11) Reflex control of circulation

Question 1

What are the two types of reflex inputs?

Answer 1

• 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

Question 2

What part of the brain deals with the reflexive inputs?

Answer 2

• The medulla

Question 3

What are arterial baroreceptors?

Answer 3

• 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

Question 4

What is the relationship between blood flow (CO), pressure (Pa) and Total Peripheral Resistance (TPR)?

Answer 4

• 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

Question 5

How do baroreceptors respond to changes in pressure?

Answer 5

• 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

Question 6

What happens to baroreceptors when we experience a continued high/low pressure?

Answer 6

• The threshold for baroreceptors activation can change

Question 7

What are the effects of increased blood pressure on baroreceptors?

Answer 7

• 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

Question 8

What are the effects of decreased blood pressure on baroreceptors?

Answer 8

• 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

Question 9

What are the different cardiac stretch receptors?

Answer 9

• 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

Question 10

Why are baroreflex so important?

Answer 10

• 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

Question 11

What are arterial chemoreceptors?

Answer 11

• 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

Question 12

What is a pressor response?

Answer 12

• 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)

Question 13

What does the vagus parasympathetic nerve do?

Answer 13

• Slows down heart rate

Question 14

What are the different parts of the medulla?

Answer 14

• Nucleus Tractus Solitarius (NTS)
• Nucleus Ambiguous (NA)
• Caudal Ventrolateral Medulla (CVLM)
• Rostral Ventrolateral Medulla (RVLM)

Question 15

What are muscle metaboreceptors?

Answer 15

• 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.

Question 16

How does the interaction between the nucleus tractus solitarius (NTS) and the RVLM occur?

Answer 16

• They are found in the medulla and receive signals from stretched baroreceptors via afferent fibres.
• They send messages to the Caudal Ventrolateral Medulla (CVLM) which sends information to the Rostral Ventrolateral Medulla (RVLM).
• This is an inhibitory signal as it causes the inhibition of sympathetic efferent nerves to the heart and vessels
• As a result we receive less sympathetic efferent signals which means less heart rate, less vasoconstriction and lower blood pressure

Question 17

How does the interaction between the nucleus tractus solitarius (NTS) and the NA occur?

Answer 17

• Loading of baroreceptors stimulates the vagus nerve which activates the NTS
• The signal from the NTS stimulates the Nucleus Ambiguous (NA)
• The NA sends vagal parasympathetic impulses to the heart which have a depressor effect

Question 18

What happens at the NTS during sinus tachycardia?

Answer 18

• First an inhibitory input is sent from the inspiratory centre
• This causes every inhalation to switch off the Nucleus Ambiguous (NA)
• Hence the inhibitory parasympathetic signal to the vagus nerve decreases and heart rate increases slightly
• This means heart rate is slightly quicker when we inhale compared to when we exhale

Question 19

How does an emotional reaction affect the Nucleus Tractus Solitarius (NTS)?

Answer 19

• The limbic centre (the emotional centre) stimulates NTS which stimulates the Nucleus Ambiguous (NA)
• This causes increased activity of the vagus nerve and a depressor effect on the AV and SA nodes
• This can lead to fainting (syncope), called a vasovagal attack, caused by decreased cerebral blood flow due to sudden drop in arterial cardiac output and blood pressure