As discussed before, flow can be of two types: "laminar" and "turbulent". Factors affect these two types of flow differently.
Effect of PRESSURE
You need a pressure difference to make flow happen. Flow occurs from an higher pressure to a lower pressure. Greater the difference, greater is the flow. In the diagram below, the person is squeezing the fluid bag to increase the flow. This works because the squeezing of the bag increases its pressure (P1). Now there is a greater difference between the pressure in the bag (P1) and the other end of the tube (P2), which thus leads to a greater flow.
In laminar flow, flow is directly proportional to the pressure difference. Double the pressure difference and the flow will double. Triple the pressure difference and the flow will triple. A graph plotted between pressure and laminar flow will show a linear (straight line) relationship.
Effect of DIAMETER
The diameter of a tube has a huge impact on the flow. If you half the diameter of the tube, for the same pressure difference, the flow will decrease by sixteen times!
Just imagine, for some sadistic reason, that you change a patients endotracheal tube from a size 8 mm to a size 4 mm. For the same ventilator pressure, you will reduce the flow by 16 times! The same effect can happen if secretions or kinks reduce the diameter.
Effect of LENGTH
Longer the length, less is the flow. The effect is less dramatic than the reduction caused by halving the diameter. For laminar flow, if you double the length of the tubing, the flow will be halved.
You might see this effect clinically. For an example, in situations where you need to be at some distance from the patient, you might use "extra" long tubing to connect fluids to the patient. This longer tubing will lessen the flow.
Effect of VISCOSITY and DENSITY
Viscosity is a measure of the "friction within the fluid". Imagine laminar flow to occur in layers. Viscosity will be the friction between these layers. This "resistance" resists flow. In laminar flow, higher the viscosity, lower is the flow.
It is important to note that since turbulent flow has a different pattern, viscosity does NOT affect turbulent flow. Turbulent flow is affected by another physical property called "density".
POISEUILLE'S EQUATION (sometimes also called "Hagen - Poiseuille's equation")
This equation is important to remember because it summarises all the above. It describes the laminar flow through a tube.
Remember, this equation is for laminar flow and not turbulent flow.
The Q with a dot on top is a symbol for flow.
Flow is directly proportional to the pressure difference (P1 - P2).
Flow is proportional to the fourth power of the radius. It is because of this fourth power, when you double the radius of the tube, the flow increases by 16 times (2 x 2 x 2 x 2 = 16 ). In a similar way, when you half the radius, the flow decreases by 16 times (1/2 x 1/2 x 1/2 x 1/2 = 1/16 ).
Don't forget the PI symbol and the number 8, both constants.
The symbol like a "n" is the Greek letter "eta", representing viscosity. Because an increase in viscosity decreases flow, viscosity appears in the denominator (the lower part) of the equation.
Similarly, because an increase in length decreases flow, it also appears in the denominator of the equation.
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difference, the flow will decrease by sixteen times!
