Mechanical properties of the heart; Starlings law

Question 1

The heart functions in two phases:

Answer 1

1. Systole (contraction)
2. Diastole (relaxation)
   The main role is to set and keep the normal blood pressure in the systemic circulation.

Question 2

What are the elements of contraction?

Answer 2

IN THE MYOCARDIUM:

1. Contractile components (CC):
   - Heart muscle fibers (working fibers
2. Non-contractile components:
   - Serially Attached Elastic elements (SEC)
   - Parallelly attached elastic elements (PEC).
   - Collagen

Question 3

What is the role of SEC and PEC?

Answer 3

Passively supports the filling of the heart

Question 4

What are the Mechanical Activity of the heart depend upon?

Answer 4

the contractile components and the elastic components together:

• Working fibers
• SEC and PEC

Question 5

What are the unique factors about Working Fibers?

Answer 5

The stretching enhances their force generating capability.

Question 6

What are the unique factors about elastic elements?

Answer 6

They passively store energy while stretched, which will be utilized as surplus energy during the next contraction.

• SEC is stretched during Systole
• PEC is stretched during Diastole

Question 7

What is the unique factors of Collagen?

Answer 7

• A Collagen fiber system prevents from overexpansion and rupture. (do not exist in skeletal muscle)
• it occurs at maximal stretch (=maximal filling in the heart)

Question 8

Major characteristics of Cardiac muscle in contrast to skeletal muscles:

Answer 8

Cardiac muscles are:

• Striated organized by sarcomers
• cells shorter than skeletal muscle
• the reddest of muscles
• more mitochondria
• less extensive sarcoplasmic reticulum and transverse tubular systems exist
• intracellular collagen network holds the ell together
• often binucleate and polyploid.
• COLLAGEN!

Question 9

How is the cell division of cardiac muscle?

Answer 9

Cells continue to divide adter actin and myosin synthesized; but cell division stops at or about birth; no myotubes formed.

Question 10

Types of contraction:

Answer 10

Isotonic, Isometric, Auxotonic, Preload and Afterload

Question 11

Phases of the Contraction of the heart:

Answer 11

1st phase - Isometric contraction:

• the weight stretches the SEC elements only.
• The weight has not moved yet.
• Stretch is present, but no shortening.

2nd phase - Isotonic contraction:

• The stretch in the SEC increases and when in balance with the weight the weight will move.
• Shortening occurs and strething force remains unchanged.

Question 12

When does heart musce show maximal tension?

Answer 12

only at increased sarcomere length.

Question 13

Differences between AP and mechanogram of cardiac and skeletam muscle:

Answer 13

• there is no plateau phase of the skeletal AP
• the skeletal AP lasts for millisec in conrast to the 200 msec AP of the heart
• the cardiac mechanogram is almost parallel with the AP
• The mechanogram of the skeletal develops only after the AP has vanished.

Question 14

Properties of a single working fiber in Cardiac muscle:

Answer 14

• The availability of Ca++ depend upon the length of the fiber (sarcomere)
• Heart muscle shows maximal tension only at increased sarcomeric length.
• Working fibers of the heart possess a stretch dependent reserve.

Question 15

Properties of a single working fiber in Skeletal muscle:

Answer 15

• whithin a borad range of arcomeric length, similar and maximal contraction can be found.

Question 16

Optimal sarcomeric length of Skeletal muscle:

Answer 16

All cross bridges are in the right place, and all Ca2+ binding sites are saturated.

Question 17

Optimal sarcomeric length of Cardiac muscle:

Answer 17

All cross bridges are in the right place, but there is not enough Ca2+ yet (so only a few binding sites are saturated)

Question 18

Upper edge of optimal sarcomeric length of Cardiac muscle:

Answer 18

All cross bridges are in the right place, and all Ca2+ binding sites are saturated, due to the increased length.

Question 19

What are the reasons for that the working fiber of the heart posesses a stretch dependent reserve?

Skeletal/Cardiac

Answer 19

1. Until 2um the Ca2+ penetration to the cross bridges increases gradually (resulting in increased contraction);
   around 2.5um enough Ca2+ is available (maximal contraction).
2. Ca2+ availability is irrespective of the length (it is aways enough), but the sensitivity of cardiac muscle fibers towards Ca2+ is greatly increased above 2um.

Question 20

What happens at very short sarcomeric length?

