## 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.