 
       
      
    In the absence of respiration, which gas alteration in the bloodstream would primarily trigger the sensation of breathlessness?
Decreasing the concentration of 
Increasing the concentration of  and decreasing the concentration of 
Decreasing the concentration of 
Increasing concentration of 
 
                
                  The respiratory center of the brain is primarily stimulated by an excess of  leading to increased signals for inhalation and exhalation to the respiratory muscles. Oxygen 
 does not have a substantial direct impact on the respiratory center of the brain in regulating respiration. Hence, the correct answer is option 4.
 
                