22-09-22 – Control of Blood Pressure Flashcards
Learning outcomes
- To identify the components of the feed-back system involved in the reflex control of mean arterial blood pressure (including the receptors, integrating centre, target effectors and associated afferent and efferent pathways), where they are located and how they contribute to regulation of MABP.
- To predict how the cardiovascular system will reflexively respond to physiological changes to maintain mean arterial blood pressure.
- To explain how tissue demands for blood flow are balanced against the requirement to maintain a mean arterial blood pressure.
Name 2 reasons why integration and control of the heart and blood vessels is necessary
- Integration and control of the heart and blood vessels is necessary to maintain tissue perfusion across the entire body:
1) Needed to keep a relatively constant arterial blood pressure (MABP)
* Too low, blood flow to organs would fail
* Too high, damage to vessels and organs would take place
2) Needed to control distribution of total cardiac output
* 5L/minute is not enough to perfuse the whole body
* Needs to respond to tissue demands
* Satisfied by local control mechanisms e.g baroreceptors and chemoreceptors (sensory cells associated with reflexes)
How rapid is sympathetic controller of arterial pressure?
- Nervous control of arterial pressure is rapid
- Can increase arterial pressure to 2x normal within 5-10 seconds
- Can decrease arterial pressure to 50% normal within 10-40s
What are the 6 fundamental components of a reflex control system?
- 6 fundamental components of a reflex control system:
1) Internal variable to be maintained
2) Receptors sensitive to change in the variable
3) Afferent pathways from the receptors
4) An integrating centre for the afferent inputs
5) Efferent pathways from the integrating
6) Target effectors that alter their activities
What is mean arterial blood pressure (MABP)?
What is the formula for MABP?
What are baroreceptors?
What do they allow? How do they achieve this?
What is this process known as?
- Mean arterial blood pressure (MABP) is an average blood pressure in an individual during a single cardiac cycle
- MABP between 70-100mmHg is considered normal
- MABP = Cardiac output x Total Peripheral resistance
- Baroreceptors are a type of stretch mechanoreceptors that allow for the relay of information about blood pressure within the autonomic nervous system.
- Information from baroreceptors is passed in rapid sequence to alter the total peripheral resistance and cardiac output, maintaining blood pressure within a preset, normalized range.
- Baroreceptors uses a negative feedback loop to control MABP
- This process is known as the baroreceptor reflex, and is a neuronal reflex
Where are the main baroreceptor locations in the arteries?
What afferent fibres are they innervated by?
What type of receptors are baroreceptors?
How does blood pressure affect the firing rate of baroreceptors?
How does this lead to blood pressure normalising?
What is the setpoint of baroreceptors?
When can it change?
What is the primary purpose of the baroreceptor reflex?
- Main arterial baroreceptor locations:
1) Walls of aorta
* Subject to high ranges of pressures
* Afferent fibres follow the vagus nerve (10th cranial nerve)
2) Carotid artery
* Subject to lower pressures
* Afferent fibres follow glossopharyngeal nerve (9th cranial nerve)
- Baroreceptors are stretch receptor, which sense the stretch in vessel walls
- When blood pressure increase causes vessel wall stretch increase, the firing rate of baroreceptors increase, the ANS can then decrease blood pressure through increasing parasympathetic activity and/or decreasing sympathetic activity
- When blood pressure decrease causes vessel wall stretch decrease, the firing rate of baroreceptors decrease, the ANS can then increase blood pressure through increasing sympathetic activity and/or decreasing parasympathetic activity
- These changes in ANS firing will alter cardiac output/total peripheral resistance, leading to an increase or decrease in MABP
- The set point of baroreceptors is the normal resting value of blood pressure that baroreceptors are sensitive around, and is the midpoint of firing rate
- The set point is different for different systems, and can be changed e.g during hypertension or during exercise
- The primary purpose of the baroreceptor reflex control is to reduce the minute-to-minute variations of arterial pulse
Baroreceptor reducing minute to minute variations of arterial pulse
- The primary purpose of the baroreceptor reflex control is to reduce the minute-to-minute variations of arterial pulse
Arterial baroreceptor flow chart
- Arterial baroreceptor flow chart
Where are cardiopulmonary baroreceptors located?
What do these receptors sense?
How does blood volume in pulmonary circulating affect baroreceptor firing?
How does this affect ANS activity?
What do most vessels not have?
- Cardiopulmonary baroreceptors are located in the atria, ventricles and pulmonary vessels
- These low-pressure baroreceptors are involved in regulating blood volume
- If a decrease in blood volume is sensed, the rate of cardiopulmonary baroreceptor firing decreases
- This change in firing increase sympathetic nerve activity to the heart and blood vessels and decreases parasympathetic activity to the heart (as most vessels don’t have parasympathetic innervation)
What is the Bainbridge reflex?
