Blood Volume

Sheet Subject physiology Done by Corrected by Doctor Blood volume: Blood is distributed in our bodies inside the arteries, heart, pulmonary vessels, capillaries and veins. Veins carry the biggest amount of blood. Blood volume is known to be the main effector on the mean systemic filling pressure. If you increase the blood volume the mean systemic filling pressure will increase and if you decrease the blood volume the mean systemic filling pressure will decrease. Since most of the blood is carried in veins, venoconstriction and venodilation are the main factors that affect the mean systemic filling pressure. Arteries on the other hand have no role in affecting the mean systemic filling pressure if they are constricted or dilated because they carry only a little amount of blood. So, the mean systemic filling pressure is affected by the change in the tone of veins. If veins constrict the mean systemic filling pressure will increase and if they dilate the mean systemic filling pressure will decrease. 1|Page 12+ 13 Pressure changes through the circulation: The pressure in the aorta and arteries is pulsatile due to the pulsatile nature of the cardiac output. The pressure in the aorta and arteries is almost constant due to the high resistance in them. If the vessels have high resistance, their blood amount will stay almost the same, so their pressure will not change. So, because of the high resistance, you will not find a pressure gradient between aorta and arteries (not that much ΔP). In the arterioles, the pressure starts to drop. The reason is that their resistance is very high which decreases the amount of blood in them which decreases the pressure in them. Veins have the lowest pressure. Arterioles can be called resistance vessels and veins can be called capacitance vessels. Changes in cross sectional area and velocity Velocity differs from pressure in that the pressure was just falling throughout the circulation. Here, velocity falls then it starts to go back up. 2|Page Velocity is inversely proportional to the cross-sectional area (the blue line in the graph). The capillaries have the lowest velocity because they have the largest crosssectional area. Flow =A (area) * V (velocity) So, velocity is inversely proportional to the area. Another way to calculate the blood flow is using Ohm’s law: F =ΔP (mean arterial pressure-right atrial pressure (which is 0))/total peripheral resistance. You can calculate the mean arterial pressure by multiplying the cardiac output to the total peripheral resistance. Cardiac output can be changed by changing the stroke volume or the heart rate or both. The total peripheral resistance can be changed by vasodilation and vasoconstriction. The heart generates a pressure gradient so that blood moves from the arterial side to the venous side in the system. Laminar flow vs turbulent flow All the laws that we have studied so far are applied to the laminar flow of blood not the turbulent flow. Turbulent flow is when the blood doesn’t have a direction and goes everywhere. It rarely happens in the body. In the laminar flow, blood adjacent to the vessel wall is slower due to the friction with the vessel wall. Blood in the middle is the fastest. Turbulent flow gives us sounds (murmur or bruit) and laminar flow is silent. As you can see that blood doesn’t move at the same velocity. 3|Page Reynold’s number: The number is used to tell if the blood flowing in a vessel is laminar or turbulent. Reynold’s number= V (velocity) D (diameter) ρ (density) η (viscosity) -ρ is read as “raw” and η is read as “eta” If Reynold’s number was <400 then the flow is laminar. If the number was more than >1000 then the flow is turbulent. So, what if the number is between 400 and 1000? It’s differs depending on the situation. From the equation, you can increase Reynold’s number by increasing the velocity, diameter, density or by decreasing viscosity. This graph shows the critical velocity, before it, the flow is laminar and after it, the flow is turbulent. Turbulent flow is not effective, the effective flow is the laminar. 4|Page Causes of the turbulent flow: 1-high velocity. 2- Sharp turns (they can cause normal turbulence (harmless)). Example: there is a sharp turn between the subclavian and the axillary arteries. This sharp turn is a constriction, and constriction is a cause of turbulent flow. 3- Rough surfaces as in atherosclerosis. 4- Rapid narrowing of blood vessels as in atherosclerosis. Turbulent flow causes murmur (abnormal sounds). You listen to the veins. If there is constriction (which causes turbulence) you will hear abnormal sounds. These sounds caused by the rush of blood around a constriction are called Bruit. So, Bruit are under the category of murmur. Murmurs or bruits are important in diagnosing vessels stenosis, vessel shunts and cardiac valvular lesions. Clinical significance of turbulence? Normally: at the branching of vessels and at roots of aorta and pulmonary arteries. Pathologically: 1. Constriction of arteries by atherosclerotic plaque. 