Arterial Blood Pressure Flashcards
What do we mean by resistance vessels?
Provide high resistance to blood flow and pressure falls - arterioles
What is the equation for mean arterial pressure?
Pa = Pd + (Ps - Pd)/3
What is the actual normal Pa and what is this usually rounded up to?
93mmHg
100mmHg
What is the main physical determinant of MAP?
Arterial blood volume (Va)
What are the physiological determinants of MAP?
Rate of inflow of blood into arteries during systole (Qh) - Cardiac output (Q)
Rate of outflow through arterioles into capillaries (peripheral runoff) - (Qr)
When is the MAP constant?
Qh = Qr
When does MAP increase?
Qh > Qr
When does MAP decrease?
Qh < Qr
What does increased resistance do to the rate of blood flow?
Decreases it
What is Total peripheral resistance?
Resistance to blood flow through entire arteriolar system
What does total peripheral runoff determine?
The rate of blood flow out of the arterial system
What raises arterial blood pressure?
Increase in total peripheral resistance
Vasoconstriction of arterioles
If there is constriction in the system, what happens to blood pressure in both upstream and downstream vessels?
Upstream - increased
Downstream - decreased
What is mean arterial pressure calculated by?
Cardiac output * total peripheral resistance = MAP
How do you calculate cardiac output?
Cardiac rate * Stroke volume
What determines the rate of change of arterial blood pressure (Pa)?
Compliance
How does compliance determine the rate of change of Pa?
Rigid arteries attain higher Pa level rapidly
In elastic arteries, increases in Pa occur at a slower rate
What will result in long term effects on Pa?
Long term increases in either CO or TPR
What are the physical factors that affect arterial blood pressure?
Arterial blood volume
Arterial compliance
What are the physiological factors that affect Arterial blood pressure?
Peripheral resistance
Cardiac output
Why is stroke volume a major determinant of pulse pressue?
During ventricular ejection, arterial blood volume increases to V2 and blood pressure increases to P2
During diastole, peripheral runoff reduces volume to V1, and pressure falls to P1
(V2 - V1) directly proportional to (P2 - P1) pulse pressure
What would an increase in stroke volume do to pulse pressure?
Increase it
Do lower compliant arteries have higher or lower pulse pressures?
Higher
Why do lower compliant arteries have higher pulse pressures?
Ejected blood exerts a higher pressure on rigid walls
Systolic pressure is increased in rigid arteries, so pulse pressure also increases
How does aortic compliance change as we age and what effect does this have on pulse pressure?
Compliance decreases so pulse pressure increases
What gives a measure of stroke volume clinically?
Magnitude of pulse pressure assuming normal arterial compliance in the patient
What condition may cause a diminished pulse pressure?
Cardiac failure as it reduces stroke volume
When is there increased pulse pressure?
Aortic valve regurgitation - drops diastolic pressure
What is used clinically to measure systolic and diastolic pressure?
Inflatable cuff and sphymomanometer
What happens when the cuff pressure supersedes systolic pressure?
Artery is entirely closed and blood flow drops
What happens after the cuff pressure overtakes systolic pressure?
Cuff pressure is slow dropped which partially opens the artery, restarting blood flow in a turbulent fashion through the pinched artery
What can be heard by auscultation during the measurement of blood pressure as the cuff pressure slow drops?
Thumping noise called Korotkoff sounds
What happens when cuff pressure is less than diastolic pressure?
Artery is fully open; blood flow is smooth and Korotkoff sounds disappear
Where is the stethoscope placed when listening for Korotkoff sounds?
Immediately distal to the cuff
What is the pressure displayed during the first Korotkoff sound taken to be?
Systolic pressure
How do we know the diastolic pressure?
When Korotkoff sounds disappear
How can MAP be modulated?
Altering stroke volume, total peripheral resistance, heart rate
What is important in maintenance of arterial blood pressure?
Arterioles
What increases and decreases blood flow in arterioles?
Contraction reduces blood flow
Relaxation increases blood flow
What controls total peripheral resistance?
Regulation of smooth muscle contraction and relaxation in arterioles
What is the Ca++ sensor in smooth muscle?
Calmodulin
What is the role of Ca++/calmodulin complex?
Binding and activating myosin light chain kinase
What determines the strength of contraction of vascular smooth muscle?
Degree of opening of Ca++ channels
What is myogenic regulation?
Rapid increase in blood pressure increases transmural pressure induces reflex contraction of arterioles
Rapid reduction in blood pressure induces a reflex dilation of these vessels
What is the mechanism of myogenic regulation?
Unclear, but stretching of vascular smooth muscle increase [Ca++]i inducing contraction of VSM cells
Explain how myogenic regulation helps blood flow when someone moves from a lying to standing position.
This movement causes rapid flow of blood to lower extremities causing a rise in both venous and arterial pressure
Increased stretch triggers reflex constriction of arterioles reducing blood flow into lower leg capillaries
What would happen while someone is standing up in the absence of myogenic regulation?
