Blood Flow, Blood Pressure, and Resistance | Anatomy & Physiology
Pressure difference is the driving force for blood flow, and resistance is impediment to flow. The relationship of flow (Q), resistance (R), and pressure difference. OHM'S LAW Pressure (P) Flow (Q) Ohm's law describes the relationship between helps understand the determinants of blood flow, pressure, and resistance. The graph shows the components of blood pressure throughout the blood vessels, . blood flow, it can help solidify an understanding of their relationships.
But let's really figure this out and make this a little bit more intuitive for us. So to do that, let's start with an equation. And this equation is really going to walk us through this puzzle. So we've got pressure, P, equals Q times R. Really easy to remember, because the letters follow each other in the alphabet. And here actually, instead of P, let me put delta P, which is really change in pressure.
So this is change in pressure. And a little doodle that I always keep in my mind to remember what the heck that means is if you have a little tube, the pressure at the beginning-- let me say start; S is for start-- and the pressure at the end can be subtracted from one another. The change in pressure is really the change from one part of tube the end of the tube. And that's the first part of the equation. So next we've got Q.
So what is Q?
This is flow, and more specifically it's blood flow. And this can be thought of in terms of a volume of blood over time. So let's say minutes. So how much volume-- how many liters of blood are flowing in a minute? Or whatever number of minutes you decide? And that's kind of a hard thing to figure out actually.
But what we can figure out is that Q, the flow, will equal the stroke volume, and I'll tell you what this is just after I write it. The stroke volume times the heart rate. So what that means is that basically, if you can know how much blood is in each heartbeat-- so if you know the volume per heartbeat-- and if you know how many beats there are per minute, then you can actually figure out the volume per minute, right? Because the beats would just cancel each other out. And it just turns out, it happens to be, that I'm about 70 kilos.
And for a 70 kilogram person, the stroke volume is about 70 milliliters. So for a 70 kilo person, you can expect about 70 milliliters per beat. And as I write this, let's say my heart rate is about 70 beats per minute. I feel pretty calm, and so it's not too fast. So the beats cancel as we said, and I'm left with 70 milliliters times 70 per minute.
So that's about 4, milliliters per minute. Or if I was to simplify, that's a 5, let's say about. So the flow is about 5 liters per minute. So I figured out the blood flow, and that was simply because I happen to know my weight, and my weight tells me the stroke volume.
And I know that there's a change in pressure. We've got to figure that out soon. And lastly, this last thing over here is resistance. And know I've said it before. I just want to point out to you again, the resistance is going to be proportional to 1 over R to the fourth.
And so just remember that this is an important issue. And that's the radius of the vessel. So let's figure out this equation.
Let's figure out the variables in this equation and how it's going to help us solve the question I asked you-- what is the total body resistance? So if I have to figure out total body resistance-- let me clear out the board-- I've got, let's say, the heart. I like to do the heart in red.
Arterial Blood Pressure
And it's pumping blood at my aorta. So blood is going out of the aorta. And then it's going and branching here. And then it's going to branch some more. An individual weighing pounds has approximately 60, miles of vessels in the body.
Gaining about 10 pounds adds from to miles of vessels, depending upon the nature of the gained tissue. One of the great benefits of weight reduction is the reduced stress to the heart, which does not have to overcome the resistance of as many miles of vessels.
In contrast to length, the blood vessel diameter changes throughout the body, according to the type of vessel, as we discussed earlier. The diameter of any given vessel may also change frequently throughout the day in response to neural and chemical signals that trigger vasodilation and vasoconstriction. The vascular tone of the vessel is the contractile state of the smooth muscle and the primary determinant of diameter, and thus of resistance and flow. The effect of vessel diameter on resistance is inverse: Given the same volume of blood, an increased diameter means there is less blood contacting the vessel wall, thus lower friction and lower resistance, subsequently increasing flow.
A decreased diameter means more of the blood contacts the vessel wall, and resistance increases, subsequently decreasing flow. The influence of lumen diameter on resistance is dramatic: A slight increase or decrease in diameter causes a huge decrease or increase in resistance. This means, for example, that if an artery or arteriole constricts to one-half of its original radius, the resistance to flow will increase 16 times.
A Mathematical Approach to Factors Affecting Blood Flow Jean Louis Marie Poiseuille was a French physician and physiologist who devised a mathematical equation describing blood flow and its relationship to known parameters. The same equation also applies to engineering studies of the flow of fluids. Although understanding the math behind the relationships among the factors affecting blood flow is not necessary to understand blood flow, it can help solidify an understanding of their relationships.