What effect does the Bainbridge reflex have on HR and contractility?
What does this reflex prevent?
What happens if the aortic/baroreceptors detect an increase in blood pressure?
What is the link between the Frank Starling mechanism and the Bainbridge reflex?
- The Bainbridge reflex is the sympathetic-mediated reflex in response to increased blood (increased stretch) in the atria
- The Bainbridge reflex increase HR (positive chronotropic effect) and increase contractility (positive inotropic effect), both of which increase blood pressure
- The Bainbridge effect prevents blood from backlogging in the veins
- If the aortic/carotid baroreceptors sense high pressure, the Bainbridge effect can override this
- The Frank Starling mechanism also increases contractility, and can be triggered simultaneously with the bainbridge reflex, leading to an even greater increase in contractility
Where is integrated control of BP?
What does this centre integrate?
What is found in the sensory area of this centre?
What is found in the lateral portion?
What is found in the medial portion?
- Integrated control of BP is in the medullary cardiovascular vasomotor control centre (MCVCC)
- This centre can integrate the baroreceptor and Bainbridge reflex signals
- In the sensory area of this centre, there is the input from baroreceptors
- In the lateral portion, there are efferent sympathetic nerves
- In the medial portion, there are efferent parasympathetic (vagal) nerves
What are the target effectors in the reflex control of MABP?
How does the ANS effect these effectors?
How do these changes affect MABP?
- The target effectors in the reflex control of MABP are the heart and blood vessels
- Heart
- The sympathetics and parasympathetics both control heart rate and normally function simultaneously
- At rest, the parasympathetics is the predominant tone
- When active, sympathetics are the predominant tone, as they can significantly increase contractility, heart rate and stroke volume through changes in calcium handling
- These changes alter cardiac output, which leads to a change in MABP
- Blood vessels
- Most vessels don’t have parasympathetic supply
- Sympathetic vasoconstrictor tone exerts a vasomotor tone on vessels, meaning they are kept partially restricted
- This continuous low-level tone in vessels affects total peripheral resistance (vascular resistance)
- If we want to reduce vascular resistance, we can take away the sympathetic tone, which allows for relaxation of the smooth muscle
- Sympathetic control also aids in venous return
- Sympathetic constrict veins, which decreases capacitance, which increases vascular resistance, which increases vascular pressure, which increases venous return, which increases stroke volume, which increases cardiac output
- These changes in total peripheral resistance and cardiac output lead to an increase in MABP
Control of blood pressure flow chart
Control of blood pressure flow chart
What is the CNS ischemic response?
What triggers this response?
What 3 changes does this response lead to?
- The CNS ischemic response is an emergency control pressure system (last ditch stand)
- This response is triggered when the blood flow to the medullary CVCC is greatly reduced
- This response leads to:
1) Increased Peripheral vasoconstriction (almost completely occludes some peripheral vessels)
2) Increased sympathetic stimulation of the heart
3) Great increase in systemic arterial pressure – as high as 250mmHg for 10 mins (normal value is 120mmhg Systolic and 90mmHg diastolic), which is the biggest increase in pressure possible
Do organs auto-regulate blood flow?
How do organs auto-regulate blood flow?
What 2 intrinsic ways can we maintain safe blood flow when blood pressure increases?
- Organs auto-regulate blood flow independent of innervation/hormonal control, despite the central decision about where we constrict vasculature to modify total peripheral resistance
- Organs auto-regulate blood flow through Active and reactive hyperaemia
- 2 intrinsic ways we can maintain safe blood flow when blood pressure increases:
1) Myogenic theory (acute flow auto-regulation)
* Stretch induces vascular depolarisation of smooth muscle due to increase in arterial pressure
* This limits the blood flow that can move through the vessel, preventing damage to the vessels
* This is a myogenic response, where stretch activated Ca2+ channels trigger the process of contraction
2) Metabolic theory (acute flow autoregulation)
* An increase in arterial pressure increases O2 and washes out local factors e.g Breakdown of ATP to ADP and AMP, which can be converted to adenosine in the blood
* High degree of oxygen delivery to smooth muscle is likely to trigger constriction mediated effects in the vasculature, leading to an occlusion of the blood supply downstream, as there is enough oxygen present
* This constriction can reduce flow and protect the vessel walls from being damaged
Cardiac output summary
Cardiac output summary
Total peripheral resistance (vascular resistance) summary
Total peripheral resistance (vascular resistance) summary
