Blood Flow, Blood Pressure, and Resistance | Anatomy & Physiology
Heart; Cardiac cycle; Arteries; Blood flow; Vasodilation; Arterial Resistance; .. Li YW, Aronow WS () Diabetes Mellitus and Cardiovascular Disease. () Relationship between Fibroblast Growth Factor 21 and Extent of Left () Acetaminophen increases blood pressure in patients with coronary artery disease. As shown in Figure 1, the difference between the systolic pressure and the diastolic . In the venous system, the opposite relationship is true. Cardiac output is the measurement of blood flow from the heart through the ventricles, and is. For the flow of blood in a blood vessel, the ΔP is the pressure difference between any two points along a given length of the vessel. When describing the flow of.
While this procedure is normally performed using the radial artery in the wrist or the common carotid artery in the neck, any superficial artery that can be palpated may be used Figure Common sites to find a pulse include temporal and facial arteries in the head, brachial arteries in the upper arm, femoral arteries in the thigh, popliteal arteries behind the knees, posterior tibial arteries near the medial tarsal regions, and dorsalis pedis arteries in the feet.
A variety of commercial electronic devices are also available to measure pulse. The pulse is most readily measured at the radial artery, but can be measured at any of the pulse points shown. Measurement of Blood Pressure Blood pressure is one of the critical parameters measured on virtually every patient in every healthcare setting.
The technique used today was developed more than years ago by a pioneering Russian physician, Dr. Turbulent blood flow through the vessels can be heard as a soft ticking while measuring blood pressure; these sounds are known as Korotkoff sounds. The technique of measuring blood pressure requires the use of a sphygmomanometer a blood pressure cuff attached to a measuring device and a stethoscope.
The technique is as follows: Although there are five recognized Korotkoff sounds, only two are normally recorded. Initially, no sounds are heard since there is no blood flow through the vessels, but as air pressure drops, the cuff relaxes, and blood flow returns to the arm.
As shown in Figure As more air is released from the cuff, blood is able to flow freely through the brachial artery and all sounds disappear. When pressure in a sphygmomanometer cuff is released, a clinician can hear the Korotkoff sounds. In this graph, a blood pressure tracing is aligned to a measurement of systolic and diastolic pressures. The majority of hospitals and clinics have automated equipment for measuring blood pressure that work on the same principles.
The patient then holds the wrist over the heart while the device measures blood flow and records pressure. Cardiac output Volume of the blood Resistance Recall that blood moves from higher pressure to lower pressure.
It is pumped from the heart into the arteries at high pressure. Since pressure in the veins is normally relatively low, for blood to flow back into the heart, the pressure in the atria during atrial diastole must be even lower. It normally approaches zero, except when the atria contract see Figure Cardiac Output Cardiac output is the measurement of blood flow from the heart through the ventricles, and is usually measured in liters per minute.
Any factor that causes cardiac output to increase, by elevating heart rate or stroke volume or both, will elevate blood pressure and promote blood flow.
Hemodynamics (Pressure, Flow, and Resistance)
These factors include sympathetic stimulation, the catecholamines epinephrine and norepinephrine, thyroid hormones, and increased calcium ion levels. Conversely, any factor that decreases cardiac output, by decreasing heart rate or stroke volume or both, will decrease arterial pressure and blood flow.
These factors include parasympathetic stimulation, elevated or decreased potassium ion levels, decreased calcium levels, anoxia, and acidosis. Compliance Compliance is the ability of any compartment to expand to accommodate increased content. A metal pipe, for example, is not compliant, whereas a balloon is.Ohm's Law and Hemodynamics (Fluid Mechanics - Lesson 9)
The greater the compliance of an artery, the more effectively it is able to expand to accommodate surges in blood flow without increased resistance or blood pressure. Veins are more compliant than arteries and can expand to hold more blood. When vascular disease causes stiffening of arteries, compliance is reduced and resistance to blood flow is increased.
The result is more turbulence, higher pressure within the vessel, and reduced blood flow. This increases the work of the heart. Blood Volume The relationship between blood volume, blood pressure, and blood flow is intuitively obvious. Water may merely trickle along a creek bed in a dry season, but rush quickly and under great pressure after a heavy rain.
Similarly, as blood volume decreases, pressure and flow decrease. As blood volume increases, pressure and flow increase.
Under normal circumstances, blood volume varies little. Low blood volume, called hypovolemia, may be caused by bleeding, dehydration, vomiting, severe burns, or some medications used to treat hypertension. It is important to recognize that other regulatory mechanisms in the body are so effective at maintaining blood pressure that an individual may be asymptomatic until 10—20 percent of the blood volume has been lost.
Treatment typically includes intravenous fluid replacement. Hypervolemia, excessive fluid volume, may be caused by retention of water and sodium, as seen in patients with heart failure, liver cirrhosis, some forms of kidney disease, hyperaldosteronism, and some glucocorticoid steroid treatments.
Restoring homeostasis in these patients depends upon reversing the condition that triggered the hypervolemia. Resistance The three most important factors affecting resistance are blood viscosity, vessel length and vessel diameter and are each considered below.
Blood viscosity is the thickness of fluids that affects their ability to flow. Clean water, for example, is less viscous than mud.
The viscosity of blood is directly proportional to resistance and inversely proportional to flow; therefore, any condition that causes viscosity to increase will also increase resistance and decrease flow. For example, imagine sipping milk, then a milkshake, through the same size straw. You experience more resistance and therefore less flow from the milkshake. Conversely, any condition that causes viscosity to decrease such as when the milkshake melts will decrease resistance and increase flow.
