HomeHemodynamics Blood flow directly affects the stroke volume. Anything that affects blood flow, like resistance, affects stroke volume. Predict what would happen to stroke volume and blood flow if there was an increase in resistance. -> CO = HR x SV • CO = HR × SV • Resistance affects blood flow → affects SV If resistance increases: • It’s harder for blood to flow forward. • The heart must work harder to push the same amount of blood out. 👉 Prediction: • Stroke Volume ↓ (decreases) • Blood Flow ↓ (decreases) 2 | Recall from Activity 1 we used the equation for flow, F = (PA - Pv)/R, where F = Flow; PA = Arteriole Pressure, Pv = Venous Pressure, and R = Resistance. We can simplify the (PA - Pv) to change in pressure (AP). Therefore, we can solve for AP = F x R (Change in Pressure = Flow x Resistance). We can develop a mathematical equation substituting cardiac output for flow to get AP = HR x SV × R which means heart rate, stroke volume, and resistance all have an impact on the blood pressure in the body. To maintain a normal blood pressure, heart rate, stroke volume, and resistance can be altered. Predict what would happen to blood pressure if resistance was increased due to vasoconstriction of the arterioles. Flow equation: F = (PA - Pv)/R • Or rearranged: ∆P = F × R • CO = HR × SV, so ∆P = (HR × SV) × R 👉 Prediction: • If resistance increases due to vasoconstriction, then blood pressure ↑ (increases). 💡 Because narrowing arterioles make it harder for blood to flow, raising the pressure inside the vessels. 3 Afterload is the amount of resistance the left ventricle must overcome to force open the aortic semilunar valve, ejecting the blood into the ascending aorta. The greater the afterload, the harder it is to open the aortic semilunar valve, and less blood is ejected into the ascending aorta. Stroke volume is the amount of blood ejected into the ascending aorta during left ventricular systole. Stroke volume can be calculated by subtracting the End Systolic Volume (ESV) from the End Diastolic Volume (EDV). SV - EDV - ESV. EDY is the amount of blood in the ventrice when the ventricle is 100% filled. ESV is the amount of blood let in the ventrice after systole (contraction) ends. The greater the afterload, the larger the BSV. Resistance is generated from the body's arterioles pushing blood back towards the heart and is called peripheral vascular resistance. When there is an increase in peripheral vascular resistance, afterload is increased. a | Predict what would happen to the ESV If peripheral vascular resistance was increased. If peripheral vascular resistance increases: • Afterload (the pressure the left ventricle must overcome) increases. • The ventricle can’t eject as much blood. 👉 ESV ↑ (increases) — more blood stays in the ventricle after contraction. b | Predict what would happen to cardiac output if the ESV amount was increased. If ESV increases: • Stroke volume = EDV - ESV → less blood ejected per beat. • Therefore, stroke volume ↓, so cardiac output ↓. 💡 Because the heart is pumping against greater pressure, it empties less efficiently. 4 | Coronary artery disease is a form of atherosclerosis, where plaques form in the coronary arteries of the heart. This causes a decrease in the radius of the arteries due to the accumulation of plaque within the lumen of the arteries. Recall the right and left coronary arteries come off the ascending aorta, just above the aortic semilunar valve. a | Predict what will happen to the resistance in the coronary vessels that contain plaques. Resistance in coronary vessels: • Plaque narrows the lumen → resistance ↑ (increases). b | What will happen to the resistance on the aortic semilunar valve? How will this impact afterload? Explain. Resistance on the aortic semilunar valve & afterload: • The narrowing in coronary arteries doesn’t directly affect the valve, but overall, the heart faces increased resistance to blood flow leaving the aorta, so afterload ↑ (increases). c | How will this impact the ability of the heart to contract (with greater or less force)? Impact on heart contraction: • With increased afterload, the heart must contract with greater force to eject blood. • Over time, this strains the heart and can lead to weaker contractions and heart failure if untreated. 5| Calcium channel blockers like verapamil are used to help the smooth muscle of the arteries relax and is often a treatment for coronary artery disease. Explain how verapamil will affect resistance in the coronary arteries. How will this affect blood pressure in the body as a whole? How will this impact the ability of the heart to contract (with greater or less force)? What verapamil does: • Relaxes smooth muscle in arterial walls → vasodilation (arteries widen). Effects: • Resistance ↓ (decreases) in coronary arteries. • Blood pressure ↓ throughout the body (less vascular resistance). • The heart doesn’t have to work as hard → afterload ↓ → contracts with less force, but more efficiently. 💡 It eases the workload on the heart by widening the arteries and lowering overall pressure