...So if i have arteriolar constriction, the velocity of blood flow to the capillary will increase. But if i have arteriolar constriction for example during sympathetic activity, and if the velocity of blood flow increases why dont i hear any bruit in that case? Since you mentioned if there is narrowing of vessels either by stenosis of increasing resistance, blood flow will be at a high velocity and there will be an abnormal sound. right?
Sorry, i forgot to mention something important about this. In general, if flow is constant, and if area is decreased, velocity is increased. Whether or not we hear a bruit depends on the
Pressure. In the case of bruits, you normally have a short area of stenosis i.e. a calcified valve or a short segment of artery with atherosclerotic plaque. In this case the high pressure behind the obstruction will cause a jet of blood to shoot through the orifice and the blood behind the obstruction will then take on a turbulent pattern in the area of lower pressure.
Recall that Blood pressure changes in this pattern depending on how far we are from the heart
1. Leaving the Heart through the Aorta ~120mmHg
2. Arterioles ~ 50 mm Hg
3. Capillaries ~20 mmHg
4. Vena Cava ~ 4 mmHg
This explains why common sites for bruits and murmurs are in the larger muscular and elastic arteries and are often heard in the carotids, abdominal aorta, renal artery, valves, and stenotic PDA. It all depends on the pressure.
Q= MAP-Right atrial pressure / TPR.
Arterioles are the site of highest resistance. Q has to remain unchanged to keep flow constant. Therefore, if TPR goes up,Mean arterial pressure has to go down. Thus, the largest drop in pressure occurs at the arterioles. Low pressure behind this area of narrowing means that no bruits will be heard.
I am actually kind of confused about this. Because its written in one Physio book, that increasing the diameter will increase the velocity. It didnt say whether its arteriolar diameter or diameter of an artery. The same book and other physio books also said if for exmaple carotid artery is narrowed/stenosed , again the velocity will increase.Only this time every book mentioned about specific part- the artery. This created a confusion.
The only situation i can think of where increasing diameter will increase velocity is say, if you had very low flow to begin with and you increase the diameter to the point where blood flow is now higher. In this scenario, blood velocity will appear to have RELATIVE increase from baseline.
In all other cases, I think F = v x A still holds true for laminar flow.
1. Any artery is a high pressure system and close to the pumping machine. So it has a very high velocity of blood flow. So if there is any sudden narrowing , velocity of blood flow will be further increased through the narrowed portion to prevent the accumulation.
You are right, high pressure will allow for higher velocity. Even though I present the equation F= v x A all the time, i am looking it in isolation without factoring in pressure.
2.Arteriole is far away pumping machine and so blood flow in an arteriole is very slow i.e. very low velocity of blood flow. So, when there is arteriolar constriction in sympathetic activity , blood flow rate will be further decreased i.e velocity is further decreased because it was already a narrowed tube and blood was going real slow and now it is further narrowed and hence velocity further decreased.
It is not so much because it is far away that blood flow is slow, it is the fact that it has such high resistance. But you are correct, increased TPR will slow blood flow and velocity will have to go down with it.
Q=delta P/TPR is another equation we use for this. Arterioles have high resistance so flow is decreased. Now you must be wondering, well if flow is decreased in cardiac output is 5 L/min, what happens to the rest of the cardiac output? This is because arterioles cannot be viewed in isolation. You have many arterioles and capillaries throughout your body and it is important to note that TOTAL CROSS SECTIONAL AREA increases as we move away from the heart to the capillaries and decreases as we move from capillaries to the heart. So sure for each arteriole, the flow will be slow. But if you add them all up, flow is constant.
http://www.sci.utah.edu/~macleod/bioen/be6000/notes/L09-hemo.pdf
This pdf has some amazing figures that show the relationship between velocity and cross sectional area.
