Blood Flow, Blood Pressure, and Resistance | Anatomy & Physiology
pressure drives blood flow. Thus, we have an inverse relationship between blood vessel resistance and the blood flow rate - the higher the resistance, the. Remember Ohm's Law from high school? V=IR Voltage=Current times Resistance If we substitute pressure instead of voltage, and flow instead. hemodynamics of blood flow. In relating Ohm's Law to fluid flow, the voltage difference is the pressure difference (ΔP; sometimes called driving pressure, perfusion pressure, graphical relationship between pressure, flow and resistance.
And this is one branch of the aorta. I didn't draw a lot of the other ones.
Hemodynamics (Pressure, Flow, and Resistance)
This is the brachial artery. And the blood is flowing from the aorta into the brachial artery. And let's say that the blood is trying to make its way out to a fingertip, for example. So on its way out there, it makes its way to an arterial. And the blood continues flowing, and it goes into the capillary bed, and the vessels are too small to draw, so I just kind of do that thing.
And it then goes into the other half of the capillary bed, where now the blood is deoxygenated. So I'm going to draw that as blue. 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.
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? Women are more affected with Atherosclerosis [ 19 ]. At times increase in blood pressure may leads to various kinds of health problems [ 2021 ]. Heart failure patients are at increased risk of sudden death due to ventricular problems [ 22 - 24 ].
Diabetes Mellitus DM is also a main risk factor for heart failure [ 25 - 27 ]. Most of the cardiovascular emergencies are caused by coronary artery disease [ 2829 ]. Echocardiography is the modality of choice for investigation of suspected congenital or acquired heart disease [ 30 - 32 ] Suspected heart disorders and related heart diseases can be investigated using Echocardiogram [ 33 - 35 ].
The frequency of the cardiac cycle is described by the heart rate [ 36 ]. There are two phases of the cardiac cycle. The heart ventricles are relaxed and the heart fills with blood in diastole phase [ 37 ]. The ventricles contract and pump blood to the arteries in systole phase [ 38 ]. When the heart fills with blood and the blood is pumped out of the heart one cardiac cycle gets complete.
The events of the cardiac cycle explains how the blood enters the heart, is pumped to the lungs, again travels back to the heart and is pumped out to the rest of the body [ 39 ]. The important thing to be observed is that the events that occur in the first and second diastole and systole phases actually happen at the same time [ 40 ].
During this first diastole phase, the atrioventricular valves are open and the atria and ventricles are relaxed. From the superior and inferior vena cavae the de-oxygenated blood flows in to the right atrium. The atrioventricular valves which are opened allow the blood to pass through to the ventricles [ 41 ].
The Sino Atrial SA node contracts and also triggers the atria to contract. The contents of the right atrium get emptied into the right ventricle.
During this first systole phase, the right ventricle contracts as it receives impulses from the Purkinje fibers [ 42 ]. The semi lunar valves get opened and the atrioventricular valves get closed.
The de-oxygenated blood is pumped into the pulmonary artery. The back flow of blood in to the right ventricle is prevented by pulmonary valve [ 43 ]. The blood is carried by pulmonary artery to the lungs. There the blood picks up the oxygen and is returned to the left atrium of the heart by the pulmonary veins [ 44 ].
In the next diastolic phase, the atrioventricular valves get opened and the semi lunar valves get closed. The left atrium gets filled by blood from the pulmonary veins, simultaneously Blood from the vena cava is also filling the right atrium. The Sino Atrial SA node contracts again triggering the atria to contract. The contents from the left atrium were into the left ventricle [ 45 ]. During the following systolic phase, the semi lunar valves get open and atrioventricular valves get closed.
The left ventricle contracts, as it receives impulses from the Purkinje fibers [ 47 ]. Oxygenated blood is pumped into the aorta. The prevention of oxygenated blood from flowing back into the left ventricle is done by the aortic valve. Aortic and mitral valves are important as they are highly important for the normal function of heart [ 48 ].
The aorta branches out and provides oxygenated blood to all parts of the body. The oxygen depleted blood is returned to the heart via the vena cavae. Left Ventricular pressure or volume overload hypertrophy LVH leads to LV remodeling the first step toward heart failure, causing impairment of both diastolic and systolic function [ 4950 ]. Coronary heart disease [CHD] is a global health problem that affects all ethnic groups involving various risk factors [ 5152 ].
Vasodilation Vasodilation is increase in the internal diameter of blood vessels or widening of blood vessels that is caused by relaxation of smooth muscle cells within the walls of the vessels particularly in the large arteries, smaller arterioles and large veins thus causing an increase in blood flow [ 53 ].
When blood vessels dilate, the blood flow is increased due to a decrease in vascular resistance [ 54 ]. Therefore, dilation of arteries and arterioles leads to an immediate decrease in arterial blood pressure and heart rate hence, chemical arterial dilators are used to treat heart failure, systemic and pulmonary hypertension, and angina [ 55 ].
At times leads to respiratory problems [ 56 ]. The response may be intrinsic due to local processes in the surrounding tissue or extrinsic due to hormones or the nervous system. The frequencies and heart rate were recorded while surgeries [ 57 ]. 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.
- Blood pressure, blood flow, and resistance
- Effects of Vasodilation and Arterial Resistance on Cardiac Output
- Putting it all together: Pressure, flow, and resistance
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.
Pressure and Blood Flow
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.
CV Physiology: Hemodynamics (Pressure, Flow, and Resistance)
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.
Effects of Vasodilation and Arterial Resistance on Cardiac Output | OMICS International
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.