How the Respiratory and Circulatory Systems Work Together

What Is the Difference Between the Respiratory System and the Circulatory System?

The difference is the job each one does with oxygen: the respiratory system gets oxygen from the air into the blood, and the circulatory system moves that blood to the tissues. One is an air-handling system, the other is a fluid-transport system, and neither can keep a cell alive without the other.

The respiratory system is built around a branching set of air tubes, from the nose and pharynx down through the trachea and bronchi to millions of tiny air sacs called alveoli. It is powered by the diaphragm and the muscles between the ribs, which change the size of the chest so air flows in and out. Its product is fresh alveolar air, rich in oxygen and low in carbon dioxide.

The circulatory system, also called the cardiovascular system, is a closed loop of blood vessels driven by the heart. Arteries carry blood away from the heart, capillaries let substances pass between blood and tissues, and veins bring blood back. Besides oxygen and carbon dioxide, blood carries nutrients, hormones, heat, immune cells and wastes, so the circulatory system serves every organ, not only the lungs.

Explained simply: think of the lungs as a loading dock and the blood as a fleet of delivery trucks. The dock loads oxygen onto the trucks and takes away the carbon dioxide they bring back. The heart is the engine that keeps the trucks moving. A dock with no trucks, or trucks with no dock, delivers nothing.

How Does the Respiratory System Work With the Circulatory System?

The respiratory system works with the circulatory system in a continuous loop: the lungs add oxygen to the blood, the heart pumps it to the body, the tissues use the oxygen and add carbon dioxide, and the heart sends the blood back to the lungs to unload it. Each trip around the loop follows the same steps.

1. Ventilation: Breathing in brings fresh air along the pathway of air into the alveoli, keeping their oxygen level high and carbon dioxide level low.
2. Gas exchange in the lungs: Oxygen diffuses from the alveoli into the blood of the pulmonary capillaries, and carbon dioxide diffuses the other way, as described on our gas exchange page.
3. Return to the heart: Oxygen-rich blood leaves the lungs in the pulmonary veins and enters the left atrium and left ventricle.
4. Delivery to the body: The left ventricle pumps the blood into the aorta and out through the systemic arteries to every organ.
5. Gas exchange in the tissues: In the body's capillaries, oxygen moves into the cells and carbon dioxide moves into the blood. This step is called internal respiration; see external and internal respiration.
6. Back to the lungs: Oxygen-poor blood returns in the veins to the right atrium and right ventricle, which pumps it through the pulmonary arteries into the lungs, and the loop starts again.

The two halves of the heart are therefore two pumps in series. The right side serves the pulmonary circulation, a short, low-pressure loop through the lungs, and the left side serves the systemic circulation, a long, high-pressure loop through the rest of the body. Every drop of blood must pass through the lungs once for each trip around the body.

Where Do the Respiratory and Circulatory Systems Meet?

The two systems meet at the walls of the alveoli, where each air sac is wrapped in a dense net of pulmonary capillaries so thin that red blood cells pass through them almost in single file. This meeting point is the only place where air and blood come close enough to exchange gases.

Between the air and the blood lies the respiratory membrane: the flat type I cells of the alveolar wall, a shared basement membrane and the capillary wall. In most places it is only about 0.5 micrometer thick, and in an adult the total surface for exchange is roughly 50 to 75 square meters. Gases cross it by simple diffusion, moving from higher to lower partial pressure, so no energy is spent on the exchange itself.

The lungs are the only organ that receives the entire output of the heart. At rest, about 5 liters of blood flow through the pulmonary capillaries every minute, and each red blood cell spends roughly three-quarters of a second there, which is normally more than enough time to fill with oxygen. The lung tissue itself is fed separately by small bronchial arteries that branch from the aorta, because the blood in the pulmonary arteries is low in oxygen.

Respiratory System vs Cardiovascular System: Key Differences

The key differences are in what each system moves, what powers it and how it is controlled: the respiratory system moves air with skeletal muscles that you can partly control, while the cardiovascular system moves blood with heart muscle that works on its own. The table below sets the two side by side.

