Systemic circulation carries blood from the left side of the heart to the body’s tissues and returns it to the right side, while pulmonary circulation carries blood from the right side of the heart to the lungs and returns it to the left side. Systemic circulation primarily delivers oxygen and nutrients to tissues, whereas pulmonary circulation allows blood to release carbon dioxide and take up oxygen.
These two pathways form a continuous circuit powered by the heart. Understanding how blood travels through them connects human anatomy with the functions of the heart, blood vessels, lungs, and other organs.
The easiest way to remember the difference is by destination: pulmonary circulation connects the heart with the lungs, and systemic circulation connects the heart with the rest of the body. Although their routes and pressures differ, both carry approximately the same volume of blood per minute under normal steady-state conditions.
What Is Blood Circulation?
Blood circulation is the continuous movement of blood through the heart and blood vessels. Your circulatory system is responsible for transporting oxygen, nutrients, and waste products throughout the body while also helping distribute hormones, immune cells, and heat.
The heart generates pressure that moves blood through arteries, smaller arterioles, capillaries, venules, and veins. Each type of vessel has structural features suited to its function. These differences are explained further in Arteries, Veins, and Capillaries.
Capillaries are especially important because their thin walls allow substances to move between blood and surrounding tissues. In the lungs, capillaries participate in oxygen and carbon dioxide exchange; in other organs, they support nutrient delivery and waste removal.
Blood circulation also works closely with breathing. The lungs bring fresh air into contact with the respiratory exchange surfaces, while pulmonary blood flow transports gases between the lungs and the rest of the body. See Respiratory System Integration with Circulation for this relationship.
The Two Main Circulatory Pathways
Human circulation is often described as double circulation because blood passes through the heart twice during one complete journey through the pulmonary and systemic circuits.
- Pulmonary circulation: Moves blood between the right side of the heart and the lungs, where gas exchange occurs.
- Systemic circulation: Moves blood between the left side of the heart and body tissues, delivering oxygen and returning blood toward the right side.
The circuits are connected in series, meaning blood leaving one normally enters the other. This arrangement allows oxygen picked up in the lungs to be delivered throughout the body.
What Is Pulmonary Circulation?
Pulmonary circulation is the movement of blood from the right ventricle through the lungs and back to the left atrium. Its main function is to support gas exchange between the blood and air inside the lungs.
Blood arriving from most body tissues has a relatively low oxygen content and a higher carbon dioxide content than blood leaving the lungs. Pulmonary circulation allows these gas concentrations to change before blood enters the systemic circuit again.
Pulmonary Circulation: Step by Step
- Oxygen-poor blood enters the right atrium from the systemic veins.
- Blood passes through the tricuspid valve into the right ventricle.
- The right ventricle pumps blood through the pulmonary valve into the pulmonary trunk and pulmonary arteries.
- Blood flows through capillaries surrounding the alveoli, where carbon dioxide diffuses into the air spaces and oxygen diffuses into the blood.
- Oxygen-rich blood returns through the pulmonary veins to the left atrium.
The pulmonary arteries are unusual because they carry relatively oxygen-poor blood away from the heart. Pulmonary veins carry oxygen-rich blood toward the heart. Their classification as arteries or veins depends on the direction of blood flow, not oxygen content.
What Is Systemic Circulation?
Systemic circulation carries oxygen-rich blood from the left ventricle through the aorta and its branches to the body’s tissues. It also returns blood through systemic veins to the right atrium.
As blood passes through tissue capillaries, oxygen and nutrients can move into surrounding cells, while carbon dioxide and other metabolic products can enter the blood. The exact exchange depends on the needs and activity of each tissue.
Systemic Circulation: Step by Step
- Oxygen-rich blood moves from the left atrium through the mitral valve into the left ventricle.
- The left ventricle contracts, pushing blood through the aortic valve into the aorta.
- Arteries and arterioles distribute blood to organs and tissues throughout the body.
- Exchange occurs across systemic capillaries, supplying tissues with oxygen and other needed substances.
- Blood returns through venules and veins, eventually reaching the superior and inferior venae cavae and entering the right atrium.
