Explain why hydrostatic pressure of blood drops as blood moves away from heart

The arteries divide into arterioles, then these divide into capillaries. You are correct that the lumen gets smaller, but this means that a larger volume of the blood flowing through these vessels is in contact with the lumen wall, so more of the blood is experiencing friction which slows down the flow of the blood.
Hope this helps!

"vessels have larger total cross sectional area - the CSA of each blood vessel/arteriole or capillary is small. But because you have many of these vessels (arterioles or capillaries) then the summation of the CSA is large. So the blood has more "pathways" to take. So there will be reduced blood flow to these capillaries as there will be less blood flowing in each capillary. This is why there is "reduced resistance to blood flow".

as with CSA relating to pressure. this equation should help you to see the link.
Pressure = force/area

so now when we had a greater total CSA, then the pressure decreases (if you consider the maths of the equation).

Thanks for your reply back! I understand now why there is reduced resistance to blood flow in capillaries compared to other blood vessels because there are lots of blood capillaries so less blood is flowing in each capillary and e.g there are less arterioles so more blood is flowing through each arteriole

However, you can also say there is increased resistance to blood flow from arteries-->arterioles-->capillaries because the lumen size decreases and there is much more friction between the blood and the capillary endothelium than the blood and the artery endothelium. So this contradicts the above.

Could you please let me know your thoughts?
Thanks a lot!

It will be useful if I introduce Poiseulle's Law in the pic.
Delta P = pressure gradient.
r = radius of lumen
n = viscosity of blood
l = length of vessel (eg capillary or arteriole etc)
Q = flow rate

Now if you consider the mathematics of the equation, we will assume a constant length for any blood vessel. So only the flow rate and radius will affect pressure gradient along the blood vessel.

What you're trying to say is that decreasing lumen size (so effectively the radius of the lumen) will increase the pressure. That's true - to some extent - because if we look at the proportionalities then decreasing r will obviously decrease the value of r^4. which in turn increases the value of delta P, so the blood pressure will rise as blood flows through narrower blood vessels (for example from the arteriole to the capillary).

However, to what extent will this hydrostatic pressure rise? Well not much, because for one thing the radius of the lumen is very small, and if we decreased the size of the lumen then that change will of course be very small in itself (so the change in r is very small) that means that the change in r^4 is going to be negligible because we are multiply a small change in radius(r) by itself 4 times. The result is that the hydrostatic pressure will increase by a very small amount due to the decreased lumen size.

What the mark-scheme and I are trying to say is that because blood will move through more capillaries, then there will be less blood in each capillary. So this means that Q will be severely reduced ( if there is much less blood moving in a blood vessel then Q must be a small value because the blood from, say the aorta, had "lots of options" as to which arteriole and which capillary to flow into simply because there are a huge number of them). Now a big reduction in Q will greatly reduce the value of hydrostatic pressure if you consider the maths of the equation. (Q is at the numerator so you could say that change in pressure is directly proportional to flow rate of blood, for the same blood vessel).
Don't forget that you also have the loss of hydrostatic pressure due to tissue fluid moving out of the capillary into the tissue so this will also reduce the value of Q.

In general, you have to look at the magnitudes of both effects on the change in hydrostatic pressure, and its that the flow rate will dominate more than the effect of the radius.

I hope this this helps and do let me know if anything else needs explaining.

Thanks a lot! This makes sense!
By "vessels have a larger total lumen" do they mean from artey-->arterioles-->capillaries, the lumen width gets bigger in proportion to the whole width of the vessel or do they mean there are more lumens because there are more vessels (in capillaries)?

because there are more vessels

because there are more vessels

Thank you!
Here are my points for the question:
-Blood vessels are smaller and there are more of them from the artery(pulmonary artery or aorta) to the arterioles to the capillaries
-This means the blood vessels have a larger total lumen(/cross-sectional area/surface area) , where capillaries have the largest total cross-sectional area as there are more capillaries than arterioles for example
-This means there is a reduced resistance to blood flow because for example, more capillaries means less blood is flowing through each capillary compared to in any other blood vessel, so we can say there is less friction between the capillary endothelium and the blood compared to the arteriole endothelium and the blood
-Also, arteries stretch/ expand which reduces the hydrostatic pressure of the blood
-Furthermore, loss of fluid/plasma from the capillaries to form tissue tissue fluid means even less blood is flowing through capillaries, so hydrostatic pressure of the blood is reduced

Just above in my last point, is it correct to say less blood is flowing through capillaries because for the 4th point of the mark scheme, it says "DO NOT CREDIT loss of blood/water" and for there to be less blood flowing through the capillary, there must've been a loss of blood from the capillary?
Furthermore, why do we ignore any reference to veins when answering this question?
Also, could you please check if my points make sense & if I can shorten/write it in a better way?I tend to waffle a bit in my answer
Thanks very very much for all the help so far btw!! I really appreciate it!!!

1. The question is for 2 marks, so you don't need to include all 5 points (1 mark from any statement from top 2 and the other mark from any mention of the statement from bottom 3 points in the MS).

1st point - fine

2nd point - fine (you could simplify by removing the "than arterioles for example" as you already said capilarries have largEST total CSA.

3rd point - fine (although you don't need to go into the detail of more capilarries so less blood in each capillary etc). Just mentioning the reduced resistance is fine to get the mark. (The explanation given was for background knowledge so no need to explain it in the question)

4th point - fine

However, this is my view only, talk to your teacher and ask for their view as theirs is more significant and follow their advice.

2. No what you wrote is completely fine, there is less blood flowing in the capillary. The MS is saying that do not credit "loss of blood/water" because its not blood that is forced out of capillaries (its the plasma component of the blood that is forced out). If you think about it, if blood was forced out then so will red blood cells, all the white cells and the plasma proteins - which cant happen as theyre too big to pass through the pores in the capillaries

3. We ignore references to veins as they return blood back towards the right atrium of the heart (and the question says why hydrostatic pressure reduces as we move away from the heart (= arteries as they transport the blood away from the heart)

Ohhh I see! So blood is made up of plasma and red blood cells, white blood cells & plasma proteins, and when plasma is forced out of the capillary then that means that now there is even less blood in the capillaries too because plasma is a big component of blood which has been forced out of the capillaries.
However, I can't say blood is forced out of the capillaries because that means both the plasma and the red blood cells, white blood cells & plasma proteins are forced out which is incorrect cause red blood cells,white blood cells & plasma proteins are too big.
Have I got this right?

Btw the rest makes sense

Yes. Plasma proteins are too big to pass through.

Just make sure you say that there is even lower hydrostatic pressure in the capillaries too (for the loss of tissue fluid- its safer to say that in the exam.

As with your attempt, just make sure you check with your teacher though.