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Physiological Acid Base Balance

To maintain proper physiological function, the acid-base balance must be maintained. A pH scale is used to measure the acid-base balance in the body.
To maintain proper physiological function, the acid-base balance must be maintained. A pH scale is used to measure the acid-base balance in the body. Despite perturbations, the pH of the blood and bodily fluids can be maintained through a variety of buffering systems. By dilating the effect of excess acid or base on hydrogen ion concentration, buffers dampen the effect of excess acids and bases. It is usually either a weak acid that absorbs hydroxyl ions or a weak base that absorbs hydrogen ions that absorbs ions.



Buffer Systems in the Body

The human body has several effective buffer systems, and they each operate at a different pace. By exhaling CO2, the respiratory tract can increase the blood pH in minutes. Chemical buffers in the blood make quick adjustments to pH. In addition to the kidney's ability to adjust blood pH, the system can excrete hydrogen ions (H+) and conserve bicarbonate, however, these processes take hours or days to take full effect. Buffer systems operating in blood serum include plasma proteins, phosphates, and carbonic and bicarbonate acids. In addition to excreting hydrogen ions, the kidneys produce bicarbonate, which helps regulate acid-base balance. A protein buffer system is mainly located inside the cell.

The Plasma and Cellular Protein Buffers

Proteins can act as buffers in almost all cases. The amino acid consists of positively charged amino groups and negatively charged carboxyl groups, whereas proteins contain negatively charged carboxyl groups. Hydrogen and hydroxyl ions can be bound to the charged regions of these molecules, which act as buffers. Blood contains two-thirds of the buffering power of proteins, and cells contain most of the buffering power of proteins.

Buffering by Hemoglobin

In red blood cells, hemoglobin is the main protein that makes up about one-third of the cell's mass. By the dissociation of oxygen, hemoglobin serves as a buffer for hydrogen ions liberated in the reaction. Hydrogen ions are therefore buffered during CO2 conversion into bicarbonate. As the CO2 diffuses into the air sacs, the buffering keeps pH stable and can be reversed in the pulmonary capillaries to create CO2. This aspect of the respiratory system will be discussed further in the chapter on respiratory system functions.

Phosphate Buffer

Sodium dihydrogen phosphate (Na2H2PO4-), a weak acid, and sodium mono-hydrogen phosphate (Na2HPO42-), a weak base, are the two types of phosphates found in the blood. With a strong acid, such as HCl, Na2HPO42- turns into Na2H2PO4- and sodium chloride, NaCl. As a weak acid, Na2HPO42- (the weak base) transforms back into a weak acid in the presence of a strong base, such as sodium hydroxide (NaOH). It has the same acids and bases as before, but it holds onto the ions.

HCl + Na2HPO4NaH2PO4 + NaCl

(Strong acid) + (weak base) (weak acid) + (salt)

NaOH + NaH2PO4 Na2HPO4 + H2O

(Strong base) + (weak acid) (weak base) + (water)


Bicarbonate – Carbonic acid Buffer

In the same way as phosphate buffers, bicarbonate, and carbonic acid work by dissolving each in the other. The phosphate ions and bicarbonate ions in the blood are regulated by sodium. The phosphate ions and bicarbonate ions in the blood are regulated by sodium. The phosphate ions and bicarbonate ions in the blood are regulated by sodium.

NaHCO3 + HCl H2CO3+NaCl

(Sodium bicarbonate) + (strong acid) (weak acid) + (salt)

H2CO3 + NaOHHCO3- + H2O

(Weak acid) + (strong base) (bicarbonate) + (water)


With the phosphate buffer, the free ions are captured by a weak acid or weak base, preventing significant pH changes. A 20:1 ratio of bicarbonate ions to carbonic acid can be found in the blood if the pH of the blood is within normal limits. This capture system can buffer acidity changes by 20 times better than carbonic acid because it contains 20 times more bicarbonate than carbonic acid. Acids, such as lactic acid and ketones, account for the majority of the body's metabolic waste. CO2 is emitted from the lungs when you breathe out. This controls the levels of carbonic acid in your blood. A consequence of carbonic anhydrase activity is the dissociation of acid in the blood, thereby causing the blood to become less acidic. Acid dissociation results in CO2 exhalation (see equation above). To maintain blood bicarbonate levels, the renal system preserves bicarbonate ions in the renal filtrate. Bicarbonate buffers, however, are the primary buffer system for IF surrounding cells in tissues throughout the body.

