6.3: Cardiovascular Mechanics

Question 1

Single ventricular cell structure

Answer 1

100 µm long and 15 µm wide
T-tubules on cell surface

Question 2

Structure of T-tubule openings

Answer 2

Openings up to 200nm diameter
Spaced so that T-tubule lies alongside each Z-line of every myofinril
Carry surface depolarisation deep into the cell

Question 3

Ventricular cell structure composition

Answer 3

Myofibrils 46%
Mitochondria 36%
Sarcoplasmic reticulum 4%
Nucleus 2%

Question 4

Excitation-contraction coupling in the heart

Answer 4

1) SAN depolarises
2) Action potential travels down t-tubules
3)Ca2+ enters cell and binds to ryanodine receptor
4) Ca2+ enters cytoplasm from cytoplasmic reticulum and brings to troponin C
5) myosin heads bind to actin
6) Muscle fibres shorten

Question 5

The relationship between force production and intercellular Ca2+

Answer 5

Sigmoidal curve, more Ca2+ greater attraction

Question 6

As muscle length increases passive force

Answer 6

Increases continuously

Question 7

What 4 things affect preload

Answer 7

Increased adrenaline secretion during fight or flight response
Decreased ventricular compliance
Decreased central venous pressure
Right atrial pressure

Question 8

Two forms of heart contraction

Answer 8

Isometric and isotonic

Question 9

In isometric contraction

Answer 9

Muscle fibres do not change length, but exert force so pressures increase in both ventricles

Question 10

In isotonic contraction

Answer 10

Shortening of fibres and blood is ejected from ventricles

Question 11

In vivo correlates of preload

Answer 11

Blood fills heart during diastole, stretching resting ventricular walls
Stretch determines the preload on the ventricles before ejection
Preload is dependent on venous return
Measures of preload include end-diastolic volume, end diastolic pressure and right atrial pressure

Question 12

In vivo correlates of afterload

Answer 12

Afterload is the load against which the left ventricle ejects blood after opening of aortic valve
Any increase in afterload decreases the amount of isotonic shortening
Measures of afterload include diastolic blood pressure

Question 13

Definition of the Frank-Starling relationship [F-S]

Answer 13

Increased diastolic fibre length increases ventricular contraction

Question 14

Consequence of the Frank-Starling relationship [F-S]

Answer 14

Ventricles pump greater stroke volume so that, at equilibrium, cardiac output exactly balances the augmented venous return

Question 15

Two factors the F-S relationship is thought to be caused by

Answer 15

Changes in the number of myofilament cross bridges that interact
Changes in the Ca2+ sensitivity of myofilaments

Question 16

How do changes in the number of myofilament cross bridges that interact affect the F-S relationship

Answer 16

At shorter lengths than optimal the actin filaments overlap on themselves so reducing the number of myosin cross bridges that can be made

Question 17

How do changes in Ca2+ sensitivity of myofilaments affect the F-S relationship

Answer 17

Ca2+ required for myofilament activation
Troponin C (TnC) is think filament protein that binds Ca2+
TnC regulates formation of cross bridges between actin and myosin
At longer sarcomere lengths, the affinity of TnC for Ca2+ is increased due to conformational change in protein
Less Ca2+ required for same amount of force

Question 18

Definition of stroke work

Answer 18

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

Question 19

Equation for stroke work

Answer 19

SW= SV x P

SV- volume of blood ejected during each stroke
P- pressure at which blood is ejected

Question 20

Law of LaPlace

Answer 20

When the pressure within a cylinder is held constant, the tension in its walls increases with increasing radius

Question 21

Wall tension equation

Answer 21

Wall tension = pressure in vessel x Radius of vessel
T= P x R

Question 22

Law of LaPlace in humans

Answer 22

Radius of curvature of walls of LV less than that of RV allowing LV to generate higher pressures with similar wall stress

Question 23

How is the law of LaPlace affected during heart failure

Answer 23

Heart becomes dilated, increasing wall stress

Question 24

What is the communication interface

Answer 24

Sarcoplasmic reticulum and ryanodine

Question 25

What is a huge intracellular calcium store in muscles

Answer 25

Sarcoplasmic reticulum

Question 26

What is heart contraction like

Answer 26

Concentric in the heart

Question 27

Preload is

Answer 27

The initial stretching of the heart muscle during diastolic filling

Question 28

Afterload is

Answer 28

The pressure against which the heart must eject to allow systolic ejection e.g ciastoliny blond pressure

Question 29

What is the law of LaPlace applicable to

Answer 29

Ventricles and blood vessels

Question 30

What does the law of LaPlace describe doesn’t happen

Answer 30

Heart doesn’t rip given high internal pressures it creates