Cardiovascular control

Question 1

High pressure baroreceptors 1

Answer 1

• The medullar is responsible for both CVS and respiratory control
• Baroreceptors (high pressure stretch receptors) are the most important regulator on a moment-moment basis
• They are located in the carotid sinus (CN IX) and aortic arch (CN X)

Question 2

High pressure baroreceptors 2

Answer 2

• They increase their firing rate due to increase in transmural pressure-> stretching of the walls
• Firing rate is highest at systole, and a max firing rate around 200 mmHg
• Response: sympathetic and parasympathetic output to heart, and sympathetic (only) output to nerves in the peripheral vasculature to monitor tone
• Both CO and TPR are affects (MAP = CO x TPR)

Question 3

High pressure baroreceptors 3

Answer 3

• Increase in afferent receptor firing (increased stretch) leads to decreased efferent sympathetics to the heart and peripheral vasculature
• There is increased parasympathetic output to the heart
• Overall CO goes down and TPR goes down, thus MAP goes down
• The opposite happens if stretch falls and afferent baroreceptor firing frequency falls

Question 4

Low pressure stretch receptors 1

Answer 4

• Are volume receptors (monitor changes in blood volume) and are not involved in moment-moment regulation
• They exert their effects on a longer time scale via affecting renal activity to change blood volume (involve neural and endocrine mechanisms)
• They are located at the venoatrial junctions on the R and L sides and within atria, sending afferents to medulla via CN X

Question 5

Low pressure stretch receptors 2

Answer 5

• The atria can release atrial natriuretic peptide (ANP) directly into the blood in response to stretching of the receptors
• ANP exerts it action in the kidneys, where it causes filtration of Na and thus H2O to reduce blood volume (diuretic)
• ANP also has a vasodilator effect on arterioles
• The receptors cause short term and long term responses, with long term being more important

Question 6

Low pressure stretch receptors 3

Answer 6

• Long term: increased stretch increases ANP release and afferent firing, both which influence the kidneys to excrete Na and water, and also regulates water intake via thirst (opposite is true if stretch decreases)
• Short term: increased stretch will increase HR, where as decreased stretch will increase arteriole tone by vasoconstriction

Question 7

Peripheral chemoreceptors 1

Answer 7

• These receptors mainly regulate respiratory function but can also affect CV function
• Located in aortic bodies and carotid bodies (near sinuses)
• They sense O2 tension (pO2) of the arterial blood and send afferents on CN IX (carotid body) and CN X (aortic body)

Question 8

Peripheral chemoreceptors 2

Answer 8

• The receptors will increase afferent firing rate in response to decreased pO2
• The response to increased afferent firing (low pO2) is to increase vasoconstriction (increase TPR), increase HR and CO
• Important note: changes in TPR are always organ specific!

Question 9

CV response to standing after sitting

Answer 9

• When standing after sitting the blood pools in the legs (venous side due to high compliance)
• Since the pooled blood isn’t returned to the heart, venous return goes down and thus EDV, SV and CO all decrease
• These together initially drop the blood pressure, but this is sensed by the high pressure stretch receptors
• In response to increased baroreceptor firing, the HR is increased to increase CO and TPR is increased (both via sympathetics) to raise the blood pressure back to normal

Question 10

Guyton model

Answer 10

• Describes how CO and RAP (or venous pressure) are related
• CO is the independent variable and RAP is the dependent variable
• Must keep in mind that compliance in venous side is much greater than compliance in arterial side

Question 11

When CO equals 0 and 1L/min

Answer 11

• If CO equals 0 there is no blood flow, but there is still venous pressure
• The pressure depends on how much blood is in the venous system, and on the compliance of the veins
• Normal RAP at 0 CO is 7mmHg
• Increasing CO to 1L/min causes blood to be moved from the venous side to the arterial side, thus decreasing the blood on the venous side
• This will lower the Pv and increase Pa (pressure thru capillaries monitored by arteriole resistance)
• The amount of pressure increases/decrease is proportional to the compliance of each side (Ca is 20x that of Cv)
• Thus Pa increases 20x more than Pv decreases

Question 12

When CO equals 5L/min (normal HR)

