11) Reflex control of circulation Flashcards
1
Q
What are the two types of reflex inputs?
A
- Excitatory inputs: Stimulation of these reflexes increases cardiac output, TPR and blood pressure. They are known as a Pressor response. It involves arterial chemoreceptors and muscle metaboreceptors
- Inhibitory inputs: Stimulation of these reflexes decreases cardiac output, TPR and blood pressure. It is known as a Depressor response. It involves arterial baroreceptors and cardiac-pulmonary receptors
2
Q
What part of the brain deals with the reflexive inputs?
A
- The medulla
3
Q
What are arterial baroreceptors?
A
- They maintain blood flow to the brain and myocardium
- They monitor blood pressure in carotid and coronary arteries
- By monitoring the blood pressure we can deduce what the flow is like as we do not have flow censors in the body
- They do this by detecting arterial wall stretch
4
Q
What is the relationship between blood flow (CO), pressure (Pa) and Total Peripheral Resistance (TPR)?
A
- CO = Pa/ TPR
- A fall in Pa means either a decrease in CO or a decrease in TPR (if CO remains the same)
- Both of these reduce blood flow to the heart and brain
5
Q
How do baroreceptors respond to changes in pressure?
A
- At rest pressure is low so the amount of pulses fired is low
- However as pressure starts to increase we have a fast firing of pulses as it reaches the threshold
- Upon reaching the threshold the pulses fired starts to slow down and remains at a level that is faster and higher than at rest.
- It has adapted to a new normal
- When there is a decrease in pressure the firing slows down proportionately
6
Q
What happens to baroreceptors when we experience a continued high/low pressure?
A
- The threshold for baroreceptors activation can change
7
Q
What are the effects of increased blood pressure on baroreceptors?
A
- When exercising there is an increase in blood pressure (called loading)
- Pulse pressure falls which means stroke volume is reduced
- Vasodilation occurs (which causes a decrease in TPR and blood pressure) along with decreased sympathetic nerve activity
- Finally activity of the vagus nerve is increased
8
Q
What are the effects of decreased blood pressure on baroreceptors?
A
- When haemorrhaging there is a decrease in blood pressure (called unloading)
- As a result there is increased sympathetic activity and decreased vagus activity
- Heart rate and force of contraction also increases to increase cardiac output
- Arterioles constrict to give increased TPR
- Venous constriction increases central venous pressure and so stroke volume and cardiac output increases (due to Starling’s law)
- There is also the release of adrenaline, ADH and activation of RAAS
- This causes constriction which will increase blood pressure and decrease capillary pressure
- Hence there is a decrease in hydrostatic pressure (so less filtration and more reabsorption)
- These effects maintain blood pressure and blood flow to vital organs
9
Q
What are the different cardiac stretch receptors?
A
- Veno-atrial mechanoreceptors: They are stimulated by increased cardiac filling/ CVP. When stretched they switch off ADH and RAAS which reduces the sympathetic activity to kidneys to increase filtration at capillaries. They also secrete atrial natriuretic peptide which increases Na+ secretion. This reduces blood volume and pressure
- Ventricular mechanoreceptors: They are stimulated by stretching of ventricles and deliver a depression response. There is a weak reflex as they cause mild vasodilation which lowers blood pressure and preload which play a protective role for vessels
- Nociceptive sympathetic afferents: Stimulated by K+, H+ and bradykinin during ischeamia. They mediate pain of angina and myocardial infarction. They cause increased sympathetic activity which results in a person turning pale, sweaty and tachycardia of angina. This allows less blood to the skin and more blood to the heart
10
Q
Why are baroreflex so important?
A
- When afferent baroreflex fibres are removed, arterial pressure varies enormously however the means were not too different
- When afferent cardiac receptor fibres are also removed atrial pressure still varies greatly however the means become different too
11
Q
What are arterial chemoreceptors?
A
- They regulate ventilation and also drive cardiac reflexes during asphyxia, shock and haemorrhage.
- They are stimulated by low O2 (hypoxia), high CO2 (hypercapnia), H+ and K+
- They are well perfused
- When blood pressure is below the range of baroreflex (maximally unloaded) the chemoreceptors are still active and may compensate
- They produce a pressor response. This means that there is increased sympathetic activity, tachycardia, increased arterial/venous constriction and increased cardiac output and blood pressure
12
Q
What is a pressor response?
A
- Occurs when atrial chemoreceptors are stimulated
- We experience increased sympathetic activity
- There is also Tachycardia along with increased selective arterial/venous constriction
- There is increased cardiac output and blood pressure (especially in the preservation of cerebral blood flow)
13
Q
What does the vagus parasympathetic nerve do?
A
- Slows down heart rate
14
Q
What are the different parts of the medulla?
A
- Nucleus Tractus Solitarius (NTS)
- Nucleus Ambiguous (NA)
- Caudal Ventrolateral Medulla (CVLM)
- Rostral Ventrolateral Medulla (RVLM)
15
Q
What are muscle metaboreceptors?
A
- They are sensory fibres in skeletal muscle that are activated by metabolites such as K+, lactate and adenosine
- They also produce a pressor response. This means that there is increased sympathetic activity, tachycardia, increased arterial/venous constriction and increased cardiac output and blood pressure
- This is important during isometric exercise (exercise where muscle is continually contracted but joint angle and muscle length doesn’t change). This produces a higher blood pressure which drives blood into contracted muscles to maintain perfusion. These muscles undergo metabolic hyperaemia allowing blood flow to contracted tissue.
