How to monitor concentrations of substances in the blood
Cells in the body can only function properly in a suitable environment. This
environment is provided by the tissue fluid which bathes the cells. Since tissue fluid
is derived from blood, the levels of various substances in the blood are critical.
The normal pH range for blood is 7.35 – 7.45.
Normally, the pH of arterial blood is about 7.45, and the pH of venous blood is about
7.35. The slightly lower pH of venous blood compared with arterial blood results
primarily form carbon dioxide (CO ) entering venous blood as a waste product of
cellular metabolism. As carbon dioxide enters the blood, some of it combines with
water (H O) and is converted into carbonic acid by carbonic anhydrase, an enzyme
found in red blood cells.
Q) Why are the pH values for arterial and venous blood different?
How blood is buffered to maintain pH within safe limits
-Buffers are chemical substances that prevent a sharp change in the pH of a fluid
when an acid or base is added to it.
-Strong acids and bases, if added to the blood, would ‘dissociate’ almost completely
and release large quantities of H or OH ions.
-The result would be drastic changes in blood pH.
-Survival itself depends on protecting the body from such drastic pH changes.
-More acids than bases are usually added to body fluids. This is because catabolism,
a process that goes on continually in every cell of the body, produces acids that enter
blood as it flows through tissue capillaries.
-Almost immediately, one of the salts present in blood – a buffer, that is – reacts with
these relatively strong acids to change them to weaker acids.
-The weaker acids decrease blood pH only slightly, whereas the stronger acids formed
by catabolism would have decreased it greatly if they were not buffered.
-Buffers consist of two kinds of substances and are therefore often called buffer pairs.
-One of the main blood buffer pairs is ordinary baking soda (sodium bicarbonate, or
NaHCO ) and carbonic acid (H CO ).
pH Imbalances
-Acisosis and alkalosis are the two kinds of pH or acid-base imbalance. In acidosis
the blood pH falls as H ion concentration increases (or because of a loss of bases).
Only rarely does it fall as low as 7.0 (neutrality) and almost never does it become
even slightly acid, because death usually intervenes before the pH drops that much.
In alkalosis, which develops less often than acidosis, the blood pH is higher than
normal. Alkalosis results from a loss of acids or an accumulation of bases.
From a clinical standpoint, disturbances in acid-base balance can be considered
dependent on the relative quantities (ratio) of H CO and NaHCO in the blood.
Components of this important buffer pair must be maintained at the proper ratio (20
times more NaHCO than H CO ) if acid-base balance is to remain normal. It is
fortunate that the body can regulate both chemicals in the NaHCO –H CO buffer
system. Blood levels of NaHCO can be regulated by the kidneys and H CO levels
by the respiratory system (lungs).
Metabolic and Respiratory Disturbances
Two types of disturbances, metabolic and respiratory, can alter the proper ratio of
these components. Metabolic disturbances affect the bicarbonate (NaHCO ) element
of the buffer pair, and respiratory disturbances affect the H CO element, as follows:
1. Metabolic disturbances:
a. Metabolic acidosis (bicarbonate deficit). Patients that have metabolic acidosis
with a bicarbonate deficit often suffer from renal disease, uncontrolled diabetes,
prolonged diahorrea, or have ingested toxic chemicals such as antifreeze (ethylene
glycol) or wood alcohol (methanol).
b. Metabolic alkalosis (bicarbonate excess). The bicarbonate excess in metabolic
alkalosis can result from diuretic therapy, loss of acid-containing gastric fluid caused
by vomiting or suction, or from certain diseases such as Cushing syndrome.
2.Respiratory disturbances:
a. Respiratory acidosis (H CO excess). The increase in H CO characteristic of
respiratory acidosis is caused most often by slow breathing, which results in excess
CO in arterial blood. Causes include depression of the respiratory centre by drugs or
anaesthesia or by pulmonary diseases such as emphysema and pneumonia.
b. Respiratory alkalosis (H CO deficit). Hyperventilation leads to an H CO deficit
caused by excessive loss of CO in expired air. The result is respiratory alkalosis.
Anxiety (hyperventilation syndrome), overinflation of patients on ventilators, or
hepatic coma can all reduce H CO and CO to dangerously low levels.
The ratio of NaHCO to H CO levels in the blood is the key to acid-base balance. If
the normal ratio (20:1 NaHCO /H CO ) can be maintained, the acid-base balance and
pH remain normal despite changes in the absolute amounts of either component of the
buffer pair in the blood.
As a clinical example, in a person suffering from untreated diabetes, abnormally large
amounts of acids enter the blood. The normal 20:1 ratio is altered as the NaHCO
component of the buffer pair reacts with the acids. Blood levels of NaHCO decrease
rapidly in these patients. The result is a lower ratio of NaHCO to H CO (perhaps
10:1) and lower blood pH. The condition is called uncompensated metabolic acidosis.
The body attempts to correct or compensate for the acidosis by altering the ratio of
NaHCO to H CO . Acidosis in a diabetic patient is often accompanied by rapid
breathing or hyperventilation. This compensatory action of the respiratory system
results in a ‘blow-off’ of CO . Decreased blood levels of CO result in lower H CO
levels. A new compensated ratio of NaHCO to H CO (perhaps 10:0.5) may result.
In such individuals the blood pH returns to normal or near-normal levels. The
condition is called compensated metabolic acidosis.
Diabetic Ketoacidosis
An important part of home care for diabetics involves monitoring the level of glucose
in the blood and, especially for patients taking insulin, carefully watching the
appearance of ketone bodies in the urine. Accumulation of these acidic substances in
the blood results from the excessive metabolism of fats, most often in those people
with uncontrolled type 1 diabetes. These individuals have trouble metabolising
carbohydrates and instead burn fat as a primary energy source. The accumulation of
ketone bodies results in a condition called diabetic ketoacidosis that causes the blood
to become dangerously acidic. As blood levels of ketones increases they ‘spill over’
into the urine and can be detected by use of appropriate reagent strips.
Activity
Vomiting can be a result of pH imbalances. Explain how and why vomiting occurs.