 Hi, welcome to Physiology Open. In this video we will discuss concepts on buffers like what is dissociation constant pKa and which are most effective buffers in physiological systems and why. An equi-solution consists of water molecule and its dissociated components. Any substance which adds protons or hydrogen ions into an equi-solution creating an excess of hydrogen ions into the solution is known as an acid while that which accepts the hydrogen ions creating an excess of hydroxyl ions is known as a base. Due to this whenever a strong acid or base is added to an equi-solution there will be a change in pH. Strong acids and bases dissociate almost completely in an equi-solution into their respective ions and hence add hydrogen ions and hydroxyl ions into the solution. Now buffer is any system which binds hydrogen ions reversibly and prevents the change in pH in case there is addition of acids or bases into the solution. So what is the chemical characteristics of these buffers? Well, weak acids and bases which do not completely ionized when dissolved in water this is unlike the strong acids. So these weak acids and bases act as buffers. Let's take an example. So here is a weak acid which weakly dissociates into hydrogen ions and its conjugate base. So in a solution partly it exists as dissociated ions and partly as un-dissociated acid. Now suppose there is addition of some strong base into the solution. Remember strong means completely ionized right? Now the excess hydroxyl ions will react with the weak acid and the reaction will proceed this way forming its conjugate base and water. On the other hand if a strong acid is added which adds hydrogen ions into solution this conjugate base that is bicarbonate ion will react with it and form weak acid. So fundamentally the weak acid acts as proton donor and its conjugate base acts as a proton acceptor. Thus weak acids and its conjugate base or for that matter even weak base and its conjugate acid act as buffers and prevent change in pH. We will focus here on weak acids and not on bases since in physiological systems weak acids and their conjugate base are predominant buffers. Now when can be a buffer most effective and how one buffer differs from other buffers? A buffer is most effective for handling changes in pH on either side when the proton acceptor that is the conjugate base and the proton donor that is the weak acid are in equal concentration. So when hydroxyl ions are added acid accept it and forms a conjugate base while when protons are added conjugate base accepts it and forms the acid. So basically a buffer will be most effective in either direction when the proton acceptor and the proton donor are in equal concentrations. But is it true for all buffers in physiological systems that they are always in equal concentrations? See we have said that weak acids do not dissociate completely to its science. Partly they remain under associated and partly they dissociate but partly means how much? Is it that their concentration in solution is half or dissociated component is more than and dissociated or vice versa? Well it depends on the dissociation characteristics of an acid that is its tendency to release hydrogen ions into a solution. The dissociation characteristics of the acids is studied using equilibrium constant or dissociation constant which is expressed by this symbol. Dissociation constant is equal to the dissociated ions that is hydrogen ions and conjugate base divided by the concentration of the un-dissociated acid in the solution. Can you tell which acids will have larger dissociation constant? Stronger or weaker acids? In simple terms when numerator is more dissociation constant will be more. So stronger acids which are largely ionized have larger dissociation constant isn't it? Now this dissociation constant can be expressed as negative logarithm just like pH. It's called pKa. So since it's negative log pKa will be larger for weaker acids. It's inverse isn't it? So if I give you three wake acids and they are pKa can you tell which is the weakest acid among these by looking at the pKa? Yes it's ammonium ion since it has a large pKa. But how does this dissociation constant help us in understanding the characteristics of the buffer and when they are most effective? That will be better understood with Henderson- Hazelback equation. Well I'm sorry for throwing equations at you but if you stay with me I assure you you will get to understand the topic much better. Okay Henderson-Hazelback equation is a little changed version of dissociation constant but it's important for understanding buffer action and acid base balance in human body. So here we have written the same dissociation constant equation. Now we'll little bit rearrange this such that we'll solve for hydrogen ions. So this will go here and this will go here. So equation will be something like this. Now we will take negative log on both sides. You might be wondering why every time we have to take negative log of values because in aqueous solution these values are very low like 10 to the power minus 8 or 10 to the power minus 4. So that's why we express them as negative log so that every time we don't speak in negative values or decimal values. Anyways we'll take negative log on both sides. Now if you remember this negative log of dissociation constant is known as pK and negative log of hydrogen ions is pH. So we'll substitute it here. Now if we invert these products we can write this equation with plus sign of log. So what are these? These are the conjugate base and the weak acid that is proton acceptor and the proton donor. And we have said before that the buffer is most effective for handling changes in pH on either side when the proton acceptor and proton donor are in equal concentrations. Okay so when they are in equal concentration this portion will become log 1. So we will rewrite the equation and since log 1 is 0 it will become pKa is equal to pH. So what we see here that at a pH which is equal to pKa of the buffer the dissociation is such that the concentrations of proton donor and proton acceptor are equal. So at this pH the buffer is most effective in either directions. What will happen if pH of the solution is more or less in which they are placed? See pH is more they will exist more as ionized form while if it is less they exist more as weak acid. Now let's see why the buffers of our body are so effective. But one thing you should remember here that body has to be more equipped to handle acids than the basis since lot of acids are being added per day. First let's see bicarbonate buffer system. So carbonic acid is the weak acid which dissociates into hydrogen ions and bicarbonate ions. pKa of this buffer is 6.1. So at pH 6.1 this buffer will exist equally in dissociated and undissociated form and will be most effective on both sides. If pH is less it will be more in undissociated form and if more it will be more in dissociated form. So our body pH what do you think it's more or less? It's more it's 7.38 to 7.42. So that means at this pH which is higher than pKa it will exist more in dissociated form. That's good or bad? Well that's good for us. Since bicarbonate ions will be available to bind to hydrogen ions and as I told before body should be more equipped to handle acids. Also this buffer has one more advantage. Carbonic acid which is formed dissociates into carbon dioxide and water and carbon dioxide is exhaled out so the reaction can proceed freely towards this side. Now let's see phosphate buffer. Phosphate buffer is present intracellularly and is useful for maintaining intracellular pH. Also it is very effective as a urinary pH. Its dissociation constant is 6.86. Again you see as far as body fluids are concerned it is also stays mostly in dissociated form and is more effective for capturing acids. Now let's come to ammonia buffer which is a urinary buffer. This one is a bit complicated. pKa of ammonia buffer is 9.25. What do you think? Is it a good buffer or a bad buffer? If you have understood the concept you might have realized that at urinary pH which is acidic and can become as low as 4 to 4.5 this ammonia buffer system mostly exists as undissociated form that is as ammonium ion. So actually it will not be able to bind with much hydrogen ions at this pH. So it's a very poor buffer per se. However the production of ammonium ion by the metabolism of glutamine also produces one bicarbonate ion which is added back into the blood. Plus in medullary interstitium this ammonium buffer exists in equilibrium with ammonia and ammonia diffuses from there into the collecting duct where it binds with secreted hydrogen ions. Not only that it is one buffer whose production is regulated by kidneys that is its production increases in acidosis. So that's how it acts as an effective buffer basically by adding bicarbonate ions into the blood and that ammonia which diffuses from the medullary interstitium into the collecting duct. I've made a video on understanding concepts on acid-base balance. Do check out that video to better understand how ammonium buffer system works. Thanks for watching the video. If you liked it don't forget to like and share the video and yes do subscribe to the channel Cosiology Open. Thank you.