23. Mechanical properties of the heart, starlings’ law

Question 1

Elements of contraction

Answer 1

• CC (contractile elements): Actin and myosin
• SEC (serial elastic component): Attached to CC, relaxes during diastole and is expanded during systole.
• PEC (parallel elastic component): Attached to CC and to the SEC. Stretched by the blood filling heart during diastole: energy is stored in these fibers. The stored energy increases the performance during the next systole.
• Col (collagen fiber system): Overexpansion and rupture of the tissue is prevented by the rich collagen fiber system.
• Isometric phase: At the beginning of the contraction the weight stretches the SEC elements only.
• 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.
• At maximal filling (/stretch) the collagen fibers are expanded and display maximal resistance to prevent rupture.

Question 2

Properties of a single working fiber:

Answer 2

• 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. The difference is the calcium supply. The entry of the calcium into the sarcomeric space is length dependent in the heart muscle.

Question 3

Properties of the total working musculature:

Answer 3

• The heart muscle can adapt itself to higher requirements automatically, without the intervention of the nervous system. This is known as the “law of the heart”, by Starling and Frank.
• Volume fractions: During contraction and relaxation the heart empties a part of its blood content towards the circulatory bed and then takes up blood from the periphery. The emptying is not complete: the remaining fraction is of big importance in the adaption.
  
  • The amount of blood found in the heart by the end of diastole is called end diastolic volume (EDV).
  • The amount of blood remaining in the heart by the end of systole is called end systolic volume (ESV).
  • The difference is called stroke volume (SV), this volume fraction passes into the aorta at each cycle.
• Cardiac output = volume of blood forwarded by the left ventricle into the aorta per unit time.
  
  • CO = SV x FR (frequency)
  • Since SV=EDV-ESV CO = (EDV-ESV) x FR
• Measuring cardiac output:
  
  • Fick’s principle: Cardiac output is equal to the total oxygen consumption divided by the arterio-venous oxygen concentration difference.

Question 4

Starlings experiment:

Answer 4

• First he increased the venous return, then he changed the peripheral resistance. Proved that the heart can adapt to the increased load. By increased venous return, the cardiac output and stroke volume increased. By changing the peripheral resistance, the stroke volume and cardiac output remained unchanged since EDV and ESV increases proportionally).
• Physiological importance of Starlings law: The heart can increase its diastolic reserves so that it can perform better:
  
  • Posture:
    
    • Mediated by the change in the venous return
    • Due to gravitational effects
  • Left-right symmetry:
    
    • Heterometric autoregulation, a continuous phenomenon.

Question 5

The work of the heart

Answer 5

• The total work of the heart is composed of outer work (mechanical work) and inner work (heat production): W_t = W_o + W_i
• W_o =Px∆V+1⁄2 mv²
• The kinetic component amounts for only 4% of the total work.
• One can express the outer work of the heart as the product of the stroke volume times the pressure difference between the aorta and the vena cava (∆P): W_o = SV x ∆P
• Since the heart gains its energy purely from the oxidative processes, the total work of the heart can be determined by measuring the total oxygen consumption of the tissue.
  ​W_t = oxygen consumption x energy equivalent of O₂.
• Knowing W_o and W_t the efficiency (E) of the heart can also be calculated: E = W_o/W_t

Question 6

Rushmer diagram

Answer 6

• Analyses the work of the heart in the pressure-volume coordinate system.

1. Mitral valve close, isovolumetric contraction
2. Aortic valves open, ejection phase
3. Semilunar valves close, isovolumetric relaxation
4. Mitral valves open, filling.

Question 7

The performance of the heart

Answer 7

• Performance=work/time W/t=PxV/t
• Since the heart maintains a close to constant peripheral pressure (constant P), the performance (W/t) is mainly determined by the volume flow.
• Performance is proportional to the cardiac output.