Final answer:
When a fluid flows through a narrowed channel, such as a tube or a pipe constriction, its speed increases, leading to a decrease in pressure.
Step-by-step explanation:
When a channel narrows, there is a pressure difference which results in a net force on the fluid.
This net force increases the fluid's kinetic energy, and as a result, the pressure drops in a rapidly-moving fluid, whether or not it is confined to a tube.
For example, when a fluid flows through a narrowed tube, such as in a nozzle or a faucet, the speed of the fluid increases due to the conservation of mass.
This increase in speed leads to a decrease in pressure according to Bernoulli's principle.
Similarly, when a fluid flows through a constriction, like a pipe with a narrower section, the fluid has to speed up to maintain the same flow rate.
This increase in speed again results in a decrease in pressure.