Determinants of Cardiac Output and Blood Pressure

Question 1

What is Cardiac Output, how is it measured and what does it indicate?

Answer 1

The volume of blood pumped from each ventricle per minute.

Stroke Volume x Heart Rate = CO
e.g. 70ml/beat x 75beats/min = 5.25L/min

Indicates adequacy of blood flow to tissues.

Question 2

Heart rate is the rhythm set by the () node and modified by?

Answer 2

Set by the SA node.

Modified by:
SNS - Epi
PNS - Ach
Hormones
Body temperature
Emotions

Question 3

The SA node is?

Answer 3

The pacemaker of the heart. Sets a spontaneous regular rhythm.

Question 4

What determines resting heart rate?

Answer 4

The PNS sets resting heart rate. Varies between individuals.

Question 5

Describe the pacemaker potential.

Answer 5

Pacemaker cells have an unstable resting membrane potential (RMP) that slowly drifts to threshold.

At -60mV slow Na+ channels open. Slow depolarization = the pacemaker potential.

At threshold fast Ca+ channels open causing rapid depolarization phase of the action potential.

Question 6

What is stroke volume and how is it measured?

Answer 6

The volume of blood pumped from the ventricles with each beat.

SV = End Diastolic Volume (EDV) - End Systolic Volume (ESV)

Question 7

What is preload and what is it proportional to?

Answer 7

The pressure in the ventricles from the stretch of cardiac muscle cells due to filling of blood.

Preload is proportional to end diastolic volume (EDV).

Question 8

What 2 factors influence preload?

Answer 8

HR:
Increased HR = decreased ventricular filling = decreased end diastolic volume (EDV)
Decreased HR = increased ventricular filling time = increased EDV.

Venous Return (VR):
- influenced by skeleton muscle activity, venous pressure and blood volume

Question 9

What is the Frank Starling Law?

Answer 9

The relationship between preload (and EDV) and stroke volume (SV) within the following:

An increase in venous return = increase preload (EDV) = increased stroke volume = increased cardiac output.

“The more you fill the heart, the better it contracts.”

Question 10

What is afterload and what measurement is a good indicator for afterload?

Answer 10

The pressure needed by the ventricles to overcome the resistance to the ejection of blood during systole.

Diastolic pressure is a good indicator of afterload.

Question 11

What are some causes of increased afterload? What is the correlation between afterload and stroke volume in healthy individuals?

Answer 11

• hypertension
• narrowing of the arteries = atherosclerosis
• stenosis (narrowing) of the aortic or pulmonary valves
• embolism

Afterload in healthy individuals is not a major determinant of stroke volume because it remains relatively consistent.

Question 12

Describe the relationship between afterload, stroke volume and hypertrophy.

Answer 12

Afterload and stroke volume (SV) are inversely related: an increase in afterload = decrease in SV.

Cardiac workload increases to meet the demands of cardiac output. Over a prolonged period of time this can lead to hypertrophy.

Question 13

What is myocardial contractility and what 3 factors is it dependent on.

Answer 13

The force of muscle contraction during systole (independent of preload).

Depends on:
1. Amount of contractile proteins in muscle cells.
2. The amount of intracellular calcium.
3. Availability of ATP.

Question 14

What inotropic agents increase myocardial contractility?

Answer 14

• sympathetic activation
• adrenal catecholamines
• thyroid hormone
• certain drugs (e.g. beta-1 agonists)

Question 15

What is blood pressure?

Answer 15

Measure of the pressure exerted by blood on arterial walls in mmHg.

Includes:
- systemic pressure
- pulmonary pressure

Question 16

What is systemic blood pressure?

Answer 16

Systolic BP = the highest arterial pressure generated by the volume of blood leaving the heart during systole. (Ventricular contraction and arteries stretch).

Diastolic BP = the lowest arterial pressure when blood returns to the heart during diastole. (Ventricular relaxation and arteries recoil).

Question 17

Why are differences in systolic and diastolic BP observed?

Answer 17

In the aorta and other majory arteries.

Question 18

What is a healthy BP reading?

Answer 18

<120/80

Question 19

What is an ‘elevated’ BP?

Answer 19

Systolic 120-129

Question 20

What is stage 1 hypertension?

Answer 20

130-139 OR 80-89

Question 21

What is stage 2 hypertension?

Answer 21

Above or equal to 140 OR above or equal to 90

Question 22

What is a hypertensive crises?

Answer 22

> 180 OR above or >120.

Question 23

What does a systolic pressure indicate?

Answer 23

Risk factors for cardiovascular disease and stroke.

Question 24

What dose the diastolic pressure indicate?

Answer 24

Workload of the heart. The pressure the LV needs to overcome to pump blood out.

Question 25

What effect does age have on BP?

Answer 25

BP increases due to vessel stiffening and narrowing.

Stiffening = decreased arterial compliance = increased BP.

Question 26

What effect does the circadian rhythm have on BP?

Answer 26

Highest in the morning and lowest between 2-5am.

Question 27

What is arterial compliance?

Answer 27

The ability of an artery to expand and accommodate an increase in blood volume with minimum effects on BP.

Question 28

What are determinants of BP?

Answer 28

Blood volume = cardiac output (which is dependent on HR and SV)

Systemic vascular resistance (SVR)

Question 29

What is vascular resistance and what 3 factors determine VR?

Answer 29

The opposition of blood flow due to friction between blood and blood vessel walls.

1. vessel diameter
2. total blood vessel length
3. blood viscosity

Question 30

What is the site of greatest vascular resistance?

Answer 30

Arterioles = contraction and relaxation of the smooth muscles in arterioles regulates blood flow into tissues.

Question 31

What causes turbulance in blood flow?

Answer 31

1. changes in vessel diameter
2. irregular surfaces (e.g atherosclerosis).

Turbulance caused by atherosclerosis is heard as a bruit.

Question 32

How does blood vessel length increase VR?

Answer 32

Increased blood vessel length = increased VR d/t increased surface area for blood to contact.

Question 33

What can cause blood vessel length increase in an adult?

Answer 33

Additional blood vessel formation in adipose tissue during weight gain.

Question 34

What is blood viscosity dependent on?

Answer 34

Ratio of blood cells, plasma proteins and fats relative to plasma volume.

Question 35

What conditions can increase blood viscosity?

Answer 35

polycythemia and dehydration

Question 36

What is systemic vascular resistance (SVR) and where is it greatest?

Answer 36

The total resistance to blood flow by all the systemic vasculature.

Greatest furthest from the heart = the peripheral vasculature.

Question 37

What is venous pressure?

Answer 37

Pressure difference from the venules to the right atrium = 15mm/Hg.

Pressure gradient will drive some blood flow but additional mechanisms are required.

Question 38

What are 4 factors that promote venous return?

Answer 38

1. skeletal muscle pump
2. respiratory pump
3. sympathetic venoconstriction
4. total blood volume

Question 39

Describe the skeletal muscle pump.

Answer 39

Skeletal muscle contraction and compression on deep veins in the legs and arms pushes venous blood toward the heart.

Veins have proximal and distal venous valves (folds of the tunica intima) - during contraction the proximal valve opens and the distal valve closes pushing blood toward the heart.

Question 40

Describe the respiratory pump.

Answer 40

During inhalation the diaphragm flattens and pushes down = increasing pressure in the abdominal cavity = abdominal veins compressed and blood pushed upwards into thoracic veins.