Flow and Poiseuille's law in operation
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Flow and Poiseuille's law in operation
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[post_date] => 2025-01-09 07:47:39
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[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Poiseuille’s Law is a description of the pressure of a fluid as it travels through a cylindrical pipe. Combined with Ohm’s law describing flow as a function of pressure and resistance, clinicians can determine the amount of fluid delivery in many different aspects of medicine, including arterial blood flow, IV administration, and airway resistance. This is readily apparent in conditions like asthma, where resistance is caused by the decrease in airway diameter due to inflammation.
In applying Poiseuille’s law to an airway, variables that are inversely related to flow such such as length of the airway and viscosity of the fluid can be ignored, as these are constant. Therefore, the airflow into the lungs then is directly dependent on the air pressure as well as the radius of the airway. In addition, because flow is related exponentially (r4) to the radius of the airway, even modest decreases in airway radius can result in respiratory distress due to lack of oxygen availability.
In an operating room, Poiseuille’s law is extremely important. For procedures requiring general anesthesia, a patient’s airway must be kept open for proper exchange of oxygen and carbon dioxide. This can be achieved via an endotracheal tube. If an improperly sized tube is provided, a patient may not receive enough oxygen and enter respiratory distress. Similarly, if a patient is rapidly losing blood, blood transfusions must be done to prevent the patient from bleeding out. If the IV needle used for transfusion is not an appropriate size, complications can occur, including simply not returning enough blood if the flow is too little, and inducing fluid overload if the flow is too high.
Attribution: https://www.gosh.nhs.uk/health-professionals/clinical-guidelines/flush-volumes-for-central-venous-access-devices-cvads/ Berry, K., Bravery, K., & Ho, A. (2014). Central venuous access devices (CVADs): flush volumes. Great Ormond Street Hospital for Children. London, UK.
[post_title] => Flow and Poiseuille's law in operation
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[question] => If a patient is critically hypotensive (has dangerously low blood pressure) and you are trying to increase their pressure as fast as possible via an IV infusion of fluid, what would the best choice to administer the most fluid?
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[answer] => 2
[description] => Reason for the Correct Answer:
To administer fluid as fast as possible, you would want to maximize the flow of the fluid.
Resistance to flow is directly proportional to the length of a tube, so the longer the tube, the more resistance, therefore you’d have less flow.
Since the radius is exponentially and directly proportional, a lumen of 2mm with a shorter length (5mm) would provide the fastest choice of infusion.
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[each_answer] => A. A needle of 7cm length and 1 mm lumen radius
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[each_answer] => B. A needle of 5cm length and 2 mm lumen radius
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[each_answer] => C. A needle of 7cm length and 2 mm lumen radius
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[each_answer] => D. A needle of 5cm length and 1 mm lumen radius
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => Waldenström’s macroglobulinemia is a condition which causes increased blood viscosity due to high protein content in the blood. How would Waldenström’s macroglobulinemia influence blood flow and pressure?
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[answer] => 4
[description] => Reason for the Correct Answer:
Viscosity is directly proportional to resistance.
Blood flow is inversely proportional to resistance.
Blood pressure is directly proportional to flow.
If you have increased viscosity, you’d have increased resistance, resulting in less flow, and consequently lower pressure.
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[each_answer] => A. Blood flow would decrease, blood pressure would increase
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[each_answer] => B. Blood flow would be increased, blood pressure would increase
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[each_answer] => C. Blood flow would be increased, blood pressure would decrease
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[each_answer] => D. Blood flow would decrease, blood pressure would decrease
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[2] => Array
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[quiz_unique_key] => 83407773
[question] => IV lines must be regularly ‘flushed’ to avoid infection and line occlusion. More turbulent flushing better removes any particles attached to a catheter wall, which decreases chances of complications. Which of the following choices would be best suited to flush an IV line?
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[answer] => 3
[description] => Reason for the Correct Answer:
Turbulence creates eddies, the higher the velocity of flow, the higher the turbulence.
A highly viscous fluid would decrease the velocity of flow, making it more laminar.
A smaller lumen would increase the velocity of flow, increasing turbulence, therefore a low viscosity fluid with a small needle would be best suited.
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[answers] => Array
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[0] => Array
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[each_answer] => A. A low viscosity fluid with a large lumen needle
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[each_answer] => B. A high viscosity fluid with a large lumen needle
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[each_answer] => C. A low viscosity fluid with a small lumen needle
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[each_answer] => D. A high viscosity fluid with a small lumen needle
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[quiz_unique_key] => 2261298308
[question] => Emphysema is a condition which results from the alveoli in the lungs losing their elasticity. Which of the following variables responsible for fluid flow would likely be affected by emphysema?
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[answer] => 4
[description] => Reason for the Correct Answer:
The length of the airway would not be affected by decreased elasticity. The dynamic diameter, however, would be affected, as its expansion and restriction would be limited.
The viscosity of air would not be changed.
The radius of the airway is determined by the bronchioles, not the alveoli. (There is some bronchoconstriction with COPD disorders, but it is not due to elasticity loss)
The pressure exerted on air by the lungs would be decreased during exhalation.
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[each_answer] => A. Radius of the airway
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[each_answer] => B. Viscosity of the fluid
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[each_answer] => C. Length of the airway
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[each_answer] => D. Pressure on the fluid
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[quiz_unique_key] => 574431310
[question] => Would you expect the vascular resistance to be higher in a vessel with a 3 cm radius and 50 cm long, or in a shorter vessel with a 1 cm radius and 1 cm long?
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[answer] => 1
[description] => Reason for the Correct Answer:
The viscosity and blood pressure in both vessels would be constant, and therefore can be ignored when calculating resistance.
While length is directly proportional, and the radius is inversely proportional, the radius is also proportional at an exponential amount (r⁴)
Even though the 3 cm radius vessel is 50 times longer, the radius decreases the resistance at 3⁴ or 81 times more. Therefore the 1 cm radius vessel would have more resistance than the 3 cm radius vessel.
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[0] => Array
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[each_answer] => A. The resistance would be higher in the shorter vessel
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[each_answer] => B. The resistance would be higher in the longer vessel
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[each_answer] => C. The resistances would be equal to one another
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[each_answer] => D. You would need to know the viscosity and blood pressure
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