Exchange of gases between alveoli and blood in pulmonary capillaries is known as a. External respirationb. Internal respirationc. Cellular respirationd. None of the above

The correct answer is a. External respiration.
External respiration refers to the process of gas exchange that occurs between the alveoli (the tiny air sacs in the lungs) and the blood in the pulmonary capillaries. This process allows oxygen from the inhaled air to pass into the blood, while carbon dioxide from the blood is transferred into the alveoli to be exhaled.
Here’s how external respiration works:
1. Oxygen Exchange: When air is inhaled, oxygen moves into the alveoli. The concentration of oxygen in the alveoli is higher than in the blood of the pulmonary capillaries. Therefore, oxygen diffuses from the alveoli into the blood.
2. Carbon Dioxide Exchange: At the same time, carbon dioxide in the blood (which is a waste product of cellular metabolism) is at a higher concentration than in the alveoli. Thus, carbon dioxide diffuses from the blood into the alveoli to be expelled from the body when exhaled.
Explanation of the other options:
1. Internal respiration (b):
o Internal respiration refers to the exchange of gases between the blood and the body's tissues, not the lungs. It occurs at the level of the capillaries in tissues and muscles, where oxygen is delivered to the cells and carbon dioxide is picked up from the cells to be transported back to the lungs.
2. Cellular respiration (c):
o Cellular respiration is the process by which cells use oxygen to produce energy (ATP) from glucose. It occurs within the mitochondria of cells and involves the consumption of oxygen and the production of carbon dioxide. While it involves the use of oxygen and release of carbon dioxide, it is not the same as external respiration.
3. None of the above (d):
o Since external respiration is the correct term for gas exchange in the lungs, "None of the above" is incorrect.
External respiration specifically refers to the exchange of gases (oxygen and carbon dioxide) between the alveoli in the lungs and the blood in the pulmonary capillaries.
Therefore, the correct answer is a. External respiration.