Please note that even if the equation looks intimidating, breaking it down into its components and following the relationships will make these relationships clearer, even if you are weak in math.
Focus on the three critical variables: It may commonly be represented as 3.
One of several things this equation allows us to do is calculate the resistance in the vascular system. Normally this value is extremely difficult to measure, but it can be calculated from this known relationship: The important thing to remember is this: Two of these variables, viscosity and vessel length, will change slowly in the body.
Only one of these factors, the radius, can be changed rapidly by vasoconstriction and vasodilation, thus dramatically impacting resistance and flow. Further, small changes in the radius will greatly affect flow, since it is raised to the fourth power in the equation.
The Roles of Vessel Diameter and Total Area in Blood Flow and Blood Pressure Recall that we classified arterioles as resistance vessels, because given their small lumen, they dramatically slow the flow of blood from arteries. In fact, arterioles are the site of greatest resistance in the entire vascular network. This may seem surprising, given that capillaries have a smaller size. How can this phenomenon be explained? Although the diameter of an individual capillary is significantly smaller than the diameter of an arteriole, there are vastly more capillaries in the body than there are other types of blood vessels.
Part c shows that blood pressure drops unevenly as blood travels from arteries to arterioles, capillaries, venules, and veins, and encounters greater resistance. However, the site of the most precipitous drop, and the site of greatest resistance, is the arterioles.
CV Physiology: Hemodynamics (Pressure, Flow, and Resistance)
This explains why vasodilation and vasoconstriction of arterioles play more significant roles in regulating blood pressure than do the vasodilation and vasoconstriction of other vessels. Part d shows that the velocity speed of blood flow decreases dramatically as the blood moves from arteries to arterioles to capillaries.
This slow flow rate allows more time for exchange processes to occur. As blood flows through the veins, the rate of velocity increases, as blood is returned to the heart. The relationships among blood vessels that can be compared include a vessel diameter, b total cross-sectional area, c average blood pressure, and d velocity of blood flow. Disorders of the…Cardiovascular System: Arteriosclerosis Compliance allows an artery to expand when blood is pumped through it from the heart, and then to recoil after the surge has passed.
This helps promote blood flow. In arteriosclerosis, compliance is reduced, and pressure and resistance within the vessel increase. This is a leading cause of hypertension and coronary heart disease, as it causes the heart to work harder to generate a pressure great enough to overcome the resistance.
Arteriosclerosis begins with injury to the endothelium of an artery, which may be caused by irritation from high blood glucose, infection, tobacco use, excessive blood lipids, and other factors.
Artery walls that are constantly stressed by blood flowing at high pressure are also more likely to be injured—which means that hypertension can promote arteriosclerosis, as well as result from it. Recall that tissue injury causes inflammation. As inflammation spreads into the artery wall, it weakens and scars it, leaving it stiff sclerotic. As a result, compliance is reduced. Moreover, circulating triglycerides and cholesterol can seep between the damaged lining cells and become trapped within the artery wall, where they are frequently joined by leukocytes, calcium, and cellular debris.
Eventually, this buildup, called plaque, can narrow arteries enough to impair blood flow.
When this happens, platelets rush to the site to clot the blood. This clot can further obstruct the artery and—if it occurs in a coronary or cerebral artery—cause a sudden heart attack or stroke.
Alternatively, plaque can break off and travel through the bloodstream as an embolus until it blocks a more distant, smaller artery. Ischemia in turn leads to hypoxia—decreased supply of oxygen to the tissues. Hypoxia involving cardiac muscle or brain tissue can lead to cell death and severe impairment of brain or heart function.
A major risk factor for both arteriosclerosis and atherosclerosis is advanced age, as the conditions tend to progress over time. However, obesity, poor nutrition, lack of physical activity, and tobacco use all are major risk factors. Treatment includes lifestyle changes, such as weight loss, smoking cessation, regular exercise, and adoption of a diet low in sodium and saturated fats.
Medications to reduce cholesterol and blood pressure may be prescribed. For blocked coronary arteries, surgery is warranted. In angioplasty, a catheter is inserted into the vessel at the point of narrowing, and a second catheter with a balloon-like tip is inflated to widen the opening.
