Reflex control of circulation Flashcards
What is the function of arterial baroreceptors
arterial baroreceptors- vital to maintain blood flow to brain and myocardium
What is the equation for blood flow (CO)
blood flow (CO)= Pa/TPR
What does a decrease in Pa reflect in other components
a decrease in Pa reflects a decrease in either CO or TPR which compromises blood flow to brain and the heart
What do blood pressure sensors do
blood pressure sensors in walls of the carotid arteries/aorta inform brain of pressure changes in these key feeder vessels
How do baroreceptors respond to changes in pressure
-there is not much firing of electrical signals at rest, then as pressure increases there is fast firing
-eventually this fast firing slows down and becomes constant but at a higher level than before
-adaptation to a new normal
-for a decrease in pressure the firing slows down proportionately
What is loading in regards to BP
Increase in BP is termed as loading
Explain the effect of increased BP on baroreflex
The effect of increased BP on baroreflex:
-pulse pressure falls (decreased stroke volume)
-vasodilation deceases TPR and BP
-Decreased sympathetic nerve activity
-Increased Vagus nerve activity
Explain the effect of decreased BP on baroreflex
Decrease in BP is termed as unloading.
The effect of decreased BP on baroreflex is:
-increased sympathetic activity and decreased vagus activity
-increased HR and force of contraction so increased cardiac output
-arteriole constriction gives increased TPR
-venous constriction increases central venous pressure, by Starling’s law this increases stroke volume and cardiac output
What can also increase Na+/H20 absorption in kidneys
Adrenaline secretion, Vasopressin (ADH) secretion and stimulation of RAAS increases Na+/H20 absorption in kidneys raising blood volume
Explain what increases absorption of interstitial fluid
Vasoconstriction decreases capillary pressure which increases absorption of interstitial fluid which also increases blood volume
Explain two different types of cardiac receptors
1) veno-atrial mechanoreceptors:
stimulated by an increase in cardiac filling.
This reduces sympathetic activity to renal arteries, this increases glomerular filtration. This secretes atrial natriured peptide.
-this increases Na+ exrection which in turns lowers blood volume reducing ADH and RAAS
2) Ventricular mechanoreceptors:
-stimulated by over distension of ventricles as a depressor response.
-weak reflex is caused by mild vasodilation, lower blood pressure and preload.
Explain one afferent nerve
1) Nociceptive sympathetic afferents:
-stimulated by K+, H+ and bradykinin during ischaemia.
-Mediate pain of angina and myocardial infarction.
-reflex increased sympathetic activity
-causes pale, sweaty, tachycardia of angina
-MI symptoms
What happens when afferent fibres from baroreceptors are removed
when afferent fibres from baroreceptors are removed arterial pressure varies enormously. however the means aren’t all that different
What happens when afferent fibres from cardiac receptors are removed
when afferent fibres from cardiac fibres are also removed arterial pressure still varies and the means have now become very different
Explain characteristics of arterial chemoreceptors
-detects chemical changes in the blood
-stimulated by low 02, high CO2, H+ and K+
-they are well supplied with blood flow around 20ml/g/min
-regulates ventilation and also drives cardiac reflexes during asphyxia (low O2/high CO2)
-When BP below the range of baroreflex, the chemoreceptors are still active and may compensate
Explain what happens during the pressor response (raising of blood pressure)
-increased sympathetic activity
-tachycardia (increased selective arterial/venous constriction
-increased cardiac output + blood pressure
Explain what muscle metaboreceptors are
Muscle metaboreceptors- sensory fibres in group IV motor fibres located in skeletal muscle.
Activated via metabolites K+, lactate, adenosine
Explain why muscle metaboreceptors are important during isometric exercise
important during isometric exercise:
-continually contracted muscle but joint angle and muscle length don’t change.
-higher BP drives blood into the contracted muscle to maintain perfusion.
-these muscles undergo metabolic hyperaemia allowing blood flow to the contracted muscle
Explain the central role of the nucleus tractus solitarius (NTS)
1) Signal from stretched baroreceptor sent via afferent fibres enter nucleus tractus solitarius (NTS)
2) This then sends information out to the Caudal Ventrolateral Medulla (CVLM)
3) The CVLM sends information to the rostral ventrolateral medulla (RVLM)
4) This results in the inhibition of sympathetic efferent nerves to heart an vessels
5) Less sympathetic efferent signals result in reduction in HR, less vasoconstriction, lower BP
Explain vagus parasympathetic impulses to the heart
-loading of the baroreceptors also stimulates the vagus nerve which again activates the NTS
-The signal from the NTS stimulates the nucleus ambiguous (vagal nuclei)
-vagal parasympathetic impulses are sent to the heart and these have a depressor effect
Explain what the sinus tachycardia does as vagal parasympathetic outflow
-inhibitory input from inspiratory centre
-each inhalation switches OFF nucleus ambiguus
-the inhibitory parasympathetic signal to the vagus decreases the heart rate increases slightly
What does the limbic system do
The limbic system (emotional centre) stimulates NTS which in turn stimulates the nucleus ambiguus causing increased activity of the vagal nerve and depressor effect on the AV and SA nodes
Explain vasovagal syncope
Vasovagal syncope:
-can lead to fainting (syncope)- vasovagal attack
-syncope caused by decreased cerebral blood flow due to sudden drop in arterial cardiac output and blood pressure
