Mechanical properties of the heart; Starlings law Flashcards
The heart functions in two phases:
- Systole (contraction)
- Diastole (relaxation)
The main role is to set and keep the normal blood pressure in the systemic circulation.
What are the elements of contraction?
IN THE MYOCARDIUM:
- Contractile components (CC):
- Heart muscle fibers (working fibers - Non-contractile components:
- Serially Attached Elastic elements (SEC)
- Parallelly attached elastic elements (PEC).
- Collagen
What is the role of SEC and PEC?
Passively supports the filling of the heart
What are the Mechanical Activity of the heart depend upon?
the contractile components and the elastic components together:
- Working fibers
- SEC and PEC
What are the unique factors about Working Fibers?
The stretching enhances their force generating capability.
What are the unique factors about elastic elements?
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
What is the unique factors of Collagen?
- 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)
Major characteristics of Cardiac muscle in contrast to skeletal muscles:
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!
How is the cell division of cardiac muscle?
Cells continue to divide adter actin and myosin synthesized; but cell division stops at or about birth; no myotubes formed.
Types of contraction:
Isotonic, Isometric, Auxotonic, Preload and Afterload
Phases of the Contraction of the heart:
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.
When does heart musce show maximal tension?
only at increased sarcomere length.
Differences between AP and mechanogram of cardiac and skeletam muscle:
- 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.
Properties of a single working fiber in Cardiac muscle:
- 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.
Properties of a single working fiber in Skeletal muscle:
- whithin a borad range of arcomeric length, similar and maximal contraction can be found.
Optimal sarcomeric length of Skeletal muscle:
All cross bridges are in the right place, and all Ca2+ binding sites are saturated.
Optimal sarcomeric length of Cardiac muscle:
All cross bridges are in the right place, but there is not enough Ca2+ yet (so only a few binding sites are saturated)
Upper edge of optimal sarcomeric length of Cardiac muscle:
All cross bridges are in the right place, and all Ca2+ binding sites are saturated, due to the increased length.
What are the reasons for that the working fiber of the heart posesses a stretch dependent reserve?
Skeletal/Cardiac
- Until 2um the Ca2+ penetration to the cross bridges increases gradually (resulting in increased contraction);
around 2.5um enough Ca2+ is available (maximal contraction). - 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.
What happens at very short sarcomeric length?
Skeletal/cardiac
both types of muscles perform less, since the optimum actin/myosin constellation is distorted.
What happens at very large sarcomeric length?
Skeletal/cardiac
only a few, or no, myosin heads are opposed by active actin binding sites: therefore the performance is small in both types of muscles.
What happens between 1.9-2.5 sarcomeric length?
Skeletal/cardiac
in both muscle types an optimal opposition of binding sites and myosin heads occur.
Still, in the heart, the maximal performance required pre-stretch.
Properties of the Total working muscle:
- 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”
definition of The Law of the Heart - Starling´s experiment:
- 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
What do EDV mean?
End Diastolic Volume
- At the end of the diastole (maximal relaxation, right before the next heart contraction), the ventricles are maximally filled.
What do ESV mean?
End Systolic Volume
- When the ventricles are maximally emptied (end of systole, just before relaxation), there is still some blood remaining in them.
What determines the mechanical properties of the heart?
The tension of its fibers, that is by its volume
- EDV
- ESV
- SV
What is SV?
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
What is Cardiac Output (CO=?
- The volume of blood pumped into the circulation by the heart in one minute
(EDV - ESV) x Frequency = CO = Sv x Frequency
Types of volume fractions:
- EDV
- ESV
- SV
- CO
Theory behind the Starling´s heart-lung
- 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.
Starling´s heart-lung preparation
- 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)
Starling´s experiments:
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
Experiment 1: increasing the venous return
- 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)
Experiment 2: increased peripheral resistance
- 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.
What is the role of Starlings Law?
The heart can increase the diastolic reserve, resulting in increased stretch and consequently increased performance.
Role:
1. Posture changes
2. Heterometric autoregulation
What happens during change of posture (Role of Starlings Law)
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.
What happens during Heterometric autoregulation (Role of Starlings law)?
- 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.
