• If acid is added to a buffered solution, some of the added H+ combines with the conjugate base and the pH is less affected than if the buffer were not present.

• The human body has several buffer systems which are a first line of defense when the body’s pH deviates from normal.

Buffer Systems

Bicarbonate Carbonic Acid Buffer System
- The main plasma buffer
Hemoglobin Buffer System
Phosphate Buffer System
Protein Buffer System

What generates H+ in our bodies?

Breakdown of fats and sugars for energy generates C02, which forms carbonic acid.
Metabolism of protein produces hydrochloric, sulfuric and other metabolic acids (ketoacids, phosphoric acids).

What removes H+ from our bodies?

Our lungs are responsible for removing C02. If C02 production is altered, we adjust our breathing to exhale more or less C02. Most of the acid produced by our bodies is in the form of C02, so our lungs excrete most of the acid.
The kidneys are responsible for excreting metabolic acids. They secrete H+ into the urine and reabsorb HC03 from urine.
Both mechanisms work together to maintain homeostasis

Henderson-Hasselbalch Equation

The Bicarbonate-Carbonic Acid buffer is the main plasma buffer. At a normal pH with a pk of 6.1, the ratio of HC03:H2C03 is 20:1.
Lab analyzers use this equation to calculate the bicarbonate result. (pH and pC02 are measured).
pC02 multiplied by 0.0306 is equal to H2C03. (0.031 is the solubility constant for pC02 and converts the unit from mmHg to mmol/L)

Acid Base Disturbances

In acid base imbalances, HCO3 is a base and we think of pCO2 as an acid, because of its close relationship with carbonic acid.
Acidosis – pH more acidic than 7.35. May be due to an excess of acid (H2C03 or pC02) or a lack of base (HC03).
Alkalosis – pH more alkaline than 7.45. May be due to an excess of base (HC03) or a lack of acid (H2C03 or pC02).
The reference range for pH in the human body is very tight.

Metabolic vs. Respiratory

HC03 relates to the metabolic component (kidney)
H2C03 (pC02) relates to the respiratory component (lungs)
The renal and respiratory systems operate jointly to maintain blood pH within normal limits. If one system is overwhelmed or not functioning, the other adjusts, automatically, to limit the disturbance. This is called compensation.

Just one little equation...

C02 + H20 H2C03 H+ + HC03-
This equation shows that when C02 is dissolved in blood it becomes an acid. The more C02 added, the more H2C03 is produced, which dissociates into free H+ ions.
It also shows that pH depends not on absolute amounts of C02 or HC03, but on the ratio of C02 and HC03. A change in C02 will not cause a change in pH if it is balanced by a change in HC03 that preserves the ratio.

Normal Acid Base Regulation at the Kidney

Kidney uses the same equation to retain the bicarbonate ion and get rid of H+
The kidney also uses ammonia and phosphate buffer systems to remove H+.
The kidney can also selectively excrete bicarbonate (HC03) when it is necessary to correct the pH in the body.

Tissue Oxygenation

Seven conditions are necessary for the tissues to receive adequate oxygenation
- available atmospheric oxygen
- adequate ventilation
- gas exchange between the lung and arterial blood
- adequate hemoglobin
- ability of hemoglobin to bind oxygen
- adequate blood circulation (cardiac output)
- release of 02 from hemoglobin to the tissues
If the tissues must make energy under anaerobic conditions, lactic acid is produced causing metabolic acidosis.

Oxygen Transport

Most 02 is transported by hemoglobin
Each adult hemoglobin can combine with up to 4 molecules of 02
The actual amount of 02 loaded onto each hemoglobin depends on: the pH, body temperature, levels of pC02 and presence of non – oxygen substances such as carbon monoxide ( detected with a cooximeter).

Calculated vs. Measured 02 Saturation

A Co-oximeter is a special spectrophotometer that uses light to measure each Hgb fraction
Measured oxygen saturation measures the absorption of at least four wavelengths, O2Hb, HHb, COHb, MetHb.
In the presence of COHb, MetHb, or other dyshemoglobins, a calculated S02 will be falsely elevated. (The hemoglobin is saturated, but not with 02)

Co-oximetry

A co-oximeter is a spectrophotometer (measures the light that passes through the sample)
Pulse Oximetry and Co-oximetry are NOT THE SAME!!!

Hemoglobin Fractions

When running co-oximetry samples, it is important to double check that the fractions of hemoglobin add up to 100%(+/- 1%).

For example:

COHb 9.7%
MetHb 0.7%
O2Hb 89.0%
HHb 0.4%

How is hemoglobin measured in a CBC analyzer???

Co-oximetry is similar, but each type of hemoglobin (fraction) absorbs light at a different wavelength (colour of light)
This is only significant if:
- The patient has been exposed to carbon monoxide
- The patient has other abnormal hemoglobins (dyshemoglobins)
Once oxygen reaches the tissues, the oxygen must get released.
The oxygen dissociation curve shows how other factors affect the affinity of hemoglobin for oxygen.
It is important to check the pO2 value because hypoxemia can cause acid base disturbance, especially below pO2 of 50 – 60 mmHg.
The Oxygen dissociation curve provides much information about tissue oxygenation and the factors which affect it.

Lactate

Lactate (lactic acid) can be measured by an electrode or by a spectrophotometer.
Lactate is produced when tissues are deprived of oxygen.
Increased lactate is a good indicator of hypoxia.

Acid Base Disorders

pH <7.35 = acidosis
pH >7.45 = alkalosis
When looking at blood gas results, HC03 is a base and we think of pC02 is an acid.
Remember the Henderson Hasselbalch equation:
pH = 6.10 + log HC03 (base)

0.0306 x pC02 (acid)

A primary change in the pC02 (respiratory component) is always referred to as respiratory acidosis or alkalosis
A primary change in the bicarbonate (metabolic component) is always referred to as non-respiratory or metabolic acidosis or alkalosis

Compensation

Whenever a pH imbalance occurs the body quickly tries to return the pH to normal, this is called compensation.
Compensation is achieved by altering the factor that was not the primary cause of the imbalance.
For example, if the primary disorder is that the lungs are not able to remove C02 (acid) then the kidney will compensate by retaining bicarbonate and removing H+
If the primary disorder is the overproduction of acid by a metabolic source the lungs will try to compensate by removing acid (done by blowing out carbonic acid in the form of carbon dioxide)
Fully compensated means the pH has returned to normal (The other two parameters will still be abnormal)
Partially compensated means the pH is close to normal (pH, pCO2 and bicarbonate will all be abnormal)
Uncompensated means the body has not yet begun to compensate. (pH and one other parameter will be abnormal)