Question

∆ Learn: Blood Flow through the Heart ∆

Answer 1

∆ Flow of blood through the heart ∆

The circulatory system is… circular; as such there is no beginning and end. However, for the purpose of this instruction we will begin with the right atrium.

Deoxygenated blood enters from systemic blood vessels into the right atrium. The electrical impulse, which regulates a healthy heart, emanates from the sinoatrial (SA) node. This causes a contraction in the atria, pushing the blood into the ventricles.

The deoxygenated blood from the right atrium passes through the tricuspid valve and enters the right ventricle. Retrograde flow (back flow) of blood through the valves is prevented by the chordae tendineae. Note that the impulse that originated from the right atrium in the SA node continues to travel down through the atrioventricular (AV) node, and then down to the AV bundle (Bundle of His). The AV node functions as a delay for the impulse, otherwise the atria and the ventricles would contract at the same time.

Once the impulse reaches the AV bundle, the impulse fires down the left and right bundle branches causing the ventricles contract. The deoxygenated blood in the right ventricle passes through the pulmonic valve to the pulmonary artery (note that this is the only artery in the body that carries deoxygenated blood, there will be more detail on this at the end).

The blood travels to the lungs where, through diffusion with the alveoli, the blood becomes saturated with oxygen (O2). This newly oxygenated blood travels back through the pulmonary veins (note that these are the only veins in the body carry oxygenated blood) until it returns to the heart through the left atrium.

At this point, the heart has gone through one cycle: a LUB (atrial contraction) and a DUB (ventricular contraction). The oxygenated blood is in the left atrium when the SA node fires an impulse causing the atria to contract again. The blood is pumped through the bicuspid (or mitral) valve, to the left ventricle.

From the left ventricle, the blood is pumped through the Aortic valve, to the Aorta and into the systemic blood vessels. The oxygenated blood travels through arteries all throughout the body, eventually the arteries decrease in diameter and become arterioles, and eventually into the individual capillaries that form the capillary beds were the diffusion of oxygen and nutrients into somatic cells occurs.

The blood then begins to return to heart by passing through the venuoles, which increase in diameter to become veins and then into the vena cava where the blood is funneled back into the right atrium. Blood from the upper portion of the systemic blood vessels is funneled through the superior vena cava, and blood from the lower portion of the systemic blood vessels is funneled in through the inferior vena cava.

From here the cycle begins again!


Extra notes:

LEFT VENTRICULAR WALLS

You may or may not have noticed in cross-sectional diagrams of the heart that the walls of the left ventricle are significantly thicker than other portions of the heart.

The reason for this is because once the blood has reached the left ventricle it is about to be pumped to the entire systemic circulation. In order that this oxygenated blood reaches every capillary in every corner of the body, it must be pumped under a very high pressure. In order to withstand the force, the walls of the left ventricle are as they are, thick. This is also why the Aorta is the largest artery in the body.

LEAFLETS IN VEINS

Blood easily reaches the capillary beds via the arteries while still under significant pressure from the initial push through the Aorta, but how does the blood return to the heart? The Veins have the task of returning the blood to the heart under substantially lower pressure. To compensate for this, there are structures within the veins known as leaflets. These tiny valves function to slightly increase the pressure within the veins by decreasing their diameter, but also prevent retrograde flow of blood.

CORONARY CIRCULATION

I’ll add more on this later, but for now know that the heart supplies blood to itself through its own closed system by means of coronary arteries.

PULMONARY ARTERY/VEIN MIXUP

You may have known that the pulmonary artery and pulmonary vein are the only to two veins who have a “flip-flop” of the oxygenated blood – deoxygenated blood pattern that we see in the rest of the hemodynamic system. This change in the rule, however forces us to redefine what we call a vein and what we call an artery.

Instead of referring to the oxygenation of the blood being carried, instead notice the correlation between the direction of blood flow, and the pressure under which the blood is moving. Arteries move blood away from the heart under high pressure, while veins return blood back to heart under low pressure.

Answer 2

For anyone who is interested, let me know if you find any errors.