CVS Session 6 Flashcards
What determines venous pressure?
Rate heart pumps blood out
Rate of blood entry
What determines arterial pressure?
Cardiac output
TPR
What do arterial pressure and venous return both affect?
Stroke volume
Heart rate
If the cardiac output is kept constant and the TPR decreases, what happens?
Body asks for more blood so pressure gradient across system has to decrease
Arterial pressure decreases
Venous pressure increases
If the cardiac output remains constant and the TPR increases, what happens?
Body needs less blood so more pressure dissipated
Arterial pressure increases
Venous pressure decreases
What does a constant TPR indicate?
No change in body demand for blood
If the TPR is kept constant and cardiac output increases, what happens?
Increased arterial pressure - extra blood must be pushed out
Decreased venous pressure - blood removed from veins
If TPR is constant and cardiac output decreases, what happens?
Arterial pressure decreases
Venous pressure increases
Less blood removed from veins
What blood pressure changes occur after consumption of a meal?
Metabolism changes –> local metabolites dilate arterioles –> TPR decreases –> arterial pressure decreases and venous pressure increases
How are TPR and the body’s need for blood related?
They are inversely proportional
What is demand-led pumping?
When the heart pumps more blood to meet increased demand and restore arterial and venous pressures to normal
What is stroke volume?
Difference b/w end diastolic volume and end systolic volume
Why is the stroke volume only a portion of its maximum value?
Needs to be variable to match demand
How do you increase stroke volume?
Fill heart more in diastole
Leave less blood remaining at the end of systole
Which is the dominant variable acting on stroke volume?
Venous pressure - increase to increase stroke volume
What variable has a small effect on stroke volume?
Arterial pressure - decrease to increase stroke volume
How does increasing venous pressure increasing heart filling in diastole?
Ventricle fills until intra ventricular pressure = venous pressure
What is the ventricular compliance curve?
Pressure vs volume of the ventricle with the curve giving end diastolic volume
What is Starling’s law of the heart?
More in = more out
By what mechanism does Starling’s law of the heart works.
Fuller heart = more stretched ventricle therefore harder ventricle contracts thus increasing stroke volume
What automatically causes an increase in stroke volume?
Increase in venous pressure
What does the Starling curve relate?
Stroke volume to venous pressure
What does the slope of the Starling Curve indicate?
Contractility of ventricle
Does contractility quantify how much the heart can contract?
No, it is itself a variable
What causes the starling Curve to level off and subsequently decrease?
Overfilling of the heart
Why does the stroke volume fall off when the heart is overfilled?
Force of contraction of the heart is impeded by the pericardium
How is the Starling Curve described?
Monotonic - increase venous pressure –> increase stroke volume
Fairly linear until close to overfilling
Which two factors determine end systolic volume?
How hard ventricle contracts
How hard blood is ejected
What is the principal determinant of end systolic volume?
How hard the ventricle contracts
What determines how hard the ventricle contracts?
End diastolic volume (Starling’s law)
Contractility
How is contractility increased?
Increase sympathetic activity by increasing NA action on cardiac myocytes
What determines how hard blood is ejected from the heart?
Aortic impedance
TPR (therefore indirectly effects end systolic volume)
What does harder ejection of blood mean for arterial pressure?
It will increase
Is the control of heart rate an intrinsic property of the heart?
No
What controls heart rate?
Autonomic outflow controlled by signals from baroreceptors
What affect does decreased arterial pressure have on parasympathetic and sympathetic activity?
Decreases parasympathetic activity - decreased vagal outflow and no ACh on pacemaker cells
Increases sympathetic activity - increase HR and contractility
Why is the amplified change of cardiac output caused by increased contractility limited?
Diastole time is limited so at >180 bpm diastole time is too short for sufficient ventricular filling for an increased cardiac output
What is the Bainbridge reflex?
Increased venous pressure sensed by R atrium –> decreased parasympathetic activity –> increased HR
How does increased venous pressure effect the heart?
Increases rate only
Is regulation of heart rate affected by the absence of the Bainbridge reflex?
Nope
What effect does increasing venous pressure have on the CVS?
Increase stroke volume
Increase HR
Therefore, increases cardiac output
What effect does decreasing arterial pressure have on the CVS?
