Difference Between External Respiration and Internal Respiration

Edited by Diffzy | Updated on: March 29, 2022


Difference Between External Respiration and Internal Respiration Difference Between External Respiration and Internal Respiration

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We all know that oxygen is necessary to live. But the processes involved in breathing oxygen and converting it into a usable form of energy to be utilized for various activities may be more complex than we think. Oxygen acquired by the body needs to undergo a set of pathways and metabolic processes to be able to be used by the body. This is mainly done by two processes: internal and external respiration.

The main components of respiration include cells of the lung, or the alveoli, capillaries, and tissue cells. External respiration consists of normal ventilation and exchange between alveoli, tissues, and capillaries. These capillaries carrying oxygenated blood are distributed to the heart, which pumps blood to the entire body. Every cell uses oxygen to release energy, following which waste products are released. The main waste gas produced is carbon dioxide. Internal respiration focuses in depth on the micro-processes that help in breaking down sugars into energy using the help of gases acquired by external respiration. Hence, both respiration and synthesis play an equally important role in providing an organism with the energy it requires.

External respiration vs. Internal respiration

External respiration usually denotes the inflow and outflow of gases that are determined by certain parameters and just includes a physical process.

Whereas, internal respiration involves a series of chemical reaction chains that the cell undergoes in order to make the process of respiration an energy-generating process. This can be more complex at a basal cellular level and is the basis of produced energy.

Difference Between External and Internal Respiration in Tabular form.

Table: External Respiration vs. Internal Respiration
Parameters of comparison
External respiration
Internal respiration
The ventilation and exchange processes between alveolar cells, tissue, and the capillaries of the lungs and back out are located from the nostrils until the bronchioles.
In prokaryotes, it occurs in the cytosol and across the plasma membrane. Whereas in eukaryotes, including humans, it occurs in cytosol and mitochondria.
Diffusion between
Exchange of gases between the blood and the environment at the level of respiratory membrane and between the blood and tissue cells at the level of tissue-blood membrane.
No gas exchange involved. Instead, the metabolic processes that depend on gas exchange are involved here.
Processes involved
It involves two processes. They are ventilation, and gas exchange.
It involves aerobic and anaerobic processes responsible for generating ATP for cell energy.
Partial pressures
Partial pressures between alveoli, oxygenated, and deoxygenated blood are given below respectively.
O2- 104, 40, 95 mm Hg.
CO2-40,  45,  40 mm Hg.
Not driven by partial pressure and there is no gas exchange.
Factors affecting
Intact respiratory pathway, normal air consumption, partial pressures, alveolar surface area, ventilation-perfusion ratios.
Intact functioning organelles of cells- involves the mainly the organelle mitochondria where most processes are localized.
Aerobic and anaerobic processes based on availability of oxygen.
Nostrils, Nasal cavity, pharynx, trachea, bronchi, bronchioles, alveolar sacs, alveoli. Alveoli are the anatomical functional units of the lungs.
90 percent of oxygen transport is made through RBC’s and rest dissolved in plasma.
CO2 transport
-70 percent converting to bicarbonate.
-25 percent binding to hemoglobin.
-Rest dissolving in plasma
Oxygen diffuses in from alveolus to blood, and carbon dioxide out from blood into alveolar air. Similarly also the oxygen and carbon dioxide exchange between blood and tissue.
Oxygen diffuses into blood, and carbon dioxide out from cells into blood.
 No metabolic reactions are involved. Only physical exchange and diffusion of gases occurs here.
Chemical breakdown using oxygen is involved in internal respiration.
Impact on body
 Affect the acid-base metabolism in the blood, and hence the body functions.
Affects the rate of energy production by every cell and, hence the metabolic rate.

What is External Respiration?

The term refers to the exchange of gases between the blood and the environment. Air from the environment enters into cells of the lung or alveoli through the respiratory tract. The alveolar membrane shares its limit with the capillary membrane through which the gases diffuse into the blood. In the blood, they are carried by the protein hemoglobin, mainly reversibly to oxygen and directly to carbon dioxide. Hemoglobin has a greater affinity to bind in decreasing order to carbon monoxide, carbon dioxide, and oxygen. Carbon dioxide transport pathways include 7 percent by dissolved plasma and the rest by other means. Carbon dioxide usually involves conversion into hydrogen carbonate to be able to diffuse across the blood.

The three main components of external respiration include partial pressure gradients, alveolar surface area, and ventilation and perfusion matching. The ratio of ventilation to perfusion should be maintained at one and is an important parameter in determining the changes made. Any increase or decrease in the ratio makes adequate adjustments to maintain the ratio of one.

Perfusion responds to changes in PO2 and ventilation responds to changes in PCO2.

For example, in cases where a patient suffers from long-term COPD (chronic obstructive pulmonary disease), it decreases the air supply to the alveoli. This decrease in ventilation is corrected by vasoconstriction of the artery that supplies the alveoli. In this way, the lungs ensure that no blood goes to the unventilated alveolus, preventing wastage and maximum use of it.

Similarly, if perfusion is blocked due to vasospasm or other causes, it reduces ventilation to those alveoli by contracting the smooth muscles that surround the bronchioles. This also ensures the maintenance of the ventilation-perfusion ratio at one.

What is Internal Respiration?

It is a set of biochemical processes that cells use to generate energy by combining glucose or sugar with oxygen, resulting in the formation of carbon dioxide, water, and ATP.s a process that releases heat and energy. Cellular respiration occurs at a more fundamental cellular level and is also known as "cellular respiration."