Systemic circulation supplies the brain, kidneys, muscles, digestive organs, skin, and most other tissues. Its many parallel vascular pathways allow blood flow to be adjusted according to the demands of different organs.
Systemic vs Pulmonary Circulation: Key Differences
Both circuits contain arteries, capillaries, and veins, but they differ in their routes, pressures, and gas-exchange functions.
| Feature | Systemic Circulation | Pulmonary Circulation |
|---|---|---|
| Main destination | Body tissues | Lungs |
| Starting chamber | Left ventricle | Right ventricle |
| Ending chamber | Right atrium | Left atrium |
| Main outgoing vessel | Aorta | Pulmonary trunk and arteries |
| Blood leaving the heart | Usually oxygen-rich | Usually oxygen-poor |
| Blood returning to the heart | Usually oxygen-poor | Usually oxygen-rich |
| Main exchange | Oxygen and nutrients delivered to tissues | Oxygen absorbed and carbon dioxide released |
| Typical pressure | Higher | Lower |
| Vascular resistance | Higher | Lower |
The oxygen descriptions in this table represent normal circulation. Special situations, including certain congenital heart defects, can alter the usual pathways and oxygen levels.
How the Heart Pumps Blood Through Both Circuits
The heart functions as two coordinated pumps. The right ventricle generates the pressure needed to move blood through the lungs, while the left ventricle generates the higher pressure required to supply systemic tissues.
Heart valves help maintain one-way flow as the chambers contract and relax. The sequence of filling, contraction, and ejection is explained in How the Heart Pumps Blood.
The heart muscle itself also needs a blood supply. Coronary arteries branch from the aorta and deliver oxygen-rich blood to the myocardium. This specialized part of systemic circulation is covered in Coronary Circulation and Heart Supply.
Under normal steady-state conditions, the right and left ventricles pump approximately equal volumes per minute. If their outputs remained substantially different, blood would accumulate in one part of the circulation.
Worked Example: How Much Blood Moves Each Minute?
Cardiac output is the volume of blood pumped by one ventricle each minute. It can be calculated by multiplying heart rate by stroke volume, the amount of blood ejected during each heartbeat.
Consider an illustrative resting adult with a heart rate of 70 beats per minute and a stroke volume of 70 milliliters per beat.
- Multiply 70 beats per minute by 70 milliliters per beat.
- The result is 4,900 milliliters per minute.
- Convert milliliters to liters: 4,900 milliliters equals 4.9 liters.
- The cardiac output is therefore 4.9 liters per minute.
In normal steady-state circulation, approximately 4.9 liters per minute would pass through each circuit, not 4.9 liters divided between the two. The same blood moves sequentially through pulmonary and systemic circulation. These values are illustrative; actual cardiac output varies with body size, activity, and physiological conditions.
Why Pulmonary Circulation Has Lower Pressure
Pulmonary blood vessels normally offer much less resistance to flow than the systemic vascular network. This allows the right ventricle to move blood through the lungs using lower pressures.
For comparison, a typical resting systemic arterial blood pressure is often described as approximately 120/80 millimeters of mercury, while normal pulmonary arterial pressure is much lower, commonly around 25/8 millimeters of mercury. These are representative textbook values rather than universal measurements.
The lower-pressure pulmonary circuit supports blood flow through delicate lung capillaries while limiting excessive fluid movement into lung tissue. The systemic circuit requires higher pressure to overcome greater vascular resistance and distribute blood among many organs.
Pressure and flow are related, but they are not interchangeable. Two circuits can carry similar blood volumes per minute while operating at very different pressures because their vascular resistances differ.
How Oxygen and Carbon Dioxide Move Between Blood and Tissues
Gas exchange occurs through diffusion, the movement of gases from regions of higher partial pressure toward regions of lower partial pressure.
In pulmonary capillaries, oxygen moves from alveolar air into the blood, while carbon dioxide moves from blood into the alveoli to be exhaled. Hemoglobin inside red blood cells binds much of the oxygen transported through the circulation.
In systemic capillaries, oxygen generally moves from blood into tissues where cells consume it during metabolism. Carbon dioxide produced by cells moves into the blood and is transported back toward the lungs.
