How the Heart Works
The heart is responsible for circulating blood throughout the body. It is about the size of your clenched fist and sits in the chest cavity between your two lungs. Its walls are made up of muscle that can squeeze or pump blood out every time the heart "beats" or contracts. Fresh, oxygen-rich air is brought into the lungs every time you take a breath. The lungs are responsible for delivering oxygen to the blood, and the heart circulates the blood through the lungs and out to the different parts of the body.
The heart is divided into four chambers or "rooms". You can compare it to a duplex apartment that is made up of a right and a left unit, separated from each other by a partition wall known as a septum (pronounced SEP-tum).
Each "duplex" is subdivided into an upper and a lower chamber. The upper chamber is known as the atrium (pronounced AY-tree-yum) while the lower chamber is referred to as the ventricle (pronounced VEN-trickle). The right atrium (RA) sits on top of the right ventricle (RV) on the right side of the heart while the left atrium (LA) sits atop the left ventricle (LV) on the left side.
The right side of the heart (RA and RV) is responsible for pumping blood to the lungs, where the blood cells pick up fresh oxygen. This oxygenated blood is then returned to the left side of the heart (LA and LV). From here the oxygenated blood is pumped out to the rest of the body supplying the fuel that the body cells need to function. The cells of the body remove oxygen from the blood, and the oxygen-poor blood is returned to the RA, where the journey began. This round trip is known as the circulation of blood.
Do you wonder why each side of the heart has two pumping chambers (atrium and ventricle)? Why not just have a ventricle to receive blood and then pump it straight out? The reason is that the atrium serves as a "booster pump" that increases the filling of the ventricle. Filling a normal ventricle to capacity translates to more vigorous contraction or emptying. You can compare this to a strong spring. Within reasonable limits, the more you stretch a spring, the more vigorously will be its contraction or recoil. More complete filling of the ventricles thus translates into more vigorous ventricular contraction (a good thing).
The figure shown above is a section of the heart, as viewed from the front. It demonstrates the four chambers. You will also notice that there is an opening between the right atrium (RA) and the right ventricle (RV). This is actually a valve known as the tricuspid valve (pronounced try-CUS-pid). It is made of three flexible thin parts, known as leaflets, that open and shut. The figure below shows the tricuspid valve, as seen from above, in the open and shut position (the other valves pictured are discussed below).
When shut, the edges of the three tricuspid valve leaflets touch each other, preventing blood from going back into the RA when the RV squeezes. Thus, the tricuspid valve serves as a one-way door that allows blood to move only in one direction - from RA to RV. Similarly, the mitral valve (pronounced my-TRULL) allows blood to flow only in one direction from the LA to the LV. Unlike the tricuspid valve, the mitral valve has only two leaflets.
In the top diagram, you will also notice thin thread like structures attached to the edges of the mitral and tricuspid valves. These chords or strings are known as chordae tendineae (pronounced cord-EYE TEND-in-eye). They connect the edges of the tricuspid and mitral valves to muscle bands or papillary muscles (pronounced PAP-pill-larry). The papillary muscles keep the valve leaflets from flopping back into the atrium. The chords are designed to control the movement of the valve leaflets similar to ropes attached to the sail of a boat. Like ropes, they allow the sail to bulge outwards in the direction of the wind but prevents them from helplessly flapping in the breeze. In other words, they allow the valve to open and shut in a given direction but not beyond a certain point.
Lets now follow the circulation of blood more closely. Oxygen-poor blood from the head, neck and arms returns to the right atrium (RA) via the superior vena cava (pronounced VEE-nah CAVE-ah) or SVC. On the other hand, oxygen-poor blood from the lower portion of the body returns to the RA via the inferior vena cava or IVC. When the RA is full, it contracts. This builds up pressure and pushes the tricuspid valve open. Blood now rushes from the RA into the right ventricle (RV). When the RV is filled, the walls of the ventricle begin to contract and the pressure within the RV rises. The increased pressure shuts the tricuspid valve and blood is pumped into the pulmonary artery (pronounced PULL-mun-narey) through the pulmonic valve (pronounced pull-MON-nick). The diagram below once again shows the four heart valves as viewed from the top of the heart, i.e., we are looking down at the two ventricles with the right atrium and left atrium removed.
The pulmonic valve is made up of three cusps or flexible cup-like structures, capable of holding blood. When the pressure in the right ventricle is low (as is the case when the RV is filling with blood) blood starts to move backward from the lungs toward the RV. The three cusps of the pulmonic valve fill with that blood and their sides touch each other, effectively shutting the valve. This prevents blood from leaking from the pulmonary artery into the right ventricle while the RV is filling. When the RV contracts to empty, the pressure within the RV rises above that of the pulmonary artery. This forces open the three cusps of the pulmonic valve and blood rushes through the pulmonary artery towards the lungs, where the red blood cells pick up oxygen.
The oxygenated blood from the lungs now returns to the left atrium (LA) via four tubes that are known as pulmonary veins (each draining a separate portion of the lungs). The pulmonary veins empty into the back portion of the LA. When the LA is completely filled it contracts. The mitral valve then opens, and blood is forced into the left ventricle (LV). When the LV is completely filled, it starts to empty its contents by contacting its walls. This increases pressure within the chamber, shuts the mitral valve and opens the aortic valve (AV, pronounced ey-OR-tick). The sequence is similar to that described for the RA, RV and pulmonic valve. The aortic valve also has three cusps.
The mitral and tricuspid valves open and the aortic and pulmonic valves shut while the ventricles fill with blood. In contrast, the mitral and tricuspid valves shut while the aortic and pulmonic valves open during ventricular contraction. This sequence ensures that the ventricles are filled to capacity before the ventricles start to pump blood and that the blood flows in only one direction.
After leaving the LV, blood now rushes through the aorta (pronounced a-OR-tah). The aorta is the main "highway" blood vessel that supplies blood to the head, neck, arms, legs, kidneys, etc. Blood is brought to these organs and limbs via branches that originate from the aorta. The cells within each part of the body pick up oxygen and nutrients from the blood. The oxygen-poor blood then returns to the RA, via the superior and inferior vena cava, and the beat goes on!!