2. In severe anemia. 3. Stenotic and incompetent cardiac valves. -In severe anemia, the number of blood cells is very low which decreases the viscosity. The peripheral resistance More than 50% of the vascular resistance lies in the arterioles. That is why arterioles are called resistance vessels. Remember that the capacitance vessels are the veins because they have more than two thirds of the blood in the body. Again. Ohm’s law: flow=ΔP/R. Cardiac output= (mean arterial pressure-right atrial pressure)/TPR (total peripheral resistance) {they are the same law}. 5|Page How to measure the mean arterial pressure: MAP=1/3 of the systolic pressure+2/3 of the diastolic pressure. Since the right atrial pressure is 0, the law becomes: CO=MAP/TPR. Accordingly, the MAP=CO*TPR. Poiseuille’s law: Poiseuille tried to see the effect of the 1-diameter of the vessel on the flow 2-length of the vessel on the flow 3-the pressure gradient on the flow 4-viscosity of the fluid on the flow To do this experiment, he fixed three of the variables and changed the one he wanted to test. For example, if he wants to see the effect of the pressure gradient (ΔP), he fixed the other three variables. When the initial ΔP was 10mmhg, he found that the flow was 1L. Then he made ΔP 20mmhg and found the flow to be 2L. Then he decreased the ΔP to 5mmhg and found the flow to be 0.5L. {The numbers not for memorization}. After this experiment, he found out that the flow is directly proportional to ΔP. After that he tried the length. He started with 10mm length for the vessel and found the flow to be 1L. Then he made the length 5mm and found the flow to be 2L. He increased the length to 20mm and found the flow to be 0.5L. After this experiment, he found out that the flow is inversely proportional to length. {In humans, the least effective variable is the length because the length of vessels stays the same. It may change when growing up, but it is still the least effective variable}. He fixed everything after that and changed the viscosity. When it was 1, the flow was 1L. When it was 2, the flow became 0.5L. So, he found out that viscosity is inversely proportional to the flow. When trying the diameter, he brought a vessel with 1mm diameter and found the flow to be 1L. when making the diameter 2mm, the flow became 16L. when the diameter was 3mm, the flow became 81L. he decreased the diameter to 0.5mm and found the flow to be 1/16. He decreased it to 1/3mm and found the flow to be 1/81L. So, he found out that the flow is directly proportional to the radius to the power 4 (r^4). 6|Page The law: since the flow=ΔP/R, by exchanging (F) in Poiseuille’s law with ΔP/R, we end up with this law: R=8ηL/πr^4. From this law we can conclude that the radius is the variable with the most effect on resistance (because it is to the power 4). A small vasodilation can increase the flow a lot and a small vasoconstriction can decrease the flow a lot. - Be aware that the radius is directly proportional to the flow and inversely proportional to resistance. Parallel and serial resistance sites in the circulation Resistance in series can be seen in figure A. In series, the total resistance is higher than the highest resistance among the serial resistances. The law is the one above in the figure. You sum all serial resistances together to get their total resistance. Resistance in parallel can be seen in figure B. In parallel, the total resistance is lower than the lowest resistance among the parallel resistances. The law is the one below in the figure. You sum the inverse of all parallel resistances together to get their inverse total resistance. (if you didn’t understand, just look at the law). -In the body, the capillaries and vessels are found in parallel form not in series. 7|Page Viscosity Factors that affect viscosity: viscosity is affected by the number of red blood cells. In order to measure the amount of red blood cells we use hematocrit (which is the percentage of the red blood cells to the whole blood). So, viscosity is affected by: 1-hematocrit 2-plasma proteins (mainly albumin) 3-diameter of the blood vessels 4-temperature. Temperature of blood is fixed everywhere in the body, so it is not considered an effector on the viscosity inside the body. The diameter effect is cancelled out by the ability of red blood cells to change shape to get through any vessel regardless of its diameter. In the end, the main effector of blood viscosity inside the body is hematocrit and to a lesser degree plasma protein. The effect of hematocrit on viscosity: The blue line represents water. Water always has a viscosity of 1 The green line represents the viscosity of plasma. In the plasma there are no red blood cells. This means that plasma proteins are the only effector on the viscosity of the plasma. The viscosity of the plasma doesn’t change because plasma proteins are made with a constant rate, so there is no change in the concentration of plasma proteins which makes the viscosity constant. 