Bernuolli's principle
http://en.wikipedia.org/wiki/Bernoulli's_principle
which applies to ideal fluids, says that as a fluid is moving from an area to another, energy in conserved (as velocity of a fluid increases, pressure decreases, and vice versa): so if you can imagine a pipe that gets smaller in size in it's second section:
duct 1
cross sectional area: smaller
fluid pressure: lower
fluid velocity: higher
duct 2
cross sectional area: larger
fluid pressure: higher
fluid velocity: lower
In an ideal fluid
-The fluid is always fluid -- in the case of blood, once you go to the capillaries, the red blood cells are passing through one by one, acting essentially as a stream of solid particles
-fluid flow is inviscid -Energy is conserved -- you lose kinetic energy (dissipated as heat) when blood flows
(
http://forums.studentdoctor.net/archive/index.php/t-480138.html)
The heart is a pump working in a closed circuit and blood is not an ideal fluid, there is energy loss at each step of a way, for this reason as we go from:
artery
cross sectional area: smaller
fluid pressure: highest
fluid velocity: highest
capillary
cross sectional area: larger
fluid pressure: lower
fluid velocity: lower
vein
cross sectional area: smaller
fluid pressure: higher
fluid velocity: higher
to understand the local factors you need to understand blood flow
-Change in pressure = pressure difference along two points in a vessel
-for blood flow = the pressure difference is considered to be the difference between Pa (arterial) and Pv (venuous)
F (flow) = delta P (Pa-Pv) / R (resistance)
Of course, one big thing to notice is that there is a variation between P in arteries and P in venuoles otherwise the system would be 'static' and not 'flow'. So when blood is distributed into different vessels, the pressure pushing from behind is the same (Pa-Pv), the length of the vessels are the same, any change in flow is pretty much determined by the lcoal resistance
Resistance to blood flow within a vascular network is determined by the
1) size of individual vessels (length and diameter),
2) the organization of the vascular network (series and parallel arrangements),
3) physical characteristics of the blood (viscosity, laminar flow versus turbulent flow),
4) extravascular mechanical forces acting upon the vasculature.
http://www.cvphysiology.com/Hemodynamics/H002.htm
Taking into consideration the simple diameter, obviously something that's wider (less R) is going to allow for greater flow, and something that is less wide (greater R) reduces the flow.
VASOCONSTRICTION:
- narrowing of the blood vessels
- smaller area
- less flow
- leads to high blood pressure
VASODILATION:
- widening of the blood vessels
- larger area
- more flow
- leads to low blood pressure
this is elegantly displayed by Poiseuille's Law
"relates the rate at which blood flows through a small blood vessel (Q) with the difference in blood pressure at the two ends (P), the radius (a) and the length (L) of the artery, and the viscosity
👎 of the blood."
http://math.arizona.edu/~maw1999/blood/poiseuille/
The law describes a "slow viscous incompressible flow through a constant circular cross-section"
Q = (k * P * r ^ 4)/(n * l)
where "Q is flow, P is the pressure difference, r is the radius, n is viscosity, l is length, and k is just a constant."
essentially relating Q (F), P, r, so while higher pressure leads to greater flow rate and so on and so forth, radius by far has the most important role (a good site for this is
http://hyperphysics.phy-astr.gsu.edu/Hbase/ppois2.html)
why is there a pressure change? to put it simply because of homeostasis. The blood system is a circuit with a finite amount of blood, and if something is obstructing its way, the blood is going to build up behind in order to allow for the same 'flow' to occur. In the same way if the system is too dilated, the pressure difference is going to drop to bring the system back to its ideal condition.
phew... that took a long time.
feel free to contact me if you have any other Qs (and if anyone catches anything wrong here please let me know)