Acid based write-up

Introduction:

The pH of blood plasma needs to be regulated. This is because enzymes are present in the blood. Enzymes are globular proteins and are sensitive to pH. A significant change in pH would cause the enzymes to denature and enzymic reactions would cease. The pH of blood plasma is in the region of 7.4, while the pH within the cells is much lower then in the plasma, of the order of pH 6.1. This is because the H+ is made within the cells and this means a high concentration of H+, hence a lower pH (more acidic pH).

Hydrogen ions in the blood plasma are dependent on two components: 1). The respiratory component and 2). The metabolic component. The respiratory component is the amount of CO2 present in the plasma. The metabolic component is the organic acids e.g. lactic acid and ketone bodies present. The CO2 factor is mediated by the lungs, while the organic acid and ketone body factor is mediated by the kidneys. The lungs and kidneys independently mediate the factors and so compensate for one another. This is important in for example respiratory depression (CO2 accumulation), when not enough CO2 is removed via the lungs. The kidneys will compensate by removing H+ in the urine.

The H+ in the blood plasma is in the form of buffers. There are three buffers:

HCO3- bicarbonate

HG- haemoglobin, both

Oxyheamoglobin and haemoglobin are

Buffers, but oxyhaemoglobin is a more

acidic buffer.

PP- Protein

1/3 PP constitutes the buffer content of plasma, while the ...

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acidic buffer.

PP- Protein

1/3 PP constitutes the buffer content of plasma, while the greater constituents are the other two

(the most important constituents). We will consider bicarbonate (HCO3). Changes in

bicarbonate buffer reflect changes in respiratory and kidney response (rate of removal of CO2 /

Organic acids etc.

The reaction below occurs in blood cells:

To blood plasma

CO2 + H2O H2 CO3 H+ + HCO3-

Fig.1

The above reaction is at equilibrium. We will assume for the experiment that it is at equilibrium at all

times.

In respiratory acidosis- CO2 levels increase, this causes the above reaction to occur in the forward

direction.

In respiratory alkalosis- the reverse will happen. In metabolic acidosis- metabolic acid levels increase, this causes the above reaction to occur in the reverse direction. Some typical values:

Respiratory acid = pH 7.4

pCO2 = 40 mmHg

[H CO3-] = 24mEqL

PH = pK + log [HCO3-] ÷ pCO2

The above equation shows pH depends on the balance between [HCO3-] and pCO2. This is effected by the respiratory and metabolic components, but is largely dominated by the metabolic component.

Method:

In the experiment a blood sample is taken by a finger-prick before and after the procedure and run through the blood gas analyser so that the two samples can be compared. The following procedures were performed 1). Hyperventilation (rapid breathing while resting on a couch) 2). Exercise for three minutes on an exercise bicycle 3). Fasting for 24 hours. The pin should be sterilised and pressure should be applied to the puncture wound. Peprin, an anti-coagulant is combined with the blood and thoroughly mixed by stirring with a magnet. The blood sample is also not exposed to air; otherwise the CO2 present within the blood will be lost to the atmospheric air. The blood samples taken are arterial samples. Venepuncture blood samples are taken from are vein. Veins carry deoxygenated blood i.e. it is low in oxygen and high in Carbon dioxide. Blood from arteries is oxygenated so the reverse is true. Thus taking Venepuncture blood samples will give different results. During hyperventilation one is breathing at an abnormally rapid rate at rest. Demands for Oxygen delivery and Carbon dioxide delivery are exceeded. This causes a reduction in Carbon dioxide concentration of arterial blood and an increase in oxygen concentration. This leads to a drop in blood pressure and proceeds to the subject feeling dizziness and a tingling sensation in the limbs. During exercise the muscles need an increased blood supply so that oxygen and glucose can be provided for aerobic respiration and waste carbon dioxide and heat can be removed. If the demand for oxygen by the muscles exceeds the amount of oxygen delivered via the lungs, anaerobic respiration occurs. This means lactic acid is accumulated. When exercise stops the oxygen debt has to be repaid. This is known as the recovery period. The oxygen demand depends on how hard the muscles are working, while the recovery period depends upon the amount of lactic acid accumulated during the exercise.

Results:

Analysis:

Hyperventilation: As the pH increases pCO2 levels decrease significantly and HCO3 levels decrease very slightly so there is virtually no change. In hyperventilation the rate of respiration exceeds demands for oxygen delivery and Carbon dioxide delivery. This means the reaction in fig.1 is shifted to the left (in the backward direction), so more CO2 is produced, this means less H+ and consequently the pH is more alkaline.

Exercise: As the pH increases pCO2 and HCO3 levels decrease significantly. The reaction occurs in the forward direction. This means more H+ in the blood plasma and the pH decreases (more acidic). The metabolic acidosis is stronger than respiratory alkalosis. Exercise causes hyperventilation hence the alkalosis, but lack of oxygen leads to build up of lactic acid (this is anaerobic respiration); this is the stronger component during exercise.

Fasting for 24 hours: There is no change in pH, pCO2 and HCO3 decrease significantly. When fasting the body’s glycogen stores get used up. The body then starts to burn up fat and produces ketone bodies. Hunger also causes hyperventilation-hence-respiratory alkalosis. The ketone body’s present cause metabolic acidosis. The acidosis compensates for the alkalosis i.e. the kidneys compensate for the lungs. The reaction occurs in the forward direction for metabolic acidosis. This means more H+ in the plasma and pH levels decrease. In respiratory alkalosis. The reaction occurs in the reverse direction. This means less H+ in the plasma and an increase in pH, thus they have compensated for each other.

Conclusion:

Any activity involving the body causes a shift in pH from the mean value. This results in either acidosis or alkalosis. The body tries to correct this via its buffers, which act as to bring the pH back to the mean value. The kidney and the respiratory system reply correspondingly. If the pH exceeds the mean value to such an extent that the buffers cannot compensate by inducing equilibrium and so ultimately death is apparent. This has to be avoided at all costs.

Evaluation:

In the experiment only one subject was tested for each procedure. This is not statistically significant. In order to be statistically accurate a larger number of subjects should be tested, around the order of a 100, so that percentages can be drawn from this and a mean value can be determined. The exercise should be carried out for a longer period of time, for example twenty minutes, so it is visually obvious the breathing rate of the subject has increased; suggesting the body muscles are active. Three minutes are not enough to achieve this. This is because different subjects have different levels of fitness so require different minimum amounts of exercise for the above to be achieved. Twenty minutes would encompass even the fittest individuals.