Increase stroke volume - sympathetic and direct effect
Increase heart rate - principal effect of baroreceptors
Therefore increases cardiac output
What effect does decreased arterial pressure have on veins and flow in the skin and gut?
Causes venoconstriction
Temporarily increases flow resistance in skin and gut tissues
How does eating a meal or carrying out light exercise affect venous and arterial pressures?
Increases venous pressure
Decreases arterial pressure
What causes the pressure changes seen in eating a meal/taking light exercise?
Release of local metabolites decreasing TPR
How does significant exercise threaten bloodflow to the brain?
Very large and rapid decrease in TPR decreases arterial pressure
How can significant exercise cause pulmonary oedema?
Large and rapid decreases in TPR increases venous pressure –> heart overfills –> L and R heart outputs not matched due to easier perfusion of lungs –> transient excess blood in lungs pushes fluid into tissues
How is overfilling of the heart in significant exercise prevented?
Pre-emptive increase in heart rate driven by the brain
How does the pre-emotive increase in heart rate seen in significant exercise vary between well-trained and untrained individuals?
Well-trained: occurs before exercise starts to maximise cardiac output
Untrained: rapid increase on starting exercise
If the heart rate increases w/no other change, what happens initially?
Cardiac output increases
What happens after the immediate rise in cardiac output seen when heart rate increases w/ no other changes?
TPR remains constant therefore venous pressure reduced –> decreased filling of heart in diastole –> stroke volume reduced to exactly same extent HR increased
Why is the heart driven by the circulation?
Increased cardiac output and HR would cause the heart to stop therefore proportions of stroke volume and HR must be altered
How are the R and L ventricles matched?
Share common pacemaker so only match by stroke volume
If RH pumps more, LH fills more –> LH pumps more
In what circumstance will blood accumulate in the lungs?
If heart is at the top of the Starling Curve - LH cannot respond to RH so ventricles are no longer matched
What must the arterial and venous pressure changes do in order to allow the cardiac control system to function without problem?
Move in different directions, irrespective of magnitude
Why does blood pool in the superficial veins of the legs upon standing?
Transmural pressure is high as these veins are surrounded by air not water which is less affected by gravity
Why does arterial pressure fall upon standing?
Central venous pressure decreases –> transient decrease in venous return so cardiac output falls
Why can the decrease in arterial pressure upon standing not be corrected by increased contraction of the heart?
Decreased venous return
How is arterial pressure restored upon standing?
Decrease is detected by baroreceptors –> stimulate increase HR but venous pressure still low –> TPR increases to defend arterial pressure
How is TPR increased to defend arterial pressure?
Briefly shut down bloodflow to skin and gut
What effect muscle-pumping have when standing up?
Increases venous return
What causes postural hypotension?
ANS vasoconstriction doesn’t work
Circulatory volume is reduced
Who is postural hypotension commonly seen in?
Elderly
What is the net effect of heamorrhage?
Blood loss from veins causing decrease in venous volume
Describe what changes take place in heamorrhage.
Decrease blood volume decreases venous pressure –> decreased cardiac output –> decreased arterial pressure –> detected by baroreceptors which increase HR and TPR
What effect does increasing the HR caused by the baroreceptors in response to decreased arterial pressure in haemorrhage have on venous pressure?
Decreases it further - makes it worse
Which pressure change does increasing the TPR in response to heamorrhage help?
Arterial
What causes the risk of loss of perfusion to vital organs in haemorrhage?
Decreased arterial pressure
What must occur for a haemorrhage to be overcome?
Increase in venous pressure
Replacement of blood volume cost
How does the body cope with losing 1 l of blood without outside help?
Venoconstriction to increase venous return
Autotransfusion to move ECF into blood
In what circumstance will both arterial and venous pressure increase?
When circulating volume is higher than it should be e.g. long term high sodium levels
Why does long term hypernatraemia cause vasoconstriction?
Increases venous pressure –> increases cardiac output and therefore arterial pressure –> over-perfusion of tissues washes away metabolites
Why does over-perfusion of tissues increase and maintain arterial pressure?
Causes increase in TPR
Why should hypertension be treated as soon as it is detected?
Avoid arterial pressure being too high for too long as this causes resistance vessels to become bigger and fatter allowing them to constrict harder, worsening hypertension
What leads to hypertension?
More blood causing high arterial pressure for as long as blood is maintained
How is modification of resistance vessels prevented?
Reduction of circulating volume
