Question

How does the lung maintain the Ventilation/Perfusion Ratio as close to 1 as possible?

Answer 1

Alveolar Ventilation is regulated to maintain constant arterial blood gas tensions (particularly of CO2), despite variable levels of O2 consumption and CO2 production.

During normal breathing, the venous blood has a lower PO2 (40 mmHg) and higher PCO2 (46 mmHg) than inspired gas which has PO2 = 150 mmHg, PCO2 usually 0. Hence there is a partial pressure gradient which drives O2 in and CO2 out of the pulmonary capillary blood.

Ventilation of the lungs is the pprocess that mixes fresh inspired gas with alveolar gas.

If there is no ventilation at all, there will be no replenishment of O2 and no removal of CO2. PO2 will then fall and PCO2 will then rise towards the venous O2 and CO2 tensions. Hence, Ventilation is a very important process that must occur to maintain the ratio of O2 and CO2 Constant. And If the ventilation is greater than is needed, then the alveolar gas tensions will be shift closer to inspired gas, which means the CO2 level will be lower, and the O2 level a little higher. So, Ventilation is a process by which O2 and CO2 are transported to and from the lungs.

Lungs performs Inspiration which is an active process by which air enters inside lungs and then it performs Expiration which is a passive process by which air exists outside the lungs.

Generally, the Inspiration to Expiration ratio( I:E ratio) at rest is 1:2 or less. On exertion, the I:E ratio becomes 1:1. Inspiration is normally an active process and Expiration is passive process, and usually longer than the time required for exhalation. When breathing spontaneously, the work of breathing is minimised by keeping inspiratory times shorterj enough to get rid of the produced CO2.

It is likely that the optimal I:E ratio is 1:1. A good starting point is a respiratory rate about 12 at an I:E ratio of 1:1. The actual Inspiratory and Expiratory time should be adjusted properly so that the gas flow curve in pressure mode should be near equilibration both in inspiration and expiration.

Now If the ratio is disturbed, assume Expiration rate is reduced. So the ratio becomes less than 1. Now, the CO2 which needs to be expelled is not expelling out properly. So, this will give a feedback to pneumatic centres at brain which will stimulate lungs to perfrom expiration property. Hence, the lungs will then change the permiability of alveoli which will allow more CO2 to leave quickly from lungs from outside. This process is called Ventilation. Hence, Expiration ratio will be increased and I:E ratio will come back to 1.

Similarly, If Inspiration ratio is decreased, the lungs will again change the permeability of alveoli which will allow more O2 to enter inside lungs to increase the Inspiration rate. This process is also called Ventilation. Hence, the ratio will again vome back to 1.

Hence, the lungs by getting stimulation from pneumatic center present in Medulla Oblongata of Brain which changes the permiability of alveoli according to the requirement and hence, the ratio comes back to 1. Hence, lungs tries to maintain a Ventilation ratio nearly to 1.