The respiratory center is located in the medulla oblongata and pons section of the brain. It consists of a group of neurons which transmit nerve impulses to the respiratory muscles allowing them to alter the size of the thorax. Within the respiratory center are sensory neurons known as chemoreceptors which monitor the levels of CO², H+ and O².
There are two types of chemoreceptors; Central chemoreceptors and Peripheral chemoreceptors. Central chemoreceptors are found located in the medulla oblongata in the CNS and react to changes found in H+ concentrations or PCO² in the spinal fluid. Peripheral chemoreceptors are found in aortic bodies and carotid bodies. The aortic bodies contain clusters of chemoreceptors and are seen to be located in the wall of the aorta. The carotid bodies are oval nodules and are found in the left and right carotid arteries. Both sets of chemoreceptors found in the aortic and carotid bodies are part of the peripheral nervous system and react to changes in PO², H+ and PCO² in the blood.
In the blood CO² is able to diffuse easily into cells, but when carbonic anhydrase is present it combines with H²O to form carbonic acid (H²CO³). An increase in H+ occurs as carbonic acid breaks down into H+ and HCO³-. Therefore an increased concentration of CO² found in the bloodstream will cause a further increase of H+ inside the cells. At the same time if there was a decrease in CO² in the blood, a low concentration of H+ would also occur.
When there is an increase in both PCO² and H+ the peripheral chemoreceptors are stimulated. The central chemoreceptors only act when there is a high concentration of PCO². When there is a low affinity of O² the peripheral chemoreceptors are activated but not the central chemoreceptors. The concentration levels of all three gases are regulated by a negative feedback mechanism.
When there is a low concentration of O² and high concentrations of CO² and H+ ventilation is increased. This is because the central and peripheral chemoreceptors are stimulated and have an affect on the inspiratory area causing it to become highly active therefore increasing the rate and depth of breathing. However when there is a severe decrease in O² and it falls below 50mmHg it depresses the activity of the central chemoreceptors and the inspiratory area. This in turn causes no response to any input and results in a decrease in the breathing rate, which further lowers the concentration in PO² levels causing it to fall lower creating a positive feedback mechanism to occur. When this happens this could have a fatal result.
The purpose of this practical is to determine which arterial gas pressure O² or CO² is the primary drive for respiration in an inactive person of good health. Also the effects of each gas mixtures on the rate of tidal ventilation will be investigated.
Materials & Method
Materials
The equipment and materials used within this practical were as follows:
- Dry flow head spirometer with anti-rebreathe valve
- 1000L Douglas bag
- Nose clip
- Mouthpiece
- PowerLab digital recording and analysis package
- Three gas mixtures: Room Air (79% N², 20% O², 0.04% CO²
100% O²
95% O²/ 5% CO²
Method
Before starting the experiment ensure that the dry spirometer and the computer have been calibrated and that there is enough gas in the Douglas bag each time a new gas mixture is being utilized. The gas selection switch should primarily be set so that the subject is breathing in room air. As soon as the subject is relaxed and seated a nose clip should be placed upon the nose and a mouthpiece in their mouth. To prevent the subject from hyperventilating a period of 2minutes in order for them to acclimatize needs to be set. Once this time is up the gas selection switch should then be set so that the subject is now breathing from the Douglas bag. To begin the recording of the subject’s tidal respiration, press START on the computer. After 5 minutes stop the recording and remove the mouthpiece from their mouth.
The experiment should then be repeated with the other two Douglas bags containing Room Air and 95% O²/ 5% CO². Once all the tidal respirations have been recorded calculate the respiration rate and minute volume.
Results
The results obtained from this experiment were as follows:
Table 1: Tidal Volume for each of the gas mixtures
Figure 1: Graph showing tidal volume for each gas mixture
Table 2: Frequency of BPM for Each Gas Mixture
Figure 2: Frequency of BPM for Each gas Mixture
Discussion
From the results obtained in doing this practical it is clear to distinguish that the gas mixture containing 5% CO² had the largest recordings for tidal volume and a high value for the frequency of BPM. The readings for room air also showed high values for tidal volume and BPM which is due to it also containing a portion, although small, of CO² molecules whereas as the 100% O² didn’t contain any at all. Therefore it is evident to say that CO² is the gas in which drives respiration the hardest and therefore is the blood gas in which is the primary drive for respiration.
This is because high concentrations of CO² increase the rate and depth of ventilation. This is due to the fact it stimulates both peripheral and central chemoreceptors which have an effect on the respiratory centre in the medulla oblongata causing it to become highly active. These chemoreceptors monitor the concentrations of CO², O² and H+ molecules within the blood. The peripheral chemoreceptors found in the aortic and carotid bodies and monitor the changes in H+ Po² and Pco² in the bloodstream.
CO² is a key causal factor in raising the level of ventilation as when high levels have been detected a series of events cause a negative feedback mechanism to occur which in turn acts to change the rate of ventilation, where in the case of high CO² increase it.
With the presence of carbonic anhydrase in the blood CO² is combined with H²O to form carbonic acid (H²CO³). If an increase of H+ molecules occur this causes the breakdown of carbonic acid into H+ and HCO³. This results in an increased amount of H+ present in the blood. When a high concentration of both CO² and H+ is detected the peripheral chemoreceptors become stimulated and initiate responses to brain to restore the concentrations back to normal. As the chemoreceptors participate in a negative feedback mechanism, they cause the inspiratory area to become highly active and produce a change in the rate and depth of breathing. Increased ventilation will occur when high concentrations of CO² are detected as it can be dangerous to have high levels within the body therefore the CO² molecules need to be exhaled out.
The frequency of ventilation for each gas had increased; this could have been as a result of the subject hyperventilating slightly due to the conditions in which they were under. Therefore it was essential that that subject was given time to acclimatize each time before they started to breathe in each of the gas mixtures. The highest BPM reading was recorded under the room air gas. The reason as to why this may have occurred is also due to the fact the subject may not have entirely acclimatized before breathing in this air which then caused them to hyperventilate slightly.