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BIOC34H3 (114)


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Biological Sciences
Stephen Reid

1Lecture 6 Stroke Volume Regulation continued Afterload and Blood Flow Regulation1 Filling Time EDV and SVIn the previous lecture we looked at factors affecting end diastolic volume the amount of blood in the ventricles just before they contract which is an important factor in determining stroke volume We ended up looking at the effects of filling time on end diastolic volume filling time is essentially the amount of time the heart spends in diastole We also discussed how increases in heart rate affect cardiac outputincreasing heart rate leads to a decrease in stroke volume by a certain amount and whilst cardiac output will increase for a time as heart rate increases it will reach a certain maximum before the decreases in stroke volume are sufficiently large and cardiac output begins to fall This limit on stroke volume can be countered by sympathetic stimulation With sympathetic stimulation the heart both beats faster and relaxes faster The increased rate is due to increased calcium mobilization from the ECF and SR the quicker relaxation time is due to the enhanced uptake of calcium by the sarcoplasmic reticulum The heart therefore spends more time in diastole then in systole giving extra time for it to fill with blood All of these effects combine to cause the heart to not only beat faster but also to pump a larger amount of blood whilst doing sothe previously discussed limitation on cardiac output is still there but the volume able to be pumped is far greater than would be allowed without sympathetic stimulation2 Coronary Blood FlowBlood flow to the heart muscle itself ie to the cardiac myocytes occurs primarily during diastole The reason for this is that during systole the hearts contraction compresses some of the coronary arteries and completely occludes themit doesnt do it do all of them but to a large portion Contraction also causes the closure of the opening of the coronary arteries In mammals the heart must be supplied with blood through the coronary arteries Although blood is constantly flowing through the atria and the ventricles there is very little diffusion of oxygen to the cardiac muscle from blood within the lumen of the heart As such the heart needs its own blood vessels to supply gases to it These coronary arteries come off from the aorta and then branch out to the rest of the heartIf we compare the trace of aortic pressure to a trace of coronary blood flow during the cardiac cycle we can see some interesting differences Coronary flow is pulsatile it moves up and down although in general the trace is roughly similar to the aortic pressure trace The main difference is that blood flow reaches its maximum during early diastole whilst it drops significantly during isovolumetric contraction down to almost nothing and remains low during ventricular ejectionIn a patient with coronary obstruction normal blood flow is altered During the systolic period blood flow actually becomes negative it flows slightly backwards out of the arteries and back into the aorta Blood still flows properly in diastole but at a significantly reduced level Both of these effects remain true during exercise as well but are enhanced compared to rest
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