The body is made up of circulatory systems that serve as pathways for blood-carrying vessels. These systems are subdivided into two major parts: pulmonary and systemic circulation. Although they are both powered by the heart, they assume different roles in the body.
Summary Table
| Pulmonary Circulation | Systemic Circulation |
| Mainly responsible for supplying oxygen and releasing carbon dioxide to and from the heart | Mainly responsible for moving blood from the heart to the cells of the body, and vice versa |
| Composed of the pulmonary trunk (also called pulmonary artery) and the pulmonary veins | Composed of the aorta and the superior and inferior vena cava |
| Carries deoxygenated blood from the right ventricle to the lungs via the pulmonary trunk | Carries oxygenated blood from the left ventricle to the cells of the body via the aorta |
| Transports oxygenated blood from the lungs to the left atrium via the pulmonary veins | Transports deoxygenated blood from the body to the right atrium via the superior and inferior vena cava |
| Uses the right ventricle and the left atrium as pathways for deoxygenated and oxygenated blood, respectively | Uses the left ventricle and the right atrium as pathways for oxygenated and deoxygenated blood, respectively |
Definitions
Pulmonary circulation is a part of the circulatory system responsible for forming a circuit of vessels that transport blood between the heart and the lungs.
Systemic circulation, on the other hand, forms a closed circuit between the heart and the rest of the body. It aids in transporting oxygenated blood, which carries nutrients, to different tissues.
Pulmonary vs Systemic Circulation
These two systems have one major similarity: they are both closed circuit systems that transport blood to and from the heart. These two cardiovascular systems also occur in many mammals. They both help to move respiratory gases, nutrients, or metabolic waste products to all the designated destinations. But while they play equally-important roles in the body, there is a huge difference between pulmonary and systemic circulation.
Both the pulmonary and systemic circulations play a role in the overall control of body temperature. When there is a decrease in blood flow to the lungs, heat is transferred from the body’s core to its periphery. In other words, when there is a lack of blood flow to the lungs, heat is transferred from peripheral tissues to core tissues.
The systemic circulation acts as a buffer for any temperature changes in the body by transferring heat from peripheral tissues to core tissues and vice versa. However, in most cases, a decrease in blood flow will be matched by an increase in perfusion pressure.
Therefore, in these cases where perfusion pressure is normal or increased it can be assumed that both temperatures are being controlled equally by the vascular system.
The Key Differences Between Pulmonary and Systemic Circulation
In the cardiovascular system, the pulmonary circulation is a system of blood vessels that transport blood from the right side of the heart to the lungs. In contrast, in the systemic circulation, which includes both the arteries and veins in the rest of the body, blood is transported from a number of organs and tissues throughout your body to a common point in your abdomen.
There are several differences between these two circulations:
Pulmonary circulation is characterized by high resistance to flow due to tissue perfusion that is limited by its size in comparison with total body perfusion.
Pulmonary circulation has three different types: arterial pulmonary circulation (the main portion), venous pulmonary circulation (the smallest portion), and mixed pulmonary circulations (between arterial and venous portions). All types are responsible for supplying oxygenated blood to all organs in the body except brain.
In pulmonary circulation, blood flows in one direction, from right to left. Pulmonary veins deliver oxygenated blood to your lungs; venous blood returns oxygen-depleted blood back to your heart. Pulmonary artery carries deoxygenated blood away from your lungs.
Pulmonary veins and arteries both empty into pulmonary artery and vein respectively. Pulmonary artery has higher resistance than pulmonary vein; therefore, pulmonary vein is able to hold more oxygenated (blood) fluid than pulmonary artery.
Lungs contain millions of tiny air sacs that fill with deoxygenated blood after it has been pumped through your heart and then emptied into a vein that returns it back to your heart.
Heart contracts repeatedly each time it pumps its supply of deoxygenated blood through pulmonary circulation; once these cycles are complete, deoxygenated blood is returned to your heart via pulmonary veins.
In systemic circulation, however, blood flows in one direction, from left to right. Arteries deliver oxygenated blood to tissues; veins return oxygen-depleted blood back to the heart. Veins are connected to arteries via the vena cava, which returns oxygenated blood from the tissues to the heart.
