Physiologyopen

Learn about the ionic basis of neuron action potential... 

Gibbs Donnan Equilibrium | General Physiology

Gibbs Donnan equilibrium explains how charged particles distribute themselves across a semipermeable membrane in the presence of another charged particle to which the membrane is not permeable. 

By understanding Gibbs Donnan equilibrium we get to know why concentration of ions is different inside the cells as opposed to its outside environment and why this leads to generation of a potential across the membrane. 

 Inside and outside of the cells are separated by a semipermeable membrane. Suppose the ions and their number are same inside and outside at the beginning, If now inside the cell proteins are added to which the cell membrane is impermeable,  the other ions to which the cell membrane is permeable will redistribute since for a charged molecule there are two forces which govern its movement 

1. Concentration gradient: depends on actual number of ions 

2. Electrical gradient: depends on the net charge. 

 So since proteins have been added which are negatively charged, even though there is no concentration gradient for the ions but the net charge on one side has changed so there is a development of an electrical gradient… So an electrical gradient for positive ions develops from outside to inside and for negative ions from inside to outside. With the movement of ions along their electrical gradient, a concentration fradient develops...so ion movement continues until their electrical gradient is balanced by their concentraion gradient when the net driving forces become zero. 

 There are certain general things which we can deuce from this distribution of ions at equilibrium: 

 1) At equilibrium,  in the presence of a non-diffusible ion, the diffusible ions distribute themselves so that at equilibrium their concentration ratios are equal. 

2) No. of total ions is more on the side of the presence of non-diffusible ions.  So basically no. of osmotically active particles are more on one side of the membrane. This will lead to movement of water from outside to inside and hence cell swelling and rupture. This doesn’t happen for a cell because of the presence of sodium potassium ATPase which constantly throws out more ions than what it brings in. 

Buffers | Dissociation constant | Henderson Hasselbalch equation | Phys...

Strong acids and bases dissociate almost completely in an aqueous solution into respective ions and hence add H+ ion and hydroxyl ions into solution causing a change in pH. 

  Buffer is any substance which binds H+ ions reversibly and prevents the change in pH in case there is addition of acids or bases into a solution. Weak acids which do not completely ionise when dissolved in water behave as buffers. Fundamentally the weak acid acts as proton donor and its conjugate base acts as a proton acceptor. 

 Now when can be a buffer most effective and how one buffer differs from other buffers ? 

 A buffer will be most effective for handling changes in pH on either side when the proton acceptor and the proton donor are in equal concentrations. . This depends on the dissociation characteristics of any acid i.e its tendency to release hydrogen ions into a solution. The dissociation characteristic of the acids is studied using equilibrium constant or dissociation constant.  Dissociation constant  is equal to the dissociated ions i.e H+ ions and its conjugate base divided by the concentration of the undissociated acid in the solution. This dissociation constant can be expressed as negative logarithm. Its called pKa. Since its negative logarithm, pKa will be larger for weaker acids. 

 But how this dissociation constants helps us in understanding the characteristics of the buffer and when they are most effective  ? 

 That is better understood  with Henderson Hasselbach equation such that we are solving dissocitaion constant equation for H+. 

 pKa + log [A-] / [HA] = pH 

 Buffer is most effective for handling changes in pH on either side when the proton acceptor and the proton donor are in equal concentrations. When they are in equal concentration, pKa = pH. So  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…

Physiological buffers

 HCO3- buffer system: 

 pKa of this buffer is 6.i, so at pH 6.1 this buffer will exist equally in dissociated and undissociated form and will be most effective on both sides. But our body pH is 7.38 - 7.42 so that means at this pH which is higher than pKa , it will exist more in dissociated form. Well that’s good for us since bicarbonate ions will be available to bind to acids. 

Acid base disorders are identified based on arterial blood gas bicarbonate concentration, paCO2 and pH 

 It involves a 3 step approach: 

 1. First see pH: If its less than 7.35, its acidosis if more than 7.45, its alkalosis

 2. Then look at the values of bicarbonate and pCO2: In acidosis either bicarbonate is less or pCO2 is high. Less bicarbonate indicates metabolic acidosis, more pCO2 indicates respiratory acidosis. In alkalosis , either bicarbonate is more (metabolic alkalosis) or pco2 is less (respiratory alkalosis) 

Acid base balance | Physiology | Biochemistry

Normal pH of extracellular fluids ranges from 7.38 to 7.42. Decrease in pH is known as acidosis while increase is known as alkalosis. Since body enzymes work best in a narrow range of pH, it is critically important that the added acids need to be handled well. Levels of handling acids and bases addition in body fluids: 

 1. Buffers: Buffers are the first line of defence and bind with the acid and bases produced to prevent immediate change in pH 

 2. Lungs excret carbondioxide 

 3. Kidneys secrete H+ ions and reabsorb bicarbonate ions.. 

 Buffers: 

Buffers bind with hydrogen ions reversibly. Most important buffer present in body is bicarbonate which can combine with H+ ions and form a weak acid…carbonic acid.  But H2co3 has one very important feature. This H2co3 can dissociate into H+ and bicarbonate ion but this is a weak dissociation…instead it can dissociate into CO2 and H20 in presence of enzyme carbonic anhydrase and this co2 in turn is excreted out by respiratory system. So what is happening that H2CO3 is preventing pH change but..in the process it is getting used up. Apart from bicarbonate , there are other buffers too..i.e proteins acts as buffer and phosphate ions also act as buffer

 Carbondioxide excretion by lungs 

 Our body has chemoreceptors which detect the partial pressure of carbondioxide..this stimulates respiration and hence cause more excretion of carbondioxide. 

 Role of kidneys