Dissecting Pressure - The Pulmonary Circuit vs. Systemic Circuit

In contrast to fetal circulation, the pressure of the pulmonary circuit in postnatal circulation is less than that of the systemic circuit. Pulmonary circulation in adults can be seen as the circulation on the right side of the body, where the goal is to oxygenate and remove carbon dioxide from the blood through gas exchange with the lungs. The systemic circulation, on the other hand, is made to ensure that each bodily tissue has oxygenated blood delivered to it.

In comparison, we see that the two circuits differ in the amount of real estate they need to supply blood to; pressure in the pulmonary circuit is lower since it only requires to send blood towards the lungs, while pressure in the systemic circuit will be greater in order to push enough blood to the whole remainder of the body. The left side of the heart is additionally made to sustain higher pressure; the walls of the left ventricle are more muscular than that of the right in order to push blood with enough force to supply the body. The right ventricle walls are less muscular in order to compensate for blood flow to solely the lungs. The left and right ventricles also differ in shape in order to satisfy their functions. The left ventricle is round, while its counterpart is crescent shaped. Functionally, these structures make sense when considering their targets.

The differences in pressure maintain a healthy ratio of blood flow and ensure that circulation within the body remains homeostatic. If pulmonary blood pressure were to be as great as that of the systemic circuit, the lungs would receive blood at a much greater rate than it should. This will likely increase respiratory rates to compensate for increased gas exchange and would overwork the left circuit by oversupplying it with oxygenated blood to serve the rest of the body.

For these reasons, it is understandable that pressure within the pulmonary circuit is lower than in the systemic circuit of the body. The two circuits are interdependent as the output of one circuit is the input to the other, despite being closed circuits. It is necessary that they individually maintain their specific circulatory pressure in order to maintain a balanced ratio of blood flow throughout the entirety of the body when they ultimately come to work together.