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Flow in small airways is laminar because: (AIPG 2011)

A Reynolds number in small airways is more than 2000.

B The total cross sectional area of small airways is small.

C The diameter of theses airways is very small.

D The linear velocity of airflow in the small airways is extremely low.

Ans. D The linear velocity of airflow in the small airways is extremely low

Generally, turbulent flow occurs when Re > 2000, and laminar flow occurs when Re < 2000.Q

Cross-sectional area as a function of airway generation:

a. The total cross-sectional area of the airway tree increases as one moves towards the lung periphery because of the dichotomous branching system of the human airway tree. Hence, the total number of parallel airways increases as one moves from the trachea (generation 1) to the lung periphery.

b. Re is related to flow velocity, which is related to tube diameter.

c. Average velocity (cm/sec) of gas in the tube is calculated by dividing the flow rate (cm3/sec) by tube area (cm2). Hence, flow is turbulent in the larger airways, but laminar in more peripheral airways, where the total cross-sectional area is greater.

d. The key point here is the TOTAL cross-sectional area at a given level of the airway tree.

e. So that, even if an individual peripheral airway has a relatively small diameter, the total cross sectional area of all the airways that comprise that airway generation, will be relatively high.

f. Consequently, the average velocity (ie. flow rate/tube area) will be a relatively small number, and according to the above equation, Re (Reynold’s number) will be low and flow will be laminar.

g. This explains why flow in peripheral airways is laminar, even though the radius of an individual peripheral airway is much smaller than that of the main conducting airways.

Poiseuille resistance (R) is thus:


The key point is that R is proportional to the tube length but inversely proportional to the fourth power of the tube radius. This means that resistance increases very rapidly as tube radius decreases.


a. The flow of blood in straight blood vessels, like the flow of liquids in narrow rigid tubes, is normally laminar (streamline).

b. Within the blood vessels, an infinitely thin layer of blood in contact with the wall of the vessel does not move.

c. The next layer within the vessel has a low velocity, the next a higher velocity, and so forth, velocity being greatest in the center of the stream. Laminar flow occurs at velocities up to a certain critical velocity.

d. At or above this velocity, flow is turbulent. Streamline flow is silent, but turbulent flow creates sounds.


a. The probability of turbulence is also related to the diameter of the vessel and the viscosity of the blood. This probability can be expressed by the ratio of inertial to viscous forces as follows:

o Where,

i. Re is the Reynolds number, named for the man who described the relationship;

ii. p is the density of the fluid;

iii. D is the diameter of the tube under consideration;

iv. V is the velocity of the flow; and is the viscosity of the fluid.

b. The higher the value of Re, the greater is the probability of turbulence. When D is in cm, V is in cm/s–1, andin poises; flow is usually not turbulent if Re is less than 2000.Q

c. When Re is more than 3000, turbulence is almost always present.

d. Laminar flow is disturbed at branching of arteries, but normally not to the point that turbulence is produced.

e. Constriction of an artery increases the velocity of blood flow through the constriction, producing turbulence, and consequently sounds, beyond the constriction.

f. Examples arebruits heard over arteries constricted by atherosclerotic plaques and the sounds of Korotkoff heard when measuring blood pressure.

g. In humans, the critical velocity is sometimes exceeded in the ascending aorta at the peak of systolic ejection, but it is usually exceeded only when an artery is constricted. Turbulence occurs more frequently in anemia because the viscosity of the blood is lower. This may be the explanation of the systolic murmurs that are common in anemia.

Capillaries have highest total cross-sectional area.Q

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