Venous Return, Venous Pressure

[sps27]

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TBR bio book 1 psg 3, page 114. This passage in on terms like venous return, venous pressure, cardiac output etc.

1) Startlings law states that an increase in cardiac output leads to an increase in venous pressure. 2) The passage also states that if the cardiac output and venous return are increased, the arterial pressure will increase while the pressure in the venous system will decrease. This is due to the increased transfer of blood from the venous to arterial system.

These two statements seem contradictory. So if I increase cardiac output by sympathetic simulation of heart, this leads to constriction of the arteries which increases mean arterial pressure, venous return (I think) will increase because more volume of blood is in play now, shouldn't venous pressure also increase as there is more blood in veins. Why does venous pressure decrease? Why are venous return and venous pressure inversely related? Also, when arteries constrict, will the veins constrict simultaneously? They both have smooth muscle lining. It is also a bit difficult to understand that veins which carry 64% of blood volume at any given time are at a lower pressure than arteries which carry about 15%.

Any thoughts, insights?

 

[sazerac]

Starling's law states that the output (for instance cardiac output) is based on the input (for instance how much the pressure in the veins is shoving blood into the heart). You seem to be stating it backwards in your (1).

The other thing you need to know is that the big clog in the vascular system is at the arterioles. Not the capillaries, because while each capillary is small, there are just so many that the pressure drop-off through the capillaries is not very much. The big drop in pressure, the big constriction if you will, is at the arterioles.

So sure, you have the veins which have a lot of blood at low pressure, a heart that brings things up to maximum pressure, and pressure declines along the path, especially at the arterioles. If your cardiac output and venous return increase, you are just taking more blood from the veins and putting it into the arteries.

Let me know if this helps. Your second paragraph is a mish-mash of confusion and some false assumptions, which I hope I have cleared up. If you need some more help, post a reply.

 

[sps27]

Starling's law states that the output (for instance cardiac output) is based on the input (for instance how much the pressure in the veins is shoving blood into the heart). You seem to be stating it backwards in your (1).

The other thing you need to know is that the big clog in the vascular system is at the arterioles. Not the capillaries, because while each capillary is small, there are just so many that the pressure drop-off through the capillaries is not very much. The big drop in pressure, the big constriction if you will, is at the arterioles.

So sure, you have the veins which have a lot of blood at low pressure, a heart that brings things up to maximum pressure, and pressure declines along the path, especially at the arterioles. If your cardiac output and venous return increase, you are just taking more blood from the veins and putting it into the arteries.

Let me know if this helps. Your second paragraph is a mish-mash of confusion and some false assumptions, which I hope I have cleared up. If you need some more help, post a reply.

Thanks for your response. I have uploaded the passage and basically my doubts were regarding question 17 and 19. Files 1 and 2 are the question stem and Files 3 and 4 are the answers to the question. Apologize for the file size. Only 640 by 640 is allowed. Hopefully you can resize it in Paint.