Transport

Hop on the crosstown bus, and you’ll transport yourself from one place to another. Engineers, biologists and physicians can all agree on that much. But that’s about where the agreement stops. What’s more, an engineer discussing transport could mean one of a number of things.

To a fluids engineer, transport means that fluids are literally moving from one place to another. Because the detailed movements of fluids are often highly complex and mathematically difficult to predict, engineers use computational fluid dynamics to predict in detail and in three dimensions how fluids will speed up, slow down, or eddy out over time. Vrishank Raghav, an assistant professor of aerospace engineering at Auburn University, uses computational fluid dynamics to examine the aerodynamics of helicopter rotors. He also uses similar methods to analyze how cardiovascular disease alters blood flow through blood vessels, and how chronic obstructive pulmonary disease alters airflow through airways in the lung. In all these cases, pressure differential transfers momentum to the bulk fluid, and the substance moves from one place to another.

When heat is transported rather than momentum, the substance itself does not move. Heat can be transferred in one of three ways. It can be transferred by conduction, flowing from one object to another that’s in direct contact, just as heat flows from the base of a cast-iron frying pan to the handle. Understanding heat conduction would be important for a biomedical engineer designing a thermal therapy, which kills cancerous tissue by heating it, but needs to leave the neighboring tissue intact.

Alternatively, the heat can be transferred by convection, the way a flowing mixture of water and antifreeze cools the engine block of a car. It can also be transferred by radiation, in which an object emits long infrared waves that transmit heat without physical contact, the way heat from a hot radiator can be felt some distance away.

All three methods of heat transfer matter in medicine. To understand heat stroke and how to avoid it, for example, a physiologist might analyze how the sun’s radiation heats a trail runner on a hot day, how that runner cools off her core when convection brings warm blood near the skin surface, how a breeze moving by convection causes sweat to evaporate, cooling her flushed, hot skin, or what happens when she sits down on the cool ground in the shade and cools off by conduction.

For biologists or physicians, transport can also refer to how drugs, nutrients, wastes or other substances circulate, or how they cross through capillary walls into or out of the bloodstream. The sugar from the energy bar the runner ate, for example, is transported from the small intestine through the bloodstream to the runner’s muscles.

For a cell biologist or molecular biologist, transport refers to something else still. To function properly, all cells regulate the concentrations of key substances inside them—by synthesizing them, destroying them, or transporting them in or out. Some molecules, including gases like oxygen and carbon dioxide, easily cross the membrane by passive transport, simply diffusing from an area of high concentration outside the cell to an area of low concentration inside the cell, or vice versa. Others diffuse across only via a process called facilitated diffusion, in which gateway proteins called channels are open. Still other molecules move from low concentration to high in a process called active transport, in which specialized proteins pump the molecules into the cell to concentrate them.

Cell biologists have spent years working out how specific channel and pump proteins regulate transport of specific substances, and how these channels are opened or pumps activated on command. If you hear a biologist talk about transport, keep listening—there’s likely to be a lot of detail you still need to grasp.