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14 Cards in this Set
- Front
- Back
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Where the blood is in the body
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9% of volume in the pulmonary circulation
7% within the heart chambers 84% of volume in the systemic circulation -64% in veins -13% in arteries -7% in capillaries |
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Why is so much blood in veins?
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The veins are very compliant
-They have a high capacitance -They can hold a lot of volume without developing a lot of pressure -They are distensible Arteries are less compliant -Notice that arterial compliance decreases with age |
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Cross sectional area vs volume
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The capillaries have the greatest cross-sectional area
But the veins contain the greatest volume of blood |
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Blood flow equations
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Pressure gradient = flow x resistance
The greater the pressure gradient, the greater the flow The greater the resistance, the less the flow will be Pressure can be measured Flow can be measured Resistance can only be calculated |
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Total peripheral resistance
Systemic vasculature resistance Pulmonary vascular resistance |
TPR = Total Peripheral Resistance
-Is the resistance of the entire vasculature -Usually taken to be synonymous with the systemic vascular resistance SVR = Systemic Vasculature Resistance -The resistance to flow across the entire systemic circulation PVR = Pulmonary Vascular Resistance -The resistance to flow across the pulmonary circulation |
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Cardiac output and venous return
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“Cardiac Output” – amount of blood pumped by the heart per unit time – given in Liters per minute (L/min)
“Venous Return” – amount of blood returning to the heart from the venous circulation At steady state, Cardiac Output = Venous Return |
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Resistance: things that alter it
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R=(8*V*L)/(pi*r^4)
Greater viscosity means greater resistance to flow -↑Hematocrit = ↑viscosity Longer vessel means greater resistance to flow Greater diameter (or radius) of vessel means much less resistance to flow (4th power!) |
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Cardiac output = venous return
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This diagram actually shows the blood going in and out of the heart twice: first from the heart to the systemic circulation, then it returns to the heart and goes back out again to the pulmonary circulation, then comes back to the heart from the lungs
Anyway, the flow is the same all the way across the graph The pressure goes up twice because the heart is pumping, but what makes the pressure go down? Then the big pressure drop (∆P) across the arterioles must be because the arterioles have a very high resistance to flow -they can also selectively constrict or dilate to alter flow to individual organs (remember deltaP=Q*R) |
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Velocity, flow and area
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V=Q/A
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Increase in velocity
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The greater the velocity, the greater the chance that blood flow will be turbulent
Turbulence causes: -Audible murmur (heart) -Audible bruit (artery) ↑velocity of flow -Caused primarily by stenosis, or anything that narrows the vessel (blood clots, artificial heart valve…) ↓viscosity of blood -Anemia – more likely to hear a murmur if anemic |
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Systole, diastole, mean arterial pressure
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Systole: heart contracts and pumps blood into the aorta
Diastole: heart relaxes Mean Arterial Pressure (MAP) is not halfway between systolic and diastolic pressure That’s because more time is spent in diastole than in systole Pulse pressure is the difference between systolic and diastolic pressures MAP = Diastolic BP + 1⁄3 Pulse Pressure MAP = Cardiac Output × Total Peripheral Resistance Cardiac output = stroke volume x heart rate So: MAP=(SV*HR)*TPR |
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Mean arterial pressure and heart rate
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Faster heart rate = more time spent in systole
-Will increase the mean pressure -Unless the faster heart rate prevents adequate ventricular filling --Then the stroke volume will be less --The two parameters (SV and HR) can work against each other sometimes |
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Aorta - elastic properties
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The aorta is elastic, and expands and contracts (passively) with each heartbeat
This tends to dampen the systolic pressure a little, and also prevents the diastolic pressure from falling very low (in the aorta) -It’s as if the aorta keeps on squeezing during diastole -Pulse pressure (difference between SBP and DBP) is actually lower in the aorta than in other large arteries |
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Abnormal arterial pressure
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If the arteries are stiffer due to atherosclerosis (hardening of the arteries):
-don’t expand as well -don’t absorb the pulse pressure as well -Result is higher systolic pressure but same (maybe lower) cardiac output If the aortic valve is stenotic, then systolic pressure is decreased |
