Topic 25 - Mechanical properties of the heart; Starling's law

Question 1

Words to include

Answer 1

• Systole
  
  • Contraction
• Diastole
  
  • Relaxation
• Conctractile components (CC)
  
  • Actin
  • Myosin
• Serial elastic components (SEC)
• Parallel elastic components
• Collagen fiber system
• Isometric phase
• Isotonic phase
• Cardiac muscle
  
  • Striated
  • Sarcomeres
  • Mitochondria (more)
  • Cells (shorter)
  • Sarcoplasmic reticulum
  • T-tubule
  • Binucleated
  • Polyploid
• Isotonic contraction
  
  • Constant tension
• Isometric contraction
  
  • Isovolumetric contraction
• Mixed contraction
  
  • Auxotonic contraction
  • Preload
    
    • Isometric contraction → Equilibrium → Isotonic contraction
  • Afterload
    
    • Isotonic contraction → blocking of contraction
• Single working fiber
  
  • Sarcomere
    
    • Optimal sarcomeric length
      
      • 1.9-2.5 micrometer
  • Stretch dependent reserve
  • Cross bridges
  • Ca²⁺
  • Maximal tension
  • Normal working range
• Total working musculature
  
  • Gap junction
    
    • Direct communication
  • Anulus fibrosus
  • Mechanical load
    
    • “Law of the heart” - Starling & Frank
• Volume fractions
  
  • End diastolic volume (EDV)
    
    • Maximally filled ventricles
  • End systolic volume (ESV)
    
    • Maximally emptied ventricles
  • Stroke volume (SV)
    
    • Aorta
    • Cycle
    • EDV ÷ ESV = SV
  • Cardiac output (CO)
    
    • Volume of blood
    • Circulation
    • CO = (EDV - ESV) x frequency = CO = SV x Frequency
    • Fick’s principle
    • Stewart’s principle
• Starling experiment
  
  • Experiment 1
    
    • Venous return ↑
    • EDV ↑
    • ESV ↑
    • SV ↑
    • Extra load
  • Experiment 2
    
    • Peripheral resistance ↑
    • Residual volume ↑
    • ESV ↑
    • SV ↓
    • SV ↑
    • Denreved
• Mechanical load ↑
• Nervous system (ø)
• Diastolic reserves ↑
• Posture changes
• Heterometeric autoregulation
  
  • Left-right symmetry
  • Automatic compensation

Question 2

Topics to include in the essay

Answer 2

1. Phases of the heart
2. Elements of contraction
3. Characteristics of caridac muscle
   
   • Types of contraction
4. Properties of a single working fiber
5. Properties of the total working musclature
   
   • Volume fractions
6. Starlings experiment
   
   • Starling’s law
   • Experiment 1
   • Experiment 2
   • Physological importance of Starling’s law

Question 3

Mechanical properties of the heart

Phases of the heart

Answer 3

• The heart functions in two phases:
  
  1. Systole (contraction)
  2. Diastole (relaxation)

Question 4

Mechanical properties of the heart

Elements of contraction

Answer 4

1. Contractile components (CC)
   
   • Actin
   • Myosin
2. Serial elastic component (SEC)
   
   • Relaxes during diastole
   • Expanded during systole
3. Parallel elastic component
   
   • Stretched by the blood filling heart during diastole: energy is stored in these fibers
   • The stored energy increases the performance during the next systole
4. Collagen fiber system
   
   • Overexpansion and rupture of the tissue is prevented
   • At maximal filling (/stretch) the collagen fibers are expanded and display maximal resistance to prevent rupture
5. Isometric phase
   
   • At the beginning of the contraction the weight stretches the SEC elements only
6. Isotonic phase
   
   • When the stretch in the SEC gets into balance with the weight, the weight begins to move
   • Shortening occurs and the stretching force remains equal to the weight of the object lifted

Question 5

Mechanical properties of the heart

Characteristics of cardiac muscle

Answer 5

• Contrasts with skeletal muscle:
  
  • Striated
  • Sarcomeres
  • Cells shorter
  • More mitochondria
  • Less extensive sarcoplasmic reticulum and transverse tubular system
  • Binucleated
  • Polyploid
  • Cells continue to divide after actin and myosin synthesized
    
    • But cell division stops after birth
  • No attachment to bone or tendons
• Types of contraction:
  
  1. Isotonic
     
     • ​​Constant tension
  2. Isometric
     
     • ​​In heart: isovolumetric contraction
     • Tension increases without any change in length
  3. Auxotonic
     
     • ​Tension and length increases
  4. Preload
     
     • ​​Starts with isometric contraction until equilibrium is reached with load, then isotonic contraction
  5. Afterload
     
     • ​​​​Begins with isotonic contraction, then blocking of contraction with a load

Question 6

Mechanical properties of the heart

Properties of a single working fiber

Answer 6

• Low performance at short sarcomeric lengths which then increases when the sarcomeric length is increased
• Morphologically both the heart and skeletal muscle get into optimal position between 1.9-2.5 micrometer sarcomeric lengths
• Compared to skeletal muscle:
  
  • The availability of Ca²⁺ is dependent upon the length of the fiber (sacromere)
  • Heart muscle shows maximal tension only at increased sarcomeric length
  • Working fibers of the heart possess a stretch dependent reserve

Question 7

Mechanical properties of the heart

Properties of the total working musculature

Answer 7

• Gap junctions among muscle fibers provide direct communication among fibers
• The atria and form two distinct unit
  
  • Separated by the anulus fibrosus
• The heart muscle can adapt to the increased mechanical load, without the intervention of the nervous system
  
  • This is known as the “law of the heart”, by Starling and Frank

Question 8

Mechanical properties of the heart

Properties of the total working musculature, volume fractions

Answer 8

1. End diastolic volume
   
   • The amount of blood found in the heart by the end of diastole
   • The ventricles are maximally filled
2. End systolic volume
   
   • The amount of blood remaining in the heart by the end of systole
   • When the ventricles are maximally emptied
3. Stroke volume
   
   1. The volume fraction passes into the aorta at each cycle
   2. EDV - ESV = stroke volume
4. Cardiac output
   
   • The volume of blood pumped into the circulation by the heart in one minute
   • CO = (EDV - ESV) x frequency
     
     • ​CO: cardiac output
     • (EDV - ESV): stroke volume
     • Frequency: heart beat, or beats per minute
   • Measuring CO:
     
     • Fick’s principle: CO equals total oxygen consumption divided by the arterio-venous oxygen concentration difference
     • Stewart’s principle​

Question 9

Starling’s law

Give Starling’s law

Answer 9

The heart muscle can adapt to the increasing mechanical load, without the intervention of the nervous system

Question 10

Starling’s law

Experiments

Answer 10

• Experiment 1:
  
  • Venous return ↑
  • Immediate reaction: EDV ↑
  • Later: ESV ↑ → SV ↑
  • End: SV (and CO) ↑ to deal with extra load
    
    • ​Extra load = stronger contraction
• Experiment 2:
  
  • Peripheral resistance ↑
  • Immediate reaction: Residual volume ↑
    
    • ​ESV ↑
    • SV ↑
  • Later: ESV and EDV ↑ = SV ↑ to the same level
  • End: SV (and CO) will be set as it was before
    
    • Heart is denerved

Question 11

Starling’s law

Physological importance

Answer 11

• The heart can increase its diastolic reserves so that it can perform better:
  
  1. Posture changes:
     
     • Mediated by the change in the venous return
     • Due to gravitational effects
  2. Heterometric autoregulation:
     
     • Left-right symmetry
     • Automatic compensation
     • Continuous phenomenon