Lecture 17.1 Acids, Bases, and Buffers PDF

Lecture 17.1: Acids, bases, and buffers A. Introduction The pH of our body fluids are a measure of the concentration of H+ ions, and pH is measured along a scale from 1 to 14. More acidic pH levels occur below 7, more alkaline pH levels occur above 7, while pH 7 is designated as a neutral...

Lecture 17.1: Acids, bases, and buffers A. Introduction The pH of our body fluids are a measure of the concentration of H+ ions, and pH is measured along a scale from 1 to 14. More acidic pH levels occur below 7, more alkaline pH levels occur above 7, while pH 7 is designated as a neutral pH. For example, our blood is slightly alkaline at a pH that ranges between 7.35-7.45. We use the pH scale to express the concentration of H+ ions because they occur at relatively low levels compared to other ions, like sodium. For this reason, a pH 7 describes a concentration of H+ ions; 0.0000010 (10-7) mmol/L. A major contributor to body fluid pH is carbon dioxide (CO2), a metabolic byproduct of aerobic metabolism (i.e. pyruvate oxidation and Krebs cycle). As CO2 moves from the intracellular fluid (ICF) to the extracellular fluid (ECF), it reaches the blood and enters our red blood cells. Inside our RBCs is an enzyme called carbonic anhydrase which catalyzes the production of carbonic acid (H2CO3) from CO2 and water. Carbonic acid is a weak acid as it dissociates reversibly in solution into hydrogen ions and bicarbonate (HCO3-): H2CO3 ↔ H+ + HCO3- Carbonic acid, being a weak acid, is a chemical buffer that helps resist pH changes in our blood as our cells produce other acids, like lactic acid, sulphuric acid, hydrochloric acid, and phosphoric acid; these are metabolic byproducts. However, let’s first examine how changes in CO2 can alter the pH of our body fluids. According to LeChatelier’s Principle, the direction of a reversible reaction is determined by the availability of compounds on either side of the chemical reaction, i.e. the reactants and products. When we produce more CO2, our body fluids become more acidic because more carbonic acid is made. A greater concentration of carbonic acid causes the reaction above to move towards the right, causing its dissociation into H+ ions and bicarbonate. When we produce less CO2, our body fluids become less acidic because less carbonic acid is made. A decrease in carbonic acid concentration causes the reaction above to move towards the left, causing H+ ions and bicarbonate to combine and form more carbonic acid. When CO2 alters the pH of our body fluids, we can adjust our ventilation rate to exhale more or less CO2. 1 Why do we label carbonic acid as a chemical buffer? Weak acids and bases help maintain the pH of our body fluids because their dissociation reactions are reversible. Carbonic acid is a weak acid, while bicarbonate is its conjugate base. Consider also ammonia (NH3): NH3 (weak base) + H+ ion ↔ NH4+ (conjugate acid) In an acidic solution, ammonia can combine with an hydrogen ion to create ammonium, resulting in the sequestration of hydrogen ions if there is a spike in acidic metabolic byproduct. In this event, ammonia sequesters hydrogen ions by combining with it to produce more ammonium. This is a crucial reaction in the kidneys because ammonium is excreted in the urine, effectively removing excess H+ ions from the body. There are additional buffer systems in the body, including monosodium phosphate: NaH2PO4 (weak acid) ↔ NaHPO4- (conjugate base) + H+ ion We will discuss this system in lecture. To recap the carbonic acid buffer system, when there is an increase in H+ ions (as when metabolism produces more acidic byproducts), the reaction is pushed to the left, forming H2CO3. When there is a decrease in H+ ions, the reaction is pushed to the right, causing H2CO3 to dissociate into H+ and HCO3- ions. Chemical buffers in the human body are commonly weak acids or weak bases. To be clear, acids and bases come in buffering pairs: H2CO3 is the weak acid while HCO3- is its conjugate base, while ammonia (NH3) is a weak base while ammonium (NH4+) is its conjugate acid. B. Regulatory mechanisms of the body There are three main mechanisms to regulate the pH of our body fluids: 1) chemical buffering (like the one above), 2) respiratory regulation, and 3) urinary regulation. We’ve already described how the first mechanism works, so let’s move onto the second and third. CO2 is transported from your peripheral tissues to the lungs in three different ways: dissolved in plasma, bound to hemoglobin (forming carbaminohemoglobin), and converted to carbonic acid in your red blood cells. Once blood reaches the pulmonary capillaries, CO2 diffuses across the respiratory membrane into the alveoli, and you exhale it with every breath. The enzyme that combines CO2 and H2O into H2CO3 catalyzes the reverse reactions in the lungs, whereby H2CO3 is broken down into CO2 and H2O. The CO2 is then exhaled. Respiratory regulation is rapid and can help readjust pH in minutes. Chemoreceptors monitor pH and pCO2 in our blood (found in the aortic and carotid sinuses) and cerebrospinal fluid; these signals are carried along the glossopharyngeal nerve (CN IX) to the medulla oblongata, which regulates our ventilation rate. When pCO2 is elevated, our breathing rate increases to help us eliminate more CO2 from our blood. 2  The kidneys also help regulate the pH of our body fluids, but these mechanisms take longer (on the order of days). In effect, the kidneys secrete H+ ions and reabsorb bicarbonate from the urine; this process begins along the proximal convoluted tubule and ends along the distal convoluted tubules. One of the major players in helping secrete H+ ions is the K+/H+ active antiporter, which exchanges potassium ions for hydrogen ions. When body fluid pH is acidic, this antiporter exchanges potassium in the tubular fluid for H+ ions, effectively secreting hydrogen ions into the urine and reabsorbing potassium. As you can probably guess, acid-base balance influences potassium concentrations, however, potassium concentrations can also influence acid-base balance. This is possible as this antiporter is also found on the surface of other cells of the body, helping move these ions between the ECF and ICF. 3 Review questions: 1. The buffering system of the blood is important in maintaining a pH level that is appropriate for the proper functioning of plasma macromolecules and those found on the membranes of the blood cells. When exercising very intensely, our muscles will release lactic acid which will acidify our blood. Which of the following statements about this situation is TRUE? A. The overall concentration of OH- ions in the blood is greater. B. The overall concentration of H+ ions in the blood is lower. C. To buffer the blood, bicarbonate will combine with hydrogen ions to create carbonic acid. D. To buffer the blood, carbonic acid will dissociate to produce bicarbonate and hydrogen ions. 2. Remember that our breathing rate will be modified according to tissue activity. Recall that our breathing rate must match blood flow, and this is controlled by regulatory mechanisms. Each of the following statements below is FALSE. Correct the underlined word to make the statement true: A. [ ___________ ] Chemoreceptors in the aortic and carotid sinus predominantly monitor oxygen levels. B. [ ___________ ] Signals from chemoreceptors are communicated to the pons. C. [ ___________ ] Information of respiratory gasses in the blood are communicated by the vagus nerve. D. [ ___________ ] During exercise, our muscle tissue produces more CO2, and this alkalizes our body fluids. E. [ ___________ ] [ ___________ ] During hypokalemia, potassium is moved inside our cells in exchange for bicarbonate ions. 4 Answer key: C, carbon dioxide, medulla oblongata, glossopharyngeal, acidifies, outside and hydrogen ions. 5