Physiology: Homeostasis Flashcards
(29 cards)
What is homeostasis?
Maintenance of a relatively constant internal environment despite changes in the external environment.
Internal environment
Cell cytosol
External environment
Ambient conditions
Presence or absence of food and drink
Level of exercise (causing excesses and demands)
Elements of the internal environment
Core temp Blood pressure Blood osmolarity Blood pH Blood glucose level
What is osmolarity?
Osmolality of blood increases with dehydration and decreases with overhydratio.
Elements of the external environment
Heart rate
Body fat percentage
etc.
Normal ranges for blood valves
Arterial pH
7.35 - 7.45
Normal ranges for blood valves
Glucose pH
75-110mg / 100ml
Why is homeostasis so important?
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.
Summary of homeostasis `
Chain
Normal homeostatic state.
Change causes loss of homeostasis,
Attempts made to compensate for change
If compensation inadequate = health risk
If compensation adequate = health maintained
What are homeostatic mechanisms?
Regulatory mechanisms that compensate for changes away from a stable condition
Homeostatic Mechanisms
Local / Intrinsic mechanisms
Regulation within a tissue involving no external systems
May be direct effect or local nerve reflexes
Homeostatic Mechanisms
Reflex / extrinsic mechanisms (usually feedback loops)
Control system outside organ or tissue being controlled.
Involves nervous or endocrine systems and CNS
Homeostatic Mechanisms
Intracellular mechanisms
Action taken within the cells
Allosteric modification of enzymes (substances bind to sites on enzymes (not active sites) that change their function).
Examples of local mechanisms
Autoregulation
e.g. kidney
Stretching the renal afferent arteriole walls causes them to constrict, reducing blood flow, and vice versa.
Process of homeostatis requires:
Receptors
Some kind of receptor to detect any changes in the internal environment
Process of homeostatis requires:
Integrating system
A control centre that monitors inputs from the receptors and makes decisions on what to do.
Process of homeostatis requires:
Effectors
Effector cells or organs that respond to the hormones or neural signals
Process of homeostatis requires:
Effects
Effects or responses that act to reduce the change / stimulus
Process of homeostatis requires:
Internal Environment compared
A pre-programmed reference point or set point
Process of homeostatis requires:
Hormones / Neurons
Carry the messages
Negative feedback (reflex)
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
Negative Feedback
What happens…
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
Set-points can be modified
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
