Renal/Pulmonary Response to Acidosis/Alkylosis

 

It is important that body fluids remain at a normal pH range. If the pH of a body fluid becomes too acidic or too alkaline (basic), this can disrupt the structure (shape) of the enzymes and other proteins in the body fluid and the proteins will no longer function (recall that the function of a protein is the result of its structure).

 

Renal/Pulmonary Response to Acidosis/Alkylosis
Review of Acids, Bases, and pH
Metabolism and Clinical Conditions
How the Body Returns the pH

Review of Acids, Bases, and pH

Molecules that are dissolved in water may dissociate into charged ions. An acid is a substance that increases the number of H+ ions in a solution. A base is a substance that decreases the number of H+ ions in a solution.

The concentration of H⁺ ions in a solution can be measured and is called the pH of the solution.

The pH of a solution can be measured using a scale that ranges from 0 to 14.  A solution of pH = 7 is neutral, a solution of pH lower than 7 is acidic, and a solution of pH greater than 7 is basic (alkaline).

The number of H⁺ ions increases as the pH number decreases (and vice versa). The difference between two successive numbers on the pH scale represents a ten-fold difference in the H⁺ ion concentration because the scale is a logarithmic scale (log of base 10). For example, a solution with a pH of 2 has 10 times more H⁺ ions as a solution with a pH of 3. A solution with a pH of 2 has 100 times more H⁺ ions as a solution with a pH of 4. 

Metabolism and Clinical Conditions Can Change the pH of Body Fluids Such as Blood

Blood pH is normally 7.35-7.45. Sometimes, however, blood pH decreases to below 7.35 and becomes acidic. This occurs normally from the byproducts released during metabolism such as carbon dioxide (CO₂), sulfuric acid, lactic acid, etc. The blood may also become acidic in patients suffering from emphysema, asthma, bronchitis, pneumonia, and pulmonary edema. In these cases, carbon dioxide increases in the blood since it cannot effectively diffuse out of the lungs. This results in a condition known as acidosis. Acidosis means that the hydrogen ions are increased ( pH is decreased).

How the Body Returns the pH of the Blood Back to Normal:

The body uses 3 different mechanisms to return the pH back to normal when the blood becomes acidic or alkaline:

1. Exhalation of Carbon Dioxide (CO₂) by the Lungs

Specific areas in the brain stem called respiration centers control the rate and depth of breathing. If the blood becomes too acidic, these areas of the brain send a signal which increases the rate and depth of breathing. This results in a more forceful exhalation, which removes CO₂ from the body. Less CO₂ in the blood means less carbonic acid being formed, making the blood pH more alkaline.

2. Excretion of Hydrogen Ions (H⁺) by the Kidneys

When the blood becomes too acidic, t he kidneys remove excess H⁺ ions from the body and excrete them in the urine. This makes the urine more acidic and the blood less acidic. Hydrogen ions are removed by the proximal convoluted tubules (PCTs) and collecting tubules (CTs) that are part of the nephrons of the kidneys. Nephrons are microscopic structures that filter the blood plasma and process it into urine.

H⁺ ions are moved out of the blood and into the PCTs and CTs in a process called secretion. At the same time as H⁺ ions are secreted, bicarbonate ions (HCO₃^– ) are reabsorbed back into the blood so that they are not lost in the urine.

3. Buffer Systems

A buffer is a chemical substance that resists changes in pH. The body has several different buffering systems that temporarily bind or release free H⁺ ions when the pH is changed.

If an acidic solution is added to a buffer solution, the buffer will combine with the extra H⁺ ions and help to maintain the pH. If a basic solution is added to a buffer solution, the buffer will release H⁺ ions to help maintain the pH. 

There are many different buffers and each one stabilizes the pH of the solution within a specific pH range. One buffer may be effective within a range of pH 2 to pH 6, while another buffer may be effective within a range of pH 10 to pH 12. 

One example of a buffering system is the carbonic acid-bicarbonate buffering system. When CO₂ diffuses into the blood, it reacts with water (H₂O) in the plasma to produce carbonic acid (H₂CO₃). This unstable acid then breaks down into H⁺ ions and bicarbonate ions (HCO₃^– ). This can be summarized as follows:

CO₂ + H₂O <===> H₂CO₃ <===> H⁺ + HCO₃^–

If the amount of CO₂ is increased, as during exercise, the increased CO₂ will cause the equilibrium to shift to the right, thereby increasing the amount of H₂CO₃ (carbonic acid), H⁺ and HCO₃^– (bicarbonate ions). The increase in H⁺ leads to a decrease in pH (that is, the blood becomes more acidic).

If the amount of CO₂ is decreased, the reaction can run in reverse. That is, the HCO₃^– binds the free H⁺ ions to produce carbonic acid and convert it into CO₂. The blood becomes more alkaline.

 

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