8|Page The red line represents the whole blood. The viscosity of blood is mainly affected by the hematocrit. As you can see from the graph, the curve is not linear; it is an exponential (or logarithmic) curve. The dangerous thing about this is that any slight change in hematocrit will cause a big change in the viscosity. For example: if the curve was linear: if the hematocrit was 20 (which is around the normal hematocrit level) and became 60, the viscosity should increase by 3 only and become 3, but this is not the case. In reality, because the curve is exponential, the viscosity will become 5. This is the danger of polycythemia. In polycythemia, the hematocrit increases a lot which increases the viscosity a lot which decreases the flow of blood. The flow becomes sluggish. Sluggish flow causes coagulation, clots and reduces the cells supply with oxygen. -The main factor that changes the flow is the resistance. The body doesn’t change the pressure in order to affect the flow; it changes the resistance. This means that the main effector on flow is resistance. Resistance can be changed by changing the diameter of blood vessels according to Poiseuille’s law. A slight increase in the diameter decreases the resistance a lot since the resistance is inversely proportional to r^4. So, if you want to increase the flow, you decrease the resistance by increasing the diameter. This is apparent in the figure above where the flow on the right is the highest because it has the least resistance compared to the others. 9|Page Laplace’s law: T=P*r - Critical closing pressure is the internal pressure at which a blood vessel collapses and closes completely. If blood pressure falls below critical closing pressure, then the vessels collapse. They close because the pressure is too low; pressure cannot overcome the tension generated by the vessel wall, so they close because of their own tension. As you can see from the graph on the right, there are two forces affecting the vessel. A force which tries to close the vessel (T) and a force that tries to open the vessel (P). Laplace’s law is T=P*r(radius). So, when the vessel closes, r becomes 0; so T becomes 0. This doesn’t mean that P=0. There might be very low pressure although the vessel collapsed, but the pressure is too low to open the vessel. Distensibility and compliance: Distensibility is the proportional increase in volume per 1mmhg increase in pressure. So, it is a ratio. So, if you want to describe it, you describe it as for example10% not 10. Compliance is how much volume needed in order to increase the pressure by 1mmhg. Distensibility equation: D=ΔV/ (ΔP*V) *100% Compliance equation: C=ΔV/ΔP. (here it is not multiplied by 100% because it is not a ratio) 10 | P a g e Compliance can also be calculated using dispensability. D=ΔV/(ΔP*V). By rearranging the equation: ΔV/ΔP= D*V. And because C=ΔV/ΔP, then compliance equals D*V. So, compliance is dispensability multiplied by the volume. -ΔV is the added volume. -V is the original volume -ΔP is the change in pressure Examples: ● If we have 100 ml volume (the original volume), if we add 10 ml, the pressure increases 1 mm Hg (10/(1*100) *100%= 10% which refers to the distensibility while the compliance here is 10 ml). ● If the original volume was 90 and we needed to add 10ml to increase pressure by 1mmhg then Distensibility equals 10/(90*1)*100%. So, the distensibility changed, but the compliance will not change in this case and it will stay 10ml because ΔV was not changed. ● If the original volume was 90 ml and the distensibility = 10%, how much do we need to achieve rise of 1 mm Hg? 9 ml (which represents the compliance). -The distensibility of veins is 6 times the distensibility of arteries. And because the volume of veins is 4 times the volume of arteries, and Compliance=Distensibility*Volume then the compliance in veins is 24 times the compliance in arteries. 11 | P a g e Pulsation waves If we count the heart rate, each beat represents one cardiac cycle thus one pulse. These pulses are waves that move in the wall of the artery (so what we feel during the heart rate counting is not due to the flow but because of the movement of the wall which is a fast movement that depends on the compliance of the vessel thus if the compliance is high, the velocity is low and if the compliance is low, the velocity is high). The pulsation we feel in arteries is caused by the pulsation wave not by blood flow. Velocities of pulsation waves in different vessels: The artery with the highest compliance among all arteries is the aorta so it has the slowest pulsation (4-5 m/s). (But not slower than veins) The speed of pulsation in large arteries (10-20 m/s) is higher than the aorta because their compliance is less. The speed of pulsation in small arteries is higher than the aorta and large arteries because their compliance is lower than the aorta and large arteries. Arterioles have the fastest pulsation speed which may reach 40 m/s as they have the lowest compliance {remember that arterioles have the highest resistance because they have the lowest compliance}. Veins have higher compliance than all arteries so they have lower pulsation speed than arteries. 