Question

Explain the three major ways the body handles potentially developing acidosis (which can be life-threatening, if not dealt with). Kidney, lungs GI tract

Answer 1

In a healthy individual, there is a stable state of the organs i.e. they maintain homeostasis with the environment. If there is a fluctuation in the temperature or pH, the body deals with that situation to restore normalcy. Being said, acidosis is a condition in the body where there is an imbalance of the pH where it further drops to the net acidic levels. Now we know that to deal with the pH fluctuations the body has its defense mechanism of buffers (specifically of bicarbonate buffer) at the place. So there is an HCO3-/CO2 buffer system that usually deals with the pH fluctuations in the body. What happens is, in acidosis this bicarbonate buffer(HCO3-) is either lost or there is a constant build-up of compounds that are much more acidic in nature(such as CO2) in the fluids. We will see further how does our organs such as kidneys lungs and GI tract helps in tackling this issue.

The kidneys help regulate the acid-base homeostasis in two ways a) by producing new HCO3- or by b) reabsorption of the HCO3- that is filtered out in the urine. Since the HCO3- is readily filtered at the glomerules in the kidneys it is the proximal convoluted tubules where the reabsorption of the HCO3- takes place. This reabsorption rate is such high that there is an almost negligible loss in the amount of bicarbonate component of the crucial buffer system in the urine. Furthermore, apart from this reabsorption the kidneys also produce some of the HCO3-. Basically this formation of bicarbonate is in response to the production of acid by the kidneys. Understand it in this way, the kidneys will secrete acid in the urine, and in order to tackle that acid, they will further produce HCO3-. Thus that physiological response will, in turn, lead to the enhancement of HCO3- secretion in the urine apart from the amount present there due to glomerular filtration. Thus the kidneys will lead to an overall increase of the HCO3- in the blood to counter any fluctuation of pH, including acidosis.

Furthermore, the presence of CO2(a substance that lowers the pH) or it can be put in other words, that the accumulation of the CO2 in the blood or fluids will lead to a drop in the pH to acidic levels thus contributing to acidosis. Since we know that gaseous exchange that takes place at the alveoli in the lungs involves CO2 exchange there are chances that this CO2 will dissolve in the blood during an exchange and thus cause a pH drop. This situation in the lungs is thus tackled by alveoli ventilation. The partial pressure of CO2 should be 40mm of mercury or Hg and that rate is maintained by gaseous exchange by the lungs (remember breathing in and out of the air here). This is further controlled by a change in pH and partial pressure of CO2 detection by the chemoreceptors present on the cells of the medulla oblongata ( a site in the brain that controls breathing patterns).
So if there are any changes in the PCO2 or the pH that will trigger the alveolar ventilation mediated by the medulla oblongata and ultimately reach homeostasis.

The GI tract, however, deals with the site-specific acidosis where the stomach acid secretion is neutralized via pancreatic and liver secretions(in the form of juices) that are rich in HCO3-. These secretions of the juices by the pancreas rich in HCO3- are triggered by the entry of the acidic chyme(food mixed with gastric juices such as acid) in the small intestine.

So if any of these three mechanisms fail to control the drop in the pH it may result in a life-threatening condition of the acidosis.