Cellular respiration refers to three main processes: glycolysis, the Krebs cycle, and the oxidative phosphorylation chain. By glycolysis, the glucose is broken down into two pyruvate molecules, which are then converted into acetyl coenzyme A and enter the TCA, or Krebs cycle. The cycle involves eight steps catalyzed by eight different enzymes and generates energy as a result. This energy, however, is stored as NAD+ or FAD molecules, which in later stages are converted into ATP.

These temporary molecules that hold energy now are used to power the downside electron flow, which releases massive amounts of energy every time the electrons are acquired by oxygen molecules. This is also referred to as the electron transport chain. Cytochromes are heme-proteins that form the basic unit of the electron transport chain. This chain is divided into multiple complexes across which the electrons flow.

Alveolar air taken up by your tissue cells can run glycolysis through two pathways. The only difference between aerobic and anaerobic respiration is the amount of energy gained from each, as well as the end products formed. Water and carbon dioxide are released as end products in aerobic respiration, whereas lactic acid or alcohol may be the end products of anaerobic respiration.

Also, aerobic processes generate more energy than anaerobic processes do. Each time oxygen is utilized by the aerobic process, it gains a total of thirty-eight ATP molecules, whereas an anaerobic process gains only two ATP molecules. The aerobic process can take place only in the presence of oxygen, whereas anaerobic is an alternative option when an organism faces low oxidation conditions. This could be because the aerobic process ensures complete oxidation of its reactants, whereas the anaerobic process undergoes incomplete oxidation.

For the same reason, it is always better than the aerobic process occurs not just for the high energy gained but also for fewer waste substances generated like lactic acid and carbon dioxide that can influence the acid-base balance of the body. Lactic acid production when tissue undergoes anaerobic metabolism results in its accumulation, which can cause instant cramps. Usually, it is observed in sportsmen who run fast that their muscles fail to receive proper oxygen supply reaching the legs. Anaerobic respiratory processes occur only in prokaryotes and in human muscle cells under ischemic conditions.

Main Differences Between External Respiration and Internal Respiration in Points


External respiration has the function of purifying and humidifying air entering the respiratory tract. It also affects the number of gases that can enter or exit the pathway in its ventilation function.

External respiration diffusions between blood, tissues, and alveoli, on the other hand, have a direct impact on the body's acid-base balance and basal metabolism.

Internal respiration, on the other hand, determines the possible energy that can be generated based on the availability of oxygen or not. This includes aerobic and anaerobic respiration. Not all organisms can use both these pathways. However, humans are given the possibility that this energy can be used even in the absence of oxygen.

Processes include

External respiration involves processes of ventilation and gas exchange.

Internal respiration involves processes that use these gases to metabolize and generate energy. There are two possible pathways. They are aerobic and anaerobic pathways. Also, other step-wise procedures include the Krebs cycle, oxidative phosphorylation, the electron transport chain, and others.

At the end of the electron transport chain, it is oxygen that powers the conversion of ADP into ATP, the energy currency of our cells.

Pressure gradient

In external respiration, the partial pressure of oxygen and carbon dioxide favors the oxygen inflow and carbon dioxide release out of the body system.

In internal respiration, there is no pressure gradient. The flow of oxygen and carbon dioxide is determined not by the pressure gradient, but by the oxygen itself, which drives or powers processes down a chain to eventually convert into molecules of energy, or ATP.

Anatomical involvement

External respiration begins at the level of the nostrils, nasal cavity, larynx, trachea, bronchi, bronchioles, and alveolar sacs. Alveolar sacs intend to increase the possible surface area for gas exchange. This includes the physiological and anatomical dead spaces in our respiratory system.

On the other hand, internal respiration processes take place in the cytosol, mitochondrial membrane and inside it. It has more microscopic anatomical locations in comparison to the visible organs mostly involved in external respiration.

Reactions involved

No chemical reactions are involved in external respiration. It denotes the physical inflow and outflow of gases following the diffusion principle across the blood-alveolar and blood-tissue membranes.

On the other hand, chemical reactions form the major part of internal respiration by which ATP is generated by undergoing several chains of breakdown processes. The three main steps include the conversion of glucose into pyruvate by glycolysis (aerobic or anaerobic), the Krebs cycle that involves a series of eight reactions by which energy is stored in temporary forms as NAD or FAD molecules.

These substances are in the next step as they flow down the complexes and cytochromes of the electron transport chain. 


Different species have different mechanisms to acquire air and metabolize it to generate energy products. Not all organisms have the spongy elastic organ we have-the lungs. For example, while fish use gills, frogs use their skin to breathe. Despite the structural differences among organisms, the processes they undergo are almost similar.

Oxygen acquired by the body needs to undergo a set of pathways and metabolic processes to be able to be used by the body. This is mainly done by two processes: internal and external respiration.

External respiration consists solely of the physical inflow and outflow of the gases oxygen and carbon dioxide via diffusion between the membranes that separate alveoli and capillaries, as well as capillaries and tissues. These processes are determined by several parameters like the partial pressure gradient, ventilation-perfusion ratio, and others.

The internal respiration involves microscopic metabolic processes and determines the energy produced by these gas exchanges. This energy produced is used by each cell to run its own metabolic activities, thereby maintaining the whole human body's metabolism. Energy generated by these processes is determined by the availability or absence of oxygen and intact mechanisms that run glycolysis, the Krebs cycle, and electron transport chain reactions.

Also, other functions of phonation and olfaction are performed during external respiration. Phonation involves a process of voice production as air is expelled from the larynx. Olfaction is a sensory function that enables you to detect smells and interpret them with your brain. 


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"Difference Between External Respiration and Internal Respiration." Diffzy.com, 2022. Tue. 24 May. 2022. <https://www.diffzy.com/article/difference-between-external-respiration-and-internal-respiration-28>.

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