Composition of Alveolar Gas

1) Explain why alveolar gas has a different composition from air. Atmospheric (external) air contains a high percentage of oxygen and nitrogen whereas alveoli gas contains a lower percentage of oxygen and a higher percentage of carbon dioxide. The differences in composition are due to the fact that gaseous exchange is taking place within the alveoli in the lungs. Oxygen is diffusing from the alveoli into the pulmonary artery and carbon dioxide is diffusing back into the alveoli from the pulmonary artery due to the concentration gradients. This is the main reason why external air and alveolar gas have such different compositions. Another reason why the gas compositions are different is due to the fact that alveoli gas contains a mixture of both atmospheric air which has been inhaled and ‘old’ air which stays in the respiratory track and is not exhaled after each breath therefore giving it a different composition. Alveolar gas has a larger percentage of water vapour than atmospheric air due to the fact that air is being warmed and moistened as it travels into the respiratory track. This is done by the mucus in the nasal cavity as the air travels past it.

|Gas |% Atmospheric Air |% Alveolar Air |
|Nitrogen |78.6 |74.9 |
|Oxygen |20.9 |13.7 |
|Carbon Dioxide |0.04 |5.2 |
|Water |0.46 |6.2 |

(Human anatomy and physiology 6th Ed, Marieb.)

2) Identify the reason for [A] myoglobin and [B] fetal haemoglobin having oxygen binding curves different from haemoglobin.
[A] Myoglobin is a single subunit and polypeptide very similar to haemoglobin. Myoglobin is a red pigment that transports oxygen in the muscles (used by some marine mammals as a substitute for haemoglobin). Its oxygen-binding curve is displaced to the left which means myoglobin has the ability to become saturated with oxygen at low pO . When exercising the pO becomes very low in the muscles causing myoglobin to release all O it is carrying. The oxygen binding curve ensures that the muscles always have a plentiful supply of oxygen when it is needed.

[B] Fetal haemoglobin has an oxygen-binding curve which is shifted to the left. This means that fetal blood is very efficient at ‘picking up’ oxygen. Fetal haemoglobin has a stronger affinity to oxygen than the mother’s haemoglobin to ensure that it receives a plentiful supply of oxygen out of the mother’s blood. Many processes will be taking place within the foetus e.g. respiration and the development of limbs and organs so that if the foetus did not have an oxygen-binding curve displaced to the left then it wouldn’t receive a plentiful supply of oxygen which could hinder the baby’s development causing problems in the pregnancy.

3) Japanese pearl divers use hyperventilation techniques to able them to stay underwater for 5 minutes or more. Why does hyperventilation allow them to do this? Hyperventilation is when the breathing rate increases and becomes ‘deeper’. This causes increased ventilation of the alveoli and as a result more carbon dioxide diffuses from the blood into the alveoli and is exhaled from the lungs in each breath. The effect of more carbon dioxide being exhaled is a decrease of carbon dioxide in the blood. Decreased CO in the blood shifts the normal oxygen-haemoglobin dissociation curve to the left. The result of this is that the haemoglobin has a stronger affinity to oxygen (increased ability to pick up oxygen) and becomes saturated with oxygen at a much lower partial pressure than normal. When the partial pressure of oxygen drops to low levels in the tissue then the haemoglobin will readily release all the oxygen it is carrying. Japanese pearl divers can stay under water for 5 minutes plus because their haemoglobin becomes saturated with oxygen due to the hyperventilation technique. This means that their main organs and tissues, i.e. the brain, will still receive a plentiful supply of oxygen as it is released during the dive when the pO drop’s to very low levels. This enables them to stay under water for long periods.