The vena cava carries deoxygenated blood away from the tissues back to the heart. Arteries are connected directly with one another via arterioles, which are smaller arteries that do not have valves. The arterioles are part of your circulation system because they are not connected with any other arteries or veins.
Therefore, arterioles only transport deoxygenated blood between different parts of your body and never return oxygenated blood back to your heart.
Your veins also have larger arteries attached, called capillaries, which form part of your circulation system because they are attached directly with other arteries and therefore only transport deoxygenated blood between different parts of your body and never return oxygenated blood back to your heart.
Pulmonary Circulation: How It Works
When you breathe in, the air enters your lungs through your mouth and nose. It then travels through the airways into the respiratory bronchioles, which are branches of your bronchi that connect to your alveoli. From there, it is carried to the pulmonary capillaries via two separate arteries.
The right pulmonary artery is larger than the left one, but both travel along the same path until they reach their destination in the right and left ventricles respectively. The pulmonary capillaries deliver oxygenated blood to all parts of your lungs, including to the epithelial cells lining these tiny blood vessels.
When you exhale, air is expelled from your lungs through your mouth and nose. This means that it now needs to be carried out of these tiny blood vessels back into your body via a branch of one or both of these arteries known as the pulmonary veins.
Blood from both systems will then return to either one or both of these two main vessels once again.
Systemic Circulation: How It Works
In contrast to pulmonary circulation, systemic circulation has a direct connection to your heart and other organs. In fact, it is comprised of three main vessels: the aorta, the pulmonary artery, and the pulmonary veins.
The aorta is one of the largest arteries in your body and begins at your heart. It then travels to the thoracic cavity where it passes through your diaphragm into your abdomen before splitting into two separate arteries known as the left and right common iliacs.
Each branch then travels up through your left and right abdominal cavities respectively before entering your thorax (the space between your chest and neck).
After traveling through the thorax, these two arteries merge together again into one single vessel called the superior vena cava (SVC).
The SVC is about three feet long before branching off into two more branches: one goes to your heart and lungs while the other passes along the left side of your abdomen.
From there, this vessel splits again; one branch travels to your liver while another returns blood back to both kidneys.
Finally, blood will continue on its way through another branch of this vessel which eventually empties into both large veins in your lower extremities known as femoral veins.
Function
The pulmonary and systemic circulation work harmoniously to maintain homeostasis, but they do so differently. Pulmonary circulation is mainly responsible for supplying oxygen and releasing carbon dioxide to and from the heart, while systemic circulation moves oxygenated blood from the heart to the cells of the body, enabling these cells to absorb nutrients and excrete waste. As oxygen is absorbed, deoxygenated blood is transported back to the heart via systemic circulatory pathways.
Transport Vessels
Pulmonary circulation involves blood-carrying vessels such as the pulmonary trunk (also called pulmonary artery) and the pulmonary veins. Systemic circulation, on the other hand, is facilitated by the aorta and the superior and inferior vena cava. The superior vena cava carries blood from the upper parts of the body, while the inferior vena cava is responsible for blood transport from the lower parts of the body.
Circulatory Pathways
Pulmonary circulation works by forming a closed circuit of blood-carrying vessels between the heart and the lungs. To supply the blood with the oxygen it needs, deoxygenated blood exits the heart via the right ventricle and the pulmonary trunk. Structurally, the pulmonary trunk is split into two main branches that carry blood to the left and the right lungs. These branches are further subdivided into smaller branches that can reach pulmonary air sacs (also called alveoli) and capillaries – the main site for oxygen absorption and carbon dioxide release. The oxygen-rich blood is then transported to the pulmonary veins, which open up to the left atrium of the heart.
Systemic circulation uses a completely different circulatory pathway. As the system begins, the heart pumps oxygenated blood, which uses the left ventricle and the aorta (the main artery of the body) as a pathway. The movement of oxygen-rich blood towards arterioles and capillary beds facilitates cellular nutrient absorption and waste excretion. Then, the deoxygenated blood, which now carries cellular waste materials, drains into veins and is transported back to the right atrium via the superior and inferior vena cava.
Conclusion
The pulmonary circuit is a single circulatory system that pumps deoxygenated blood from the right side of the heart to the lungs. The systemic circuit is a system of blood vessels that transports oxygenated blood from many different organs and tissues throughout your body to a common point in your abdomen.
