Define PH and buffer. How and why blood PH is maintained at a normal level

Def: P^H is the negative logarithm of hydrogen ion in a particular solution.

Normal pH: 7.35 – 7.45

Compatible with life: 6.9 – 7.7

Optimum p^H: Optimum p^H is that p^H at which the activity of an enzyme is maximum.

Importance of normal p^H:

1.      For proper activity of an enzyme p^H should be maintained within normal range.

2.      To maintain appropriate form of biomolecules for any biochemical reaction.

3.      To maintain the normal activity of vital organs in the body.

p^K is defined as the negative logarithm of the [H⁺] at which half the acid molecules are undissociated and half are dissociated.

Buffers:

Def: Buffer is a mixture of weak acid and its conjugate base that tends to maintain the p^H or hydrogen ion concentration of a solution within normal range in spite of the addition of acid or base in moderate amount.

Body buffers:

ECF buffers:

1)     Plasma buffers

a)      Bicarbonate buffer

b)     Protein buffer

c)      Phosphate buffer

2)     RBC buffers

a)      Hemoglobin buffer

b)     Phosphate buffer

c)      Bicarbonate buffer

3)     Interstitial fluid buffers

a)      Bicarbonate buffer

b)     Phosphate buffer

Tissue buffer:

a)      Protein buffer

b)     Phosphate buffer

c)      Bicarbonate buffer

How blood P^H is maintained at a normal level:

a)      Chemical buffering by the extracellular and intracellular buffers

b)     Changes in alveolar ventilation to control the P_co2 (Respiratory System)

c)      Alterations in renal H⁺ excretion to regulate the plasma HCO₃ concentration (Renal system)

Body Buffers: Acts within seconds. Maintain p^H by combination of H⁺ with a blood buffer or an intracellular buffer.

Respiratory system: Acts within minutes. Maintain p^H by reduction of carbonic acid by elimination of CO₂ through lungs.

Renal system: Acts within hours. Maintain p^H by reduction of noncarbonic acid by renal elimination of H⁺.

Respiratory regulation of acid base balance:

Arterial p^H depends on the ratio of [HCO₃] and CO₂ tension in the blood. CO₂ that are formed in the tissues as a result of metabolism, transported in plasma in three forms.

·        HCO₃

·        Carbamino compounds

·        Physically dissolved in plasma

At the tissue level CO₂ enters into the capillary blood from tissues. This process is accompanied by the reduction of oxyhemoglobin.

CO₂               Diffusion                capillary blood.

HbO₂                                              Hb^-- + O₂

CO₂ enters into the RBC, hydrated to form H₂CO₃, which dissociates into H⁺ and HCO₃

H⁺: Buffered by hemoglobin

HCO₃: Shifted to plasma in exchange for chloride.

At the level of the lungs HCO₃^-- combines with potential protons (HHbO₂) to form H₂CO₃, which finally gives to H₂O and CO₂.

HHb+O₂                                        HHbO₂

HCO₃^-- + HHbO₂                                          HbO₂ + H₂CO₃

H₂CO₃                                         H₂O + CO₂

 CO₂ is exhaled out and thus proton (H⁺) is eliminated in the form of CO₂.

An increase in the CO₂ concentration in the body fluids decreases P^H, which in turns stimulates pulmonary ventilation, and excess H⁺ is washed out in the form of CO₂. The opposite effect occurs when CO₂ concentration is decreased.

Renal regulation of acid base balance:

The kidney is crucial in the response to change in H⁺ concentration. To maintain the normal P^H of the body fluid the kidneys perform two major functions:

·        HCO₃ conservation or excretion.

·        H⁺ secretion.

Acid base regulation by body buffers:

The most important body buffers involved in the regulation of the normal acid-base balance are:

Plasma                                    : CO₂ bicarbonate system

R.B.C.                                      : Hemoglobin buffer

Kidney                                                : Phosphate and ammonia buffer system

Bone                                       : Hydroxyapatite buffer system

The most important buffer system is the CO₂-bicarbonate system. Arterial blood H⁺ concentration depends on the concentration of

1.      Carbonic acid

2.      CO₂

3.      Bicarbonate.

H⁺+ HCO₃^- ↔H₂CO₃ ↔ H₂O + CO₂

If H⁺ ions are added to the blood the equation is pushed to the right, and ventilation increase via stimulation of chemoreceptor to blow off CO₂. If the [CO₂] rises, the equilibrium is pushed to the left, increasing [H⁺]. To compensate the kidney must excrete more H⁺ and regenerate more HCO^—to set up a new equilibrium.

In contrast if H⁺ ions are lost, the equation is pulled to the left. To bring the [H⁺] back to normal, more CO₂ is needed, and [CO₂] rises through reduced ventilation by lungs. Voluntary or involuntary hyperventilation leads to a fall in [CO₂], and the equilibrium is pulled to the right with a fall in [H⁺].