What is homeostasis?
maintenance of a steady state/internal environment
What can alter homeostasis?
Either a physiological or pathological challenge causes a change in body homeostasis
What responds to maintain homeostasis?
Body’s physiological systems
What are the components of a feedback system?
Regulated factor
- set point (via hypothalamus), operating [range], “error
signal
Detector/sensor
- afferent path
Comparator/control centre
- determines set point of variable, compares and maintains
variable at set point
- Intrinsic: local – cell or tissue autoregulates
- Extrinsic: endocrine system, nervous system
Effector
- returns variable to set point (efferent path)
Response
Explain how a negative feedback loop works
If [factor] gets too high, it is detected by sensors and a signal is sent to the control centre to limit the release of that factor
How does the negative feedback loop affect a [low] factor?
If [factor] is low there is no negative feedback, positive response continues until levels get too high
What are the controlled chemical substances in the body?
Circulating concentrations of chemical substances
- Ions e.g. Na+, Ca2+
- Nutrients e.g. blood glucose concentration
- Hormones
What are the controlled physical entities of the body?
Physical entity
- Blood pressure
- Core temperature
How does the body respond to being too cold?
shivering, vasoconstriction, increased metabolism (↑)
How does the body respond to a rise in temperature?
vasodilatation, sweating (↓)
Describe the physiological control of core body temperature
36.5 degrees is set point
The hypothalamus receives signals from cutaneous and hypothalamic temp. receptors detecting either heat loss or heat gain
Effect caused by sending signals to effector organs
once set point achieved, negative feedback loop activated to limit response as back to normal
What is the pathological effect on core body temperature?
Pathological → With infection the set point is changed and body temperature rises
How do pyrogens (bacteria/virus) affect body temperature?
Pyrogens (bacterial or viral infections) change the set point to a higher level resulting in fever
What are the benefits of a higher temperature during infection?
inhibits bacterial growth
speeds up metabolic reactions
Increases delivery of white blood cells to infection sites
Describe how body temperature is increased due to pathology?
Blood flow shifted to core to conserve heat
Increased muscle activity (shivering)
Chills stop when high temp reached
Explain how body temperature changes and is maintained during a fever
The actual body temperature lags behind the rapid shift in set point and even though regulation is maintained during the fever it is less precise
What is the consequence of long term elevated BP?
Chronic elevated BP can lead to hypertension → stroke /CVS diseases
Describe the series of events that follow an increase in BP
- Increased BP
- Detected by baroreceptors (stretch)
- Signal sent to CVS control centre - medulla
- ↑p/s input - decreased CO
- ↓ sympathetic activity - decreased CO + TPR
What is hypertension?
The resetting the sensitivity of the baroreceptors
What is the role of the neurones in the hypothalamus?
Neurones in the hypothalamus synthesise and release hormones from the posterior pituitary
What do neurosecretory cells in the hypothalamus do?
Release their hormones into portal capillaries in which they are transported directly to endocrine cells of the anterior pituitary gland
Name some hormones released by neurosecretory cells of the hypothalamus
Vasopressin (ADH)
CRH (corticotropin-releasing hormone) stimulates ACTH secretion
ACTH (adrenocorticotropic hormone) stimulates cortisol secretion
What is the effect of a haemorrhage on BP?
After a haemorrhage blood volume and hence blood pressure are reduced
What homeostatic control systems are activated to help restore BP after a haemorrhage?
- Baroreceptor reflex - increases CO + TPR
- Vasopressin (ADH) secretion to increase blood volume
What stimuli activate pituitary to release ADH?
Angiotensin II
Hyperosmolarity
Decreased atrial receptor firing
Sympathetic stimulation
What are the effects of Vasopressin (ADH)?
Renal fluid absorption - increases BV
Vasoconstriction
These increase the arterial pressure
Which system integrates feedback loops in the control of Na balance, BP and fluid volume?
Renin-Aldosterone-Angiotensin II system
RAAS
How is circulating [cortisol] controlled?
- Hypothalamus secretes CRH
- Activates anterior pituitary to secrete ACTH
- ACTH acts on adrenal cortex to secrete cortisol
- If adrenal cortex over-secretes, negative feedback loop
to hypothalamus to inhibit CRH release
What factors increase [cortisol]?
Cortisol concentrations increased by stress and circadian rhythms (body clock)
Describe the effect of a negative feedback loop
Increase in a controlled variable causes a decrease in a controlled variable
Define a positive feedback loop
An increase in a controlled variable stimulates further increase in a controlled variable
Name systems operating via a positive feedback mechanism
blood clotting, ovulation, childbirth
Explain how the positive feedback mechanism of oxytocin controls uterine contractions in labour
- Labour oxytocin stimulates contraction of uterine
muscles - Cervix dilates and activate stretch receptors
- Action potentials signal to hypothalamus
- Stimulates further release of oxytocin