Pressure and Fluidity



                                                                                                  heart pump


                                                        Extending out of the top of the heart is a large blood vessel called the Aorta.  The Aorta is the largest blood
                                                        vessel in the body, pumping through it all of the blood that leaves the heart, and transporting it to all of your
                                                        periferal vessels.  Given the fact that all of the blood goes through this vessel it must be quite large to accomodate
                                                        the shear pressure exerted on it.  To give some perspective, an average human aorta is so large you would be able
                                                        to put two of your fingers side by side into it.
  
                                                        Given the fact that the aorta is so large, a large volume of blood may pass through.  This vessel in turn branches out
                                                        millions of times into smaller and smaller vessels until it eventually is at the capillary size.  Some capillaries are so small
                                                        that only on red blood cell can fit through at a time.  Since there is this size gradient, that means that there is also a pressure
                                                        gradient.  To be able to force blood throughout the entire body, and all the way down into tiny vessels covering every part of
                                                        the body and then having the blood pushed back to the heart takes a great amount of pressure. 
   
                                                       There are several things that affect blood return however, not just the pressure coming out of the heart and pushing through
                                                       the aorta.  Once blood oxygen has been exchanged through the capillaries, they blood then begins its journey back to the heart.
                                                       To help push the now lower pressure blood back to the heart the body uses several main devises to complete this journey.  The
                                                       two main ways the body does this though is through one way valves in all the returning vein in the body, and also through your
                                                       own muscle structure.  Just by moving or flexing, and with the valves your body is able to return blood to the heart.  There are
                                                       several other ways the body does this, but I will not be disscussing them here.         
                                                                                    

                                               aorta diagram                                   valves and muscles


                                               However, since we have talked about the physiology, we will now talk about some physics.
                                               Blood obviously is a fluid, this means that it does not maintain a specific shape, but takes on
                                               the shape of its container.  In the case the container is the blood vessels. 
                                               
                                                    

Density and pressure

The density (ρ) of a substance of uniform composition is its mass per unit volume: ρ = m/ V. In the SI system, density is measured in units of kilograms per cubic meter.

Imagine an upright cylindrical beaker filled with a fluid. The fluid exerts a force on the bottom of the container due to its weight. Pressure is defined as the force per unit area: P = F/ A , or in terms of magnitude, P = mg/A, where mg is the weight of the fluid. The SI unit of pressure is N/m2, called a pascal. The pressure at the bottom of a fluid can be expressed in terms of the density (ρ) and height (h) of the fluid: (giancolli)

                      
                  
                                                  equation                            car
 

Now if you are to think of the blood as the fluid and the vessels as the container, you can see how you would be able to derive the equation P=F/A.  Given different bodies types, weights and many other conditions, you will be able to see how the pressure of the blood can change depending on many factors, but you will be able to see the bassis of why and how the blood of the body has pressure.


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