the set point.

Reaching the threshold triggers messages to be sent to the effectors to take action.

The action of the effectors causes the value to start to fall

Once the value falls below a threshold under the set point, it will trigger the effectors to take a different action

The result is OSCILLATION either side of the set point

Question 24

Set-points can be modified

Answer 24

e.g. Thermoregulation: hypothalamus (integrator)

Increase in set-point for core body temperature during fever.

e.g. one form of acclimatization to environmental oxygen level - some people who have spent a very long time at altitude have a lower set point for PaO2

Question 25

The efficiency of the negative feedback loop can be altered

Sensitivity

Answer 25

More than one stimulus can sensitise the loop.

e.g. low PaO2 and high PaCO2 both trigger increased ventilation, however the response when both occur together is greater than the sum on when they occur independently.

Question 26

The efficiency of the negative feedback loop can be altered

Anticipation

Answer 26

Feed-forward control

The response can be triggered before the threshold has been reached.

e.g. increase in heart rate before exercise i.e. before any changes in blood gases have occured

Question 27

Positive feedback

Cycle

Answer 27

Receptors
Integrating centre*
Effectors
Effects
Change to external environment**

• Internal environment compared to a ‘set point’
• *The effects result in a change FURTHER AWAY from the norm.

Question 28

Positive Feedback

What happens

Answer 28

The value changes in some way (can be up or down)

The change triggers a further change in the same direction

An external influence is required to break the cycle

This is rarely a homeostatic response

Question 29

Oxytocin and childbirth

Answer 29

Mechano-receptors

Hypothalamus

Hypothalmic nuclei

Release of oxytocin

Contraction of uterus

increases

Pressure on the cervix