What is the approximate pO2 in the pulmonary vein?
a) 60 mmHg
b) 100 mmHg
c) 40 mmHg
d) 45 mmHg
e) 5 mmHg
The correct answer and explanation is:
The correct answer is b) 100 mmHg.
Explanation:
pO2 stands for the partial pressure of oxygen, which is a measure of the amount of oxygen gas dissolved in the blood or air. It is usually measured in millimeters of mercury (mmHg).
The pulmonary vein is the blood vessel that carries oxygenated blood from the lungs to the left atrium of the heart. Understanding the oxygen partial pressure here is important because it reflects how much oxygen the blood has picked up in the lungs before it circulates to the rest of the body.
Why is the pO2 about 100 mmHg in the pulmonary vein?
- Gas exchange in the lungs: In the alveoli (tiny air sacs in the lungs), oxygen from the inhaled air diffuses into the blood, and carbon dioxide diffuses out of the blood to be exhaled.
- The alveolar oxygen partial pressure (pO2) is roughly 100 mmHg under normal conditions.
- As blood flows through the pulmonary capillaries surrounding the alveoli, oxygen rapidly diffuses into the blood, equilibrating its pO2 with that of the alveoli.
- Therefore, by the time blood leaves the lungs through the pulmonary veins, it is fully oxygenated, meaning the pO2 in the pulmonary vein is close to the alveolar pO2 — approximately 100 mmHg.
What about the other options?
- a) 60 mmHg — This is approximately the pO2 of blood in the pulmonary artery, which carries deoxygenated blood from the right ventricle to the lungs. It’s lower because the tissues have consumed oxygen.
- c) 40 mmHg — This is close to the typical pO2 in the venous blood returning to the lungs (mixed venous blood).
- d) 45 mmHg — This is more like the typical partial pressure of carbon dioxide (pCO2) in venous blood, not oxygen.
- e) 5 mmHg — This is far too low for any physiological oxygen partial pressure in blood.
Summary:
The pulmonary vein carries freshly oxygenated blood with a pO2 approximately 100 mmHg, reflecting efficient oxygen transfer from the alveoli to the blood during pulmonary circulation. This high oxygen partial pressure is crucial for delivering sufficient oxygen to the body’s tissues.