Increased capillary blood pressure (hydrostatic pressure)
Elevated hydrostatic pressure increases capillary filtration causing oedema in feet and lower legs
What does endothelial-mediated regulation do?
Releases substances that trigger contraction/relaxation of VSM
What does increased blood flow result in by endothelial-mediated regulation?
Increased blood flow causes shear stress to endothelium and induces release of nitric oxide
What is nitric oxide synthesised from?
L-arginine
What does NO do once diffused to VSM cells?
Activates guanylyl cyclase increasing the intracellular concentration of cGMP
What does cGMP do to VSM?
Decreases [Ca++]i inducing dilation of blood vessels
What is release of NO induced by?
Accelerated blood flow
What does metabolic activity regulate in the circulatory system?
Capillary blood flow
What happens if O2 levels are inadequate in capillaries?
Metabolites with vasodilator activity are produced
What do metabolites with vasodilator activity do?
Diffuse to VSM inducing relaxation and increasing capillary blood flow
What is active hyperemia?
Rise in blood flow caused by increased tissue activity
What are the possible metabolites responsible in metabolic regulation?
Phosphate
K+
Adenosine
(Breakdown products of ATP)
What innervates VSM of arterioles and veins?
Post-ganglionic sympathetic nerves
What does norepinephrine bind to on VSM cells and what does this induce? (Most prominent)
Alpha 1 adrenergic receptors
Induces vasoconstriction
What does adrenaline bind to on VSM cells and what does it induce?
Beta 2 adrenergic receptors in skeletal blood vessels
Induces vasodilation
What do alpha 1 adrenergic receptors do when activated?
Activate phospholipase C produces IP3 which increases [Ca++]i
Results in contraction
What do activated beta 2 adrenergic receptors do?
Activates adenylyl cyclase and increases [cAMP]i which inhibits MLCK
Is there a high level of sympathetic neural activity delivered to arterioles?
Yes
How often is noradrenaline released from sympathetic nerves and why?
Constantly to induce a degree of vasoconstriction
Tonically active/vasoconstrictor tone
What does increased sympathetic nerve activity do to peripheral blood flow?
Induces further vasoconstriction increasing TPR, decreasing rate of blood flow from arteries into tissues
What is the purpose of sympathetic neural vasoconstriciton?
To increase MAP
Where is a high percentage of blood?
Venules and small veins in tissue
At basal SN activity, what is the degree of venous vascular tone?
Low in most tissue, especially skin
What does sympathetic nerve stimulation do to arteries?
Reduces blood flow into tissues by constriction of arterioles
What does sympathetic nerve stimulation do to venous circulation?
Decreases tissue blood volume by constriction of venules
Increases venous return
Redistribution of blood from venous into arterial system
What does sympathetic nerve stimulation do to capillaries?
Reduces capillary hydrostatic pressure
Absorption of interstitial fluid into capillaries increases
What does vasoconstriction do?
Mobilises blood from capacitance vessels and returns it to heart
Physiologically, what is very important during exercise?
Increased venous return
Increased EDV
Increased SV
Increased CO
What maintains normal arterial pressure during severe haemorrhage?
Vasoconstriction of resistance and capacitance vessels
Do parasympathetic nerves have high or low vasodilatory effects?
Low
What is regulation of peripheral resistance mediated by?
Sympathetic nerves
Where is the vasomotor centre located?
Reticular substance of the medulla and the lower third of the Pons
What does the vasomotor centre do?
Regulates impulses of the vagus nerve
What section of the vasomotor centre regulates sympathetic nerve fibers?
Rostral ventrolateral medulla (RVLM)
What does the RVLM do?
Sends fibers to spinal cord which excite pre-ganglionic neurons of the SNS
Maintains both sympathetic cardiac and vasoconstrictor tone
What does stimulation of the RVLM do?
Increases BP and HR
What happens if the RVLM is destroyed?
BP decreases to 40mmHg
Essential for maintaining normal BP
What are other names for the RVLM?
Pressor region
Vasoconstrictor area
What is the role of the caudal ventrolateral medulla?
Fibres project to the RVLM and inhibit its activity causing vasodilation
What is the role of the caudal ventrolateral medulla?
Fibres project to the RVLM and inhibit its activity causing vasodilation
What other names exist for the CVLM?
Depressor region
Vasodilator area
What are the nucleus ambiguus (NA, AMB) and dorsal motor nucleus of vagus?
Origin of vagal projections to the heart
Vagal centre
What is the role of the nucleus of the tractus solitarius?
Receives sensory signals from circulatory system mainly through the vagus and glossopharyngeal nerves
What are the vagus and glossopharyngeal nerves stimulated by?
Baroreceptors and cardiopulmonary recpetors
Where does the NTS send output to?
CVLM
NA
What does stimulation of the NTS do?
