The Atrium – Storage Chamber and Booster Pump
In an earlier article, I’ve discussed the four heart chambers – the two upper chambers called atria, and the two lower ones called ventricles.
Let’s talk a little more about the atria.
The atria [singular : atrium] are thin walled upper chambers of the heart. There are two atria, one on the right and the other on the left.
The right atrium is located along the right border of the heart. The two great veins of the body – called the superior and inferior vena cava [SVC and IVC] – drain impure blood from the upper and lower halves of the body respectively. These two great veins ultimately empty into the right atrium.
From here, blood flows into the right ventricle across the tricuspid valve and thence into the lungs to be purified. The right atrium thus primarily functions as a storage chamber for “impure” venous blood en route to the lungs.
The left atrium is situated behind the right atrium, and slightly to its left. It is thicker walled than the right, in order to withstand the higher pressure of blood on the left side of the heart.
There are four large blood vessels, called the pulmonary veins, that return purified blood from the lungs back to the heart. These pulmonary veins drain into the left atrium. Purified blood from the left atrium then passes into the left ventricle across the mitral valve, and is then pumped out to the rest of the body.
The left atrium thus stores “pure” blood before it is pumped to the other organs of the body.
What function do the atria serve ?
As we just saw, the atria are storage chambers for blood returning to the heart.
During the time that the ventricles are contracting [called systole], pumping blood into the circulation, the valves connecting the atria to the ventricles are closed. So, the ventricles cannot receive blood returning from the atria during this time. Temporarily, then, this blood stagnates within the atria.
In addition, the atria also act as weak assist or booster pumps for the ventricles.
The ventricles, being thick walled and muscular, are the major pumps to forcibly push blood into the arteries and towards the rest of the body. At the end of each contraction, the ventricles relax, and the mitral and tricuspid valves open. Blood from the atria flow passively into the ventricles. Towards the end of this relaxation period the atria contract and thus actively force a little more blood into the ventricle cavity.
Because the atria are thin walled chambers, the force of contraction is much weaker than the ventricles. However, even this weak force is enough to enhance the flow of the pooled blood into the ventricles to some extent.
In other words, the atrial contraction acts as a booster pump to fill the ventricles optimally, and actually contributes about one-fourth of the total blood volume entering the ventricles.
How are the atria helpful to the pediatric cardiologist ?
The atrium has an important role in studying cardiac morphology. It is a useful chamber in helping to describe and classify birth defects of the heart.
The structure of the two atria are distinct from each other, and the difference can be easily studied non-invasively using the cardiac ultrasound test (or echocardiography). Different abnormalities of heart position and connections can be thus made out simply by studying the morphology and location of the two atrial chambers, a fact that is extremely helpful in diagnosing and grouping birth defects of the heart.
The atrium and the conduction system of the heart
The heart is a muscular pump that contracts and relaxes, squeezing blood out from within its chambers and into the blood vessels for distribution to other parts of the body. This contraction of heart muscle is made possible by a weak electric current generated in the heart itself.
Shocking news, isn’t it?
The portion of the heart where electric impulses originate is the Sino-Atrial Node (or SA node) which is located at the junction of the superior vena cava with the right atrium. From here, the impulses travel in poorly defined pathways, and finally cross into the ventricles. There, the electric impulse activates the heart muscle, making it contract.
The atria, being thin-walled chambers, enlarge very easily when the volume of blood inside them increases. Such a dilatation may produce changes or distortion in the pattern of transmission of electrical impulses inside the heart.
In the condition called atrial fibrillation, the electrical activity of the heart becomes chaotic. The atrium does not contract effectively, causing a greater stagnation of blood in the atrial chamber than normal. When blood remains stationary inside one chamber without circulating, it has a tendency to clot.
A clot is a rock-like clump of hardened blood. When blood clots inside the atrium, it may become dislodged at anytime by the heart’s contraction and may get flushed into the blood stream. It can then be washed along into any other blood vessel, and potentially can block blood flow to critical parts of the brain, kidneys or any other organ, causing a stroke or other complications.
Another drawback of atrial fibrillation is the loss of the “booster” pump effect of atrial contraction. This may actually cause a sudden precipitation of heart failure in a patient with other heart diseases.
In these patients, existence is dependent on the small augmentation provided by the weak atrial contraction, and the loss of even this small fraction of heart output may tilt the balance.