PH of extracellular fluid

Cells and organisms maintain a specific and constant cytosolic pH, keeping biomolecules in their optimal ionic state, usually near pH 7. In multicellular organisms, the pH of extracellular fluids is also tightly regulated. Constancy of pH is achieved primarily by biological buffers mixtures of weak acids and their conjugate bases. 

The pH of extracellular fluid is kept within very narrow limits (7.35-7.45) by buffering mechanisms (see also Chapter 1), the lungs, and the kidneys. These three systems do not act independently. For example, in acute blood loss release of ADH and aldosterone restores the blood volume and renal regulation of the pH leads to shifts in K+ and Na+ levels. 

Uric acid, if prolongedly overproduced, fulfils all the criteria of such an acid the pK value is less than that of bicarbonate and dissociation at the pH of extracellular fluid is nearly 100%. Quantitatively, uric acid is one of the main urinary organic acids. Daily molar excretion of uric acid is 0.47 to 5.80 and is comparable to that of citric acid (0.47-4.34), lactic acid (0.76-4.54), and ketone bodies (0.10-0.97). On average, 50-90% of urinary urate is in ionic form and loads to this extent the isohydry. 

If there is insufficient bicarbonate to compensate for the extra acid, acidosis can occur. Formally, acidosis is a significant decrease in pH of extracellular fluid. This condition can occur due to both respiratory and metabolic abnormalities. Respiratory acidosis occurs when breathing abnormalities result in CO2 retention and an elevation in Pco2 in alveoli and arterial blood (known as hypercapnia). The term Pco2 refers to the partial pressure of CO2 in the pulmonary alveoli during respiration. Retention of CO2 can result from inadequate ventilation during anesthesia, certain conditions that result from central nervous system disease, or from drug use, and it is observed with emphysema. Metabolic acidosis occurs with starvation, uncontrolled diabetes mellitus with ketosis, and with electrolyte and water loss due to diarrhea.il... 

Formally, alkalosis is a significant increase in the pH of extracellrdar fluid.ii Both respiratory alkalosis and metabohc alkalosis are known. Respiratory alkalosis results from h5q)erventilation, which produces lowered Pco2 and higher pH of extracellular fluids.ii Anxiety is the usual cause of hyperventilation. Metabohc alkalosis can result from the loss of gastric juices that are rich in HCl fi um excessive sodium bicarbonate ingestion, and it is associated with potassium ion deficiency. ... 

The pH-buffering of extracellular fluid depends in part on the carbon dioxide/ bicarbonate equilibrium so that the intake of sodium bicarbonate is followed by a brief alkalosis and an increased excretion of sodium carbonate in the urine. Depending on its carbonate concentration, the pH of the urine may rise to 8.07. Large doses (80—100 g/day) of sodium bicarbonate were needed if the pH of stomach contents was to be maintained at 4 or over in patients with duodenal ulcers8. Oxidation of organic anions in the body to carbon dioxide and water permits the use of sodium citrate, lactate or tartrate instead of sodium bicarbonate. In an analogous manner the ingestion of ammonium chloride induces a brief acidosis as a result of the metabolic conversion of ammonia to urea and lowers the pH of the urine. 

The composition and volume of extracellular fluid are regulated by complex hormonal and nervous mechanisms that interact to control its osmolality, volume, and pH. 

In humans, the pH of extracellular and interstitial fluids is maintained by the dicarbonate buffer system (HaCOg/HCOgJ ultrafiltration some CO jg, ogt jq... 

Transcellular fluids may differ greatly in their pH values Gastric juice is very acidic (pH 1.5) the content of the small intestine is alkaline with a pH around 8 urine is usually slightly acidic with a pH of 5. Very few data are available about the pH of cells it is generally somewhat lower than that of extracellular fluids. 

