both valves are closed during cardiac cycle during isovolumic contraction.
when are both valves closed during cardiac cycle?
isovolumic contraction
the cardiac cycle involves four major stages of activity: 1) "isovolumic relaxation", 2) inflow, 3) "isovolumic contraction", 4) "ejection". (see wiggers diagram, which presents the stages, label-wise, in 3,4,1,2 order, left-to-right.) moving from the left along the wiggers diagram shows the activities within four stages during a single cardiac cycle. (see the consecutive panels labeled, at bottom-right, "diastole" then "systole").
cardiac cycle
velocity-encoded mri is based on the detection of changes in the phase of proton precession. these changes are proportional to the velocity of the protons' movement through a magnetic field with a known gradient. when using velocity-encoded mri, the result is two sets of images, one for each time point in the cardiac cycle. one is an anatomical image and the other is an image in which the signal intensity in each pixel is directly proportional to the through-plane velocity. the average velocity in a vessel, i.e., the aorta or the pulmonary artery, is quantified by measuring the average signal intensity of the pixels in the cross-section of the vessel then multiplying by a known constant. the flow is calculated by multiplying the mean velocity by the cross-sectional area of the vessel. this flow data can be used in a flow-versus-time graph. the area under the flow-versus-time curve for one cardiac cycle is the stroke volume. the length of the cardiac cycle is known and determines heart rate; "q" can be calculated using equation (). mri is typically used to quantify the flow over one cardiac cycle as the average of several heart beats. it is also possible to quantify the stroke volume in real-time on a beat-for-beat basis.
cardiac output