Flow Down Gradients
Flow is the movement of “stuff” from one point in a system to another point in the system.
- Molecules and ions in solution move from one point to somewhere else.
- Fluids (blood, chime) and gases (air) move from one point to another.
- Heat moves from one place to another.
Flow occurs because of the existence of an energy gradient between two points in the system.
- Differences in concentration (concentration gradients) cause of molecules and ions in solution to move toward a region of lower concentration.
- Differences in electrical potential (potential gradients) causes ions in solution to move.
- Differences in pressure (pressure gradients) between two points in a system cause substances to move toward a region of lower pressure.
- Differences in temperature (temperature gradients) between two points cause heat to flow.
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.
More than one gradient may determine the magnitude and direction of the flow.
- Osmotic (concentration gradient) and hydrostatic pressures together determine flow across capillary walls.
- Concentration gradients and electrical gradients determine ion flow through channels in cell membranes of neurons and muscle cells.
There is resistance or opposition to flow in all systems.
- Resistance and flow are reciprocally related, the greater the resistance the smaller the flow
- Resistance is determined by the physical properties of a system
- Some resistances are variable and can be actively controlled
- ion channels in a membrane can open and close (increasing resistance)
- arterioles and bronchioles can constrict and dilate
- piloerection can increase the resistance to heat flow in many mammals
Misconceptions for Flow Down Gradients
Partial summary of misconceptions about flow from the October 2011 team meeting.
Component Idea II (gradient):
Component Idea III (magnitude of flow):
- To some students, the magnitude of the numbers (values), not the magnitude of the difference, is what matters.
- To some students, a larger number means greater flow.
- When given two numbers, students may think that only the higher number is important, not the lower number.
- Students may consider the magnitude of the gradient, but not the direction.
- Students may think that only the upstream number matters.
Component Idea IV (more than one gradient):
- Some students have difficulty considering two elements at the same time.
Component Idea V (resistance):
- Students may think that only the gradient matters and may not consider resistance.
- Some students cannot transfer concepts such as permeability and conductance into resistance.
- The inverse relationship between resistance and conductance is challenging for students.
- Students have a hard time with "why aren't capillaries the site of greatest resistance" because they think of one capillary, not the total cross-sectional area of the capillary bed.
- Students may have a hard time understanding that lipid bilayers are more permeable to lipid-soluble molecules than to ions.
- Students may not understand that resistance is defined by solubility.
- Students may not understand that viscosity doesn't change much but it can change.