July 27, 2009
BY 409L - LW
Lab Partners: Jennifer Rastegar and John Riopka
This experiment was designed to observe a baseline heart rate and blood pressure and to examine various modifications of the two. This experiment utilized both the BiopacPro and PhysioEx computer programs. The PhysioEx program was used to simulate a heart rate and modifications to it. The BiopacPro program was used to monitor a volunteer’s blood pressure; while PhysioEx was again used to monitor the modifications to it. Heart rate was shown to decrease as temperature decreases, increase in the presence of epinephrine, and fluctuate with addition of various ions. It was also found that increased pressure and blood vessel diameter result in increased blood flow.
In humans, the pumping of blood is essential to life. A consistent blood flow is achieved by the cardiac cycle. This cycle is controlled by the autorhythmic tissues of the heart (Wibbels & Vickery 2007). The cardiac cycle consists of two phases: the systole, or contraction phase, and the diastole, or relaxation phase. The wave of depolarization across the heart’s tissues is regulated by these autorhythmic tissues; this is what produces a consistent blood flow (Wibbels & Vickery 2007). Along with these autorhythmic tissues, the human heart also contains contractile tissues. Following a systole, these two tissues undergo a refractory period known as the diastole in which they cannot be excited into another systole. This allows the heart’s ventricles and atria to refill with blood and reset for the next systole (Wibbels & Vickery 2007). The QRS complex on the electrocardiogram denotes each systole.
The neurons of the heart are not directly controlled by the human body’s central or peripheral nervous systems. Even without stimulation of the nerves, the heart will continue to beat at a steady rate (Wibbels & Vickery 2007). That is to say that the waves of depolarization across the heart are held as close to constant as possible. The consistency that is needed is carefully maintained by some specific autorhythmic tissues such as: “the SA [sinoatrial] node, the AV node, the bundle of His, and the Purkinje fibers” (Wibbels & Vickery 2007). The heart’s rate is maintained by the SA node which has the highest rate of depolarization at 70 to 80 depolarizations per minute (Wibbels & Vickery 2007). However, the SA node is not solely responsible for the heart rate. In the absence of a functioning SA node, other autorhythmic tissues may compensate and continue contractions of the heart. When the Purkinje fibers take over for the SA and/or AV node(s) and contract the ventricles, this is called “ventricular escape” (Wibbels & Vickery 2007). Ventricular escape excites the sympathetic nervous system and allows for the contraction of the ventricles in the absence of atrial stimulation (Wibbels & Vickery 2007).
The human heart forces blood through a “closed circuit of blood vessels” (Wibbels & Vickery 2007). The blood is also forced one way in and one way out. This calls for a very organized system of events to prevent backflow of blood and to provide the most ideal environment for our bodies’ cells. Wibbels and Vickery give a great explanation of the flow of blood through the human heart:
“During relaxation of the ventricles (ventricular diastole) the atrioventricular valves open and the semilunar valves close, allowing the ventricles to fill with blood. During contraction of the ventricles (ventricular systole) the atrioventricular valves close and the semilunar valves open, allowing the ventricles to eject blood into the arteries. (Wibbels & Vickery 2007)” The expulsion of blood from the ventricles is not a constant event. The delay between the contractions results in an increase in blood pressure during systole and a decrease during diastole resulting in a pulse. When one...