TABLE 1.

Unpacking the core concept of flow down gradients^a

F1. Flow is the movement of “substances” from one point in the system to another point in the system.
F1.1	Molecules and ions can diffuse through a solution.
F1.2	Fluid (blood, chyme) and gases are transported through tubes.
F1.3	Heat can move through objects.
F2. Flow occurs because of the existence of an energy gradient between two points.
F2.1	Differences in concentration (concentration gradients) cause molecules and ions in solution to move down a gradient from high to low concentration.
F2.2	Differences in electrical potential (potential gradient) cause ions in solution to move from high to low electrical potential.
F2.3	Differences in pressure (pressure gradient) between two points in a system cause substances to move toward a region of lower temperature.
F2.4	Differences in temperature (temperature gradient) between two points cause heat to flow from the hotter to the cooler location.
F3. The magnitude of the flow is a direct function of the magnitude of the energy gradient that is present—the larger the gradient, the greater the flow.
F4. More than one gradient can determine the magnitude and direction of flow.
F4.1	Osmotic (concentration gradient for water) and hydrostatic pressure gradients together determine flow across the capillary wall.
F4.2	Concentration gradients and electrical gradients determine ion flow through channels in cell membranes.
F5. There is resistance or opposition to flow in all systems.
F5.1	Resistance and flow are reciprocally related—the greater the resistance, the smaller the flow.
F5.2	The resistance to flow is determined by the physical properties of the system.
F5.3	Some resistances can be varied and can be actively (physiologically) controlled.
F5.3.1 Ion channels in a membrane can open and close, changing resistance (decreasing and increasing, respectively).
F5.3.2 Arterioles and bronchioles can constrict (increasing resistance) and dilate (decreasing resistance).
F5.3.3 Pilo-erection can increase the resistance to heat flow in many animals.

^aThis is Table 6.1 in Michael et al. (2017).