The key difference between internal and external respiration is that the internal respiration refers to the set of metabolic reactions that occur within the cells to produce energy via the oxidation of glucose and other organic molecules while external respiration refers to the process of moving oxygen from external environment to the cells and moving carbon dioxide from the body to the external environment.
The gas exchange between blood and the metabolizing tissue is referred to as internal respiration. Tissues like skeletal muscles require oxygen in order to carry out cellular respiration by which the cells produce energy in the form of ATP by burning food, mainly glucose. ATP is used to drive cellular functions. Cellular respiration occurs in the mitochondria. Carbon dioxide is produced as a waste during cellular respiration. So, metabolizing cells in the tissue have a high demand for oxygen while carbon dioxide is to be removed from the cells. That is, the partial pressure of oxygen is low and that of carbon dioxide is high in the tissue. But in the blood, the partial pressure of oxygen is high and that of carbon dioxide is low. Therefore, oxygen diffuses out of the blood into the tissue while carbon dioxide diffuses out from the tissue into the blood. The partial pressure of oxygen (PO2) is 100 mmHg and it is 40 mmHg in the tissue cells. The oxygen exchange continues until equilibrium occurs on the either sides of the respiratory membrane in the tissue. Hence, the final PO2 in the blood becomes 40 mmHg. The gas exchange occurs in blood capillaries of the tissue. This oxygen-depleted blood is carried out to the lungs through the heart by veins. Internal respiration at metabolizing tissue is shown in figure 1.
Figure 1: Internal respiration
The gas exchange in the lungs is referred to as external respiration. One side of the respiratory membrane, alveolar air is situated outside of the body. Oxygen-depleted blood, which is transported from metabolizing tissues, flows through the pulmonary capillaries where oxygen diffuses from the alveolar air into the blood. Carbon dioxide diffuses out of the blood into the alveolar air. The partial pressure of oxygen (PO2) in the blood increases up to 100 mmHg. The partial pressure of carbon dioxide (PCO2) in the blood is 45 mmHg while that of the alveolar air is 40 mmHg. Therefore, the exchange of carbon dioxide occurs from the blood into the alveolar air. Both exchanges of oxygen and carbon dioxide occur until the equilibrium of each gas is established. The final PO2 is 100 mmHg and PCO2 is 40 mmHg in the blood which leaves the lungs. Thus, the blood which leaves the lungs is called oxygen-rich blood. This oxygen-rich blood flows to the metabolizing tissues, responding to their high demands of oxygen. Both oxygen and carbon dioxide are transported through the blood by binding with hemoglobin, which is found in red blood cells. Some of the carbon dioxide is transported by dissolving in the plasma as well. The external respiration at lungs is shown in figure 2.
Figure 2: External respiration
Internal Respiration: Internal respiration refers to the gas exchange across the respiratory membrane in the metabolizing tissues.
External Respiration: External respiration refers to the gas exchange across the respiratory membrane of lungs.
Internal Respiration: Oxygen diffuses out from the blood into tissue during internal respiration.
External Respiration: Oxygen diffuses from alveolar air into the blood during external respiration.
Internal Respiration: The PO2 in the blood is reduced from 100 mmHg to 40 mmHg during internal respiration.
External Respiration: The PO2 in the blood is increased from 40 mmHg to 100 mmHg during external respiration.
Internal Respiration: Carbon dioxide diffuses into the blood from the tissue during internal respiration.
External Respiration: Carbon dioxide diffuses out from the blood into the alveolar air during external respiration.
Internal Respiration: The PCO2 in the blood is increased from 40 mmHg to 45 mmHg during internal respiration.
External Respiration: The PCO2 in the blood is reduced from 45 mmHg to 40 mmHg during external respiration.
Internal Respiration: Internal respiration only correlates with the internal environment.
External Respiration: External respiration correlates with both internal and external environment.
Internal and external respiration are two processes where the exchange of oxygen and carbon dioxide occur. Internal respiration occurs in the respiratory membrane of metabolizing tissue. Inside the mitochondria, cellular respiration, which produces energy in the form of ATP, occurs. Cellular respiration requires oxygen and carbon dioxide is produced as a waste. Thus, in order to maintain cellular processes in a balanced condition, continuous flow of oxygen should be maintained by blood. Oxygen is provided by the internal respiration from the blood into tissues. Oxygen-depleted blood is transported into lungs, where external respiration takes place. Oxygen diffuses from the alveolar air into the blood. The main difference between internal respiration and external respiration is in the direction of gas exchange and in the location where each of the processes takes place.