So here's a picture of a heart:
I really like this picture because it shows you the atria, ventricles, and valves, as well as the attached vasculature. I got this from an NIH-NIAAA website.
So here we go: BLOOD. We've got about 18 liters of the stuff (I've said it before and I'll say it again: I HATE English measurements. USA, get into the 21st century already and get with metric!) all circulating around. It goes from out heart to our extremities, drops off nutrients, exchanges gases, and comes back to the heart. First it goes into the right side of the heart, which pumps it into the lungs. From the lungs it goes into the left side of the heart and then it gets sent out to the extremities again.
The diagram shows you how it works: into the right atrium via the inferior and superior vena cava, (these are veins, which return to the heart) into the right ventricle, and then to the lungs via the pulmonary artery (pulmonary = lungs. Cardiopulmonary system means the heart (cardio) and lungs (pulmonary)) then into the left atria from the pulmonary veins, then into the left ventricle and then out into circulation via the aorta. Arteries carry blood away from the heart.
Our heart is one big pump. And it generates a lot of pressure, almost 120 times as much pressure as the atmosphere at sea level. (EDIT: ACK! This is so wrong! Sea level pressure is 760 mm Hg. Pressure inside the aorta just after systole is 120 mm Hg. So it is quite a bit of pressure, just not as much as atmospheric. Game on.) And because fluid movements are generally dictated by differences in pressure, our blood gets coaxed along where it needs to go via a gradient of pressures, between our heart, lungs, veins, and arteries.
Let's look at the left side of the heart. It works exactly the same on the right side but it's a bit simpler to look at the left side of the heart. Blood is coming into the pulmonary vein from the lung. The pressure goes up because the volume goes up. (If you aren't familiar with Boyle's law, it might make sense for you to go look at that for a moment and then come back. Bear in mind that in physics, gases and liquids in motion are known as fluids) As the pressure goes up, a gradient between the pulmonary vein and the left atrium is building. Finally, a high pressure is reached, the valve between the vein and the atrium opens, and blood rushes into the atrium.
See the valve on the diagram between the atrium and the ventricle? It has 3 names. Annoying, isn't it? It is the atrioventricular valve, the bicuspid or the mitral valve. (because it looks like a mitre, a bishop's hat. Stupid religious imagery) It has 2 cusps or flaps of skin that open and close the valve, and are attached to collagenous strings called chordae tendonae. These strings anchor the valve to the heart muscle at little nipple-like protrusions called papillary muscles. This setup reinforces the valve. When the pressure gradient between the atria and ventricle is at a certain point, the valve opens, and blood rushes into the ventricle.
Point of interest: the valve between the right atrium and the right ventricle is the tricuspid or atrioventricular valve. Instead of two flaps of skin opening and closing the valve, it has 3, though it still only has 2 chordae tendonae.
Now the ventricle is filling, and when it can hold all that it can hold (about 135 mL of blood) the mitral valve closes. The ventricle now has greater pressure than the pulmonary veins, but the mitral valve, due to its anchoring system, prevents blood from going back into the atrium. As we age and if we are genetically predisposed to heart failure, these valves don't close very well and don't prevent backflow. This can lead to fluid building up in the lungs, a condition known as congestive heart failure.
As the blood hits the mitral valve, wanting to go back to the atrium, we hear a 'lub' sound with a stethoscope.
The left ventricle wall contracts, and the blood pressure forces the aortic semilunar valve, the valve between the left ventricle and the aorta, to open and the blood is pushed out at rather high pressure. The pressure is now greater in the aorta than in the ventricle, but the valve has two cups on either side that is forced closed with the weight of blood. The sound blood makes when it hits the cups is 'dub'. You can hear it with a stethoscope.
The amount of blood that is in the left ventricle when it is done filling (during the heart relaxation phase, diastole) is 135 mL. The amount of blood that is forced out with each contraction (systole) is 70 mL. This is the stroke volume. 65 mL of blood is left in the left ventricle after each heartbeat. This is the end systolic volume. That means that the heart operates at 55% efficiency, also known as the ejection fraction. Cardiac output is the measure of how many milliliters of blood is pushed out of the heart every minute. So if the average human's heart rate is 60 beats per minute, and each heartbeat (or stroke) pushes 70 mL out of the heart, 4200 mL of blood/1 minute would be the cardiac output.
I think that tomorrow after my exam I'm going to talk a little bit about the autonomic nervous system control of heart rate. Till then, wish me luck on my exam!
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