" BLOOD IS MORE VISCOSE THAN WATER!"

The information that follows should be useful in preparing 
for the first hour exam in human anatomy.

We will trace a drop of blood named Dan on its journey through an adult human's 
circulatory system with special attention given to the flow through the 
chambers and other structures directly associated with the heart.  
We will use color to indicate the oxygen and carbon dioxide levels of the blood.  
BLUE will indicate a low level of oxygen and higher levels of carbon dioxide.  
RED will indicate oxygenated blood that has low levels of carbon dioxide.
The path will begin with one of the two venae cavae, 
the superior vena cava or the inferior vena cava, or  the coronary sinus.   
Remember that the coronary sinus is not a vessel because it does not have 
the anatomy of a vessel.  These structures bring the blood into the first 
heart chamber it enters, the right atrium.  
The blood then flows passively through  the  tricuspid valve 
(see the Dr. J MNEMONICS to help remember the order of the AV valves) 
that is situated in the right atrioventricular orifice.  Because of its location it 
is sometimes referred to as an atrioventricular (AV) valve.
The blood then enters the right ventricle.   The blood, all of Dan's friends, 
accumulate there and eventually the right atrium contracts adding an 
additional volume of blood.  Shortly after the right atrium contracts it begins to 
relax and the right ventricle begins to contract.  The tricuspid valve closes as the 
blood starts flowing toward the right atrium and pressure in the ventricle builds.  
Eventually that pressure becomes greater than the opposing pressure in 
the pulmonary trunk and the pulmonary semilunar valve opens, 
allowing blood to flow into the  pulmonary trunk.  This flow continues until the 
right ventricle relaxes.  When the ventricle relaxes, pressure in it drops and 
blood begins to flow back toward the ventricle, thereby closing 
the pulmonary semilunar valve. 
Pressure in the pulmonary trunk is maintained because the walls of 
the vessel are contracting against the stretch they experienced due to the 
pressure pulse from the ventricular contraction.  Therefore, the blood flows 
into the pulmonary arteries and eventually to the lungs.   In the capillaries of the 
 lungs the blood undergoes exchange of the respiratory gases that brings 
the oxygen concentration up and the carbon dioxide concentration down. 
Dan the drop returns to the heart by way of the pulmonary veins, 
there are two from each lung.  The blood from the pulmonary veins 
enters the left atrium and then passes through the bicuspid (mitral) valve 
to enter the left ventricle.  
As was the case in the right side of the heart, Dan and his friends accumulate 
there and eventually the left atrium contracts.  The last volume of blood 
enters the left ventricle and just as the left atrium begins to relax, t
he left ventricle begins to contract.  The elevated pressure causes blood to 
flow toward the left atrium causing the bicuspid valve to close.
Pressure continues to elevate until the aortic semilunar valve 
opens and the blood is ejected into the aorta.    

The aorta stretches to accommodate the pressure and contracts against the pressure.  
When the left ventricle relaxes, the pressure in the aorta causes blood to move 
toward the left ventricle and close the aortic semilunar valve.  
The blood continues on its path until it gets back to the right atrium.  
Dan is tired - what a journey!  I hope this has helped.

More details of 
"The Heartbeat of Rock and Roll Hoochiecoo"
 will be covered in lecture or lab.