Pressure and Blood Flow
Pulse can be palpated manually by placing the tips of the fingers across an artery The relationship between blood volume, blood pressure, and blood flow is. of perfusion pressure and flow on coronary resistance. Resistance in a coronary branch By observing the relationship of coronary resistance to perfusion pressure at this time, .. the perfusion cannula tip and the distal polyethylene cannula. Oxygen from the pulmonary capillaries is transported by the blood flow to the tissues. In the main pulmonary artery, mean blood pressure is about 15 mmHg, the systolic It follows that the vascular resistance in the pulmonary circulation is about 10 The pulmonary circulation differs in many ways from the systemic one .
That's the part where now the blood is without oxygen. And then it continues to go and get collected into a venule, which sounds a little bit like the arterial on the other side, right? And we've got a vein over here. And then finally, the blood gets collected in a large vein called the vena cava.
And there are actually two vena cavas, so this'll be the superior vena cava. There's also an inferior vena cava. And the blood flow through this half is, as you would guess, continues to go around. And if I was to try to figure out the pressures, the blood pressures, at different points along the system, I'm going to choose some points that I think would be interesting ones to check.
So it would be good probably to check what the pressure is right at the beginning. And then maybe at all the branch points. So what the pressure is as the blood goes from the aorta to the brachial artery. Maybe as it ends the brachial artery and enters the arterial. Maybe the beginning and the end of the capillaries.
Also from the venue to a vein, and also, wrapping it up, what the pressure is at the end. Now, these numbers, or these pressures, can be represented as numbers, right? Like what is the millimeters of mercury that the blood is exerting on the wall at that particular point in the system? And earlier, we talked about systolic versus diastolic pressure, and there we wanted to use two numbers, because that's kind of the range, the upper and the lower range of pressure.
But now I'm going to do it with one number. And the reason I'm using one number instead of two, is that this is the average pressure over time. So the average pressure over time, for me-- keep in mind my blood pressure is pretty normal. It's somewhere around over 80 in my arm. So the average pressure in the aorta kind of coming out would be somewhere around 95, and in the artery in the arm, probably somewhere around Again, that's what you would expect-- somewhere between 80 and So 90 is the average, because it's going to be not exactlybecause remember, it's spending more time in diastole and relaxation than in systole.
Arterial Blood Pressure
So it's going to be closer to 80 for that reason. And then if you check the pressure over here by this x, it'd probably be something like, let's say And then as you cross the arterial, the pressure falls dramatically. So it's somewhere closer to And then here it's about Here it's about Let's say 10 over here. And then at the very end, it's going to be close to a 5 or so. Let me just write that again. And the units here are millimeters of mercury.
So I should just write that. Pressure in millimeters of mercury. That's the units that we're talking about. So the pressure falls dramatically, right? So from 95 all the way to five, and the heart is a pump, so it's going to instill a lot of pressure in that blood again and pump it around and around. And that's what keeps the blood flowing in one direction.
So now let me ask you a question. Let's see if we can figure this out. Let's see if we can figure out what the resistance is in all of the vessels in our body combined. So we talked about resistance before, but now I want to pose this question. See if we can figure it out.
CV Physiology: Hemodynamics (Pressure, Flow, and Resistance)
So what is total body resistance? And that's really the key question I want to try to figure out with you. We know that there is some relationship between radius and resistance, and we talked about vessels and tubes and things like that. 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. It is initiated by the contraction of the ventricles of the heart. If we consider the entire cardiovascular system, blood flow equals cardiac output. Ventricular contraction ejects blood into the major arteries, resulting in flow from regions of higher pressure to regions of lower pressure. This section discusses a number of critical variables that contribute to blood flow throughout the body.
It also discusses resistance which is due to factors that impede or slow blood flow. As noted earlier, hydrostatic pressure is the force exerted by a fluid due to gravitational pull, usually against the wall of the container in which it is located.
One form of hydrostatic pressure is blood pressure, the force exerted by blood upon the walls of the blood vessels or the chambers of the heart. Blood pressure may be measured in both the systemic and pulmonary circulation; however, the term blood pressure without any specific descriptors typically refers to systemic arterial blood pressure—that is, the pressure of blood flowing in the arteries of the systemic circulation.
In clinical practice, this pressure is measured in mm Hg and is usually obtained using the brachial artery of the arm.
Arterial Blood Pressure Arterial blood pressure in the larger vessels varies between systolic and diastolic pressures. Pulse pressure and mean arterial pressure are calculated values based upon the systolic and diastolic pressures Figure Systolic and Diastolic Pressures When systemic arterial blood pressure is measured, it is recorded as a ratio of two numbers e. The systolic pressure is the higher value typically around mm Hg and reflects the arterial pressure resulting from the ejection of blood during ventricular contraction, or systole.
The diastolic pressure is the lower value usually about 80 mm Hg and represents the arterial pressure of blood during ventricular relaxation, or diastole. The graph shows blood pressure throughout the blood vessels, including systolic, diastolic, mean arterial, and pulse pressures.
Pulse Pressure As shown in Figure For example, an individual with a systolic pressure of mm Hg and a diastolic pressure of 80 mm Hg would have a pulse pressure of 40 mmHg. Generally, a pulse pressure should be at least 25 percent of the systolic pressure. A pulse pressure below this level is described as low or narrow. This may occur, for example, in patients with a low stroke volume, which may be seen in congestive heart failure, stenosis of the aortic valve, or significant blood loss following trauma.
In contrast, a high or wide pulse pressure is common in healthy people following strenuous exercise, when their resting pulse pressure of 30—40 mm Hg may increase temporarily to mm Hg as stroke volume increases. A persistently high pulse pressure at or above mm Hg may indicate excessive resistance in the arteries and can be caused by a variety of disorders such as atherosclerosis. Chronic high resting pulse pressures can degrade the heart, brain, and kidneys, and warrant medical treatment.
Mean is a statistical concept and is calculated by taking the sum of the values divided by the number of values. Although complicated to measure directly and complicated to calculate, MAP can be approximated by adding the diastolic pressure to one-third of the pulse pressure or systolic pressure minus the diastolic pressure: Normally, the MAP falls within the range of 70— mm Hg.
If the value falls below 60 mm Hg for an extended time, blood pressure will not be high enough to ensure circulation to and through the tissues, which results in ischemia, or insufficient blood flow.
A condition called hypoxia, inadequate oxygenation of tissues, commonly accompanies ischemia. The term hypoxemia refers to low levels of oxygen in systemic arterial blood.
Neurons are especially sensitive to hypoxia and may die or be damaged if blood flow and oxygen supplies are not quickly restored. Pulse After blood is ejected from the heart, elastic fibers in the arteries help maintain a high-pressure gradient as they expand to accommodate the blood, then recoil to keep pressure on the blood.
This expansion and recoiling effect, known as the pulse, can be palpated manually or measured electronically. Although the effect diminishes over distance from the heart, elements of the systolic and diastolic components of the pulse are still evident down to the level of the arterioles. It is recorded as beats per minute. Both the rate and the strength of the pulse are important clinically.
A high or irregular pulse rate can be caused by physical activity or other temporary factors, but it may also indicate a heart condition. The pulse strength indicates the strength of ventricular contraction and cardiac output. If the pulse is strong, then systolic pressure is high.
If it is weak, systolic pressure has fallen, and medical intervention may be warranted. Pulse can be palpated manually by placing the tips of the fingers across an artery that runs close to the body surface and pressing lightly. 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.
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. 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.Poiseuille's Law - Pressure Difference, Volume Flow Rate, Fluid Power Physics Problems
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.