# Why does blood pressure drop as you get farther from the left ventricle?

## Main Question or Discussion Point

Blood is in continuous contact with itself. Thus any pressure on artery blood is pressure on vein blood. So they have the same pressure. What am I missing here?

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DaveC426913
Gold Member
Which question do you want an answer to? Change in BP at a distance from the ventricle, or change in BP between arterial and veinous blood?

If the latter, examine the physical proximity between arterial blood and veinous blood. They are certainly not in continuous contact with each other.

You didn't quite understand the question. Blood IS in contact with itself. There is no gap in blood. I have a straw and I blow water from one end to the other end. Why is the pressure of water in the far end of the tube lower than that in the end closer to my mouth? Pressure is imparted on the entire fluid as a whole.

Edit: pushing a line of marbles from one end causes a push of the whole line of marbles. To say that pressure decreases is like saying that the marbles closer to the push are faster, which is phsically impossible because they don't go through each other.

The blood pressure decreases as the blood travels farther away from the heart BECAUSE: the heart is a pump. Imagine with me a water pump. The water pump is pumping water through a smooth straight length of pipe. Each big "pump" makes the pressure go up to 80 to 120mmHg. The inside pressure that is there at all times inside the pipe because it is so full of water is always staying around 45 to 70 mmHg. These numbers are known as systolic and diastolic pressures. Okay? So. If the pipe is really long and not straight but very curvy sometimes then the water pressure at the end of the plumbing will necessarily be lower, will it not? There is less blood pressure in the artery of your big toe, for example than in your left arm.

DrGreg
Gold Member
You didn't quite understand the question. Blood IS in contact with itself. There is no gap in blood. I have a straw and I blow water from one end to the other end. Why is the pressure of water in the far end of the tube lower than that in the end closer to my mouth? Pressure is imparted on the entire fluid as a whole.

Edit: pushing a line of marbles from one end causes a push of the whole line of marbles. To say that pressure decreases is like saying that the marbles closer to the push are faster, which is phsically impossible because they don't go through each other.
The answer is friction. Blood is a viscous fluid and rubs against the side of the vessels it passes through. The vessels exert a force on the fluid opposing its motion and reduce the pressure. In the wide, main arteries, the pressure doesn't drop much (but gravity has some effect). In the narrower arteries and arterioles the effect is much greater. The veins (which operate at much reduced pressure) also have valves in them which have an effect, too.

Wikipedia said:
Mean blood pressure decreases as the circulating blood moves away from the heart through arteries, capillaries and veins due to viscous losses of energy. Mean blood pressure drops over the whole circulation, although most of the fall occurs along the small arteries and arterioles. Gravity affects blood pressure via hydrostatic forces (e.g. during standing) and valves in veins, breathing, and pumping from contraction of skeletal muscles also influence blood pressure in veins.
Wikipedia said:
Resistance. In the circulatory system, this is the resistance of the blood vessels. The higher the resistance, the higher the arterial pressure upstream from the resistance to blood flow. Resistance is related to vessel radius (the larger the radius, the lower the resistance), vessel length (the longer the vessel, the higher the resistance), blood viscosity, as well as the smoothness of the blood vessel walls. Smoothness is reduced by the build up of fatty deposits on the arterial walls. Substances called vasoconstrictors can reduce the size of blood vessels, thereby increasing blood pressure. Vasodilators (such as nitroglycerin) increase the size of blood vessels, thereby decreasing arterial pressure. Resistance, and its relation to volumetric flow rate (Q) and pressure difference between the two ends of a vessel are described by Poiseuille's Law.
Wikipedia said:
Pressure drops gradually as blood flows from the major arteries, through the arterioles, the capillaries until blood is pushed up back into the heart via the venules, the veins through the vena cava with the help of the muscles. At any given pressure drop, the flow rate is determined by the resistance to the blood flow. In the arteries, with the absence of diseases, there is very little or no resistance to blood. The vessel diameter is the most principal determinant to control resistance. Compared to other smaller vessels in the body, the artery has a much bigger diameter (4mm), therefore the resistance is low.[47]

In addition, flow rate (Q) is also the product of the cross-sectional area of the vessel and the average velocity (Q=AV). Flow rate is directly proportional to the pressure drop in a tube or in this case a vessel. ∆P α Q. The relationship is further described by Poisseulle’s equation ∆P=8µlQ/πr^4.[48] As evident in the Poisseulle’s equation, although flow rate is proportional to the pressure drop, there are other factors of blood vessels that contribute towards the difference in pressure drop in bifurcations of blood vessels. These include viscosity, length of the vessel, and radius of the vessel.
Wikipedia contributors, "Blood pressure" Wikipedia, The Free Encyclopedia, (accessed 2 April 2012). Text is available under the Creative Commons Attribution/Share-Alike License

But if pressure is lower in the veins, wouldn't that mean a build up of blood in the veins?

256bits
Gold Member
You didn't quite understand the question. Blood IS in contact with itself. There is no gap in blood. I have a straw and I blow water from one end to the other end. Why is the pressure of water in the far end of the tube lower than that in the end closer to my mouth? Pressure is imparted on the entire fluid as a whole.

Edit: pushing a line of marbles from one end causes a push of the whole line of marbles. To say that pressure decreases is like saying that the marbles closer to the push are faster, which is phsically impossible because they don't go through each other.
A fluid in a pipe encounters friction along the pipe walls. If you lengthen the pipe you need more pressure for the fluid to move at the same flow rate.The pressure is greater at the pump than at the farther end, contrary to waht you have stated.

Your marble analogy is not correct. All marbles have some sort of friction while rolling. The marble you are pushing on has to overcome the rolling friction of all of the marbles down the line. The last marble has to overcome only its rolling resistance, so it has the least amount of force ( or pressure ) on it, while the one you are pushing has the most.

DaveC426913
Gold Member
You didn't quite understand the question. Blood IS in contact with itself. There is no gap in blood. I have a straw and I blow water from one end to the other end. Why is the pressure of water in the far end of the tube lower than that in the end closer to my mouth? Pressure is imparted on the entire fluid as a whole.

Edit: pushing a line of marbles from one end causes a push of the whole line of marbles. To say that pressure decreases is like saying that the marbles closer to the push are faster, which is phsically impossible because they don't go through each other.
I understood the question. You mentioned arterial and veinous blood. They are separated by a significant amount of passage through capillaries. Lots of friction.

In your water and straw analogy, consider what would happen if the middle third of the straw were constricted to 1/10 the normal diameter. You'd have to jam on the water to get it through the constriction at any reasonable flow rate. and even at that it would be at much lower pressure when it came out the other side. Like a trickle.

In your marble analogy, imagine if the marbles had to all be pushed uphill. And that the tube they were being pushed through was flexible.

Make sense now?