A surgical implant is constantly bathed in extracellular tissue fluid. Basically water, this fluid contains electrolytes, complex compounds, oxygen and carbon dioxide. Electrolytes present in the largest amounts are sodium (Na ) and chloride (Cl ) ions. Most of the fluids existing in the body (such as blood, plasma and lymph) have a chloride content (and pH) somewhat similar to that of sea water (about 5 to 20g/l and pH about 8) . 

A 0-9% salt solution is considered to be isotonic with blood. Other electrolytes present include bicarbonate ions (HCOj ) and small amounts of potassium, calcium, magnesium, phosphate, sulphate and organic acid ions. Included among the complex compounds and present in smaller amounts are phospholipids, cholesterols, natural fats, proteins, glucose and amino acids. Under normal conditions the extracellular body fluid is slightly alkaline with a pH of 7-4. ... 

Plasma consists of water, electrolytes, metabolites, nutrients, proteins, and hormones. The water and electrolyte composition of plasma is practically the same as that of ail extracellular fluids. Laboratory determinations of levels of Na, K+, Ca, CL, HC03, PaC02, and of blood pH are important in the management of many patients. 

Along with the respiratory system, the renal system maintains acid-base balance by altering the excretion of hydrogen and bicarbonate ions in the urine. When the extracellular fluid becomes acidic and pH decreases, the... 

The process of tubular reabsorption is essential for the conservation of plasma constituents important to the body, in particular electrolytes and nutrient molecules. This process is highly selective in that waste products and substances with no physiological value are not reabsorbed, but instead excreted in the urine. Furthermore, reabsorption of many substances, such as Na+, H+, and Ca++ ions, and water is physiologically controlled. Consequently, volume, osmolarity, composition, and pH of the extracellular fluid are precisely regulated. 

Furthermore, pH determination has been used in other clinical research, both alone and in combination with other measurements. This research includes studies into the relationship between extracellular and intracellular pH in an ischemic heart [6, 7], the pH of airway lining fluid in respiratory disease [8], the study of pH as a marker for pyloric stenosis [9], malnutrition in alkalotic peritoneal dialysis patients [10], pH modulation of heterosexual HIV transmission [11, 12], and wound prevention and treatment [13], In addition, pH changes due to blood acidosis have been used to trigger and pace the ventricular rate of an implanted cardiac pacemaker [14], Research using pH measurements... 

The normal pH range for most body chemistry is small and close to neutral. Extracellular fluid, which is about 20% of body water (typically 14 liters), has a normal pH of 7.4 [141], This value is close to that of blood and is slightly alkaline. On the... 

Moreover, several buffer systems exist in the body, such as proteins, phosphates, and bicarbonates. Proteins are the most important buffers in the body. Protein molecules contain multiple acidic and basic groups that make protein solution a buffer that covers a wide pH range. Phosphate buffers (HPO T /H2P07) are mainly intracellular. The pK of this system is 6.8 so that it is moderately efficient at a physiological pH of 7.4. The concentration of phosphate is low in the extracellular fluid but the phosphate buffer system is an important urinary buffer. Bicarbonate (H2C03/HC0 3) is also involved in pH control but it is not an important buffer system because normal blood pH 7.4 is too far from its pK 6.1 [144],... 

Under some circumstances, lysosomal hydrolases may fail to be properly packaged in the TGN, so they enter the default pathway to the cell surface, where they are secreted. Although these hydrolases do little harm at the nearly neutral pH of most extracellular fluids, they can also be returned to lysosomes by a pathway known as receptor-mediated endocytosis. In this pathway, M6P receptors are sent to the plasma membrane, where they bind escaped lysosomal hydrolases and bring them back to lysosomes through the early and late endosomes. Receptor-mediated endocytosis is a major component of the endocytic pathways for trafficking of membrane proteins and merit more detailed consideration. 

Metabolic acidosis is characterized by decreased pH and serum HC03 concentrations, which can result from adding organic acid to extracellular fluid (e.g., lactic acid, ketoacids), loss of HC03 stores (e.g., diarrhea), or accumulation of endogenous acids due to impaired renal function (e.g., phosphates, sulfates). 

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