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Lecture 4

BIOC33-34 Lecture 4.docx

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University of Toronto St. George
Zachariah Campbell

Lecture 4 – The Cardiac Cycle, Heart Pressures and Cardiac Output Heart Rate Regulation 1. The Cardiac Cycle: (quick summary of lecture 3) • The cycle is the changes in volume and pressure and the opening and closing of the AV and semilunar valves that occur during a single contraction and relaxation of the heart • During late diastole, the ventricles are filling with blood passively  Blood flows from vena cava, through the right atria and into the right ventricle  Blood also flows from the lungs through the pulmonary veins, into the left atria and into the left ventricle o In the final phase of the diastolic part of the cardiac cycle, the atria contract, pumping more blood into the ventricles • The heart then enters its systolic phases which includes two parts: o First phase, isovolumetric phase, involves the heart contracting but no blood actually leaves the heart since all the valves are closed o Second phase, pressure differentials between the ventricles and the aorta/pulmonary artery cause the valves to open (aortic valve on the left, pulmonary valve on the right)  Blood is then pumped throughout the systemic circulation (on the left) or the pulmonary circulation (on the right) • During early diastole o We then have an isovolumetric relaxation phase, when the muscle tension in the heart begins to dissipate, while the volume stays the same (due to the closing of the valves) o Finally we return to the initial stage (mid to late diastole), the pressure differential between the atria and the ventricles cause the AV valve to open  Blood beings to flow back into the ventricles, setting the scene for the cycle to being again • Looking at the cardiac cycle using a diagram which displays 4 lines of data o These lines are the atrial pressure, ventricular pressure, aortic pressure (pulmonary pressure on the right) and ventricular volume o Key points in this chart: (they mark the opening and closing of the various valves)  When ventricular pressure goes above or below atrial pressure  When ventricular pressure goes above or below aortic pressure (or pulmonary) 2. A Closer Look at Aortic Pressure: • Dicrotic notch – the abrupt increase in aortic pressure that occurs during the isovolumetric relaxation phase • First important point about the aortic pressure trace, is the slow, steady decrease during diastole (when the semilunar valves are closed) o This reduction in aortic pressure occurs because blood is leaving the aorta and entering the systemic circulation. It is not being pumped into the aorta. • Blood can continue flowing, even during the heart’s relaxation phase, because arteries are what we call pressure reservoirs o The larger the artery, the greater the pressure differential between the large artery and the smaller arteries that are found downstream • When the heart is contracting, a very large movement of blood is flowing from the left ventricle to the aorta. Under healthy conditions, the aorta is fairly elastic and so it expands as blood enters it o So when the heart is contracting, blood flows into the systemic circulation but some also remains in the aorta, causing it to bulge outward o As the heart relaxes, the aorta reverts back to its regular size, forcing blood forward again  This allows for blood to constantly flow from the heart into the systemic circulation even when the heart is in diastole • A large artery that acts as a pressure reservoir and helps to maintain a constant flow of blood is called a Windkessel vessel • In the ventricular ejection phase, and in the isovolumetric contraction before it, we see increases in ventricular pressure o However, aortic pressure only begins to increase in the ventricular ejection phase o This rapid rise in aortic pressure is due to blood flowing from the left ventricle into the aorta at a far greater rate than blood is flowing out of it • In the ventricular ejection phase, as blood is pumped throughout the aorta, pressure will rise until it reaches its maximum value, before it beings to fall quite gradually as the phase ends o When ventricular pressure falls below aortic pressure at the start of the isovolumetric relaxation phase, the aortic semilunar valve will close, prompting an interruption in the smooth decrease of aortic pressure o This blip, a little increase in aortic pressure, is called the dicrotic notch  It is essentially a pressure noise that is associated with the closing of the aortic valve o When the valve snaps shut, a little shock wave is created in the blood in the aorta • After the dicrotic notch, aortic pressure begins to decrease in an essentially linear fashion, until the cycle begins again 3. Systolic and Diastolic Pressure; Pulse Pressure; Mean Arterial Pressure: • When we look at blood pressure regulation, we talk about mean arterial pressure o The mean arterial pressure that the body is trying to maintain and regulate is a mixture of maximum and minimum pressures • When we look at the aortic pressure trace we see that aortic pressure is at its minimum level just before the start of the ventricular ejection phase o This minimum value is called the diastolic pressure (lowest pressure value despite being in systole) o The maximum pressure, which occurs halfway through the ventricular ejection is called the systolic pressure  These values, in a typical healthy human are approximately 120 mmHg for systolic pressure and 80 mmHg for diastolic pressure • This is where you get the 120/80 number for a typical value of blood pressure, we use these two numbers when calculating mean arterial pressure • The difference between systolic pressure and diastolic pressure is referred to as pulse pressure o This value can be used in the calculation of mean arterial pressure but it can also be used to diagnose cardiovascular disease such as atherosclerosis (hardening of the arteries) • So if we have a normal blood pressure of 120/80, then we have a normal pulse pressure of 40 o This is determined by how well arteries can expand and contract when filled with blood o Arteries need to be elastic in order to handle differing volumes of blood. However, as they get older, arteries begin to lose their elasticity  This is referred to as hardening (caused by buildup of fatty deposits and plague inside the arteries)  This leads to an increase in blood pressure; blood systolic and diastolic values increase, but especially systolic, since it means more blood is trapped in the aorta waiting to flow into the systemic circulation • An increase in pulse pressure then is an indicator of atherosclerosis or simply the normal hardening of the arteries due to age • Mean arterial pressure (MAP) – the blood pressure value we are most concerned with when looking at cardiovascular regulation o The task of almost all cardiovascular regulators is to prevent blood pressure from falling too low o MAP is not just the arithmetic mean of systolic and diastolic pressure, it is more complicated because aortic pressure is closer to its minimum value for much longer than it is closer to the maximum value • Mean arterial pressure is generally calculated with the formula: (1/3 of systolic pressure) + (2/3 of diastolic pressure) (example on page 4) • Another way to calculate MAP is (1/3 of pulse pressure) + diastolic pressure o Pulse pressure = SP – DP (systolic minus diastolic pressure) o Mean arterial pressure is the driving force for blood flow in the systemic circulation, so these calculations are important • Blood pressure is rarely measured by a catheter placed directly into the aorta o Instead, a cuff is placed upon the upper arm, and pressure is measured in the brachial artery, it gives a good approximation because the upper arm is at the same level as the aorta • Any significant difference in blood pressure measurement from the normal 120/80 can be a sign of hypertension (elevated blood pressure) or hypotension (decreased blood pressure)
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