S2: CVS Reflexes Flashcards
What are the 2 types of reflexes?
- Pressor Response
2. Depressor Response
Describe the Pressor Response
Excitatory inputs e.g. arterial chemoreceptors, muscle metaboreceptors (work). They switch on reflexes to increase CO/TPR/BP.
For example: Chemoreceptors sense low O2 levels which switches on reflexes increasing BP which increases blood flow to the lungs increasing O2 saturation in blood
What sensory receptors detect pressor response?
Arterial chemoreceptors Muscle metaboreceptors (work)
What sensory receptors detect depressor response?
Inhibitory inputs e.g. Arterial Baroreceptors
Switch off reflexes to decrease CO/TPR/BP
What is the role of arterial baroreceptor?
To sense blood pressure.
They are found in the walls of the carotid arteries and aorta which allows the specific and indirect sensing of blood flow to brain and heart.
These sensors detect arterial wall stretch called baroreceptors.
They are a type of mechanoreceptors.
How can blood flow indirectly be measured?
The body has no blood flow sensors so the body instead measures blood pressure (CO = BP/ TPR)
Decrease in BP therefore reflects a difference in CO/TPR which ultimately tells us about changes in blood flow.
How do baroreceptors respond to changes in pressure?
What happens to them during hypertension?
Blood vessel stretched is sensed by baroreceptor endings and an AP is sent to the brain.
In the face of continued high or low pressure, the threshold for baroreceptor changes. During an increase in pressure, the baroreceptors start of firing rapidly but they slow down an ‘adaptation’ period as constant high BP decreases their sensitivity.
E.g. In hypertension where baroreceptors are less activated as threshold becomes much greater à high BP maintained
Consequently, we don’t regulate our BP as well
What is the effect of increased BP on baroreflex (Loading)?
e.g. Stress
Increased BP stimulates carotid sinus nerve which activates baroreceptors initiated the depressor.
This reduces HR and SV which decreases blood pressure.
In addition, sympathetic NS is switched off so loss of sympathetic vagal tone causes vasodilation of arterioles which decreases TPR consequently BP.
What is the effect of decreased BP on baroreflex (unloading)?
e.g. Haemorrhage, standing up
Decreasing BP causes the baroreflex to:
- Increase sympathetic activity and decrease in vagus activity
- Increase HR and force of contraction which increases cardiac output
- Arteriolar constriction = Increases TPR
- Venous constriction = Increases CVP, SV, CO
Pressor reflex: maintains blood pressure/blood flow to vital organs
What happens with severe decrease in BP e.g. haemorrhage?
- Adrenaline secretion
- Vasopressin (ADH) secretion due to decrease in blood volume sensed by the heart
- Stimulation of RAAS-Ang II production
- Vasoconstriction of arterioles decreases capillary pressure so absorption of intersisital fluid increases blood volume.
- Aldosterone secretion causes Na+/H2O reabsorption in kidneys so there is an increase in blood volume
Drop in blood pressure is very serious so the body needs to adapt very quickly to maintain blood to and from the heart
What are the 3 types of cardiac receptors?
- Nociceptors –> sympathetic afferents that detect products of pain
- Veno-atrial mechanoreceptors –> tell us how much right atrium is filling
- Ventricular mechanoreceptors –> detect stretch
Describe the cardiac receptor: Nociceptive sympathetic afferents
What reflex?
Chemosensitive ventricular afferent fibres (C-fibres that lie with sympathetic nerves). It is stimulated by:
- K+
- H+ (from lactate)
- Bradykinin (during ischaemia and they are products of pain and inflammatory mediators). They mediate pain/symptoms of angina and myocardial infarction.
Fibres converge onto same neurones in the spinal cord as somatic afferents (these nerves stimulate other body parts) - basis of referred pain.
~ Acute pressor reflex - Increase sympathetic system: Pale (vasoconstriction), sweaty (Ach –> Musc), Tachycardia (increase HR)
Describe the cardiac receptor: Veno-atrial mechanoreceptors
What reflex?
Stimulated by increase in cardiac filling/CVP
- In RA
~ Initial pressor reflex - increase sympathetic system - tachycardia. The bainbridge effect occurs.
~ Long term depressor reflex - Increases the amount of blood going to the kidney so more blood filtered. This increases diuresis which decreases blood volume. There is a feedback loop with changes in ADH, ANP, RAAS which is a reflex that makes less blood fill the heart.
What is the Bainbridge effect?
Reflex tachycardia due to rapid infusion of volume into venous system (veno-atrial stretch receptors and pacemaker disension). This is an reflex (initial pressor reflex) that increase HR to get rid of more blood.
Describe the cardiac receptor: Ventricular mechanoreceptors
What reflex?
Stimulated by over distension of ventricles.
It is linked to MI –> Poor contractility –> decrease ejection –> CO decreases
~ Depressor reflex - Heart senses and switches off reflex which protects the heart from using too much O2 leading to a reduction of HR. It role is however not well known and it may override veno-atrial reflex.
