Pulmonary circulation is the dedicated segment of the cardiovascular system responsible for moving blood between the heart and the lungs. This specific pathway ensures that the blood can be refreshed with oxygen, which is necessary for the entire body’s functioning. The circulatory loop begins when blood, depleted of oxygen after traveling through the body, is directed from the right side of the heart toward the respiratory organs. Its primary function is to facilitate the necessary exchange of gases that sustain life, removing carbon dioxide and restoring the oxygen supply within the bloodstream.
Tracing the Blood Flow Circuit
The journey of blood through the pulmonary circuit begins at the right ventricle of the heart, which receives all the deoxygenated blood returning from the body’s tissues. This blood enters the ventricle from the right atrium, having passed through the tricuspid valve. A muscular contraction then propels the blood through the pulmonary valve and into the pulmonary trunk, which is the large artery leading away from the heart.
The pulmonary trunk quickly divides into the right and left pulmonary arteries, which carry the blood toward the respective lungs. Unlike all other arteries in the body, the pulmonary arteries uniquely transport blood that is low in oxygen and high in carbon dioxide. These vessels branch extensively within the lungs, diminishing in size to become arterioles and eventually forming a dense, fine network of pulmonary capillaries that surround the air sacs.
Once the blood has passed through the capillaries, it has completed its gas exchange, becoming rich with oxygen. It then enters the venules, which progressively merge into larger vessels known as the pulmonary veins. Ultimately, the collected blood drains directly into the left atrium of the heart, finishing its circuit through the lungs. From the left atrium, the blood will pass to the left ventricle, ready to be pumped out to the rest of the body to begin the next circulatory phase.
The Essential Process of Gas Exchange
The entire purpose of the pulmonary loop is fulfilled at the microscopic interface between the blood and the inhaled air, a process known as external respiration. This exchange takes place within the lungs’ tiny air sacs, called alveoli, which are enveloped by the thin walls of the pulmonary capillaries. The vast number of alveoli, estimated at around 300 million in a human lung, provides an immense surface area for this exchange to occur efficiently.
Gases move across the alveolar and capillary membranes by simple diffusion, a passive process that requires no energy. This movement is governed by the difference in the concentration, or partial pressure, of the gases on either side of the membrane. The deoxygenated blood arriving from the right heart has a higher partial pressure of carbon dioxide than the air inside the alveoli.
This pressure difference causes carbon dioxide to diffuse out of the blood and into the alveoli, from where it is expelled during exhalation. Simultaneously, the air in the alveoli has a significantly higher partial pressure of oxygen than the blood in the capillaries. Oxygen therefore diffuses from the alveoli across the thin respiratory membrane and into the bloodstream, binding to hemoglobin in the red blood cells.
How Pulmonary Circulation Differs from Systemic
The body operates with two main circulatory routes, and the pulmonary circuit stands in contrast to the systemic circulation, which feeds oxygenated blood to all other tissues. One primary distinction is the purpose: the pulmonary circuit focuses exclusively on gas exchange, while the systemic circuit delivers oxygen and nutrients and collects waste products throughout the body.
A fundamental difference lies in the pressure dynamics within each system. The pulmonary circuit is a low-pressure, low-resistance system, with a mean arterial pressure typically ranging from 9 to 16 millimeters of mercury (mmHg). This lower pressure helps protect the delicate capillaries surrounding the alveoli from damage. In contrast, the systemic circulation is a high-pressure system, averaging around 93 mmHg, necessary because it must pump blood against higher resistance to reach the body’s extremities.
The vessels reflect this difference; pulmonary arteries have thinner walls and less muscle compared to the thicker, more robust walls of systemic arteries. The naming convention for vessels is also reversed in the pulmonary circuit based on blood content. Pulmonary arteries carry deoxygenated blood away from the heart, while pulmonary veins carry oxygenated blood toward the heart. This defies the systemic rule where arteries carry oxygenated blood and veins transport deoxygenated blood.