Introduction: Acid-Base Balance, Buffers, and the CO2 Bicarbonate System

Definitions: What is an acid?

a molecule that releases hydrogen ions (H+) into a solution

• an acidic solution has a relatively high concentration of H+ (6-1 pH)

Definitions: What is a base?

a molecule that combines with H+ and removes it from a solution

• a basic (alkaline) solution has a relatively low concentration of H+ (8-14pH)

Definitions: What is pH?

• what type of scale is it?

a measure of H+ concentration of a solution

• an inverted scale that multiplies ten fold

If the pH decreases that means?

means in increase in H+

If the pH increases that means?

means a decrease in H+

Definitions: What is a buffer?

a compound or mixture of compounds that moderates changes in pH of the solution

Definitions: What is acidosis?

a condition of lower than normal blood pH (<7.35)

Definitions: What is alkalosis?

a condition of higher than normal blood pH (>7.45)

How often does water undergo dissociation?

infrequent, 1 in 554 water molecules loose a proton (H+) to an H+ bond partner at 25C

• H2O → H+ + OH-

What is an acid-base balance?

acid-base balance refers to the regulation of hydrogen ion concentration in the body

Why is the acid-base balance essential?

H+ in extracellular fluids are low but its essential to regulate H+ within the narrow limits because it affects the structure and functions of proteins

What is the normal pH balance?

• what is the acceptable range? what does it mean if pH goes above or below ranges?

Normal pH of arterial blood is 7.4

• it has a narrow range between 7.35 to 7.45

• any deviation outside this range signals an acid-base disturbance

What are the three main mechanisms to defend against changes in body fluid pH?

1) chemical buffering

2) adjustments in pulmonary ventilation to regulate blood CO2 levels

3) adjustments in renal excretion of acids and bases

What is a buffer system?

• How does it work?

is a mixture of two compounds that minimizes changes in pH in a solution

• if H+ is added to the solution, the buffer absorbs much of the excess H+; if H+ is removed from the solution, the buffer releases H+ into the solution

What is the major buffer system of extracellular fluid?

• what are some additional ones?

The CO2- bicarbonate system

• phosphate compounds

• blood proteins (ex) hemoglobin)

CO2-Bicarbonate System

• How is the reaction created?

  • What is the equation?

Carbon dioxide combines with water in blood to form carbonic acid (H2CO3), which dissociates to form H+ and bicarbonate ions (HCO3-), in a reversible reaction

• CO2 + H2O = H2CO3 = H+ + HCO3-

What enzymes catalyzes the reaction?

carbonic anhydrase

• present in many cells and tissues

How does the reaction process If CO2 concentration in a solution increases?

the reaction proceeds to the right, H+ are formed and [H+] increases (pH increasdecreases)

• thus carbon dioxide acts as a weak acid

How does the reaction process If HCO3- concentration in a solution increases?

the reaction moves to the left, decreasing [H+, (pH increases)

• thus bicarbonate ions act as a weak base

What is the importance of CO2 levels regulation?

plays a primary role in acid-base regulation

How is the partial pressure of CO2 (PCO2) regulated?

• what is the normal range?

• what is it controlled by?

The PCO2 of arterial blood is regulated to match the rate of CO2 production of the body

The normal PCO2 of 40mmHg is regulated by the respiratory center of the medulla oblongata

The PCO2 of arterial blood is regulated to match the rate of CO2 production of the body

• what is proportional to this?

metabolic rate

What other compounds besides CO2 can act as acids?

metabolic acids, such as lactic acid from anaerobic metabolism or ketone bodies from excess fat and protein catabolism

What happens when metabolic acids are added to the blood?

• metabolic acid bases disturbances affect?

the H+ released from the acid combines with HCO3- in the ECF, decreasing HCO3-

• thus affect the bicarbonate concentration of the blood

What is the normal HCO3- of arterial blood?

• what organ plays a major role? how?

24mM

kidney plays a major role in regulating HCO3- by adjusting the rates of H+ secretion into the renal tubules and HCO3- reabsorption back into the blood

In summary, regulation of blood pH involves what two main components?

• regulation of PCO2 by the respiratory system

• regulation of HCO3- by the kidneys