Week 7- Cardiovascular Mechanics

Question 1

How is the heart imaged?

Answer 1

MRI

Question 2

What chemical is required to allow contraction of the heart?

Answer 2

Calcium

Question 3

What are the dimensions of ventricular muscle cells?

Answer 3

100 microns long

15 microns wide

Question 4

How big are the t tubule openings on ventricular cells?

Answer 4

200 nm

Question 5

How are t tubules spaced on ventricular cells?

Answer 5

Approx 2 microns apart, lie alongside each Z line of every myofibril

Question 6

What is the rough composition of ventricular cells?

Answer 6

45% myofibrils

35% mitochondria- v high content to provide lots of ATP

Question 7

What is the calcium channel in ventricular cells called?

Answer 7

L type calcium channel

Question 8

Where does most calcium bind after entering ventricular cells? What does it cause?

Answer 8

SR release channel, causes a conformational change allowing calcium release into the cytosol

Question 9

What is the process of calcium release in ventricular cells called?

Answer 9

Calcium induced calcium release

Question 10

What happens to calcium after it is used in ventricular cells?

Answer 10

Carried back to SR via ATP
Same amount that entered to trigger calcium release is pumped out via Na/Ca carrier (no ATP is required the gradient of sodium is used)

Question 11

What happens to force generated as muscle is stretched?

Answer 11

Force generated increases

Question 12

How does the length tension relation differ in skeletal vs cardiac muscle?

Answer 12

Much higher passive force generated in cardiac muscle so higher total force

Question 13

Out of cardiac and skeletal muscle which is more resistant to stretch? Why is this?

Answer 13

Cardiac due to properties of ECM and cytoskeleton

Question 14

What are the 2 forms of heart contraction?

Answer 14

Isometric

Isotonic

Question 15

Describe isometric contraction

Answer 15

Muscle fibres dont change length but pressure in both ventricles increases

Question 16

Describe isotonic contraction

Answer 16

Fibres shorten and blood is ejected from ventricles

Question 17

Which comes first out of isotonic and isometric contraction in the cardiac cycle?

Answer 17

Isometric

Question 18

What is the preload?

Answer 18

Weight that stretches muscle before it is stimulated to contract

Question 19

What is the afterload?

Answer 19

Weight only encountered when the muscle has started to contract

Question 20

As weight to lift increases what happens to muscle shortening?

Answer 20

It decreases

Question 21

What happens to shortening as preloading increases?

Answer 21

Increased shortening

Question 22

What determines the preload in the heart?

Answer 22

The amount of blood that returns to heart and fills the heart during diastole before excitation occours

Question 23

What are some measures of preload in the heart?

Answer 23

End diastolic volume
End diastolic pressure
Right atrial presure

Question 24

What is the afterload in the heart?

Answer 24

The load against which the left ventricle ejects blood after opening the aortic valve

Question 25

What happens to isotonic shortening and velocity of shortening as afterload decreases?

Answer 25

Isotonic shortening: decreases

Velocity of shortening: decreases

Question 26

How is afterload measured?

Answer 26

Diastolic blood pressure?

Question 27

Define the Frank Starling relationship

Answer 27

Increased diastolic fibre length increases ventricular contraction

Question 28

What are the 2 factors that cause the FS relationship?

Answer 28

Changes in no of myofilament cross bridges that interact

Changes in the calcium ion sensitivity of the myofilaments

Question 29

Define stroke work

Answer 29

Work done by the heart to eject blood under pressure into the aorta and pulmonary artery

Question 30

What is the equation for stroke work?

Answer 30

Volume of blood ejected during each stroke (SV) multiplied by the pressure at which the blood is ejected (P)

Stroke work = SV x P

Question 31

What is the law of LaPlace?

Answer 31

When the pressure in a cylinder is held constant, the tension in its walls increases as the radius is increased

Question 32

What is the equation for wall tension?

Answer 32

Wall tension= pressure in vessel x radius of vessel

Question 33

How does the LV generate more pressure than the RV according to the law of LaPlace?

Answer 33

Radius of the walls of LV is less than the walls of the RV so the LV can generate higher pressures with similar wall stress

Question 34

Describe how muscle fibers in the heart shorten?

Answer 34

SAN depolarises
Action potential travels down t tubules
Ca2+ enters cells
Ca2+ binds to ryanodine receptors
Ca2+ enters cytoplasm from SR
Ca2+ binds to troponin 
Myosin head binds to actin
Muscle fibres shorten

Question 35

What happens to the Frank Starling curve during exercise?

Answer 35

The graph is steeper as cardiac output increases, it also plateaus at a higher level

Question 36

What happens to the Frank Starling curve during heart failure?

Answer 36

It is less steep as cardiac output falls and plateaus at a lower point

Question 37

What is the equation for wall tension when you incorporate wall thickness?

Answer 37

Wall tension= (pressure in vessel x radius of vessel) / wall thickness

Question 38

What happens to vessels during sustained hypertension according to LaPlace?

Answer 38

Tension increases, vessel walls thicken, lumen decreases in size, total radius of the vessel stays the same