Starling’s law states that the more the ventricle is filled with blood during diastole (preload), the greater the volume of ejected blood will be during the resulting systolic contraction.  This law give us great insight into the dynamics of blood pressure.  Anything that will decrease preload will decrease stroke volume (amount of blood ejected during systolic contraction).   This is why fluid boluses are sometimes effective at treating hypovolemia.  Before giving a fluid bolus, make sure you listen to the lungs fields to ensure that the patient is not also experiencing pulmonary edema.  Fluid boluses are dangerous when given to patients with signs of pulmonary edema.

The vascular system is comprised of the heart and blood vessels.  The largest blood vessel is the aorta, which is comprised of three parts, the ascending (up the neck and into the head), the thoracic (chest), and abdominal.  After blood passes through those great vessels, it moves through arteries and then arterioles (as they get smaller).   From arterioles (which are microscopic at this point), the blood flows into capillaries.  It is at the capillaries that oxygen is offloaded from the vessel and into the surrounding tissue (at the same time, waste products like carbon dioxide are loaded into the capillary – the process is called diffusion).   After diffusion has occurred at the cell – capillary level, the blood begins its’ trip back to the heart by entering venules and then passing through veins.  All of the veins in the body converge onto another great vessel called the vena cava.  The vena cava is comprised of two parts, the superior and inferior.    So how does blood actually return to the heart after passing through all of those arteries and veins (keeping in mind that veins are not powered by the heart)?  There are four main mechanisms:

• Skeletal muscle pump – the movement of arms and legs move the blood through the venous system.
• Thoracoabdominal pump – changes in pressure in the thoracic and abdominal cavities create a vacuum that sucks the blood right back to the heart.  Any chest injury or gastric distention can affect the thoracoabdominal pump.
• Respiratory cycle – Using the mechanism of the thoracoabdominal pump, the amount of blood and pressure in the vena changes with respiration.  Any interruption in breathing causes a change in the volume or pressure in the great vein.
• Gravity.

The heart like any other muscle in your body requires a constant supply of oxygenated blood and a way to get waste products out.  To bring the oxygen in, the heart has several coronary arteries:

• Left coronary artery
• Anterior descending branch (Left Anterior Descending  a.k.a. Widowmaker)
• Circumflex
• Right coronary artery

Like every other muscle in your body, the heart has veins.  The veins of the heart are the coronary sinus and the great cardiac vein, which return blood back to the venous network