The heart: starlings law & the control of cardiac output Flashcards
isolated heart-lung preparation to measure the relationship between muscle stretch and force
The larger the diastolic volume of the heart the greater the energy of the contraction
-> energy of contraction is proportional to the resting muscle fiber length
-> direct relationship between diastolic and systolic pressure
end diastolic and filling pressure (end-systolic pressure)
mimic the resistance of arterioles with starling resistor
- measured CO by collecting the blood pumped around the circuit
- varying height of venous reservoir -> the greater the pressure (=strength) the greater the CO
relationship between end-diastolic volume & intraventricular pressure from isolated heart preparation
- Extend of preload will affect the force of contraction and stroke volume during systole
- Preload: volume of blood filling the heart during diastole generates tension in the muscle fibres
- Systolic pressure rised until diastolic volume reaches 30ml
Cardiac output
controlled by: SV and HR -> CO= SVxHR
- CO varies from 4 to 7 L per min depending on body size and fitness
- When sleeping it falls to its lowest value and is raised in pregnancy
- Athletes can increase their CO by up to six fold by increasing both the HR and to a limited extent the SV
ventricular function curve (Starling curve)
look at it- describe what it looks like
- Ascending part of curve, the heart compensates for the increase in venous pressure (stroke volume proportional to filling pressure or fibre length)
- Once the curve reaches a plateau, decompensation occurs and the heart is less efficient at following the Frank-Starling law
length-tension relationship
Energy of contraction is proportional to resting fiber length -> refelcts degree of overlap between myosin and actin fibers
a. stretched muscle with poor overlap
b. maximum overlap of filaments =maximum force
c. stretched muscle too short to form overlap
Frank Starlings law of the heart
what goes in must go out
- balance output from both sides = 2 sides of the heart stay in balance
-> Greater filling = greater contraction:
end-diastolic volume (volume in ventricle at end of relaxation) =stronger contraction = larger stroke volume
Balancing venous pressure & arterial pressure
- Preload of the ventricles is provided by central venous pressure (CVP)
- Afterload of the ventricles is provided by arterial blood pressure (BP)
- Input into the right and output from the left side of the heart must stay exactly in balance or blood would be transferred into the pulmonary circuit which has limited capacity
What would happen if output in the left side of the heart was increased by 1% compared to the right ?
More blood will go through systemic circulation & pulmonary circulation would be completely emptied
What might cause CO to change ?
Exercise but the heart can adapt to it
Starling curve is a family of curves
In Starling’s heart-lung preparation, the nervous control of heart contraction is not present
- In the intact system there are a family of Starling curves under the influence of sympathetic nerve activity
-> for any given length the cardiac output depends on which curve the heart is operating on
- The cardiac output can be adjusted depending on need (e.g. exercise)
Neural control of the heart rate by sympathetic and parasympathetic activity
The heart rate is a balance between:
1. the influence of the sympathetic and parasympathetic nervous system on the heart rate
2. the natural rate of depolarisation of the SA node
Noradrenaline (NA) released by sympathetic nerves increases heart rate
Acetylcholine (ACh) released by parasympathetic nerves (vagus) slows heart rate
Sympathetic nerve activity heart rate influences
NA released by sympatehtic nerves increases heart contraction
Adrenaline released from adrenal medulla and reaches the heart via the blood stream can also increase the HR
->increased sympathetic activity will increase heart rate/muscle contraction (shift curve up and left)
-> Decreased sympathetic activity will decrease heart rate/muscle contraction (shift curve down and right)
