External and Internal Respiration

What Is the Difference Between External and Internal Respiration?

The difference between external and internal respiration is where the gas exchange happens and which way the gases move: external respiration takes place in the lungs and loads the blood with oxygen, while internal respiration takes place in the tissues and unloads that oxygen into the cells. Carbon dioxide does the reverse, entering the blood in the tissues and leaving it in the lungs.

External respiration definition

External respiration, also called pulmonary gas exchange, is the diffusion of oxygen from the air in the alveoli into the pulmonary capillary blood and of carbon dioxide from that blood into the alveoli. It is called external because it is the exchange between the body and the outside world, with the alveolar air standing in for the outside air. Our page on gas exchange in the lungs covers the alveolar side of this stage in full detail.

Internal respiration definition

Internal respiration, also called systemic or tissue gas exchange, is the diffusion of oxygen from the blood in the systemic capillaries into the tissue cells and of carbon dioxide from the cells into the blood. It happens in every organ that has a blood supply, from the brain and heart muscle to the skin and bones.

The verdict in one line

External respiration refills the blood with oxygen in the lungs, and internal respiration hands that oxygen to the cells everywhere else, so one cannot be useful without the other.

Explained simply

Picture a delivery truck. At the warehouse, which is the lungs, it loads fresh boxes of oxygen and drops off a pile of carbon dioxide trash. At each house, which is a body cell, it drops off oxygen boxes and picks up new trash. External respiration is the warehouse stop, and internal respiration is every house stop along the route.

What Is External Respiration?

External respiration is the stage in which blood passing through the lungs picks up oxygen from alveolar air and releases carbon dioxide into it, turning oxygen-poor blood into oxygen-rich blood in less than a second. It depends on three things working at once: fresh air delivered by breathing, blood delivered by the right side of the heart, and a thin, moist barrier between them.

Where it happens

External respiration takes place across the respiratory membrane, the barrier formed by the alveolar wall, a fused basement membrane and the capillary wall, usually only about 0.5 micrometer thick. The blood comes from the right ventricle through the pulmonary arteries and leaves through the pulmonary veins toward the left atrium, as traced on our pulmonary circulation page.

The partial pressures at the lung

Alveolar air has an oxygen partial pressure (PO2) of about 100 mmHg and a carbon dioxide partial pressure (PCO2) of about 40 mmHg. The blood that arrives from the body has a PO2 of about 40 mmHg and a PCO2 of about 45 mmHg. Oxygen therefore moves into the blood down a gradient of about 60 mmHg, while carbon dioxide moves out down a gradient of only about 5 mmHg, which is enough because carbon dioxide dissolves about 20 times more readily than oxygen.

How quickly the blood is loaded

At rest a red blood cell spends about 0.75 second in a lung capillary, yet its oxygen level usually matches the alveolar air within roughly the first third of that time. The spare time is a safety margin that keeps arterial oxygen almost unchanged when blood speeds up during exercise. Matching air to blood flow matters too, a balance described by the ventilation-perfusion ratio.

What Are the Steps of External Respiration?

The steps of external respiration are fresh air reaching the alveoli, oxygen diffusing into the blood, carbon dioxide diffusing out of the blood, and the reloaded blood being carried away while the waste air is breathed out. These four steps repeat with every breath and every heartbeat.

1. Ventilation refreshes the alveolar air. During inhalation, new air mixes with the air already in the alveoli and keeps its oxygen pressure high and its carbon dioxide pressure low.
2. Oxygen diffuses into the blood. Oxygen dissolves in the fluid lining the alveolus, crosses the respiratory membrane and the plasma, and binds to hemoglobin inside red blood cells.
3. Carbon dioxide diffuses out of the blood. Bicarbonate in the plasma is turned back into carbon dioxide gas inside red blood cells, and the gas crosses the membrane into the alveolus.
4. Blood leaves and air is exhaled. Oxygen-rich blood flows to the left heart, and exhalation removes the carbon dioxide so the gradients are ready for the next cycle.

The four processes of respiration

Many textbooks list a different set of four steps that covers the whole journey of the gases, not just the lung stage. These are pulmonary ventilation, external respiration, transport of gases in the blood, and internal respiration. Ventilation is the mechanical part described on our mechanics of breathing page; the other three are diffusion and transport, and the diagram below shows how they connect to cellular respiration inside the cells.

What Is Internal Respiration?

Internal respiration is the stage in which oxygen leaves the blood in the systemic capillaries and enters the tissue cells, while carbon dioxide made by those cells enters the blood. It is the exact mirror of external respiration: the same diffusion, driven by gradients that point the other way.

Where it happens

Internal respiration takes place across the thin walls of the systemic capillaries and the interstitial fluid that bathes the cells. Oxygen-rich blood arrives from the left ventricle through the arteries and arterioles, and oxygen-poor blood leaves through the venules and veins toward the right atrium. Gases cross the capillary wall, the tissue fluid and the cell membrane, and oxygen then travels on to the mitochondria.

The partial pressures in the tissues

Arterial blood reaching the tissues has a PO2 of about 95 to 100 mmHg and a PCO2 of about 40 mmHg. Tissue cells at rest have a PO2 of about 40 mmHg or lower and a PCO2 of about 45 mmHg or higher, because they use oxygen and make carbon dioxide all the time. Oxygen therefore diffuses out of the blood, carbon dioxide diffuses in, and the blood leaves with a PO2 of about 40 mmHg and a PCO2 of about 45 mmHg.

