Red blood cells, white blood cells and platelets are constantly being made in your bone marrow.
Ask Madie King when her life changed forever and she will quickly rattle off September 8, 2010.
That was the day the 11-year-old found out she had precursor b-cell lymphoblastic lymphoma - a complicated-sounding cancer diagnosis which started as a lump in her neck.
She had just finished a netball tournament when she told her mother, Mary-Lee Rolleston-King, "I've got a lump".
What followed were numerous trips to the doctor, biopsies and surgeries.
"We found out she had cancer, but they were not too sure what sort," Mrs Rolleston-King said.
She was soon in Christchurch, with her family, for seven and a-half weeks of chemotherapy and the years of treatment began.
Madie regularly receives intramuscular and intrathecal (spinal) chemotherapy injections, blood and platelet transfusions and a "huge amount" of other drugs via a port in her side.
She has more than 400 beads of courage and faces another two years of treatment.
One of the most memorable of them so far was an emergency blood transfusion on Christmas Eve.
"As soon as she had it, her wee cheeks just went lovely and rosy and red. For us, that gave Madie some cheer and joy to get through Christmas," Mrs Rolleston-King said.
June 14th is World Blood Donor Day and the family will take the time to acknowledge what donors do for others.
"Without them, for Madie, her recovery would not be as fast. Her quality of life would not be what it is if we did not have marvellous, beautiful people to donate blood."
Dunedin donor recruiter Leita Mackay said the day was about thanking donors and encouraging others to set aside one day in the year ahead to give blood.
One donor, Claire Quested (26), said she began giving blood about 10 years ago.
When she started studying, she had more time on her hands and began giving plasma once a month.
"I'm the hugest promoter of donating blood. A lot of people want to give to charity, whereas this is something free, you just give your time. It's such a good feeling to know that you are saving lives," she said.
[Story provided courtesy of Otago Daily Times (external link) ]