• What flows: air, a mixture of gases, flows in and out of the same tubes; blood, a liquid, flows one way around a closed circuit.
• The pump: breathing depends on the diaphragm and the muscles of respiration, which are skeletal muscles; the heart is cardiac muscle with its own built-in pacemaker.
• Control: you can hold your breath or speed it up for a short time, but you cannot stop your heart by choice. Both are normally run automatically by the brainstem.
• Pressure: air moves because pressure in the lungs falls slightly below or rises slightly above the air outside, while the heart creates much higher pressures to push blood through the vessels.
• Reach: the respiratory system ends in the lungs; the circulatory system reaches almost every tissue in the body.

Typical resting rates show the difference in rhythm: an adult breathes about 12 to 20 times a minute (see respiratory rate), while the heart beats about 60 to 100 times a minute.

What Do the Respiratory and Circulatory Systems Have in Common?

Both systems are transport networks of branching tubes that end in microscopic, thin-walled spaces where exchange takes place. The airways divide again and again until they reach the alveoli, and the arteries divide again and again until they reach the capillaries, so both pack a huge exchange surface into a small volume.

• Branching design: the bronchial tree and the arterial tree both narrow and multiply at each division.
• Smooth muscle control: smooth muscle in the walls of bronchioles and arterioles adjusts how much air or blood reaches each region.
• Rhythmic pumping: both depend on a repeating cycle, the breathing cycle and the heartbeat.
• Brainstem control: centers in the medulla oblongata and pons set both breathing and heart rate.
• Shared organs in the chest: the heart and lungs sit side by side in the thoracic cavity, separated by the mediastinum.

Both systems also help keep the body's internal conditions stable. Breathing controls how much carbon dioxide stays in the blood, which sets blood pH, and blood flow spreads oxygen, heat and buffers evenly through the body. Our page on respiratory system homeostasis explains this balance in detail.

How Does the Body Keep Breathing and Blood Flow Matched?

The body keeps breathing and blood flow matched through shared sensors and control centers that raise or lower both together. When cells need more oxygen, both the breathing rate and the heart rate climb, so more air and more blood reach the lungs at the same time.

Chemoreceptors and the brainstem

Sensors called respiratory chemoreceptors track carbon dioxide, pH and oxygen. Central chemoreceptors near the medulla respond mainly to carbon dioxide through changes in pH, and peripheral chemoreceptors in the carotid bodies and aortic bodies respond to low oxygen as well. Their signals reach the brainstem, which adjusts the depth and rate of breathing and, through the autonomic nerves, heart rate. The full feedback loop is covered on our control of breathing page.

Matching inside the lungs

Within the lungs, air and blood are also matched region by region. When a group of alveoli receives little air, the small pulmonary arteries supplying it narrow, a response called hypoxic pulmonary vasoconstriction, which sends blood toward better-ventilated areas. The balance between air and blood in each region is described by the ventilation-perfusion ratio.

During exercise

During hard exercise, the amount of air breathed per minute and the heart's output can each rise several times above resting levels. Our page on exercise and the respiratory system shows how both systems respond.

Common Conditions That Affect Both Systems

Because the heart and lungs depend on each other, a problem in one system often shows up in the other. The conditions below are listed for education only; they are explained further in our respiratory diseases section.

• Pulmonary embolism: a blood clot, usually from a leg vein, blocks a pulmonary artery, so part of the lung is ventilated but not supplied with blood.
• Fluid in the lungs from heart disease: when the left side of the heart cannot pump well, pressure backs up into the lung capillaries and fluid can leak into the lung tissue, making gas exchange harder.
• Lung disease straining the heart: long-term low oxygen, as in severe COPD, narrows lung blood vessels and makes the right ventricle work harder.
• Anemia: the lungs may work normally, but blood with too little hemoglobin carries less oxygen, which can lead to hypoxia in the tissues.

Breathlessness and chest pain can come from either system, so they are a reason to see a doctor rather than to guess the cause.