Physiology
Respiration
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.

Generally, turbulent flow occurs when Re > 2000, and laminar flow occurs when Re < 2000.^{Q}
Crosssectional area as a function of airway generation:
a. The total crosssectional 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 crosssectional area is greater.
d. The key point here is the TOTAL crosssectional 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:
R=8nL/πr4
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.
Capillaries have highest total crosssectional area.^{Q}
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