Homeostasis (Endocrinology) Flashcards
What are the different aspects that are part of the homeostatic control mechanisms?
Challenge produces a change in body status
Physiology responds to maintain homeostasis
Components of a feedback system:
-Regulated factor/controlled variable (they have a set point, operating range, error signal) e.g. blood glucose
-Detector/sensor- detect when the controlled variable moves away from the set point (afferent path)
-Comparator/control center – determines set point of variable, compares and maintains variable at set point:
- Intrinsic: local – cell or tissue autoregulation
- Extrinsic: endocrine system, nervous system
Effector – returns variable to set point (efferent path)
Response
How is a feedback loop formed?
We start with the set point which is predefined and is linked to our control centre
Our control centre then signals to an effector that keeps that controlled variable within its operating range
If it changes that is detected by a sensor that feeds into the control centre
What are the examples of regulated factors/controlled variables?
Physical entities:
Blood pressure
Core temperature
Circulating concentrations of chemical substances:
Ions e.g. Na+, Ca2+
Nutrients e.g. blood glucose concentration
Hormones
How does the human body regulate body temperature?
If we are too cold we have shivering, vasoconstriction, increased metabolism (↑)
If our body gets too warm we have vasodilatation, sweating (↓)
How is core body temperature controlled through a negative feedback loop?
Core body temperature is detected by temperature receptors either in the brain, as hypothalamic temperature receptors or within the skin, as cutaneous temperature receptors.
They feed into the hypothalamus
And depending on whether we need to warm up, where shivering, vasoconstriction and metabolism will occur, or cool down, where we would sweat and out blood vessels would vasodilate to return the core body temperature back to the set point.
With pathology such as an infection the set point is changes and body temperature can rise (a fever occurs)
What are the 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
How does the baroreceptor reflex control blood pressure? What happens in the case of hypertension?
Our blood pressure is kept as a set point and that is controlled by the medulla
If the BP gets too high or too low it gets detected by the sensors which in this case are baroreceptors (stretch receptors) that detect changes in pressure
They are usually found in the aortic arch and the carotid sinuses
They signal to the medulla to either increase or decrease the heart rate and peripheral resistance
In the case of hypertension, there is a resetting of the sensitivity of the baroreceptors leading to an increase in blood pressure
What role does the hypothalamic-pituitary axis play in homeostasis through hormones (namely vasopressin and cortisol)?
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
One of these hormones is vasopressin (ADH)
Vasopressin is released from the posterior pituitary
CRH (corticotropin-releasing hormone) gets synthesised in the hypothalamus stimulates ACTH secretion in the anterior pituitary
ACTH (adrenocorticotropic hormone) stimulates cortisol secretion in the adrenal cortex
What is the role of vasopressin in the control of blood pressure? Why is this important?
Vasopressin has two major effects:
Causes vasoconstriction which causes increased arterial pressure
Increases renal fluid reabsorption which leads to increased blood volume thereby increasing arterial pressure
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
How is cortisol concentration controlled?
When cortisol is too low the hypothalamus releases CRH which acts on the anterior pituitary gland to release ACTH which acts on the adrenal cortex to release cortisol
If the cortisol is too high the pathway is stopped through a negative feedback loop
Physiologically cortisol is regulated by our circadian rhythm meaning it is high in the morning but decreases at night in fact the spike in cortisol is what wakes you up
Stress increases the levels of cortisol changing the set point to much higher
What are two examples of positive feedback control?
Positive feedback control is rare because … how would you stop it?
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
Positive feedback: control of uterine contractions in labour by oxytocin
1. In labour oxytocin stimulates contraction of uterine muscles
2. Cervix dilates and activates stretch receptors
3. Action potentials signal to the hypothalamus
4. Stimulates the further release of oxytocin