12 | P a g e So, in the order of compliance: Veins>Aorta>Large arteries>Small arteries>Arterioles. In the order of pulsation speed: Arterioles>Small arteries>Large arteries>Aorta>Veins. Pulse pressure As we know, there are two pressures in the single pulse which are systolic and diastolic. The difference between the systolic and the diastolic is called the pulse pressure. (Systolic pressure – Diastolic pressure). Pulse pressure is the cause of the pulsation wave in blood vessels. Example: if the measured pressure is 110/70, then how much is the pulse pressure? It is the difference between systolic and diastolic, so it is 110-70 which equals 40. Factors affecting the pulse pressure: 1-Stroke volume: the higher the stroke volume, the higher the pulse pressure. Why? When stroke volume is increased, the blood pressure inside the aorta increases (because there is more blood inside the aorta). This means an increase in the systolic pressure. Increasing the systolic pressure will increase the difference between it and the diastolic pressure. This will increase the pulse pressure. 13 | P a g e 2-Compliance: high compliance means that if you increase the blood volume in the vessel, the vessel wall will expand with the blood so there will be no big change in the pressure. But if the vessel has low compliance, the wall will not expand with the increased blood volume so a small increase in the blood volume will increase the pressure a lot. A high compliant vessel will have lower pulse pressure than a low compliant vessel. As you can see, the pulse pressure also depends on the ratio between the stroke volume and compliance. If you increase the stroke volume without increasing the compliance, the pulse pressure will increase. If you increase the compliance without changing the stroke volume, the pulse pressure will decrease. You can change stroke volume and compliance without affecting the pulse pressure if you increase or decrease both of them proportionally. For example: if they were 2/4 and increased both of them to 3/6 the ratio between them will not change, so there will be no change in the pulse pressure. Damping of Pulse Pressures in the Peripheral Arteries The intensity of pulsations becomes progressively less in the smaller arteries. Pulsations travel along the wall of the arteries. The degree of damping is proportional to the resistance of small vessels and arterioles and the compliance of the larger vessels. 14 | P a g e Abnormal Pressure Pulse Contours 1-Aortic regurgitation: aortic regurgitation means the return of blood from the aorta back to the left ventricle. So, if the stroke volume before the regurgitation was 70ml and the aorta returned 30ml back to the left ventricle; the stroke volume in the next cycle will be 70+30 which equals 100ml. The stroke volume increased, the systolic pressure is increased and therefore the pulse pressure is increased. -Aortic regurgitation is also associated with low diastolic pressure. When the blood goes back from the aorta to the left ventricle, the aorta will have less blood during diastole so the diastolic pressure will decrease. Less diastolic pressure means higher pulse pressure. -Also, in aortic regurgitation there will be no incisura. Incisura is caused by the turbulence of blood when return to the closed semilunar valve. In this case, the valve will be open and there will be no turbulence of the blood thus no incisura. 2-Patent ductus arteriosus: normally in the embryo, there is an artery between the pulmonary artery and the aorta called the ductus arteriosus. It connects them together so blood can go directly from the pulmonary artery to the aorta. This is because the embryo is not using his lungs, so he doesn’t need blood going to the lungs. After birth, this artery will close. If it stays open, then this condition is called patent ductus arteriosus. In this condition, blood from the right ventricle will skip the pulmonary circulation and go to the aorta. This will increase the stroke volume then the pulse pressure will increase. 3-Arteriosclerosis–decreases compliance of arterial tree, thus leading to increase in pulse. In conclusion, stroke volume is directly proportional to pulse pressure and compliance is inversely proportional to pulse pressure. 15 | P a g e