Normally the viscosity of blood does not change over short periods of time. The two primary determinants of blood viscosity are the formed elements and plasma proteins. Since the vast majority of formed elements are erythrocytes, any condition affecting erythropoiesis, such as polycythemia or anemia, can alter viscosity.
Since most plasma proteins are produced by the liver, any condition affecting liver function can also change the viscosity slightly and therefore decrease blood flow. Liver abnormalities include hepatitis, cirrhosis, alcohol damage, and drug toxicities. While leukocytes and platelets are normally a small component of the formed elements, there are some rare conditions in which severe overproduction can impact viscosity as well. Blood vessel length is directly proportional to its resistance: As with blood volume, this makes intuitive sense, since the increased surface area of the vessel will impede the flow of blood.
Likewise, if the vessel is shortened, the resistance will decrease and flow will increase. The length of our blood vessels increases throughout childhood as we grow, of course, but is unchanging in adults under normal physiological circumstances. Further, the distribution of vessels is not the same in all tissues. Adipose tissue does not have an extensive vascular supply.
One pound of adipose tissue contains approximately miles of vessels, whereas skeletal muscle contains more than twice that. Overall, vessels decrease in length only during loss of mass or amputation.
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. Vasodilation and Arterial Resistance The relationship between mean arterial pressure, cardiac output and total peripheral resistance TPR gets affected by Vasodilation.
Vasodilation occurs in the time phase of cardiac systole while vasoconstriction follows in the opposite time phase of cardiac diastole [ 63 ]. Cardiac output blood flow measured in volume per unit time is computed by multiplying the heart rate in beats per minute and the stroke volume the volume of blood ejected during ventricular systole [ 64 ].
TPR depends on certain factors, like the length of the vessel, the viscosity of blood determined by hematocrit and the diameter of the blood vessel.
Vasodilation works to decrease TPR and blood pressure through relaxation of smooth muscle cells in the tunica media layer of large arteries and smaller arterioles [ 6566 ]. A rise in the mean arterial pressure is seen when either of these physiological components cardiac output or TPR gets increased [ 67 ].
Vasodilation occurs in superficial blood vessels of warm-blooded animals when their ambient environment is hot; this diverts the flow of heated blood to the skin of the animal [ 68 ], where heat can be more easily released into the atmosphere [ 69 ]. Vasoconstriction is opposite physiological process. Systemic vascular resistance SVR is the resistance offered by the peripheral circulation [ 72 ], while the resistance offered by the vasculature of the lungs is known as the pulmonary vascular resistance PVR [ 73 ].
Vasodilation increase in diameter decreases SVR, where as Vasoconstriction i.
CV Physiology | Hemodynamics (Pressure, Flow, and Resistance)
The Units for measuring vascular resistance are dyn. This is numerically equivalent to hybrid reference units HRUalso known as Wood units, frequently used by pediatric cardiologists. To convert from Wood units to MPa. Calculation of Resistance can be done by using these following formulae: Calculating resistance is that flow is equal to driving pressure divided by resistance.
The systemic vascular resistance can therefore be calculated in units of dyn. The basic tenet of calculating resistance is that flow is equal to driving pressure divided by resistance.
Cardiac Output Cardiac output CO is the quantity of blood or volume of blood that is pumped by the heart per minute. Cardiac output is a function of heart rate and stroke volume [ 75 ]. It is the product of stroke volume SV; the volume of blood ejected from the heart in a single beat and heart rate HR; expressed as beats per minute or BPM [ 76 ]. Ivabradine IVB is a novel, specific, heart rate HRlowering agent which is very useful [ 7778 ]. Increasing either heart rate or stroke volume increases cardiac output.
Most of the strokes are caused by atrial fibrillation [ 79 ]. The cardiac output for this person at rest is: Treatment for multiple congenital cardiac defects usually refers to open-heart surgery or a combination of medical treatment and open heart surgery [ 80 - 82 ]. The timing and outcomes of cardiovascular diseases are linked with surrounding power fields also [ 83 ].
Control of Heart Rate: With the activity of both sympathetic and parasympathetic nerve fibers, Sino Atrial node of the heart gets enervated [ 84 ]. The parasympathetic fibers release acetylcholine, under rest conditions which slows the pacemaker potential of the Sino Atrial node, thus reducing the heart rate [ 85 ]. The sympathetic nerve fibers release norepinephrine, under physical or emotional conditions which speeds up the pacemaker potential of the Sino Atrial node, increasing the heart rate [ 86 ].
Epinephrine is released from adrenal medulla by the activity of Sympathetic nervous system [ 87 ]. Epinephrine enters the blood stream, and is delivered to the heart where it binds with Sino Atrial node receptors.
Binding of epinephrine leads to further increase in heart rate. Control of Stroke Volume: The heart does not fill to its maximum capacity, under rest conditions. If the heart were to fill more per beat then it could pump out more blood per beat, thus increasing stroke volume.
The heart could pump out more blood per beat if the heart were to contract more strongly [ 88 ]; in other words, a stronger contraction would lead to a larger stroke volume.
During the exercise time or exercise periods, the stroke volume increases because of these mechanisms; the heart contracts more strongly and the heart fills up with more blood [ 89 ]. The Stroke volume is increased by 2 mechanisms: A larger end-diastolic volume will stretch the heart [ 90 ]. Stretching of the heart muscles optimizes the length and strength relationship of the cardiac muscle fibers, which results in stronger contractility and greater stroke volume [ 91 ].
Increase in sympathetic system activity increases the Stroke Volume: Release of norepinephrine by sympathetic nerve fibers causes an increase in the strength of myocardial contraction, thus increasing the stroke volume [ 92 ]. 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.
Putting it all together: Pressure, flow, and resistance
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. 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.
And you can see where this is going.