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Physiology 2130
Anita Woods

Acid-Base Balance Section 12.1 – Objectives By the end of this section, you should be able to: • Define acid and base. • Describe pH, how it is calculated, and the normal pH of the body. • Describe and understand the relationship between free hydrogen ion concentration, acid, and pH. • Explain what a volatile acid is, what a nonvolatile acid is, and where they come from. • List the three ways the body regulates against increases in free hydrogen ions (H ). + • Explain how buffers work to help maintain pH when the levels of H change. • Explain how the respiratory system regulates the levels of volatile acids in the body. + • Describe how the kidneys regulate pH by excreting H and reabsorbing bicarbonate ions. • Define alkalosis and acidosis. • Describe the types of alkalosis and acidosis and their causes Section 12.2 – Introduction + • In this module, we will look at the relationship between hydrogen ions (H ) and acidity. • We will also define what acids and bases are, what the pH scale is, and why it is important to maintain H at precise levels. • We will also look at the different mechanisms in the body for regulating H + concentrations (and thus pH) and we will also learn about some of the things that can alter the pH of the body. Section 12.3 – Why is this Important? • Many chemical reactions that take place in the body, and most of the machinery inside all the cells, are very sensitive to the presence of hydrogen ions. + • Hydrogen ions (H ) can alter the shape of proteins that act as enzymes that speed up chemical reactions. • As a result, any change in the concentration of H will affect the activity of almost every cell. + • Therefore, it is essential that H concentration be very carefully regulated. Section 12.4 – The HydrogenAtom and Hydrogen Ion 1 • In order to better understand what an acid is we must first look at the hydrogen atom and hydrogen ion. • The hydrogen atom:  Has a single proton, which is positively charged, and a single electron, which is negatively charged.  The result is an electrically neutral element. • Ahydrogen ion (H ): +  It’s a hydrogen atom that has lost its electron, leaving only the positively charged proton.  In some situations, the hydrogen ion may simply be called a proton. Section 12.5 – What AreAcids and Bases? • An acid:  It’s any molecule that will release hydrogen ions when put in a solution.  For example, hydrochloric acid (HCl) will break apart (or dissociate) into free hydrogen ions (free H ) and chloride ions (Cl ) as shown at right. + o It is the presence o+ the free H that makes a solution acidic – the more free H , the more acidic the solution, and visa versa.  Astrong acid:  Will dissociate rapidly and release large amounts of H in solution. HCl is a strong acid. • Abase:  Any molecule that will accept a hydrogen ion.  For example, bicarbonate ions (HCO , the most important base in + 3 the body) will bind with H to form carbonic acid (H CO2), a3 shown at right. 2 +  Bases lower the concentration of free H in solution by combining with the H .  With less free H , the acidity of the solution will decrease and become more basic or alkaline. Section 12.6 – The pH Scale + • It’s very important to know the concentration of free H in solution – in the body you don’t want too many H ions or too few. • The pH scale: +  Away of quantifying the concentration of H in any solution. • pH = the negative logarithm (to the base 10) of the hydrogen ion concentration. +  Essentially, because it is a negative logarithm, the more free H , the lower the pH and vice versa. • It is very important not to get confused between the terms H concentration, acidity, and pH. • Keep in mind that the more free H there are in solution, the more acidic the solution and the LOWER the pH.  Definition: pH = - log [H10 + +  Low [H ] Alkaline/Basic  High pH  High [H ] Acidic  Low pH Section 12.7 – The pH Scale (cont’d) • The pH scale:  Goes from 0 to 14 with neutral pH at 7.  Asolution with a pH below 7 is considered acidic (remem+er, the lower the pH the more acidic the solution, or more free H ).  Asolution with a pH above 7 is alkaline or basic (the higher the pH the more basic the solution, or less free H ).  The normal pH of body fluids varies slightly between 7.35 and 7.45 and has an average of 7.4.  Note that this is slightly alkaline.Arterial blood has a pH of 7.45 while venous blood has a pH of 7.35. • Acidosis:  Describe body fluids when the pH is below 7.4 • Alkalosis:  Occurs when the pH is above 7.4.  When the pH of body fluids is below 6.8 or above 7.8 for long periods of time, death will occur. 3 Section 12.8 – The Source of Acid in the Body • Since maintaining the proper pH in the body is essential for life, we must now consider the sources of acids in the body – what are they and where do they come from? • When cells in the body make energy (ATP) they will produce CO as a byp2oduct. • As we saw in module 10 – The Respiratory System, this CO can, wi2h the help of the enzyme carbonic anhydrase, combine with water in red blood cells to produce carbonic acid, H C2 , 3s shown in the reversible reaction below. • The carbonic acid will dissociate into free H and bicarbonate ions, HCO . − 3 • In the lungs, the reaction will then reverse – carbonic acid will reform, it will convert to CO and H O and the CO will be removed and exhaled. 2 2 2  As a result, there is generally no NET increase in free H in the plasma.  Because the carbonic acid reforms into CO , which is then removed at the 2 lungs, carbonic acid is known as a volatile acid. • The metabolic breakdown of various proteins will produce a number of acids including sulphuric acid, phosphoric acid, lactic acid, and other organic acids. • Also, the stomach is a large source of hydrochloric acid.  These acids cannot be removed by the lungs and are therefore called nonvolatile acids. +  These acids are a significant source of free H and are constantly being produced throughout the body. • As a result, they must be dealt with in order to maintain a constant pH. 4 Section 12.9 - Regulation of Hydrogen Ion Concentration • Because of the various sources of acids mentioned on the previous page and the negative effects that these acids can have on cellular reactions, the body must + have a way to deal with potential increases in free H . • The body regulates free H concentrations by the three mechanisms shown below: 1. buffers, 2. the respiratory system 3. kidneys 5 Section 12.10 – Regulation of H Concentration: Buffers • Abuffer:  It’s any molecule that can reversibly bind (or release) free H . + + +  Because buffers bind free H , and thereby reduce the amount of free H in solution, they help to stabilize the pH. +  The general reaction between a buffer and a free H is:  X + H   XH • This reaction shows that the buffer, called “X,” combines with free H to make XH.As a result, with less free H , the pH of the solution is stabilized.  It is very important to note that buffers do not prevent the pH from changing; they only help to minimize any pH change until the free H can + be removed from the body by either the lungs or kidneys.  Some of the buffers we have already seen include bicarbonate ions and hemoglobin (Hb). Section 12.11 – Regulation of H Concentration: Buffers (con’d.) • Free H can bind with buffers in both the intracellular and extracellular fluid. 6 • Intracellular buffers include:  Phosphates (which won’t be covered here) and  Intracellular proteins, such as hemoglobin (Hb) inside red blood cells. • The most powerful extracelluar buffer is the bicarbonate ion, HCO . 3− • Hemoglobn:  Recall that inside red blood cells, Hb can reversibly bind with free H to + help stabilize the acidity inside the RBCs.  Hemoglobin can also bind CO to reduc2 the potential acidity should the CO c2mbine with H O to 2orm carbonic acid, H CO . 2 3 • Recall that the buffers mentioned above do not directly remove free H from the body. • Buffers only tie up the free H until they can be removed in another manner, in this case by the respiratory system. Let’s look at this regulatory system now. + Section 12.22 – Regulation o
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