Transcription
We will begin this lecture with the factors that determine blood flow in vessels. But before we start with that, let's first clarify what the title of the lecture itself means. The title is hemorology, and it is a field of medicine and biology. One could also say partly physics, that studies blood as a fluid. In other words, it examines how blood flows, how its viscosity changes, and how its circulation and other properties change within blood vessels. So, it focuses on the physical properties of blood flow. It must be mentioned that from a physics perspective, blood is one of the most complex fluids to work with. However, we will try to look at it in a simplified way, hopefully making it understandable without involving overly complex physics. But we will also start immediately with physics: the Ohm's law adopted for blood flow.
Let us imagine a blood vessel through which blood flows. If we want to determine how much blood passes through this vessel in a given unit of time, we use Ohm's law adapted to circulation. The formula is as follows: Blood flow in a vessel is proportional to the pressure difference between the two ends of the vessel, and it is measured in L per minute. Thus, Q represents the volume of blood that flows through the vessel per unit of time, in this case, per minute. There are two main factors that influence this: First, pressure difference between the two ends of the vessel, and secondly, resistance of the vessel. As with all fluids, blood flows from higher pressure to lower pressure. This means that if at one end of the vessel the pressure is P1 and on the other end it is P2, then for the blood flow in this direction, P1 definitely must be greater than P2.
For example, in arteries, let's say P1 could be 100 mm mercury and P2, let's say, 80 mm mercury. In this case, that means that delta P would be 20 mm mercury because that's the difference between P1 and P2. But at the same time, let's say in the veins, P1 could be 20 mm mercury and, let's say, P2 is 0 mm mercury. Now, in this case, as you see, delta P again is 20 mm mercury. In both cases, the pressure difference is the same. This shows that the pressure difference, not the absolute pressure, is the one that determines blood flow. And in a case that resistance is the same in both my situations, then the volume of blood that flows through the vessel per minute would be the same because delta P, or the pressure difference, is also the same.
Now, the second important factor is vascular resistance, denoted with the great R. This represents a complex friction force between the blood and the vessel wall. So now that you already know the two main factors that determine blood flow in vessels: one is the pressure difference between two ends of the vessel, and the other is vascular resistance.
Next, it should be mentioned that vascular resistance in circulation is divided into elastic resistance and peripheral resistance. Elastic resistance, as the name suggests, depends on the elasticity of the blood vessel. If a vessel is elastic, it can expand well and then return to its original shape and volume, offering less resistance to blood flow. In practice, this type of resistance is mainly relevant in large arteries such as the aorta, the pulmonary trunk, and their major branches. In the rest of the circulatory system, which makes up the majority, we refer to peripheral resistance. This is also the main type of resistance that determines blood flow in vessels, or more precisely, that limits blood flow as described by Ohm's law. The greater the resistance, the more it opposes blood flow.
To describe peripheral resistance in numbers, a specific formula is used, called Pu's law. And this is the formula, the Poise's law. Poise originally worked with glass tubes, which is why elasticity was not included as a factor in this equation. However, it is still very suitable for describing flow in muscular arteries, arteries, and similar vessels because they are not highly elastic. According to this law, resistance depends on three variable factors, since 8 and pi are already constants and do not change. So, what are these three variables that influence resistance and therefore blood flow?
Let's start with viscosity because it is especially important in your field. Viscosity in the human body mainly depends on the number of blood cells in the blood. For example, if a vessel contains more iritthraittytes, they create more friction against the vessel wall, increasing resistance and slowing blood flow. This corresponds to more viscous blood. On the other hand, if there are fewer aritroides, there is less friction, resistance decreases, and the blood becomes less viscous, allowing it to flow more easily. Of course, the examples shown here are somewhat simplified and overexaggerated to illustrate the concept clearly rather than reflect exact real conditions in blood vessels. In summary, the higher the viscosity, the slower the blood flow and the higher the resistance. The lower the viscosity, the faster the blood flow and the lower the resistance.
In a healthy individual, viscosity is relatively stable since the number of blood cells does not change significantly. However, it can be slightly influenced by hydration. For example, drinking fluids can dilute plasma and reduce viscosity, while dehydration can increase it. Blood viscosity is usually described in medicine using the hematocrit index because it reflects the amount of formed elements in blood and, as you know, mainly aritroytes. A normal hemocrit is approximately 45%; that is what we see also here. In anemia, hematocrit index decreases, meaning there are fewer red blood cells, and viscosity is lower. In contrast, in such conditions as polymia, where the bone marrow produces more blood cells, especially in this case, hematocrit index increases, and so does blood viscosity.
If we compare this to a graph, here you see the y-axis represents viscosity and the x-axis represents hematocrit. For comparison, the blue line, water, has a viscosity about 1, and plasma without any blood cell elements in it has a viscosity about 1.3 till 1.4 4 times that of water, mainly due to plasma proteins. Blood at a normal hematocrit (45%), which you see at this point here, has a viscosity about four times higher than water, and as hematocrit decreases, blood viscosity decreases too. As hematocrit increases, blood viscosity also increases. So, viscosity is largely dependent on the number of red blood cells in the circulation.
