Homeostasis Flashcards
(20 cards)
Describe components of a feedback system
Regulated factor/controlled variable (set point,
operating range, error signal)
* Detector/sensor that senses change (afferent path)
* Comparator/control centre – determines set point
of variable, compares and maintains variable at set
point. Initiate the response
* Intrinsic: local – cell or tissue autoregulates
* Extrinsic: endocrine system, nervous system
* Effector – returns variable to set point (efferent
path)
* Response
Explain components of a negative feedback loop
Set point ——> control centre —-> effector —-> controlled variable —> sensor
What is an error signal
error signal- value of the controlled variable minus the set point
Explain elements of homeostatic control of room temperature
Set point —-> thermometer —-> thermostat—-> boiler —- room temperature
-the value of a controlled variable will oscillate around a set point because of the time delay between sensing a change and its correction
what are examples of controlled variables/regulated factors
Physical entities
Blood pressure
Core temperature
Circulating concentrations of chemical substances
Ions e.g. Na+, Ca2+
Nutrients e.g. blood glucose concentration
Hormones
Explain what happens in an environment that is not thermoneutral
In an environment that is not thermoneutral:
the body uses 4 mechanisms for heat
exchange to maintain homeostasis:
- Conduction: The transfer of heat from
two objects e.g. transfer of heat from skin
to chair - Convection: is the transfer of heat to the
air surrounding the skin - Radiation: is the transfer of heat via
infrared waves e.g. sun warms the skin - Evaporation: is the transfer of heat by
the evaporation of water e.g. sweating
What happens when there’s heat loss and heat gain
heat loss- sweating, vasodilation
Heat gain- shivering, vasoconstriction, metabolism
What are benefits of an increase in body temperature
Pyrogens (bacterial or viral infections) change the set point to a higher level resulting in fever
➢ Benefits of a higher temperature
● Inhibits bacterial growth
● Speeds up metabolic reactions
● Increases delivery of white blood cells to infection sites
➢ How is temperature increased?
● Blood flow shifted to core to conserve heat
● Increased muscle activity (shivering)
● Chills stop when high temp reached
Explain the baroreceptor reflex
The baroreceptors in the walls of blood vessels(carotid sinus & aortic arch) detect an increase in BP
* Brain receives an input and signals blood vessels & heart
- Arterioles dilate and HR decreases leading to decreases in BP
Explain hypertension
in hypertension, the body’s “normal” setting for blood pressure, the set point, might be higher.
It also mentions “resetting the sensitivity of the baroreceptors.”
This means that the baroreceptors might become less sensitive to higher blood pressure, so they don’t signal the brain to lower it as effectively as they should.
Explain the hypothalamic pituitary axis
Neurones in the hypothalamus
synthesise and release hormones
from the posterior pituitary
Other neurosecretory cells in the
hypothalamus release their
hormones into the portal
capillaries in which they are
transported directly to endocrine
cells of the anterior pituitary gland
What happens after a haemorrhage
After a haemorrhage blood volume and hence blood pressure are reduced
To help restore blood pressure several homeostatic control systems are activated - these include:
- The baroreceptor reflex to increase cardiac output
and total peripheral resistance - Stimulation of vasopressin (ADH) secretion to
increase blood volume
Explain the role of vasopressin in the control of blood pressure
Water Retention (Kidney Action):
Vasopressin is released from the posterior pituitary when blood pressure drops or when blood osmolarity (salt concentration) increases.
It acts on the kidneys, specifically on the collecting ducts, making them more permeable to water. This leads to increased
water reabsorption back into the bloodstream, which raises blood volume — and as a result, increases blood pressure.
Vasoconstriction (Blood Vessel Action)
At higher concentrations, vasopressin can bind to V1 receptors on vascular smooth muscle, causing vasoconstriction — the narrowing of blood vessels. This increases systemic vascular resistance, which also helps raise blood pressure.
Explain integrated feedback loops in the controlled variable minus of sodium balance, blood pressure and fluid volume
The kidneys: The glomerulus or the granule cells in the kidney will secrete Renin
- Renin converts angiotensinogen into
angiotensin 1 - Angiotensin 1 under the action of ACE gets
converted to Angiotensin 2 - Angiotensin 2 has many effects:
a) Adrenal Cortex: secretes Aldosterone
b) Hypothalamus: Secretes ADH and
acts on the thirst Centre in the brain
c) On the smooth muscles cells of the
arteries to constrict
Explain the hypothalamic pituitary adrenal axis
Neurones in the hypothalamus
synthesise and release hormones from the posterior pituitary
Other neurosecretory cells in the
hypothalamus release their
hormones into the portal capillaries
in which they are transported directly
to endocrine cells of the anterior
pituitary gland
CRH (corticotropin-releasing
hormone) stimulates ACTH secretion
ACTH (adrenocorticotropic hormone)
stimulates cortisol secretion
How does the body control the amount of cortisol circulating the blood
Think of it like a thermostat controlling the 2temperature in a room:
1) There’s a set point for how much cortisol your body likes to have.
2) Stress can change this set point, making your body want more cortisol.
Your internal body clock (circadian rhythm) also affects the set point, causing cortisol levels to naturally go up and down throughout the day (as shown in the graph on the right, with higher levels in the morning).
3) When your body needs to increase cortisol, the hypothalamus (in your brain) releases CRH.
CRH tells the anterior pituitary gland to release ACTH.
4) ACTH travels to the adrenal cortex, which then releases cortisol into your blood.
5) The circulating cortisol then has effects on different parts of your body (labeled as “Effectors,” but not detailed here).
6) Finally, like a thermostat, the level of cortisol in your blood sends a signal back to the hypothalamus and anterior pituitary to reduce the release of CRH and ACTH when cortisol levels are high enough.
7) This is a negative feedback loop that prevents cortisol levels from getting too high.
Describe control of blood glucose
Blood sugar is a very tightly controlled in the body, the normal range is about 4.4 to 6.1 mmol/L (79 to 110
mg/dL)
➢ Our blood sugar will be subject to change after eating and digesting a meal- Blood sugar will rise
- The Beta cells of the pancreas will sense an increase in blood sugar and will secrete insulin
- Insulin circulates in the blood and will cause adipose tissue, muscle and liver to take up and store glucose -blood sugar drops
➢ Without eating our blood sugar level will fall – - The Alpha cells of the pancreas can secrete glucagon
- Glucagon stimulates gluconeogenesis through glycogenolysis, and glucose release from liver - increasing blood sugar
Explain the principles of negative and positive feedback control
positive feedback:
-increase in a controlled variable
Negative feedback:
-decrease in a controlled variable
Describe positive feedback control (haemostasis)
- Less common
physiologically as it is like
a “runaway train” (less
control) - The response of the effector
output reinforces the
stimulus e.g. blood
clotting, ovulation,
childbirth
Explain the control of uterine contractions in labour by oxytocin (positive feedback)
- In labour oxytocin stimulates contraction of uterine muscles
- Cervix dilates and
activate stretch
receptors - Action potentials
signal to hypothalamus - Stimulates further
release of oxytocin
