Overview of the Circulation; medical Physics of Pressure, Flow and resistance

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The circulation is a complete circuit. 


Because blood flows around and around in the same vessels, any change in flow in a single part of the circuit transiently alters the flow in the other parts. 
Another feature of circulation is that sudden constriction of a blood vessel must always be accompanied by opposite dilatation in another part, as the blood itself is not compressible.


Most of the blood volume is distributed in the veins of the systemic circulation. 
84% Systemic Circulation

  • 64% Veins
  • 13% Arteries
  • 7% Arterioles and capilaries
7% Heart
9%  Pulmonary circulation



Velocity of blood flow is inversely proportional to vascular cross-section area.
Pressures differ in different parts of the circulation
Pulse Pressure 40mmHg - (120 - 80) mmHg. 


Pressures in the pulmonary circulation are much lower than in those in the systemic circulation. 25/8 mmHg MAP = 16 mm Hg


The blood flow from each tissue of the body is precisely controlled relative to tissue needs.
The cardiac output is the sum of all local tissue flows
In most instances, the arterial pressure is controlled independently of local blood flow or cardiac control. 


Blood flow through a vessel is determined by the pressure gradient and vascular resistance.


Q=dP/R, note difference in pressure, dP=QR and R=dP/Q.  
The total pulmonary vascular resistance is much lower than the systemic vascular resistance. The pulmonary pressure gradient is lower than systemic circulation, and blood flow Q is the same.


Vessel diameter has a very large effect on resistance to blood flow - Poiseuille's Law


File:Illu capillary microcirculation.jpg

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