Since we are discussing viscosity, another important related physiological concept to consider is hemodilution. Dilution, meaning thinning, essentially refers to, of course, blood dilution. And when blood is diluted, the hematocrit, of course, decreases, not because the number of veritraittes is reduced, but because the proportion relative to the total fluid volume becomes lower. As a result, oxygen content in the blood decreases since there are fewer red blood cells per unit volume. This triggers some compensatory mechanisms. One of these is vasoddilation, where blood vessels widen in response to reduced oxygen levels. At the same time, a lower hematocrit means lower viscosity, and lower viscosity means lower peripheral resistance, and lower resistance means blood flows more easily. And because resistance decreases, venus return to the heart can increase, which leads to increased cardiac output. Due to vasoddilation and increased cardiac output, both local and overall tissue perfusion improves. However, it is important to note that if hematocrit becomes too low, oxygen transport capacity is reduced so much that improved perfusion can no longer compensate for the lack of oxygen. And this is something also described by Ohm's law. So, if the resistance decreases, the blood flow will increase.
And one more point, if we continue talking about viscosity, not only in terms of vessel diameter, but also tube size in general, including artificial tubes, not just capillaries or artery release. It is important to note that viscosity depends on the diameter of the tube. Looking at this graph here, the y-axis represents viscosity and the x-axis represents tube diameter. The percentages shown here indicate hematocrit levels in the blood. You can see that regardless whether the hematocrit is 20% (so low), normal (45%), or high (60%), there is a certain range of tube diameters where the viscosity becomes quite similar, actually relatively independent of the diameter. So that means hematocrit and thus the number of blood cells only becomes a significant factor when the tube diameter is large enough, and according to the graph, it seems that hematocrit has very little influence when blood flows through extremely narrow tubes.
Now, why does it happen? It is important to remember that real blood is not an ideal fluid. It is not like water. It is a suspension. In other words, it contains red blood cells. And these red blood cells are not spherical; they are deformable. Red blood cells can sometimes arrange themselves in rows. Under certain conditions, they can separate from plasma also. And when blood flows through very small tubes, viscosity changes very little because of the ferrris lining effect. This describes what happens when blood flows through narrow vessels when diameter is less than 300 micrometers. In such vessels, aertoytes concentrate in the center of the flow. Near the vessel wall, the plasma layer increases, and since plasma has a relatively low viscosity, the fluid near the wall has less resistance. As a result, the overall flow behaves more like a low viscosity fluid despite the presence of red blood cells in it. The effect is due to the unique properties of red blood cells and explains why viscosity becomes less dependent on hematocrit in very small vessels, like, for example, in my graph. So, 300 micrometers are approximately here, and you see that lesser and lesser the viscosity here depends on hematocrit. If we want to observe viscosity that is strongly dependent on hematocrit, we need to look at vessels or tubes with a diameter greater than 300 micrometers.
So, returning to Pu's law, we have not yet covered everything. So, the first factor, ETA (and remember this is ETA, the Greek letter, not N or anything connected with N; this is ETA), is blood viscosity, which influences vascular resistance. The next factor is the length of the vessel, denoted by L. The longer the vessel, the more friction occurs between the blood and the vessel wall, resulting in a higher resistance. You can imagine this like crawling through a tunnel. If the tunnel is short, you pass through quickly with little resistance. If it is long, it takes more effort, and resistance is higher. Similarly, longer blood vessels have greater resistance. However, in a healthy adult, vessel length is relatively constant, except for changes during growth in children. Also, when comparing circulation loops, the pulmonary circulation has lower resistance because it is shorter than the systemic circulation.
And the final factor is the radius of the vessel. And unlike length, the radius can change significantly even in a healthy individual, since vessels can both constrict and dilate under the influence of autonomic nervous system, hormones, and local factors. Because radius changes frequently throughout the day, it is the most important factor influencing vascular resistance. So, remember this most important factor. If the vessel constricts, resistance increases. If the vessel dilates, resistance decreases. An especially important point is that the equation radius is raised to the fourth power here. This means that even small changes in radius have a very large effect on resistance. For example, if we have a radius and it doubles, the actual resistance will increase 16 times because of the fourth power we have here. This means that even small changes, as I said, can affect blood flow. And this is also why radius is considered the most critical determinant of vascular resistance.
And finally, an important limitation: B's law applies only to laminer flow. We will discuss flow types in our next section. But for now, remember all these relationships hold true only under laminer flow conditions.
So now we have finally listed all the main factors that determine blood flow in vessels. If we want to combine Ohm's law, which we discussed at the beginning, with PSA's law, which describes peripheral resistance, we can express blood flow by substituting resistance with the PSA's formula, specifically for peripheral vessels. At this point, you are probably starting to worry about complicated formulas and calculations. Well, let me reassure you and clarify what you actually need to know. You should be able to write the classical Ohm's law and understand the relationship between its variables. No calculations are required for PSA's law. You should also be able to write the formula (so the classical previous one) and understand the variables involved. Again, no calculations will be required. And most importantly, you need to know the key parameters, especially the variable ones, and how each of them affects vascular resistance. The good news is that you will not be expected to perform calculations, at least not in this subject.