Putting it all together: Pressure, flow, and resistance
To prevent subsequent collapse of the vessel, a small mesh tube called a stent is often inserted. In an endarterectomy, plaque is surgically removed from the walls of a vessel.
This operation is typically performed on the carotid arteries of the neck, which are a prime source of oxygenated blood for the brain. In a coronary bypass procedure, a non-vital superficial vessel from another part of the body often the great saphenous vein or a synthetic vessel is inserted to create a path around the blocked area of a coronary artery.
Venous System The pumping action of the heart propels the blood into the arteries, from an area of higher pressure toward an area of lower pressure. If blood is to flow from the veins back into the heart, the pressure in the veins must be greater than the pressure in the atria of the heart.
Two factors help maintain this pressure gradient between the veins and the heart. First, the pressure in the atria during diastole is very low, often approaching zero when the atria are relaxed atrial diastole. These physiological pumps are less obvious. Skeletal Muscle Pump In many body regions, the pressure within the veins can be increased by the contraction of the surrounding skeletal muscle.
This mechanism, known as the skeletal muscle pump Figure As leg muscles contract, for example during walking or running, they exert pressure on nearby veins with their numerous one-way valves. This increased pressure causes blood to flow upward, opening valves superior to the contracting muscles so blood flows through. Simultaneously, valves inferior to the contracting muscles close; thus, blood should not seep back downward toward the feet.
One millimeter of mercury is equivalent to the hydrostatic pressure exerted by a 1-mm-high column of mercury on an area of 1 cm2. In a system in which fluid is flowing, pressure falls over distance as energy is lost because of friction. In addition, the pressure exerted by moving fluid has two components: Pressure within our cardiovascular system is usually called hydrostatic pressure even though it is a system in which fluid is in motion. Some textbooks are beginning to replace the term hydrostatic pressure with the term hydraulic pressure.
Hydraulics is the study of fluid in motion. Pressure Changes in Liquids Without a Change in Volume In the walls of a fluid-filled container contract, the pressure exerted on the fluid in the container increases.
You can demonstrate this principle by filling a balloon with water and squeezing the water balloon in your hand. Water is minimally compressible, and so the pressure you apply to the balloon is transmitted throughout the fluid. As you squeeze, higher pressure in the fluid causes part of the balloon to bulge. If the pressure becomes high enough, the stress on the balloon will cause it to pop.
The water volume inside the balloon did not change, but the pressure in the fluid increased. In the human heart, contraction of the blood-filled ventricles is similar to squeezing a water balloon: This high-pressure blood then flows out of the ventricle and into the blood vessels, displacing lower-pressure blood already in the vessels.
The pressure created in the ventricles is called the driving pressure because it is the force that drives blood through the blood vessels. When the walls of a fluid-filled container expand, the pressure exerted on the fluid decreases. Thus, when the heart relaxes and expands, pressure in the fluid-filled chambers falls.
Pressure changes can also take place in the blood vessels. If blood vessels dilate, blood pressure inside them falls. If blood vessels constrict, blood pressure increases. Volume changes of the blood vessels and heart are major factors that influence blood pressure in the cardiovascular system.
- Hemodynamics (Pressure, Flow, and Resistance)
- Pressure and Blood Flow
- Blood pressure, blood flow, and resistance
Blood Flows from an Area of Higher Pressure to One of Lower Pressure As stated earlier, blood flow through the cardiovascular system requires a pressure gradient. This pressure gradient is analogous to the difference in pressure between two ends of a tube through which fluid flows Fig. This relationship says that the higher the pressure gradient, the greater the fluid flow. A pressure gradient is not the same thing as the absolute pressure in the system.
Fore example, the tube in Figure b has an absolute pressure of mm Hg at each end. However, because there is no pressure gradient between the two ends of the tube, there is no flow through the tube. On the other hand, two identical tubes can have very different absolute pressures but the same flow. The top tube in Figure c has a hydrostatic pressure of mm Hg at one end and 75 mm Hg at the other end, which means that the pressure gradient between the ends of the tube equals 25 mm Hg.
The identical bottom tube has a hydrostatic pressure of 40 mm Hg at one end and 15 mm Hg at the other end. This tube has lower absolute pressure all along its length but the same pressure gradient as the top tube - 25 mm Hg. Because the pressure difference in the two tubes is identical, the fluid flow through the tubes is the same.
Resistance Opposes Flow In an ideal system, a substance in motion would remain in motion. However, no system is ideal because all movement creates friction.