Not all oxygen or carbon dioxide is transported in the same form. Most oxygen is carried bound to hemoglobin, while much of the carbon dioxide is transported as bicarbonate in the blood.
What Happens When Either Circulatory Pathway Is Disrupted?
Because the two circuits operate in series, disturbances in one can affect the other. Problems may involve the heart, blood vessels, lungs, or mechanisms controlling blood flow.
Problems Affecting Pulmonary Circulation
Pulmonary hypertension involves abnormally elevated pressure in the pulmonary circulation and can increase the workload on the right ventricle. Pulmonary embolism occurs when material, commonly a blood clot, obstructs a pulmonary artery or one of its branches, potentially reducing blood flow to part of the lungs.
Problems Affecting Systemic Circulation
Systemic hypertension increases pressure within systemic arteries and can contribute to damage involving the heart, brain, kidneys, and blood vessels. Arterial narrowing or blockage can also reduce blood supply to particular tissues, potentially causing ischemia.
Circulatory problems can produce symptoms such as breathlessness, chest discomfort, swelling, fatigue, or dizziness, depending on their cause. Sudden severe chest pain, significant difficulty breathing, or fainting can require emergency medical attention.
Common Misconceptions About Blood Circulation
Misconception 1: All Arteries Carry Oxygen-Rich Blood
Arteries are defined by carrying blood away from the heart, not by oxygen concentration. Pulmonary arteries normally carry oxygen-poor blood from the right ventricle toward the lungs.
Misconception 2: The Lungs Pump Blood Through Pulmonary Circulation
The right ventricle generates the pressure that drives blood through the pulmonary arteries and lung capillaries. The lungs provide the structures where gas exchange occurs but are not the pump for pulmonary blood flow.
Misconception 3: The Two Circuits Carry Different Amounts of Blood Each Minute
In normal steady-state conditions, the two ventricles pump approximately equal blood volumes per minute. Pulmonary and systemic circulation operate at different pressures, but their average flow rates must remain closely matched.
Misconception 4: Blood Becomes Completely Deoxygenated After Passing Through the Body
Venous blood normally retains a substantial amount of oxygen. Tissues extract only part of the oxygen delivered, and the amount removed varies with metabolic activity and local blood flow.
Frequently Asked Questions
What Is the Main Difference Between Systemic and Pulmonary Circulation?
Systemic circulation carries blood between the heart and body tissues to deliver oxygen and nutrients, while pulmonary circulation carries blood between the heart and lungs for oxygen uptake and carbon dioxide removal.
Which Side of the Heart Controls Pulmonary Circulation?
The right ventricle pumps blood into the pulmonary arteries, sending it toward the lungs. Oxygen-rich blood then returns through pulmonary veins to the left atrium.
Which Side of the Heart Controls Systemic Circulation?
The left ventricle pumps blood into the aorta and systemic arteries. Blood returning from most body tissues enters the right atrium through the superior and inferior venae cavae.
Why Is Systemic Blood Pressure Higher Than Pulmonary Blood Pressure?
Systemic circulation has greater vascular resistance and requires higher driving pressure to distribute blood throughout the body. Pulmonary vessels have lower resistance, allowing similar blood flow at much lower pressure.
Does Blood Pass Through the Heart Twice During One Complete Circuit?
Yes. In double circulation, blood passes through the right side of the heart before traveling to the lungs and through the left side before traveling to systemic tissues. These connected pathways form one continuous circulation.
Pulmonary circulation allows blood to exchange gases in the lungs, while systemic circulation distributes oxygen and nutrients to tissues and carries blood back toward the heart. Their different pressures and pathways reflect the distinct demands of the lungs and body organs. Together, the two circuits maintain continuous blood flow and connect breathing with cellular metabolism.
This article is for general education and is not medical advice. Talk to a qualified health professional about any medical concern.
Sources: OpenStax, Anatomy and Physiology 2e; Guyton and Hall, Textbook of Medical Physiology; National Heart, Lung, and Blood Institute (NHLBI), How the Heart Works; Tortora and Derrickson, Principles of Anatomy and Physiology.