Answer 12

• As more blood is moved to the arterial side, the Pa increases and Pv decreases b/c it is taken from the venous side
• Plotting CO on the y-axis vs RAP on x-axis gives the guyton curve
• The curve shows that at 0 CO the RAP is 7mmHg, and at 7L/min CO the RAP is 0
• The curve will change if blood volume or venous tone is altered
• To know which is altered, check the x-int (RAP at 0 CO)

Question 13

Guyton curve w/ varying blood volumes

Answer 13

• If the blood volume falls (hemorrhage) of increases (transfusion), the guyton curve will move either up and right (increase in blood volume) or down and left (decrease in blood volume)
• These curves are parallel to the normal curve because the rate of blood flow (the slope of the curves) is not altered since resistance has not been changed
• The x-ints will be different than the normal curve (increased for transfusion and decreased for hemorrhage), which is the best way to tell if the change is blood volume based or vascular tone based

Question 14

Guyton curve w/ varying vascular tones 1

Answer 14

• As vasodilation occurs, there is more blood moved from the arteries to the veins, and thus Pv goes up (curve angles up and to the right)
• Since at 0 CO there is equal amount of blood in the system for all scenarios the x-axis is equal (7 mmHg RAP at 0 CO)
• Vasodilation increases the blood flowing to the veins from the arteries, thus the slope of the curve increases and there is higher RAP for any given CO
• This is shown by a steeper curve plateauing at a higher CO but reaching the same max venous pressure @ 0 CO

Question 15

Guyton curve w/ varying vascular tones 2

Answer 15

• When vasoconstriction occurs, there is less blood moved from arterial side to venous side
• With the lower blood volume in the venous system the RAP will be lower for any given CO
• This means the angle of the curve is less than normal, indicating a lower max CO at 0 RAP
• Thus the curve is angled down and to the left (with the same x-int)

Question 16

Guyton curves and CO curves: myocardial contractility 1

Answer 16

• Superimposing a guyton curve with a CO curve shows the two intersecting at the equilibrium point, which represents the venous pressure that exists for the given CO
• Enhancing myocardial contractility (sympathetic stimulation) leads to a new CO curve that is shifted up and to the left of the original one

Question 17

Guyton curves and CO curves: myocardial contractility 2

Answer 17

• This is because right when the stimulation occurs, the CO increases and RAP remains the same, but the RAP will eventually fall back to the new equilibrium point (where the new CO curve intersects the guyton curve) as the CVS reaches a steady state w/ the elevated CO
• Therefore, increasing myocardial contractility results in a greater CO and a lesser RAP (equilibrium shifts along the guyton curve up and to the left)

Question 18

Guyton curves and CO curves: blood volume changes

Answer 18

• Changing blood volume does not affect the CO curve, but does affect the guyton curve
• A new guyton curve is drawn on the CO curve (one that is shifted up and to the right if blood volume increases, or a shift down and to the left if blood volume decreases)
• For a transfusion, this means that both CO and RAP are increased (equilibrium point moves up and to the right on the CO curve)
• The opposite would be true for hemorrhage

Question 19

Guyton curves and CO curves: acute vs chronic heart failure 1

Answer 19

• Heart failure complicates things, because both the CO curve and guyton curve can change
• We first must consider if the heart failure is acute or chronic
• If it is acute then there is no change in blood volume (guyton curve does not change)
• If it is chronic then blood volume will be elevated (guyton curve shifted up and right)

Question 20

Guyton curves and CO curves: acute vs chronic heart failure 2

Answer 20

• Even though guyton curve may not change, the CO curve will always change for heart failure
• The CO curve will be shifted down and to the right, the degree of the shift depends on the degree of heart failure
• Moderate heart failure has a smaller decrease in the CO curve, where as severe heart failure has a large decrease in the CO curve

Question 21

Guyton curves and CO curves: acute vs chronic heart failure 3

Answer 21

• Therefore, the new equilibrium point depends on acute vs chronic and moderate vs severe
• The new equilibrium point will always be to the right of the normal equilibrium point (at a given CO there is a higher RAP b/c the heart isn’t pumping as well)
• The new equilibrium point will often be down and to the right (if there is severe heart failure, or acute severe heart failure), indicating that even at lower CO there is still higher RAP