SCS 2472 - BIOCHEMISTRY WITH A MEDICAL PERSPECTIVE

Clinical Insight: Water and pH Balance

Dr. Robert Murray

Basic Points About Water

Water varies from 60-70% of body weight It is questionable whether life can occur in

the absence of water Humans cannot survive without water for

much longer than a week Hunger strikers have gone without food for

over 50 days

Water Balance

A healthy adult takes in about 2.5L of water per day

About the same amount is lost each day via urine, feces, sweat and some other sources

The obligatory loss can be reduced to about 1.5L per day, but not much less

Medical Conditions Involving Water Balance

Inadequate Intake: When water is scarce or not available

Excessive Loss: This may occur by vomiting, diarrhea, urinary losses, excess sweating and some other causes

A spectacular example of excessive loss is cholera

I.V. replacement of water as saline, glucose or other solutions is widely used

Structure of Aquaporins

Aquaporins are specialized water pores present in the plasma membrane of certain cells

Abnormalities of aquaporin-2 in the kidney lead to diminished reabsorption of water causing one type of diabetes insipidus

Urinalysis

This is an important area of clinical medicine, as it indicates whether kidney function is normal or abnormal, but also gives important information on non-kidney problems

Urine can be tested for volume, specific gravity, protein, ketone bodies, various metabolites, drugs, other compounds and pH

pH

Disorders affecting pH are quite frequent Blood pH is normally held within the narrow limits

of 7.37-7.43 Variation of over 0.5 pH units on either side of pH

7.4 are often fatal Changes of pH affect ionic bonds and H bonds; this

can affect the overall structures of DNA, proteins, causing loss of function and even death

DNA denaturation by the addition of base

Assessment of Disorders of pH

Basic measurements: Blood pH, pCO2 and bicarbonate

This helps to decide whether acidosis or alkalosis is present and also assists in diagnosis of the cause

Causes of Acidosis and Alkalosis

Many causes: usually classified as metabolic, respiratory or mixed

Vomiting can cause a metabolic alkalosis and uncontrolled diabetes mellitus can cause a metabolic acidosis

Respiratory acidosis can be caused by underventilation of the lungs, and hyperventilation can cause respiratory alkalosis

Summary

A sound knowledge of water balance and pH and its regulation by buffers is important

Become familiar with the Henderson-Hasselbalch equation and various buffers (bicarbonate, phosphate, haemoglobin and ammonia production)

Appropriate treatment for problems of water balance and pH is based on a thorough understanding of water balance and pH

References

MedlinePlus (Medical Encyclopedia). Acidosis

Nobelprize.org : Peter Agre (autobiography and Nobel Lecture on Aquaporin Water Channels).