pH of Human Body

The normal pH of the human body typically ranges from 7.35 to 7.45, which is slightly alkaline. The body regulates this range, especially in the blood, to maintain homeostasis and proper cell and organ functioning.

1.0Introduction

• Because H+ concentration is normally low and these small numbers are cumbersome, it is customary to express H+ concentration on a logarithmic scale using pH units.
• For instance, the normal hydrogen ion concentration ([H+]) is 40 Eq/L (0.00000004 Eq/L). Using the formula pH = −log [H+], this corresponds to a normal pH of 7.4. pH is inversely related to [H+], so a low pH indicates a high [H+], and a high pH indicates a low [H+].
• Arterial blood typically has a pH of 7.4, while venous blood and interstitial fluids have a slightly lower pH of around 7.35 due to higher CO2 levels forming H2CO3. Acidosis is diagnosed when the pH falls below 7.4, and alkalosis when it rises above this level.
• The pH of arterial blood must stay within a narrow range to sustain life, with an upper limit of about 8.0 and a lower limit of around 6.8. Intracellular pH is usually slightly lower than plasma pH because cellular metabolism produces acids like H2CO3. Intracellular pH typically ranges between 6.0 and 7.4, depending on the cell type. Conditions like tissue hypoxia and poor blood flow can lead to acid accumulation and lower intracellular pH.
• Urine pH varies from 4.5 to 8.0, reflecting the acid-base status of extracellular fluid. The kidneys adjust the extracellular fluid H+ concentration by excreting acids or bases at variable rates. An extreme example of acidic fluid is the hydrochloric acid (HCl) secreted by the parietal cells of the stomach. This acid has a hydrogen ion concentration approximately 4 million times greater than that in blood, resulting in a pH of about 0.8.

2.0Defending Against Changes in H⁺ Concentration

Buffers, Lungs, and Kidneys 

Three Main Systems Regulate H+ Concentration to Prevent Acidosis or Alkalosis:

1. Chemical Buffer Systems: These systems in body fluids quickly neutralize excess acids or bases to stabilize H+ concentrations and prevent drastic changes.

2. Respiratory System: This system regulates the removal of CO2 (and thus H2CO3) from the extracellular fluid, helping to manage H+ levels within minutes.

3. Kidneys: The kidneys adjust the H+ concentration by excreting either acidic or alkaline urine, restoring balance over a longer period, from hours to days.

The buffer systems react within seconds to minimize changes in H+ concentration, though they don’t remove or add H+ but stabilize them until equilibrium is restored. The respiratory system provides a second layer of defence by eliminating CO2 and H2CO3 within minutes. The kidneys, while slower, offer the most powerful regulation of acid-base balance over hours to days.

3.0Respiratory Acidosis

• Causes of Decreased Ventilation and Increased Pco2
• As discussed, any factor that reduces pulmonary ventilation leads to a rise in Pco2 in the extracellular fluid. This increase elevates H2CO3 and H+ levels, leading to acidosis. Since this type of acidosis is due to a respiratory issue, it is called respiratory acidosis.

Occurrence of Respiratory acidosis: 

• Conditions Impairing Respiratory Function: Certain pathological conditions can damage the respiratory centers or reduce the lungs' ability to expel CO2. These include blockages in the airways, pneumonia, and emphysema.
• Decreased pulmonary membrane surface area, as well as any factor that interferes with the exchange of gasses between the blood and the alveolar air.

In respiratory acidosis, the compensatory responses available are:

(1) The buffers of the body fluids.

(2) The kidneys, which require several days to compensate for the disorder. 

4.0Respiratory Alkalosis 

Results from Increased Ventilation and Decreased Pco₂ Respiratory alkalosis is caused by excessive ventilation by the lungs.

Occurrence of Respiratory Alkalosis: 

• Physical pathological conditions. 
• Psychoneurosis can occasionally increase breathing to the extent that a person becomes alkalotic.
• A physiologic type of respiratory alkalosis occurs when a person ascends to high altitude. The low oxygen content of the air stimulates respiration, which causes loss of CO₂ and development of mild respiratory alkalosis. 

In respiratory alkalosis, the compensatory responses available are:

• The chemical buffers of the body fluids 
• The ability of the kidneys to increase HCO₃ − excretion.

5.0Metabolic Acidosis 

• Results from Decreased Extracellular Fluid HCO₃ − Concentration.
• The term metabolic acidosis refers to all other types of acidosis besides those caused by excess CO₂ in the body fluids. 

Causes of Metabolic acidosis:

• Failure of the kidneys to excrete metabolic acids normally formed in the body.
• Formation of excess quantities of metabolic acids in the body, 
• Addition of metabolic acids to the body by ingestion or infusion of acids.
• Loss of base from the body fluids.
• Renal Tubular Acidosis. 
• Diarrhea.
• Vomiting of Intestinal Contents
• Diabetes Mellitus.
• Ingestion of Acids. Rarely are large amounts of acids ingested in normal foods.
• Some of these include acetylsalicylics (aspirin) and methyl alcohol.
• Chronic Renal Failure.

6.0Metabolic Alkalosis

Metabolic Alkalosis Results from

• Increased Extracellular Fluid HCO₃ − Concentration
• Administration of Diuretics (Except the Carbonic Anhydrase Inhibitors)
• Excess Aldosterone.
• Vomiting of Gastric Contents. Vomiting of the gastric contents alone, without vomiting of the lower gastrointestinal contents, causes loss of    the HCl secreted by the stomach mucosa. The net result is a loss of acid from the extracellular fluid and development of metabolic alkalosis.This type of alkalosis occurs especially in neonates who have pyloric obstruction caused by hypertrophic pyloric sphincter muscles. 
• Ingestion of Alkaline Drugs. A common cause of metabolic alkalosis is ingestion of alkaline drugs, such as sodium bicarbonate, for the treatment of gastritis or peptic ulcer. 

7.0Treatment for Acidosis and Alkalosis

• The best treatment for acidosis or alkalosis is to correct the condition that caused the abnormality.
• This is often difficult, especially in chronic diseases that cause impaired lung function or kidney failure.
• In these circumstances, various agents can be used to neutralize the excess acid or base in the extracellular fluid.

Treatment of Acidosis:

• To neutralize excess acid, large amounts of sodium bicarbonate can be ingested by mouth. The sodium bicarbonate is absorbed from the gastrointestinal tract into the blood and increases the HCO3 − portion of the bicarbonate buffer system, thereby increasing pH toward normal.
• Sodium bicarbonate can also be infused intravenously, but because of the potentially dangerous physiologic effects of such treatment, other substances are often used instead, such as sodium lactate and sodium gluconate.

For the treatment of alkalosis:

• Ammonium chloride can be administered by mouth.
• Ammonium chloride occasionally is infused intravenously, but NH₄ + is highly toxic and this procedure can be dangerous.
• Another substance used occasionally is lysine monohydrochloride.

8.0Mechanisms That Maintain pH of Body Fluids

• Buffer systems: Most consist of a weak acid and its salt, which functions as a weak base. They prevent drastic changes in body fluid pH. 
• Proteins: The most abundant buffers in body cells and blood. Hemoglobin inside red blood cells is a good buffer. 
• Carbonic acid– bicarbonate: Important regulator of blood pH. The most abundant buffers in extracellular fluid (ECF). 
• Phosphates: Important buffers in intracellular fluid and urine.
• Exhalation of CO₂: With increased exhalation of CO₂, pH rises (fewer H). With decreased exhalation of CO₂, pH falls (more H). 
• Kidneys: Renal tubules secrete H into urine and reabsorb HCO3  so it is not lost in urine.