11.2 Applications and skills

11.2.3 Applications of osmoregulation and the kidney

This page relates to many of the themes that you have already encountered in this course. Try to make theoretical connections between these applications and your knowledge of biological principles, especially immunity and homeostasis.

The consequences of dehydration and overhydration

  • Dehydration occurs when water intake is insufficient to cover losses. Excessive perspiration, diarrhea and vomiting are common causes.
  • Overhydration occurs when too much water intake disrupts osmotic balance. It is rare, but can occur as a result of:
    • poor kidney function
    • drugs that interfere with the secretion of ADH
    • psychiatric disorders such as schizophrenia.
    • Kidneys are able to excrete about one litre of water per hour. Drinking too much water only rarely leads to overhydration. 

Consequences of dehydration

Consequences of overhydration

Headache, nausea, lethargy

Confusion, headache, bloating

Low blood plasma volume with high sodium concentration

High blood plasma volume with low sodium concentration

Low blood pressure

High blood pressure

Muscle fatigue due to inability to clear wastes from tissues

Pain due to swelling of nerve cells in hypotonic environment

Multiple organ failure which may be fatal

Water intoxication (swelling of the brain or cerebral edema) which may be fatal

Urinalysis is used to detect blood cells, glucose, protein and drugs

  • Urine tests strips (also known as ‘dip tests’) are important medical diagnostic tools. Test strips change colour when glucose or proteins are present in the urine outside of the normal, healthy range.

Figure 11.2.3a – Urine ‘dip test’ strips Figure 11.2.3a – Urine ‘dip test’ strips 

Test result

Possible diagnosis


• Diabetes

Albumin and creatine (proteins)

• Diabetes and/or hypertension

• High albumin indicates physical kidney damage

• High creatine indicates kidney failure

Low pH (urine is acidic)

• Kidney stones (uric acid in the kidneys)

  • Test strips can also be used to detect metabolites of different classes of drugs:
    • THC (marijuana)
    • cocaine
    • opiates (morphine, heroin)
    • amphetamine (including prescription ADHD medications)
    • methamphetamine
    • MDMA (ecstasy)
    • alcohol.

Figure 11.2.3b – Drug testing using urinalysis: a positive test is indicated when two lines appear in the result window.Figure 11.2.3b – Drug testing using urinalysis: a positive test is indicated when two lines appear in the result window.

  • Blood cells are detected by direct observation of urine samples using a microscope:

Cell type detected

Possible diagnosis


• Urinary tract infection (or other infectious disease)

• Side effect of acetaminophen drugs


• Kidney damage

Treatment of kidney failure by hemodialysis or transplant

  • Kidney damage can result from an acute injury, or can progress over time as a result of diseases like diabetes.
  • When the kidneys are no longer able to filter blood, the result is kidney failure.
  • Treatment of kidney failure is hemodialysis. The patient’s bloodstream is connected to a machine that acts as an artificial kidney.
  • The dialysis machine removes wastes from the blood using a selectively permeable membrane and dialysis fluid that has the same osmolarity as blood plasma. Filtered blood is then returned to the body.

Figure 11.2.3c – Principle of hemodialysisFigure 11.2.3c – Principle of hemodialysis

  • A patient may need up to 12 hours of hemodialysis every week. Transplants are the preferred treatment for kidney failure, but donors must be matched by blood type.
  • For a successful transplant, immunosuppressive drugs need to be administered for up to six months. These drugs prevent the patient’s immune system from rejecting the donor kidney.
  • Waiting lists for kidney transplants are usually very long, as this is the most common type of organ transplant worldwide.

Figure 11.2.3d – Cerebral edemaFigure 11.2.3d – Cerebral edema
Acetaminophen (APAP) interferes with ADH secretion. Overdose can lead to cerebral edema (shown in this CT scan image).

Language help

  • Overhydration is also called hyponatrema. From Greek hypo = low and Latin natrium = sodium.
  • Dehydration is also called hypohydration. From Greek hypo = low and hudor = water.
  • THC, tetrahydrocannibinol, is the psychoactive component of marijuana.

Figure 11.2.3e – White blood cellsFigure 11.2.3e – White blood cells
White blood cells in the urine are a sign of infection.

Food for thought

  • In 2013, a Dutch student died of water intoxication four days after taking MDMA at a music festival.
  • Another woman died from water intoxication after participating in a water-drinking contest in Brazil in the same year.
  • Morphine is processed from poppy plants. Although the concentration of opiates in unprocessed plants is very low, urinalysis may show false positive results after a person ingests poppy seeds.

Figure 11.2.3f – Poppy seed muffinFigure 11.2.3f – Poppy seed muffin
You might want to have a blueberry muffin for breakfast instead!

In the lab

  • Dip tests can be set up in the classroom on samples of simulated urine with varying glucose and protein concentrations.
  • If you do not have access to urine test strips, try these simple tests:
    • Benedict’s reagent changes from blue to orange/red in the presence of glucose (and other monosaccharides)
    • Biuret’s reagent changes from blue violet in the presence of peptide bonds.

Please don’t use real urine! 

Figure 11.2.3g – HemodialysisFigure 11.2.3g – Hemodialysis
Hemodialysis requires many hours of sitting almost completely still.

Course link

  • Learn more about how drugs affect the brain and nervous system in Option A: Neurobiology and behaviour. See 12.2.5.
  • Urine test strips use monoclonal antibodies to detect proteins. See 11.2.1.
  • Dialysis fluid needs to have the same osmolarity as blood plasma. See 1.1.4.
  • Kidneys may be rejected even if there is blood type compatibility. Review blood types and antigen formation in 11.1.1.