4 Cardiac Output

Question 1

In what 3 ways can cardiac output be modulated?

Answer 1

HR
EDV
ESV

Question 2

What is stroke volume equal to?

Answer 2

SV = EDV - ESV

Question 3

What is the ESV usually?

Why?

Answer 3

50ml

this is maintained by Starling’s Law

Question 4

How can contraction force be increased on a cellular level?

Answer 4

increasing the Ca2+ sensitivity of the contractile apparatus

increasing Ca2+ concentration in the cell

Question 5

What is the principle behind Starling’s Law of the Heart?

Answer 5

‘Force of ventricular contraction is dependent on the length of ventricular muscle fibres in diastole’

so, when the ventricle fills with more blood in diastole, its’ contraction force in systole will be greater, forcing more blood out, maintaining that ESV of 50ml

Question 6

How can we measure sarcomere length?

Answer 6

we can’t, so we just use EDV as a proxy measurement instead

Question 7

How does stroke volume change with EDV (ie stretching) ?

Answer 7

proportionally(ish)

an increase in EDV of 30ml will increase stroke volume by 30ml

Question 8

Can the sarcomere keep stretching?

Answer 8

no, and at this theoretical point, stroke volume will not be increased by EDV

Question 9

What does Starling’s Law of the heart ensure?

Answer 9

R SV = L SV

Question 10

How can right side venous return affect left side SV?

Answer 10

as right venous return increases, right ventricular EDV increases, increasing R SV

after a couple of pumps, this will have gone through pulmonary circulation, and there will hence be more blood and a higher pressure on the Left side of the heart

Question 11

Why is it important that R SV = L SV?

Answer 11

prevent blood pooling in either the systemic or pulmonary circulations

Question 12

Name 5 main factors affecting venous return to the right ventricle

Answer 12

Blood Volume
skeletal muscle pump
respiratory pump
venous tone
gravity

Question 13

how does blood volume affect venous return to the right ventricle?

Answer 13

increased BV leads to increased VR (renal failure)

decreased BV leads to decreased VR (haemorrhage / dehydration)

Question 14

How does a skeletal muscle pump increase venous return to the right ventricle?

Answer 14

the presence of semilunar valves means that skeletal muscle contraction forces blood proximally, not distally,

this only requires a small bit of skeletal muscle

Question 15

How does the respiratory pump affect venous return to the right ventricle?

Answer 15

inspiration increases abdominal pressure and decreases thoracic pressure (diaphragm moves down)

this pushes blood from the abdominal vena cava to the thoracic vena cava

Question 16

How does venous tone affect venous return to the right ventricle?

Answer 16

mediated by sympathetic activity, veins can mobilise some blood stored in venous circulation by contracting to move it along (semilunar valves important here)

Question 17

How does gravity affect venous return to the heart?

Answer 17

when you stand up, blood accumulates in the lower body as less is being returned to the heart, this causes thoracic venous volume to decrease, decreasing central venous pressure

Question 18

Name 2 minor factors contributing to Pre-Load

Answer 18

Atrial contraction

HR (>180bpm)

Question 19

How does atrial contraction increase venous return to the venricle

Answer 19

innervated sympathetically, increasing total atrial force, and increasing EDV

Question 20

Why is atrial contraction important?

Answer 20

important for maintaining EDV at high heart rates when the passive filling phase of diastole is shortened

Question 21

What happens when HR >180

Answer 21

passive filling and atrial contraction are no longer enough to maintain EDV, and stroke volume won’t be able to be maintained

Question 22

How can ESV be regulated?

Answer 22

increased contractility / inotropy

After-load

Question 23

How does increased inotropy work?

Answer 23

sympathetic nerve / ciruclating NAd stimulated beta-1 receptors, increases Ca influx —stronger contraction, ESV decreases, stroke volume increases

Question 24

How does after-load affect ESV?

Answer 24

peripheral resistance will increase aortic pressure, increasing ESV as the ventricle has to work harder to pump a given amount of blood into the aorta

Question 25

Why do patients with hypertension get left ventricular hypertrophy?

Answer 25

the left ventricle has to overcome a greater aortic pressure created by increased peripheral pressure