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External and Internal Respiration in the Lungs: Definition & Process

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  • 0:05 Respiration Is Gas Exchange
  • 1:12 Gas Exchange Is Diffusion
  • 3:18 External Respiration in Lungs
  • 4:34 Internal Respiration…
  • 6:48 Lesson Summary
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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

This lesson explores the process by which oxygen and carbon dioxide get into and out of the blood located in the lungs and in our metabolizing tissues. The partial pressure gradient for each gas determines both the direction and rate of diffusion across the respiratory membrane.

Respiration is Gas Exchange

While pulmonary ventilation is the process by which oxygen enters and carbon dioxide exits the alveoli, respiration is the process by which oxygen and carbon dioxide diffuse in and out of the blood. Respiration is also referred to as gas exchange, and it occurs in two areas of the body. External respiration refers to gas exchange across the respiratory membrane in the lungs. Internal respiration refers to gas exchange across the respiratory membrane in the metabolizing tissues, like your skeletal muscles, for example. In the following discussion of gas exchange, imagine yourself as an oxygen molecule going to work. You enter the body via the lungs, travel through the body by the bloodstream, and ultimately enter a cell to go to work. You're a hard worker, and as such, you get dirty and become carbon dioxide. To go home from work, you leave the cell and travel to the lungs via the bloodstream, the same way you got to work.

Gas Exchange is Diffusion

Gas exchange occurs by diffusion, which is the movement of a substance from a high to low concentration of that substance. Have you ever heard the expression 'If you smelt it, then you dealt it?' That's because the gas will reach the person closest to the source first. The gas will spread away from the area of high concentration until it achieves equal concentration throughout the room. So, if you have to fart, do it in a location where there is plenty of room for the odorous molecules to disperse.

Partial pressure equation
Partial Pressure Equation

Fortunately, we don't have to inhale foul odor all the time. The air we breathe is a mixture of gases - nitrogen, oxygen, carbon dioxide, and even water - and each gas diffuses according to its own concentration gradient. The rate of diffusion is directly proportional to the concentration gradient of each gas. Additionally, the rate of diffusion is directly proportional to pressure, which helps to push the gas into solution. The partial pressure of a gas is the pressure contributed by a single gas in a mixture of gases. The equation for partial pressure is Pp = Pt x C, where Pp = partial pressure of the individual gas, Pt = total pressure of the mixture of gas, and C = the concentration of the individual gas. For example, the partial pressure of oxygen, or PO2, in the alveolar gas is '760 mmHg x 0.13 = 100 mmHg,' which is 13% of the total pressure. It's important to note that gas exchange is directly proportional to the partial pressure gradient across the respiratory membrane, as partial pressure takes into account both concentration and pressure.

External Respiration in Lungs

Gas exchange in the lungs is referred to as external respiration as one side of the respiratory membrane - that is, the alveolar air - is actually outside of the body. As blood flows through the pulmonary capillaries, oxygen diffuses into the blood and carbon dioxide diffuses into the alveolar gas. Each gas diffuses down its own partial pressure gradient - that is, from a high to low partial pressure. The partial pressure of oxygen is 100 mmHg in alveolar air compared to only 40 mmHg in the blood entering the lungs. The partial pressure of carbon dioxide is 40 mmHg in the alveolar air and 45 mmHg in the blood entering the lungs. The partial pressure gradient of each gas equilibrates as blood flows through the pulmonary capillaries. Therefore, the partial pressure of oxygen is 100 mmHg and the partial pressure of carbon dioxide is 40 mmHg in the blood leaving the lungs. This oxygen-rich blood will then be delivered to the tissues, where oxygen is needed to make ATP as a source of energy.

Internal Respiration in Metabolizing Tissues

Gas exchange in the metabolizing tissues is referred to as internal respiration. Compared with external respiration, the gases now move in opposite directions. That is, oxygen diffuses out of the blood into the tissues, and carbon dioxide diffuses out of the tissues into the blood. Let's take a look at this, taking into account partial pressure gradients. Just like in the lungs, partial pressure gradients drive gas exchange in the tissues. The oxygen-rich blood in the capillaries has a PO2 of about 100 mmHg, compared with only 40 mmHg in the metabolizing cells. Oxygen diffuses into the cells and does so until equilibrium is achieved across the respiratory membrane. Therefore, the PO2 in the blood drops to 40 mmHg as the blood flows through the capillary.

Oxygen diffuses into the cells until equilibrium is reached.
Cell Internal Respiration Diagram

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