Transport of CO2 from tissues:
PCO2 is higher in the tissues than in capillary blood, so CO2 will diffuse to capillary blood and will be transported in blood through the following 3 ways:
1- Dissolved in plasma d inside RBCs; represent 10 % of all CO2 transported in blood (notice tht the CO2 is more soluble than O2)
2- Bound to hemoglobin; represent 30 % of total CO2 transported in blood, CO2 bind to HB when it become free from O2 (after delivery of O2), so the HB has some affinity to bind with CO2 if it is Deoxyhemoglobin, but if it is bound to O2 (oxy hemoglobin), it has less affinity to CO2 (this process occur inside RBC)
3- As bicarbonate (HCO3-); most important way to transport CO2 represent 60 % of total CO2 transported in blood, it occur inside RBCs due to the presence of carbonic anhydrase enzyme as following:
* CO2 will react with water and this reaction catalyzed by carbonic anhydrase enzyme to produce carbonic acid (H2CO3), which will dissociate reversibly to bicarbonate (HCO3-), which is base and hydrogen ion (H+) that will be combined with free HB (like CO2 do) and by this buffering the effect of H+ so that not increase the acidity, and the bicarbonate (HCO3-) will get out side RBCs in exchange with CL- ions.
Note: when bicarbonate released to plasma it will contribute to slight alkalinity of blood PH (7.4)
When blood reach alveoli:
* All the steps will be reversed as follow:
1- Dissolved CO2: will directly diffuse to alveoli because the PCO2 in pulmonary blood is higher than the alveolar PCO2 in alveoli
2- CO2 bound to hemoglobin: CO2 will dissociate from HB because HB has stronger affinity to O2 (in alveoli there is high PO2) than CO2
3- CO2 present as bicarbonate (HCO3-): HCO3 will diffuse back to RBC in exchange to CL-, then it recombine with H+ (when it released from HB cause HB now prefer to bind with O2) to form H2CO3 which will dissociate to form CO2 and H2O, CO2 will diffuse with dissolved CO2 to alveoli to be expired to outside
Control of respiration:
* In order we can breath rhythmically there must be centers controlling respiration, and you notice that we can still breath even when we are sleeping, and some times we decide to breath voluntarily
In the following description it will be discussed simply although this subject has a lot of details that may be considered in other stage of your study
* Respiratory centers present in brain stem they are generally classified to inspiratory and expiratory centers
Inspiratory centers: they are just like pace maker generating electricity (action potentials) to be conducted to respiratory muscles to contract, when these centers fire action potentials to inspiratory muscles that will contract to produce inspiration and when these centers doesn’t fire action potentials or (inhibited) the inspiratory muscle relax and the expiration will occur passively
Expiratory centers: they inhibit inspiratory centers rhythmically and when they do so, the inspiratory muscles will relax and thus passive expiration will occur
Q. What is the importance of respiratory centers?
A. It is to produce rhythmical respiration and to adjust the rate and depth of breathing (ventilation) according to the body needs to take up O2 and to remove CO2, and this mission is accomplished by detecting amount of certain chemicals in blood (PO2, PCO2, and H+) by chemoreceptors which will send information to respiratory centers to take action accordingly
* Certain chemoreceptors are designed to detect PO2, PCO2 and H+ in blood and body fluids; they send impulses to respiratory centers that will take action to modulate ventilation accordingly.
They are generally classified to (1) peripheral chemoreceptors and (2) central chemoreceptors
* Peripheral chemoreceptors represented in carotid bodies in the carotid sinus and in aortic bodies found in aortic arch
* These peripheral receptors respond to dissolved O2 and not to O2 carried by Hb also they are responsive to H+ in blood and weak responsive to PCO2
* Central chemoreceptors are responsive to H+ in CSF which is resulted from CO2 reaction with water to produce (HCO3-) and (H+)
Influence of (PO2, PCO2, H+) on respiration:
(1) Decrease in PO2 in arterial blood: this decrease in PO2 will stimulate peripheral chemoreceptors only if PO2 is below 60 mmHg, this parameter (PO2 fall) is not concerned in minute-to-minute regulation of respiration (because it requires large decrease in PO2 so that respiratory centers will take act).
* When there is large decrease in PO2 it will depress respiratory centers itself making vicious cycle of decrease PO2 lead to respiratory depression, which will lead to further decrease in PO2 until breath will cease.
(2) Increase in PCO2: it has weak direct effect on peripheral chemoreceptors, but, when PCO2 increase in blood the same increase will happen in CSF (because CO2 is able to cross blood brain barrier) and we know that CO2 in CSF is not directly detected but it will combine with water to produce (HCO3- ) and (H+), the (H+) will affect on central chemoreceptors which will stimulate ventilation (by increasing ventilation we can remove more CO2 from blood).
* Central chemoreceptors are very sensitive to small fluctuations in H+ ions in CSF (which is resulted from CO2 reaction with water).
* thus PCO2 is the most important factor in regulation of ventilation at rest (minute to minute regulation).
* If PCO2 increased to large degree above 80 mmHg it will depress directly the respiratory centers.
(3) Increase in H+ in blood: we know that H+ cannot cross blood brain barrier so if H+ ions increased in blood it will not affect on central chemoreceptors but it affect on peripheral chemoreceptors making them stimulate respiratory centers to increase ventilation.
* Notice that even if H+ ions increased in blood from causes other than increase CO2, also the same effect will happen (increased ventilation) and this because when we increase ventilation we are removing more CO2 thus reducing more H+ ions whatever their source so this increase in H+ ions in blood will stimulate only peripheral receptors and this is very important in acid –base balance