Question

Using partial pressures for oxygen and carbon dioxide explain the gas exchange that occurs in the lungs duin g external respiration and the exchnage that occurs at the tissue level during internal respiration.

Answer 1

External respiration means gas exchange. It is the bulk flow of air into and out of the lungs and the transfer of oxygen and carbon dioxide into the bloodstream through diffusion. While the bulk flow of air from the external environment happens due to pressure changes in the lungs, the mechanisms of alveolar gas exchange are more complicated.Partial pressure gradients (differences in partial pressure) allow the loading of oxygen into the bloodstream and the unloading of carbon dioxide out of the bloodstream. These two processes occur at the same time.

External respiration is a result of partial pressure gradients, alveolar surface area, and ventilation and perfusion matching.Oxygen has a partial pressure gradient of about 60 mmHg (100 mmHg in alveolar air and 40 mmHg in deoxygenated blood ) and diffuses rapidly from the alveolar air into the capillary. Equilibrium between the alveolar air and capillaries is reached quickly, within the first third of the length of the capillary within a third of a second. The partial pressure of oxygen in the oxygenated blood of the capillary after oxygen loading is about 100 mmHg.

The process is similar in carbon dioxide. The partial pressure gradient for carbon dioxide is much smaller compared to oxygen, being only 5 mmHg (45 mmHg in deoxygenated blood and 40 mmHg in alveolar air). Based on Henry’s law, the greater solubility of carbon dioxide in blood compared to oxygen means that diffusion will still occur very rapidly despite the lower partial pressure gradient. Equilibrium between the alveolar air and the capillaries for carbon dioxide is reached within the first half of the length of the capillaries within half a second. The partial pressure of carbon dioxide in the blood leaving the capillaries is 40 mmHg.

The exchange of gas and blood supply to the lungs must be balanced in order to facilitate efficient external respiration. While a severe ventilation–perfusion mismatch indicates severe lung disease, minor imbalances can be corrected by maintaining air flow that is proportional to capillary blood flow, which maintains the balance of ventilation and perfusion.

Internal respiration refers to two distinct processes. The first is the exchange of gases between the bloodstream and the tissues. The second is the process of cellular respiration, from which cells utilize oxygen to perform basic metabolic functions.

Gas exchange occurs in the alveoli so that oxygen is loaded into the bloodstream and carbon dioxide is unloaded from the bloodstream. Afterwards, oxygen is brought to the left side of the heart via the pulmonary vein, which pumps it into systemic circulation.

Red blood cells carry the oxygen into the capillaries of the tissues of the body. Oxygen diffuses into the cells of the tissues, while carbon dioxide diffuses out of the cells of the tissues and into the bloodstream.

The factors that influence tissue gas exchange are similar to the factors of alveolar gas exchange, and include partial pressure gradients between the blood and the tissues, the blood perfusion of those tissues, and the surface areas of those tissues. Each of those factors generally increase gas exchange as those factors are increased (i.e., more oxygen diffusion in tissues with more blood perfusion).

Regarding the partial pressure gradients in systemic capillaries, they have a PaO₂ of 100mmHg and a PaCO₂ of 40mmHG within the capillary and a PaO₂ of 40 mmHg and PaCO₂ of 45 mmHg inside issue cells, which allows gas exchange to occur.