CO2 + H2O H2CO3 H+ + HCO3

The Balance between Acid-base and Respiration

Reducing carbonic acid levels in the blood allows the respiratory system to maintain the body's acid-base balance. A blood sample contains carbonic acid and CO2, but their concentrations are in equilibrium. Carbonic acid is formed when CO2 reacts with water. Excess CO2 in the blood (which occurs when you hold your breath) reacts with water to make carbonic acid, decreasing blood pH. You can also remove more CO2 by increasing breath rate and/or depth (which you might instinctively do after holding your breath). Carbonic acid levels in the blood rise as a result of carbon dioxide loss from the body, resulting in a normalized pH value. It also works in the opposite direction, as you have probably guessed. By excessively exhaling deep and rapidly (as in hyperventilation), CO2 is removed from the bloodstream, reducing carbonic acid levels, resulting in too alkaline blood. In the case of this brief alkalosis, rebreathing air exhaled into a paper bag can remedy the situation. After exhaling, rebreathing air will quickly lower the pH of the blood.



When blood travels through the pulmonary capillaries of the lungs, it undergoes chemical reactions that affect CO2 and carbonic acid levels. It is usually sufficient to change the amount of CO2 exhaled to adjust the pH of the blood with minor adjustments in breathing. If you double your breathing rate for less than a minute, the pH of your blood would rise by 0.2. Over an extended period, you will experience this problem if you exercise strenuously. When exercising beyond your aerobic threshold, you would produce additional CO2 (and lactic acid) to meet your energy needs. The respiration rate is increased to remove CO2 to counteract the acid production increase. By doing so, you prevent acidosis from occurring.

Through chemoreceptors, which primarily sense CO2 as a signal, the body controls the respiratory rate. Aortic and carotid arteries are equipped with sensors that measure peripheral blood flow. A rise or fall in CO2 levels triggers these sensors to tell the brain to adjust the respiratory rate immediately. There are also sensors inside the brain itself. In response to changes in pH, the medulla oblongata modulates breathing rate in response to changes in the CSF pH.

Hypercapnia is caused by abnormally high blood CO2 levels in conditions that impair respiratory function, for example, pneumonia and congestive heart failure. Furthermore, hypercapnia can also be caused by holding one's breath for extended periods, including when one is taking drugs such as morphine, barbiturates, or ethanol. Symptoms of hypopnea, or abnormally low blood CO2, occur whenever patients are hyperventilating in a way that causes CO2 to escape, such as salicylate toxicity, elevated room temperatures, or hysteria.

The Balance between Acid and Base by the Renal System

The kidneys' role in maintaining body acid-base balance contributes to the buffering system's metabolic component. Comparatively to the respiratory system and breathing centers in the brain, which control carbonic acid levels in the blood with exhalation, the renal system controls bicarbonate levels in the blood with reabsorption. Decreases in blood bicarbonate can occur as a result of the inhibition of carbonic anhydrase by diuretics or due to diarrhea causing excessive bicarbonate loss. In addition to people with Addison's disease (chronic adrenal insufficiency), where aldosterone is reduced, and people with renal damage such as chronic nephritis, blood bicarbonate levels tend to be lower. Last but not least, low blood bicarbonate levels can be caused by uncontrolled diabetes mellitus, where the high levels of ketones bind the bicarbonate in the filtrate and cause its loss.

It is important for the bicarbonate buffer system that bicarbonate ions, HCO3-, are present in the filtrate. However, the tubule cells do not permit bicarbonate ions to pass. According to Figure, adding bicarbonate ions to the system consists of the following steps:



Step 1 - An apical membrane lines the renal tubules, and absorbs sodium ions as it exchanges hydrogen ions with the cells.

Step 2 - Bicarbonate ions are generated by the cells and are transported to peritubular capillaries.

Step 3 - In the presence of CO2, the reaction produces carbonic acid. Bicarbonate ions and hydrogen ions are formed when this acid dissociates.

Step 4 - Once in the peritubular capillaries, bicarbonate ions are transferred back into the circulatory system. Hydrogen ions are secreted into the filtrate, where they can contribute to new water molecules and be reabsorbed from there, or they can be excreted in the urine.

As well as salts, such as phosphates or sulfates, hydrogen ions may be captured by the filtrate as well. Therefore, the bicarbonate ions will not have access to hydrogen ions for combining with and producing CO2. The filtrate does not conserve bicarbonate ions, which will also result in acidosis and pH imbalance. The exchange of potassium and sodium in renal tubules also involves hydrogen ions. When potassium is present in greater amounts than normal, rather than hydrogen ions, potassium is exchanged, increasing the potassium level in the filtrate. There is also less conservation of bicarbonate when fewer hydrogen ions participate in the conversion of bicarbonate into CO2 in the filtrate. More hydrogen ions will enter the filtrate if potassium levels are lower, allowing more sodium to replace hydrogen ions in the filtrate.

An important function of chloride ions in the body is to neutralize positive ionic charges. As a substitute for chloride ions lost by the body, bicarbonate ions are used instead. As a result, the kidneys reabsorb more bicarbonate due to lost chloride.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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