A post of mine from a long time ago....
Preload is stretch
. The amount of volume being returned to the right side of the heart from systemic circulation.
Afterload is squeeze
. The amount of resistance the left side of the heart has to overcome in order to eject blood.
The right side of the heart, composed of the right atrium and ventricle, collects and pumps blood to the lungs through the pulmonary arteries. The lungs refresh the blood with a new supply of oxygen, making it turn red.
Oxygen-rich blood, "red blood," then enters the left side of the heart, composed of the left atrium and ventricle, and is pumped through the aorta to the body to supply tissues with oxygen.
Four valves within your heart keep your blood moving the right way. The tricuspid, mitral, pulmonary and aortic valves work like gates on a fence. They open only one way and only when pushed on. Each valve opens and closes once per heartbeat — or about once every second.
A beating heart contracts and relaxes. Contraction is called systole, and relaxing is called diastole.
During systole, the ventricles contract, forcing blood into the vessels going to your lungs and body — much like ketchup being forced out of a squeeze bottle. The right ventricle contracts a little bit before the left ventricle does. The ventricles then relax during diastole and are filled with blood coming from the upper chambers, the left atria (contains oxygenated blood from the lungs/luminary vein)and right atria (from the Superior/inferior vena cava). Then the cycle starts over again.
The heart is nourished by blood vessels called coronary arteries extend over the surface of your heart and branch into smaller capillaries. The heart also has electrical wiring, which keeps it beating. Electrical impulses begin high in the right atrium and travel through specialized pathways to the ventricles, delivering the signal to pump. (PQRST) The conduction system keeps the heart beating in a coordinated and normal rhythm, which in turn keeps blood circulating. The continuous exchange of oxygen-rich blood with oxygen-poor blood is what keeps us alive.
ECG Learning Center - An introduction to clinical electrocardiography
Preload is about volume. If the water pressure is low, the water output will be a trickle, not enough to water and sustain your pretty garden. If the volume is too much, it will back up your plumbing system (Right-sided heart failure engorged liver, systemic edema, etc, or left-sided failure pulmonary edema.)
Afterload is about pressure or resistance. If there is a link or narrowing in your garden hose, the volume will back up AND the output will drop. Think of it like a garden hose.........
A good analogy is that your hose has gone form a garden hose to a fire hose or developed huge leaks and it will either get more blood to the right places easier (decreasing afterload so the heart doesn't have to work as hard and will therefore work better) ...or you will not get the blood to where it need to go until you "patch it up" (constrict the blood vessels to a more normal size like the vasodilation that occurs with sepsis)or stop the bleeding by sealing the leak(the source of hemorrhage)....OR....
Gross analogy, but it works...Think about flushing a toilet......you flush, and then flush again right away......nothing happens right? This is because the tank doesn't have time to fill....PRELOAD is decreased in the tank.
What if the toilet is plugged up? When you flush, it backs up....this is too much afterload.
Preload = Volume
Afterload = Pressure/Resistance
CV Physiology | Pressure/BP010.htm