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Lecture 6

BIOC33-34 Lecture 6.docx

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University of Toronto St. George
Zachariah Campbell

Lecture 6 – Stroke Volume Regulation (continued) and Heart Failure Factors Affecting End Diastolic Volume: Starling’s Law/Effect and Starling Curves: • Starling’s Law can be illustrated by what we call Starling curves, which are graphs of stroke volume against end diastolic volume o This generates a long, gentle arch upwards, showing how EDV increases, so does SV o The greater the amount of blood entering the heart during diastole, the greater the amount of blood that must leave the heart during systole  This Law prevents an increase in heart volume with increased venous return • The Starling effect is mediated by the position of the thick (myosin) and thin (actin) filaments in the contractile cells of the cardiac muscle o Muscle contraction is caused by the formation of cross-bridges between the thick and thin filaments, causing them to slide over each other • Cardiac muscle is slightly different from skeletal muscle because the resting position of the thick and thin filaments is not optimal for cross-bridges formation o Cardiac muscle fibers are shorter than optimal length, the sarcomeres overlap so the full potential for the thick and thin filaments to slide over one another (producing contraction) is not realized • Stretching of the cardiac muscle, due to an increase in end diastolic volume, increases the potential for cross-bridge formation o Stretched cardiac muscle can create a greater degree of contractile force than non- stretched muscle, because there is a greater range of motion for the thick and thin filaments to slide over each other o However, if the muscle stretches too much then it can’t contract at all because the thick and thin filaments would have no degree over overlap and therefore couldn’t form cross bridges  However, given that the muscle fibers are below the optimal length for the generation of contractile force, a little stretching actually improves cardiac muscle strength  This is why we have the Starling effect, as the end diastolic volume increases, the muscles stretch, contraction increases and SV increases  Keep in mind that over the years however, having cardiac muscle too stretched can be disadvantageous, as this cause the muscle to gradually lose the ability to contract properly • We can also look at the Starling curves under different conditions, such as with increased or decreased sympathetic activity o With increased or decreased sympathetic stimulation, the curves remain roughly similar in shape, although they are either raised (with increased sympathetic activity) or lowered (with decreased activity) o Stroke volume, then, can be regulated by altering end diastolic volume or by altering the amount of sympathetic activity Summary of Stroke Volume Regulation: • Look at page 10 for picture summary 2. Heart Failure: • Heart failure is any reduction in the heart’s function that prevents it from maintaining adequate cardiac output o If there is no longer sufficient blood flowing to the organs, then what may have previously been a reduction in output becomes a serious health issue o There can be many reasons for heart failure • The most common causes of heart failure is myocardial infarction, or heart attack o This occurs when cardiac myocytes die due to a blockage of coronary arteries o These blockages can be caused by fatty deposits or plaque, and generally begin to show effects as ischemia or reduced blood flow o This can lead to the death of cardiac muscle tissue due to lack of oxygen, and this necrotic tissue inhibits heart function, causing a heart attack • Long term high blood pressure can also cause heart failure as can stenosis (failure of the semilunar valves to open properly) o High blood pressure increases cardiac workload, causing cardiac hypertrophy, a loss of ventricular compliance, a reduction in end-diastolic volume and a decrease in both str
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