Violations of the acid-base state

One of the main constants of the body is the constancy of the concentration of hydrogen ions (H ⁺ ) in the extracellular fluid, which in healthy individuals is 40 ± 5 nmol / l. For convenience, the concentration of H ^{+ is} most often expressed as a negative logarithm (pH). Normally, the pH of the extracellular fluid is 7.4. Regulation of pH is necessary for the normal functioning of body cells.

The acid-base state of the body includes three main mechanisms:

• functioning of extra- and intracellular buffer systems;
• respiratory regulatory mechanisms;
• kidney mechanism.

Violations of the acid-base state - pathological reactions that are associated with a violation of the acid-base state. Isolate acidosis and alkalosis.

Buffer systems of the body

As the buffer systems are organic and inorganic substances that prevent a sharp change in the concentration of H ⁺ and, respectively, the pH value when adding acid or alkali. These include proteins, phosphates and bicarbonates. These systems are located both inside and outside the body cells. The main intracellular buffer systems are proteins, inorganic and organic phosphates. Intracellular buffers compensate almost all the load with carbonic acid (H ₂ CO ₃ ), more than 50% of the load by other inorganic acids (phosphoric, hydrochloric, sulfuric, etc.). The main extracellular buffer of the organism is bicarbonate.

[1], [2], [3], [4]

Respiratory mechanisms of pH regulation

They depend on the work of the lungs, which are able to maintain the partial pressure of carbon dioxide (CO ₂ ) in the blood at the required level, despite the large fluctuations in the formation of carbonic acid. Regulation of the release of CO ₂ occurs due to changes in the speed and volume of pulmonary ventilation. An increase in the minute volume of respiration leads to a decrease in the partial pressure of carbon dioxide in the arterial blood and vice versa. Lungs are considered as the first line in maintaining the acid-base state, as they provide a mechanism for the immediate regulation of the release of CO ₂.

Renal mechanisms to maintain the acid-base state

The kidneys are involved in maintaining the acid-base state, excreting in the urine an excess of acids and preserving the base for the organism. This is achieved through a number of mechanisms, the main of which are:

• reabsorption by buds of bicarbonates;
• formation of titrated acids;
• the formation of ammonia in the cells of the renal tubules.

Kidney bicarbonate reabsorption

In the proximal tubules of the kidneys, almost 90% of HCO ~ is absorbed by not through the direct transport of HCO through the membrane, but through complex metabolic mechanisms, the most important of which is the secretion into the lumen of the nephron H ⁺.

In the cells of the proximal tubules from water and carbon dioxide, under the action of the enzyme carbonic anhydrase, unstable carbonic acid is formed, which rapidly decays into H ⁺ and HC0 _3. "The hydrogen ions formed in the tubules cells enter the luminal membrane of the tubules, where they are exchanged for Na ⁺, in whereby H ⁺ enter the tubule lumen, and sodium cation -. A cell, and then the blood exchange takes place via a special transfer protein - Na ⁺ -H ⁺. Receipt exchanger into the lumen of the nephron hydrogen ions activates reabsorption in blood Hc0 ₃ ~. Simultaneously, in the lumen of the tubule hydrogen ion quickly connected to constantly filtered Hc0 ₃ to form carbonic acid. With the assistance of carbonic anhydrase exerted on the luminal side of the brush kaomki, H2C0 ₃ is converted into H ₂ 0 and CO _z In this carbon dioxide diffuses back in proximal tubular cells where it joins the H ₂ 0 to form carbonic acid, and this completes the cycle.

Thus, the secretion of H ⁺ ion provides reabsorption of bicarbonate in an equivalent amount of sodium.

In the Henle loop, about 5% of the filtered bicarbonate is reabsorbed and in the collection tube - another 5%, also due to the active secretion of H ⁺.

Formation of titrated acids

Some weak acids that are in the plasma are filtered and serve as urine buffer systems. Their buffer capacity is called "titratable acidity." The main component of urine buffers protrudes NR0 ₄ ~, which after addition of hydrogen ion is converted into dvuzameschonny phosphoric acid ion (NR0 ₄ ² + H ⁺ = H ₂ PO ~) having a lower acidity.

[5], [6]

Formation of ammonia in the cells of the renal tubules

Ammonia is formed in the cells of the renal tubules during the metabolism of keto acids, especially glutamine.

At neutral and especially at low values of the pH of the tubular fluid, ammonia diffuses from the tubular cells into its lumen, where it combines with H ⁺ to form an anion of ammonium (NH ₃ + H ⁺ = NH ₄ ⁺ ). In the ascending section of the loop, the reabsorption of NH ₄ ⁺ cations takes place , which accumulate in the brain substance of the kidney. A small amount of ammonium anions dissociates into NH, and hydrogen ions that are reabsorbed. NH ₃ can diffuse into the collecting tubes, where it serves as a buffer for H ⁺ secreted by this nephron unit.

The ability to increase the formation of NH ₃ and NH ₄ ⁺ excretion is considered as the main adaptation reaction of the kidneys with increasing acidity, which allows the removal of hydrogen ions by kidneys.

[7], [8], [9]

Violations of the acid-base state

At various clinical conditions the concentration of hydrogen ions in the blood may deviate from the norm. There are two main pathological reactions associated with the violation of the acid-base state, acidosis and alkalosis.

Acidosis is characterized by low blood pH (high concentration of H ⁺ ) and a low concentration of bicarbonates in the blood;

Alkalosis is characterized by high blood pH (low concentration of H ⁺ ) and high concentration of blood bicarbonates.

There are simple and mixed variants of the violation of the acid-base state. In primary or simple forms, only one violation of this equilibrium is observed.

Simple variants of acid-base disorder

• Primary respiratory acidosis. Associated with increased p _and CO ₂.
• Primary respiratory alkalosis. Occurs as a result of a decrease
• Metabolic acidosis. Due to a decrease in the concentration of HCO ₃ ~.
• Metabolic alkalosis. Occurs when the concentration of HCO _{3 is} increased .

Quite often, the above disorders can be combined in a patient, and they are designated as mixed. In this textbook, we will focus on simple metabolic forms of these disorders.