The human heart has four chambers. On each side of the heart there is one atrium and one ventricle, thus referred to as the left atrium, left ventricle, right atrium and right ventricle. To ensure that blood does not flow backwards, the heart uses several valves, limited in movement by the papillary muscle. The mitral and tricuspid valves are what allow blood to flow from the atria to the ventricles but not in the other direction. The tricuspid does this for the right side and the mitral for the left. The other two valves work for the ventricles.
The pulmonary valve makes sure blood continues in a forward motion through the pulmonary arteries and out of the heart. The aortic valve works for the left ventricle and also guarantees that blood stays on a forward path out of the heart. However, the blood leaving each of these places has a very different destination, to be discussed later. The final component of the heart is the septum. The septum, from the Latin “saeptum,” meaning a dividing wall or enclosure, does just that; it divides the right and left sides of the heart so the blood does not mix. As mentioned above, there are two sides of the heart; a right side and a left side.
Each side has a corresponding atrium and ventricle. The right atrium gathers de-oxygenated blood from the upper body/brain and the lower body by means of the superior (upper body) and inferior (lower body) vena cava. The oxygen-poor blood then travels through the tricuspid valve to the right atrium, where it is pumped through the pulmonary arteries to the lungs. This is called pulmonary circulation. Once in the lungs, the blood undergoes a gas exchange by receiving oxygen and leaving behind carbon dioxide. Now, the oxygenated blood travels through the pulmonary veins to the left side of the heart.
The blood is pumped into the left atrium which propels it into the left ventricle after it passes through the mitral valve. When the left ventricle becomes filled with oxygen rich blood, being the strongest chamber in the heart, it thrusts the blood into the aortic valve. The aorta, the largest artery, uses its strength to pump the blood through the rest of the body. This is also known as systemic circulation. Because their job is so crucial, let’s re-examine the valves. In order to keep blood flowing in one direction, the valves open and close according to pressure differences inside of the heart and with the help of the aforementioned
papillary muscle. Again, the four valves of the heart are the tricuspid, mitral, pulmonary and aortic. Once the right atrium is full of blood the tricuspid valve opens allowing it to flow into the right ventricle. Once the ventricle becomes full, a pressure change closes the tricuspid valve allowing the pulmonary valve to open and the blood to flow out of the heart to the lungs. Upon returning to the heart, the blood enters the left atrium. When this chamber becomes occupied by the oxygen-rich blood, the mitral valve bursts open pouring the scarlet red blood into the left ventricle.
Again, the heart senses a pressure change and the mitral valve seals as quickly as it opened. Now, the final aortic valve opens and the blood streams onward to its target, the limbs. Now that we know the path that blood takes during its continuous cycle through the body and back to the heart, let’s discover how. The actual heartbeat is a two-part pumping action. As the atria fill with blood, the sinoatrial node (SA node), also known as the pacemaker, generates an electrical signal from the break down of the negatively charged fibers in its tissue.
This signal tells the atria to contract, and the blood then travels to the ventricles. Diastole is the word for this part of the heartbeat. The second part, knows as systole, occurs when the impulse created by the SA node travels via the internodal tracts to the AV node. The AV node is very important to the electrical conduction system as it causes a delay. This delay makes sure that the atria and ventricles do not contract simultaneously. From the AV node, the impulse travels downward to the bundle of his. This bundle splits at the bottom into a left and right branch, each different branch serving its corresponding side.
Next, the action potential travels down the very tips of the bundle to the purkinje fibers which excite individual myocardial cells to contract. During this part of the heartbeat, the atria are re-polarizing in preparation for the next impulse. After each heartbeat, the ventricles are re-polarized. This is the entire path of an electrical impulse through the heart. The contraction that each step causes is known and felt as the heartbeat. So while this process is what maintains the rhythmic contractions of the heart, there are two parts of the autonomic nervous system that affect the rate of this beat.
The accelerans nerve releases the hormone norepinephrine (triggered by stress) to increase blood flow and the heart rate. The opposite of these nerves would be the vagus nerves. The vagus nerves transmit a signal from the brain to the heart telling the heart to slow down. The impulse sent to the brain originates in the arteries which recognize increased blood flow and create warning impulses to slow contractions. When the heart is in good condition all of these processes work together with the circulatory system to maintain a normal heartbeat and a crucial supply of blood to every tissue in the body.