Why the tissues take only part of the oxygen

At rest, the tissues remove only about a quarter of the oxygen carried in arterial blood, so venous blood is still about 75% saturated. That reserve is released when it is needed: working muscle has a lower PO2, more carbon dioxide, more acid and a higher temperature, all of which make hemoglobin let go of oxygen more easily, a shift known as the Bohr effect. In strenuous exercise, active muscle can extract most of the oxygen in its blood supply, and extra capillaries open to shorten the distance oxygen has to travel.

External Respiration vs Internal Respiration: Side by Side

External respiration vs internal respiration comes down to opposite directions in different places: the lungs load oxygen and unload carbon dioxide, and the tissues unload oxygen and load carbon dioxide. The table below lines up every feature of the two stages, including the typical partial pressures at each site.

Notice that the blood leaving one stage is exactly the blood entering the other. Blood leaves the lungs with a PO2 of about 100 mmHg and reaches the tissues at about 95 mmHg, slightly lower because a small amount of oxygen-poor blood from the bronchial and heart veins mixes in. Blood leaves the tissues at about 40 mmHg and arrives at the lungs at the same value. This continuity is why physiologists treat the two stages as one loop rather than two separate events.

What Do External and Internal Respiration Have in Common?

External and internal respiration share the same physical mechanism, the same two gases and the same carrier, the blood, and both happen across capillary walls only a cell or two thick. The list below sums up what they have in common.

• Both are passive diffusion. Neither stage uses energy from the cells doing the exchange; each gas simply moves from higher to lower partial pressure.
• Both involve oxygen and carbon dioxide moving in opposite directions at the same time.
• Both happen in capillaries, the only blood vessels with walls thin enough for gases to cross quickly.
• Both depend on hemoglobin. It loads oxygen in the lungs and releases it in the tissues, and it also carries part of the carbon dioxide.
• Both rely on carbonic anhydrase, the red blood cell enzyme that turns carbon dioxide into bicarbonate in the tissues and back again in the lungs.
• Both speed up together when the body is more active, because the circulation and breathing are adjusted as one system.

The Haldane effect ties the two stages together chemically: oxygen loading in the lungs helps hemoglobin release carbon dioxide, and oxygen unloading in the tissues helps it pick carbon dioxide up.

How Is Internal Respiration Related to Cellular Respiration?

Internal respiration delivers oxygen to the cells and removes their carbon dioxide, while cellular respiration is the chemical process inside the cells that actually uses that oxygen to release energy from food. Internal respiration is physics, the diffusion of gases; cellular respiration is chemistry, a chain of enzyme reactions.

What the mitochondria do with the oxygen

Inside the mitochondria, oxygen is the final electron acceptor at the end of the electron transport chain, where it combines with hydrogen ions to form water. This step lets the cell make most of its ATP, the molecule that powers muscle contraction, nerve signals and nearly every other job. The overall reaction for glucose is often summarized as glucose plus six oxygen molecules giving six carbon dioxide, six water and energy.

Where the carbon dioxide comes from

The carbon dioxide that internal respiration carries away is made in the mitochondria too, when pyruvate is broken down and during the citric acid cycle (Krebs cycle). An adult at rest uses about 250 milliliters of oxygen per minute and produces about 200 milliliters of carbon dioxide, a ratio of about 0.8 called the respiratory quotient. Our page on why we need oxygen explains what happens when this supply fails.

How Do External and Internal Respiration Work Together?

External and internal respiration work together as two ends of one loop: the circulation carries oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs, so each stage creates the conditions the other needs. If the tissues use more oxygen, the blood returns to the lungs emptier, and external respiration loads more.

Transport links the two stages

Between the two exchanges the gases travel in the blood. About 98 to 99% of oxygen rides on hemoglobin, as described on our oxygen transport page, and its loading and unloading follow the S-shaped oxygen-hemoglobin dissociation curve. Carbon dioxide travels mostly as bicarbonate, with smaller shares bound to proteins and dissolved in plasma, as explained under carbon dioxide transport.

Control keeps them in balance

The brainstem matches breathing to the rate at which the tissues make carbon dioxide. Respiratory chemoreceptors sense the carbon dioxide, acid and oxygen levels of the blood and adjust the depth and rate of breathing, a system described on our control of breathing page. The heart speeds up at the same time, so more blood passes through both the lungs and the tissues every minute. This partnership is the subject of our guide to the respiratory and circulatory systems.

Common Conditions That Affect External and Internal Respiration

External respiration is limited by problems in the lungs, while internal respiration is limited by problems with the blood, the circulation or the cells themselves. Our respiratory diseases section covers lung conditions in more depth, and anyone with breathing symptoms should see a doctor.

• Lung conditions such as pneumonia, emphysema and pulmonary fibrosis reduce external respiration by filling, destroying or thickening the exchange surface.
• Low inspired oxygen, as at high altitude, lowers alveolar PO2 and weakens the gradient that drives oxygen into the blood.
• Anemia and carbon monoxide leave external respiration intact but reduce how much oxygen the blood can carry to the tissues.
• Poor blood flow, from a blocked artery or a failing heart, starves internal respiration in the affected tissue even when the blood itself is full of oxygen.
• Cyanide poisoning blocks the use of oxygen inside the mitochondria, so oxygen reaches the cells but cellular respiration cannot use it.

When either stage fails, tissue oxygen falls, a state described on our hypoxia and hypoxemia page.