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Question

The most powerful respiratory stimulus for breathing in a healthy person is


A

loss of oxygen in tissues

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B

increase of carbon dioxide

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C

pH (acidosis)

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D

pH (alkalosis)

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Solution

The correct option is

A

increase of carbon dioxide

An increase in the accumulation of carbon dioxide in the tissues means less availability of oxygen for cellular respiration. This serves as a powerful stimulus for breathing. Conversely, when the carbon dioxide concentration in the blood is low, the brain decreases the frequency and depth of breaths.


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The importance of pH maintenance in blood.
Maintenance of the pH in blood and in intracellular fluids is absolutely crucial to the processes that occur in living organisms. This primarily because the functioning of enzymes-catalysts for these processes - is sharply pH dependent. The normal pH value of blood plasma is 7.4. Severe illness or death can result from sustained variations of a few tenths of pH unit.
Among the factors that lead to a condition of acidosis, in which there is a decrease in the pH of blood are heart failure, kidney failure diabetes mellitus, persistent diarrhoea or a long term high protein diet.A temporary condition of acidosis may result from prolonged, intensive exercise. Alkalosis, which causes increase in pH of blood may occur as a result of severe vomiting over breathing or exposure to high altitudes.
Several factors are involved in the control of the pH of blood. A particularly important one is the ratio of dissolved HCO3 to H2CO3. CO2(g) is moderately soluble in water and in aqueous solution reacts only to a limited extent to produce H2CO3.

CO2+H2OH2CO3(aq)
H2CO3+H2OHCO3+H3O+ , pKa1=6.11

HCO3+H2OCO23+H3O+, pKa2=10.25
In the H2CO3, HCO3 buffer system, we deal only with the first ionization step (Ka1): H2CO3 is a weak acid and HCO3 is the conjugate base (salt). CO2 enters the blood from tissues as the by -product of a metabolic reaction. In the lungs, CO2(g) is exchanged for O2(g), which is transported throughout the body by the blood.Important diagnostic analysis in the blood is :
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