Reduces sympathetic outflow and increases vagal outflow
Where does the NTS receive sensory impulses from?
Baroreceptors
cardiopulmonary receptors
Chemoreceptors
Other brain regions
Name an example of a detrimental effect of blood pressure diverging from normal limits.
Perfusion of cerebral circulation
What are baroreceptors?
Stretch-sensitive mechanoreceptors
Where are baroreceptors located?
Walls of the carotid artery and the aortic arch
How do baroreceptors become more active?
An increase in blood pressure causing the walls of the carotid artery and the aortic arch
Where exactly are stretch receptors located?
Carotid sinuses and aortic arch
What are the afferent pathways of the arterial baroreceptors?
Carotid sinus nerve (Hering’s nerve) > Glossopharyngeal nerve > NTS
Vagus nerve > NTS
“Buffer nerves” as they buffer abrupt changes in BP
At what values of BP is vagal activity depressed/activated?
Activated - >100mmHg
Depressed - <100mmHg
What do increases in arterial pressure enhance?
Firing of baroreceptor terminals
How is firing of baroreceptor terminals diminished?
Reductions in arterial pressure
What does the baroreceptor reflex allow the cardiovascular system to do?
Respond to rapid pulsatile changes in blood pressure
Which baroreceptors are more important for regulating arterial pressure?
Carotid sinus baroreceptors
What viscus is responsible for long term regulation of blood pressure?
Kidney
Can the baroreceptor reflex correct long term changes in arterial pressure?
No, they are only responsible for acute regulation of BP
At 100mmHg does a slight change in pressure cause a weak or strong change in the baroreflex signal?
Strong as it wants to readjust arterial pressure back toward normal
What will a sudden reduction in blood pressure do?
Rapid increase of sympathetic nerve activity
Reduction in vagal activity will diminish inhibitory effect of HR
What will an increase in sympathetic nerve activity do?
Increase cardiac output
Contract venous smooth muscle promoting venous return
Stimulate contraction of arterial smooth muscle to increase peripheral resistance
Vasoconstriction in kidney arterioles also minimises urine formation
What will a sudden elevation in blood pressure do?
Stimulate the vagus nerve
Inhibit sympathetic nerves
Kidneys are also stimulated to excrete more water as urine, thereby reducing total blood volume
What does stimulation of the vagus nerve do?
Reduce cardiac output by slowing heart rate
What does inhibiting sympathetic nerve activity do?
Reduces cardiac output
Causes relaxation of arterial smooth muscle to decrease peripheral resistance aka vasodilation
What do cardiopulmonary baroreceptors do?
Initiate a reflex that lowers arterial blood pressure in response to changes in blood volume
What does the reflex of cardiopulmonary baroreceptors mainly act on and to do what?
Kidneys to reduce blood volume by increasing urine output
How does the reflex of the cardiopulmonary baroreceptors achieve this?
Reduction of sympathetic nerve activity to kidneys
Inhibition of angiotensin, aldosterone and ADH
What is the Bainbridge reflex?
Increased blood volume increases atrial pressure causing a rapid increase in HR
What is the Bainbridge reflex mediated by?
Atrial stretch receptors
Where are the signals for the Bainbridge reflex transmitted in to and from the NTS?
vagus nerve afferently
vagal and sympathetic nerves efferently
What do the sympathetic and vagal nerves do in the Bainbridge reflex?
Increases HR and contractility
Transfer of blood into arterial circulation
Prevents damming of blood in veins, atria and pulmonary circulation
Are Bainbridge and baroreceptor reflexes agonistic or antagonistic?
Antagonistic
If the blood volume is high which reflex predominates?
Bainbridge reflex
If the blood volume is low which reflex predominates?
Baroreceptor reflex
What makes the decision of whether the Bainbridge reflex or the Baroreceptor reflex predominates?
NTS
Where are chemoreceptors located?
Medulla
Sinuses
Aortic arch
What do chemoreceptors do and what is their role?
Respond to changes in arterial PO2, PCO2 and pH
Set appropriate rate and depth of ventilation
What may peripheral chemoreceptor afferents do?
Increase blood pressure
Increase heart rate
What does the hypothalamus do?
Regulates appetite, body temperature, fluid balance, endocrine secretion
What does the anterior/preoptic hypothalamus do?
Decreases HR and BP
What does the posterior and lateral hypothalamus do?
Involved in the alerting reaction in response to impending danger
Increases BP, HR
Vasodilation in skeletal muscle
Vasoconstriction in skin and splanchnic organs
How does the cerebral cortex affect the circulatory system?
Vasoconstriction of skin, splanchnic and renal vessels
Vasodilation in skeletal muscles
What does the limbic area of the cerebral cortex do?
Vasodilation of facial blood vessels
Triggers cardiovascular responses to pain and anxiety
What is meant by compliance?
The ability of the blood vessels to distend (swell) in response to blood pressure
