Determining Heart Rate and Stroke Volume Flashcards
What makes up stroke volume?
End Diastolic volume - End systolic volume
How do you calculate cardiac output?
CO = HR x SV CO = HR x (EDV - ESV)
What is average cardiac output volume?
5L.min-1
What is the SAN
The heart pacemaker which regulates heart rate via autonomic innervation.
What does sympathetic innervation of the SAN do?
Increases HR
Decreases AVN delay
Increase force of contraction atria and ventricles and thus increase SV
What receptors are involved in sympathetic innervation of the heart?
Beta-1 receptors (GPCRs) stimulation increases Ca channel opening via PKA mediated phosphorylation.
What does parasympathetic innervation of the SAN and AVN do?
Decreases HR
increase AVN delay
There is tonic vagal activity at the SAN.
How is the heart rate increased?
Sympathetic innervation is increased and parasympathetic innervation is reduced which increases the heart rate.
How is heart rate controlled?
Changes in the shape of the sino-atrial node (SAN) action potential bring about changes in heart rate (covered in more detail next year).
Why is the heart rate lowered at rest compared to the original heart rate set by the SAN?
Hyperpolarise the cell
Threshold is increased
Decreased electrical gradient so it takes longer to reach threshold.
How do you calculate stroke volume and what are some average measurements for it?
SV = EDV - ESV
Systole (70ml) = End diastole (120ml) - End systole (50ml)
What does calcium ions regulate in the heart?
Regulates the number of actin-myosin cross-bridges formed and therefore the force of contraction.
The more cross-bridges formed, the stronger the force of contraction.
How can force of contraction be increased?
Increasing the calcium sensitivity of the contractile apparatus
Increases calcium concentration in the cell.
Explain the process of excitation-contraction coupling!
During the action potential there is an influx of calcium through L type calcium channels.
This stimulates the Ryanidine receptors on the sarcoplasmic reticulum.
The Ca channels open and calcium is releasd from the SR.
The calcium interacts with the contractile apparatus to cause contraction.
This process elongates the action potential.
What is starlings law of the heart?
Force of ventricular contraction is dependent on the length of ventricular muscle fibres in diastole.
Therefore more blood going into the heart, the more blood comes out of the heart.
What affect does stretching the sarcomere have?
Increased sarcomere length increases the force of contraction due an increase in the number of cross-bridges formed.
This is an intrinsic property of cardiac muscle so no external force is required.
What does starlings law of the heart ensure?
ensures that the right stroke volume = left stroke volume
any factors which affect right ventricular EDV and SV, after a couple of beats will affect left ventricular filling from the pulmonary circulation so left SV will match right SV
What would happen if the RSV > LSV?
congestion of the pulmonary circulation since less blood is being pumped out of the left ventricle per beat.
What would happen if the LSV> RSV?
congestion of systemic circulation since less blood is being pumped out of the right ventricle per beat.
What factors affect venous return to the right ventricle?
Blood volume Skeletal muscle pump Respiratory pump Venous Tone Gravity
What affect does blood volume have on the venous return to the right ventricle?
Increased blood volume might occur during renal failure, will lead to an increase in venous return. (therefore have a higher EDV and SV)
Decrease in blood volume will lead to a decrease in EDV and a decrease in SV.(this can be caused be a haemorrhage or dehydration)
What affect does the skeletal muscle pump have on the venous return to the right ventricle?
Valves ensure unidirectionality of blood through the heart.
Blood is propelled through the valves by the skeletal muscle pump.
When the skeletal muscle contracts it squashes the venous vessels pushing blood forwards towards the heart and prevented backflow via valves.
What affect does the Respiratory muscle pump have on the venous return to the right ventricle?
The action of breathing also helps to return blood to the right heart.
When we breathe in, the diaphragm flattens and presses down on the abdomen causing an increase in pressure in the abdomen.
At the same time the chest wall expands and decreases pressure with the thorax.
This pressure gradient essentially helps to “suck” blood from the abdominal vena cava into the thoracic vena cava and into the right ventricle.
This increases the filling of the heart and therefore EDV and SV.
What affect does the venous tone have on the venous return to the right ventricle?
Venous circulation holds up to 70% of blood volume at any one time.
Increasing the tone will lead to a smaller volume in the veins for a given pressure and thus more blood is moved. (venoconstriction mediated by sympathetic activity)
This is the only mechanism which we have reflex control over.
What affect does the gravity have on the venous return to the right ventricle?
Lying down:uniform distribution of blood across the body so CVP and VR is maintained.
Standing: Redistribution of blood due to gravity. venous pooling in lower extremities reduction in thoracic blood volume. CVP and VR fail EDV and SV fall.
What is Pre Load
Any factor which influences the stretch of cardiac muscle cells before contraction.
Which period of the cardiac cycle shortens when increasing HR?
Diastole
How can a greater stroke volume be achieved with the same volume of filling?
By increasing the amount of blood ejected during systole.
This however will leave less blood in the ventricles after systole and therefore lead to a lower ESV.
What is the mechanism that increases SV at a given EDV by decreasing ESV?
By increasing contractility / ionotropy:
- Increase sympathetic innervation stimulating B1 receptors
- Leads to increased Ca influx during Ap and thus more Ca released from SR
- Increassed cross bridge formation and therefore greater binding of Ca to Troponin
- Increased force of contraction leading to a greater SV
- Thus ESV is decreased.
What is a change in contractility?
a specific change in stroke volume brought about by a change in the number of cross-bridges made between actin and myosin at a give EDV.
What is after load?
The load against which the heart must contract to eject the stroke volume
Eg. high aortic pressure makes it more difficult to eject SV
How can SV be increased?
By starlings law of the heart:
Ca2+ influx/release from SR
length-dependent increase in sensitivity of contractile apparatus to Ca2+
increase cross-bridge formation
increase force of contraction
increase SV
By increasing contractility:
increase Ca2+ influx/release from SR
length-independent increase in sensitivity of contractile apparatus to Ca2+ ( increased TN-C binding affinity).
increased cross-bridge formation
increased force of contraction
Increased SV
What would an increase in EDV cause the SV to do on a pressure-volume loop?
Increases SV by widening the loop to the right
What would an decreasein ESV cause the SV to do on a pressure-volume loop?
Increases SV by widening the loop to the left
What regulates ESV?
Contractility and inotropy.
What regulates cardiac output?
HR – which is determined by the balance between the influence of the parasympathetic nervous system and the sympathetic nervous system on the SAN.
EDV – which is mainly determined by venous return to the heart which determines central venous pressure and ventricular filling (intrinsic influences of the heart).
ESV – which is determined by the contractility/inotropy of the heart which is influenced by extrinsic factors on the heart including sympathetic nerve activity and circulating hormones.
What keeps the heart rate down at rest?
At rest the parasympathetic nervous system is keeping the heart rate down.
What receptors are involved in parasympathetic innervation of the heart?
M receptor stimulation increases K channel opening
How can stroke volume be increase?
Increase End diastolic volume
Decrease End systolic volume
How can increased Calcium in the sarcomere lead to increased SV?
Increased calcium would lead to an increased number of cross bridges forming and therefore a greater force of contraction meaning that the end systolic volume is decreased as more blood is pumped oout of the heart.
How does an increased End diastolic volume increase SV?
More blood filling the heart during diastole stretches the muscle fibres.
Greater stretch in fibres increases sarcomere length and thus number of cross bridges formed leading to a greater force of contraction and more blood being pumped out (increased SV)
