Organismal Physiology Lecture No. 9: Vascular Transport I
Tuesday October 9 2012h
-Vascular transport is the movement of fluids (including gases) through tubes. The major difference in
vascular transport between plants and animals is that animals push, while plants push their vital fluid
through their vascular system.
Flow Rate Through A System:
-The flow rate (Q) is defined as the difference in pressure between the entry and exit to the system. It is
quantified with the following equation: Q = ∆P / R , where ∆P = p – p 2nd R1is the resistance in the
system. Flow rate can be determined by the pressure at the start of the system, the pressure loss in the
system, or the resistance in the system. Also note that pressure is proportional to both Q and R.
-Resistance is calculated by the following equation: R = 8 L η / π r , where L is the length of the tube, η is
the viscosity coefficient and r is the radius (diameter) of the tube. In terms of patterns associated with
determining resistance, longer systems typically have more resistance, higher viscosity means greater
resistance, and resistance being proportional to the 4 power of the tube’s radius (an important
component of vascular systems as it greatly reduces resistance).
-Thus by combining the original flow rate formula and the resistance formula we receive a combined
flow rate equation of: Q = ∆P π r / 8 L η
-The difference in pressure is affected by density (ρ), acceleration due to gravity (g) and the height
difference across the system (∆H). An equation would look as follows: ∆P = ρ g ∆H
-The difference in height across the system is proportional to the pressure gradient.
-Pressure gradients are helpful in driving fluid transport. Animals use positive pressure to push fluids
through pipes referred to as veins and arteries. Plants use negative pressure to pull fluids through pipes
referred to as xylem. In both cases, vascular fluids move down a pressure gradient (from a higher to a
Vascular Transport In Plants:
-As the plant’s leaves transpire, the evaporation creates a low pressure system at its extremities. The
establishment of this low pressure area forces water to be drawn up from the roots (a less negative,
higher pressure system) and pulled up through the plant’s vascular tubing. Capillary Action (Capillarity):
-The problem with plants pulling water from their roots is because the atmospheric pressure of the
Earth can only support a rise of water by about 10.3m. How is it possible then, that plants such as the
gigantic Californian Redwood trees can engage in vascular transport?
-Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of, and in
opposition to external forces like gravity. Plant capillaries have what are known as “wettable” walls,
where there is strong adhesion between the liquid and the walls of the vessel. There is also surface
tension between water molecules, which allows for further adhesion. Generally, a decrease in diameter
increases the height of the water column supported. Hydrogen bonds cause elasticity and wettable
walls. As xylems possess all of these features, capillary action can account for a few extra metres in
-Hydrogen bonds cause cohesion between water molecules and adhesion between water and xylem.
The stretching of hydrogen bonds as water evaporates causes tension and thus, negative pressure.
Tension is propagated from the leaves, through the xylem, to the soil. There are tiny pores in between