The body has three primary lines of defense against changes in hydrogen ion concentration in the body fluids.

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Bertha Miles
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1 ph and Nucleic acids Hydrogen Ion (H+) concentration is precisely regulated. The H+ concentration in the extracellular fluid is maintained at a very low level, averaging Eq/L. normal variations are only about 3 to 5 neq/l. Because the hydrogen ion concentration in extracellular fluid is extremely low and because these small numbers are difficult with which to work, the H+ concentration is usually expressed as ph units. The ph is the logarithm of reciprocal of H+, expressed as equivalents per liter. ph= log 1/[H+] = -log [H+] Arterial blood has a normal ph of 7.4, whereas the ph of venous blood and interstitial fluids is about A person is considered to have acidosis when the arterial ph falls significantly below 7.4 and have alkalosis when the ph rises above 7.4. The lower limit of ph at which a person can live for more than a few hours is about 6.8 and the upper limit is about 8.0. The body has three primary lines of defense against changes in hydrogen ion concentration in the body fluids. 1. The chemical acid base buffer systems of the body fluids 2. The respiratory system 3. The kidneys Defenses against change in Hydrogen Ion concentration: Buffers Lungs and Kidney The body has three primary lines of defense against changes in hydrogen ion concentration in the body fluids. The chemical acid base buffer systems of the body fluids which immediately combine with acid or base to prevent excessive changes in hydrogen ion concentrations The respiratory system, which regulates the removal of carbon dioxide and therefore carbonic acid (H 2 CO 3 ) from the extracellular fluid. This mechanism operates within seconds to minutes and acts as a second line of defense.

2 The kidneys, which excrete either alkaline or acidic urine, thereby adjusting the extracellular fluid hydrogen ion concentration toward normal during alkalosis or acidosis. The mechanism operates slowly but powerfully over a period of hours or several days to regulate the acidbase balance. Buffering of Hydrogen Ions in the body fluids A buffer is any substance that can reversibly bind H +. The general form of a buffering reaction is as follows Buffer + H + H Buffer In this example, free H + combines with the buffer to form a weak acid (H buffer) when the H + concentration increases, the reaction is forced to the right and more H + binds to the buffer for as long as available buffer is present. When the H + concentration decreases, the reaction shifts toward the left, and H + is released from the buffer. Among the most important buffer systems in the body are proteins in the cells and, to a lesser extent proteins in the plasma and interstitial fluid. The phosphate buffer system (HPO 2-4 /H 2 PO - 4 ) is not a major buffer in the extracellular fluid but is important as an intracellular buffer and as a buffer in renal tubular fluid. The most important extracellular fluid buffer is the bicarbonate buffer system (HCO - 3 /Pco 2 ) primarily because the components of the system, CO 2 and HCO - 3 are closely regulates by the lungs and kidneys respectively. Bicarbonate buffer system The bicarbonate buffer system consists of a water solution that has two main ingredients: a weak acid, H 2 CO - 3 and a bicarbonate salt such as NaHCO 3. H 2 CO 3 is formed in the body through the reaction of CO 2 and H 2 O CO 2 + H 2 O H 2 CO 3 H 2 CO 3 ionizes to form small amounts of H + and HCO 3 - : H 2 CO 3 HCO H+

