a measurement of quantity over a specified period of time. Unlike a STOCK, which is not a function of time, a flow measures quantity passing per minute, hour, day or whatever. A common analogy is to a reservoir. The water entering and leaving the reservoir is a flow but the water actually in the reservoir at any one point in time is a stock. INCOME is a flow but WEALTH is a stock.
5] Even in teenagers who have smoked only a few years, maximum expiratory flow volume curves demonstrate decreases in flow rates at small lung volumes  yet another expression of small airway obstruction.