West's zone 2 (starling resistor), flow throw a flexible tube

[coolia]

My question is in regards to a starling resistor. A starling resistor is where a flexible tube passes through a box that can have it's pressure changed. If fluid passes through the tube then flow through the box will be determined by the pressure differences from the two sides of the box provided the pressure on both sides of the tube outside the box are greater than the pressure in the box.

If pressure in the box becomes greater than the downstream pressure of the tube coming out of the box then flow will be determined by the difference between the upstream pressure and the pressure in the box. The tube will collapse at a point where the pressure in the tube becomes less than the pressure in the box (this is a pressure of 6 in the diagram below). The downstream pressure will not affect flow through the tube. I attached an image that goes along with the above explanation. Pa(arterial)=upstream pressure, Pv(venous)=downstream pressure and PA(alveolar)=pressure in the box. The above occurs physiologically in bodies most popularly in the capillaries of the lungs (zone 2). My question is how is the upstream pressure - pressure in the box the driving pressure (as they say it is) when the pressure driving flow through a tube should be difference between the pressure at the two ends of the tube? Does the collapse occur gradually, in other words is end closest to the box a smaller opening than more upstream. Also, I'm assuming the resistance increases in the tube because of the collapse and this would be the cause of the decreased flow and not a change in pressure because the downstream and upstream pressure are the same. There would also be a greater drop in pressure from the collapsed ends than from the open ends, due to the collapsed ends having a greater resistance. Is this correct?
Thank you.

 

[Valerie Park]

Yes, the upstream pressure minus the pressure in the box is the driving pressure. The collapse can occur gradually, depending on the pressure gradient between the upstream and downstream pressures. The resistance of the tube does increase as a result of the collapse, causing a decrease in flow. This is due to the increased resistance of the collapsed ends, which causes a greater drop in pressure from the collapsed ends than the open ends.

 

[charng]

I can provide some insights into the concept of a starling resistor and its role in fluid flow through a flexible tube.

Firstly, it is important to understand that the driving force for fluid flow through a tube is the pressure difference between the two ends of the tube. This is known as the pressure gradient. In the case of a starling resistor, the driving pressure is indeed the difference between the upstream pressure and the pressure inside the box. This is because the pressure inside the box acts as an additional resistance to the flow of fluid through the tube.

It is also important to note that the collapse of the tube occurs when the pressure inside the tube becomes lower than the pressure inside the box. This happens because the pressure inside the tube is not strong enough to keep the tube open against the resistance from the box. This collapse can occur gradually, with the end closest to the box collapsing first due to the higher resistance.

In terms of the resistance increasing in the tube, this is correct. The collapse of the tube creates a smaller opening, which increases the resistance to flow. This increased resistance is what leads to a decrease in flow, not a change in pressure. And you are correct in noting that there will be a greater drop in pressure from the collapsed ends compared to the open ends, due to the increased resistance.

In summary, the concept of a starling resistor is an important one in understanding fluid flow through a flexible tube, and it is applicable in physiological systems such as the capillaries of the lungs. The driving pressure for flow in this case is the pressure difference between the upstream pressure and the pressure inside the box, and the collapse of the tube is due to the increased resistance from the box. I hope this helps answer your questions.