3 The second component system, bicarbonate salt occurs mainly as sodium bicarbonate (NaHCO 3 ) in the - extracellular fluid. NaHCO 3 ionizes almost completely form HCO 3 and Na + NaHCO 3 HCO Na + Putting the entire system together, we have the following CO 2 + H 2 O H 2 CO 3 H + + HCO3 - + Na + When strong acid is added to this buffer solution the increase hydrogen ions are buffered by HCO3 - H + - HCO 3 H 2 CO 3 CO 2 + H 2 O The opposite reaction takes place when a strong base such as sodium hydroxide is added to bicarbonate buffer solution. NaOH + H 2 CO 3 NaHCO 3 + H 2 O In this case the OH - from NaOH combines with H 2 CO 3 to form additional HCO3 -. The weak base NaHCO 3 replaces the strong base NaOH. At the same time the concentration of H 2 CO 3 decreases (because it reacts with NaOH) causing more CO 2 to combine with H 2 O to replace H 2 CO - 3. Because the lungs expel CO 2 from the body rapid ventilation by the lungs decreases the concentration of CO 2 in the blood, which in turn decreases the carbonic acid and H + concentrations in blood. Conversely a decrease in pulmonary ventilation increases CO2 and H + concentrations in blood. The kidneys control the acid base balance by excreting either acidic urine, which reduces the amount of acid in extracellular fluid, or basic urine, which removes base from the extracellular fluid. Five Different Nucleotides Are Used to Build Nucleic Acids Two types of chemically similar nucleic acids, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), are the principal information-carrying molecules of the cell. The monomers from which DNA and RNA are built, called nucleotides, all have a common structure: a phosphate group linked by a phosphoester bond to a pentose (a five-carbon sugar molecule) that in turn is linked to a nitrogen- and carbon-containing ring structure commonly referred to as a

4 base. In RNA, the pentose is ribose; in DNA, it is deoxyribose. The bases adenine, guanine, and cytosine are found in both DNA and RNA; thymine is found only in DNA, and uracil is found only in RNA. Adenine and guanine are purines, which contain a pair of fused rings; cytosine, thymine, and uracil are pyrimidines, which contain a single ring. The bases are often abbreviated A, G, C, T, and U, respectively; these same single letter abbreviations are also commonly used to denote the entire nucleotides in nucleic acid polymers. In nucleotides the 1 carbon atom of the sugar (ribose or deoxyribose) is attached to the nitrogen at position 9 of a purine (N9) or at position 1 of a pyrimidine (N1). The acidic character of nucleotides is due to the phosphate group, which under normal intracellular conditions releases a hydrogen ion (H+), leaving the phosphate negatively charged. Nucleotides and nucleic acids are biological molecules that possess heterocyclic nitrogenous bases as principal components of their structure. The biochemical roles of nucleotides are numerous; they participate as essential intermediates in virtually all aspects of cellular metabolism. Serving an even more central biological purpose are the nucleic acids, the elements of heredity and the agents of genetic information transfer. Just as proteins are linear polymers of amino acids, nucleic acids are linear polymers of nucleotides. Like the letters in this sentence, the orderly sequence of nucleotide residues in a nucleic acid can encode information. The two basic kinds of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Complete hydrolysis of nucleic acids liberates nitrogenous bases, a five-carbon sugar, and phosphoric acid in equal amounts. The bases of nucleotides and nucleic acids are derivatives of either pyrimidine or purine. Pyrimidines are six-membered heterocyclic aromatic rings containing two nitrogen atoms. The atoms are numbered in a clockwise fashion, as shown in the figure. The purine ring structure is represented by the combination of a pyrimidine ring with a five-membered imidazole ring to yield a fused ring system. The pyrimidine ring system is planar, while the purine system deviates somewhat from planarity in having a slight pucker between its imidazole and pyrimidine portions. Both are relatively insoluble in water, as might be expected from their pronounced aromatic character. Most nucleic acids in cells are associated with proteins, which form ionic interactions with the negatively charged phosphates. Cells and extracellular fluids in organisms contain small

5 concentrations of nucleosides, combinations of a base and a sugar without a phosphate. Nucleotides are nucleosides that have one, two, or three phosphate groups esterified at the 5 hydroxyl. Nucleoside monophosphates have a single esterified phosphate; diphosphates contain a pyrophosphate group: and triphosphates have a third phosphate. The nucleoside triphosphates are used in the synthesis of nucleic acids. Among their other functions in the cell, GTP participates in intracellular signaling and acts as an energy reservoir, particularly in protein synthesis, and ATP is the most widely used biological energy carrier.

Alkalosis or alkalemia arterial blood ph rises above Acidosis or acidemia arterial ph drops below 7.35 (physiological acidosis)

Acid-Base Balance Normal ph of body fluids Arterial blood is 7.4 Venous blood and interstitial fluid is 7.35 Intracellular fluid is 7.0 Alkalosis or alkalemia arterial blood ph rises above 7.45 Acidosis