Q4.

Doctors sometimes need to know how much blood a patient has.

They can find out by using a radioactive solution.

The doctors use a radioactive substance called Technicium 99-m which loses half of its radioactivity every six hours.

After measuring how radioactive a small syringe-full of the solution is they inject it into the patient's blood.

They then wait for 30 minutes so that the solution has time to become completely mixed into the blood.

Finally, they take a syringe-full of blood and measure how radioactive it is.

Example: If the doctor injects 10 cm3 of the radioactive solution into the patient and this is found to have been diluted 500 times by the blood there must be 10 × 500 = 5000 cm3 of blood

(a)     After allowing for background radiation:

10 cm3 of the radioactive solution gives a reading of 7350 counts per minute;

a 10 cm3 sample of blood gives a reading of 15 counts per minute.

Calculate the volume of the patient's blood. (Show your working.)

(4 marks)

(b)     The doctor's method of estimating blood volume will not be completely accurate. Write down three reasons for this.

(3 marks)

(c)    Explain why Technicium 99-m is a suitable radioactive substance to use.

(2 marks)

(Total 9 marks)