Question

Describe gas exchange during external respiration and internal respiration, including how the partial pressure gradients determine the diffusion of the gases.

Answer 1

The exchange of gas happens in two parts of the body:

1. In lungs: where there is the intake of oxygen and release of carbon dioxide.(External Respiration)
2. At tissues: Where there is the intake of carbon dioxide and release of oxygen.(Internal respiration)

EXTERNAL RESPIRATION: It is the exchange of gases that occurs in the lungs' alveoli (External environment). The deoxygenated blood is carried by the pulmonary artery from the heart to the lungs. After reaching the lungs pulmonary artery becomes pulmonary capillaries after it branches into a network of the capillary. A respiratory membrane is being created by the pulmonary capillaries along with the alveoli. The exchange of gases occurs as the blood is being passed/pumped through the capillary network so formed. The oxygen is then being picked by RBCs(Red blood cells) that bind to hemoglobin(a type of protein). Now the hemoglobin which is containing the oxygen( basically oxygenated hemoglobin) is being transferred to the heart through pulmonary veins. The carbon dioxide takes the opposite path as traveled by oxygen and it is released in the alveoli from the blood.

The exchange of gases occurs because of partial pressure differences between the blood present in the pulmonary capillaries and alveoli, for both oxygen and carbon dioxide. The Partial pressure difference between the oxygen present in the capillaries and alveoli is approx 64mmHg, which is so high that it causes oxygen to rapidly rush into the blood from the alveoli.

Similarly for carbon dioxide, the partial pressure differences between the blood present in the pulmonary capillary and alveoli are approx 5 mmHg, which causes carbon dioxide to move from the blood to alveoli.

INTERNAL RESPIRATION: The exchange of gases that occurs at tissue level is called internal respiration. In the case of internal respiration also the partial pressure gradient plays an important role. Though the partial pressure gradient is opposite as in the case of external respiration.

As the oxygen is being used by the cells for cellular respiration, the partial pressure for oxygen in tissues is comparatively low. And the partial pressure which is there in blood for oxygen is comparatively higher. this causes a pressure gradient and leads to the dissociation of oxygen from hemoglobin, and released out of the blood to reach the tissues

similarly for carbon dioxide, as the partial pressure is higher in tissue and lower in blood. The gradient causes carbon dioxide to go out of the tissue and reach the blood. which then combines with the hemoglobin and carried to the lungs for excretion of carbon dioxide outside the body.