Skeletal/cardiac

Answer 20

both types of muscles perform less, since the optimum actin/myosin constellation is distorted.

Question 21

What happens at very large sarcomeric length?

Skeletal/cardiac

Answer 21

only a few, or no, myosin heads are opposed by active actin binding sites: therefore the performance is small in both types of muscles.

Question 22

What happens between 1.9-2.5 sarcomeric length?

Skeletal/cardiac

Answer 22

in both muscle types an optimal opposition of binding sites and myosin heads occur.
Still, in the heart, the maximal performance required pre-stretch.

Question 23

Properties of the Total working muscle:

Answer 23

• Gap junctions among muscle fibers provide direct communication among fibers.
• The atria and form to distinct units - separated by the anulus fibrosus.
• The heart muscle can adapt to the increasing mechanical load, without intervention of the nervous system = Starling´s “Law of the heart”

Question 24

definition of The Law of the Heart - Starling´s experiment:

Answer 24

• The cardiac muscle automatically adapt to the increased mechanical load, without intervention of the nervous system.
• Increased stretch results in increased contraction irrespecitvely to the innervation of the heart

Question 25

What do EDV mean?

Answer 25

End Diastolic Volume
- At the end of the diastole (maximal relaxation, right before the next heart contraction), the ventricles are maximally filled.

Question 26

What do ESV mean?

Answer 26

End Systolic Volume
- When the ventricles are maximally emptied (end of systole, just before relaxation), there is still some blood remaining in them.

Question 27

What determines the mechanical properties of the heart?

Answer 27

The tension of its fibers, that is by its volume

• EDV
• ESV
• SV

Question 28

What is SV?

Answer 28

Stroke volume
- this volume fraction passes into the aorta at each cycle (plays an important role in the adaptation of the heart o the increased stretch)
= EDV - ESV

Question 29

What is Cardiac Output (CO=?

Answer 29

• The volume of blood pumped into the circulation by the heart in one minute

(EDV - ESV) x Frequency = CO = Sv x Frequency

Question 30

Types of volume fractions:

Answer 30

• EDV
• ESV
• SV
• CO

Question 31

Theory behind the Starling´s heart-lung

Answer 31

• Heart can adapt to the increased load due to mechanical reasons.
• this adaptation is observed in isolated heart as well, therefore it is not related to the neural or hormonal functions.

Question 32

Starling´s heart-lung preparation

Answer 32

• The arterial side of the systemic circulation was replaced by an artificial “peripheral resistance” segment (adjustable rubber tubes)
• The venous side was replaced by a “reservoir”
• Neural connections are separated (= heart is denerved)

Question 33

Starling´s experiments:

Answer 33

Experiment 1: Increasing the venous return

Experiment 2: Increasing the peripheral resistance

Volume fractions where measured: EDV, ESV, FR and parameters where calculated: SV and PTF

Question 34

Experiment 1: increasing the venous return

Answer 34

• Immediate reaction: Increased EDV
• Lafter, aftr a few cycle: ESV increases –> SV increased
• Finally: SV and CO will increase in order to deal with the extra load.

Extra load led to stronger ontraction (heart rate is unchanged due to the denervation of the heart)

Question 35

Experiment 2: increased peripheral resistance

Answer 35

• Immediate raction: residual volume increases (ESV up and SV down)
• Lafter, after a few cycle: ESV and EDV up proportionally = SV down to the same level
• Finally: SV and CO will be set as it was before

heart is denreved.

Question 36

What is the role of Starlings Law?

Answer 36

The heart can increase the diastolic reserve, resulting in increased stretch and consequently increased performance.
Role:
1. Posture changes
2. Heterometric autoregulation

Question 37

What happens during change of posture (Role of Starlings Law)

Answer 37

Laying down from upright:

• More blood gets into the ventricle, leading to dilation, resulting in increased performance (immediately).
• Later on: the nervous system sets the heart rate, so the heart is adapted to the resting conditions.

Question 38

What happens during Heterometric autoregulation (Role of Starlings law)?

Answer 38

• The volume flow through the Right and Left heart should be +- the same.
  Slightly differences exist, called HETEROMETRY.

BUT The increased amount of blood leaving the right compartment will stretch, dilate the other (left) side, therefore the Starling mechanism is activated leading to the automatic compensation between the two parts -> HETEROMETRIC AUTOREGULATION.