Pope John Paul II is said to be in a critical state after further health complications.
The Pope's condition is grave
The BBC News website looks at the Pope's health.
What is the Pope's current state of health?
The latest communique by the Vatican says the Holy Father's heart and kidneys are failing, his blood pressure and pulse are unstable, and his breathing is shallow.
Vatican officials have been making regular statements since the Pope's health worsened with a high fever caused by an infection.
On Thursday he received the Saint Viaticum, the Catholics' last rites for the sick and dying, a spokesman said.
Why is he ill?
The 84-year-old pontiff's health declined rapidly on Thursday when he developed a high fever brought on by a urinary infection.
POPE'S HEALTH SCARES
May 1981 - shot in stomach and hand during assassination attempt in St Peter's Square
July 92 - undergoes surgery to remove intestinal tumour
92 - symptoms of Parkinson's disease appear
Apr 94 - suffers broken right thigh bone
Dec 95 - misses Christmas mass for the first time because of flu
Oct 96 - undergoes surgery to remove appendix
Feb 05 - has week in hospital after falling ill with flu, readmitted after relapse two weeks later and fitted with a tracheotomy to help him breath
March 05 - fitted with a feeding tube in his nose to help boost his nutrition
March 05 - developed a urinary infection, which caused fever, septic shock and then heart and circulatory failure
His body has gone into a state of shock and "cardio-circulatory collapse set in", said a spokesman.
What does this mean?
This means his heart is not working properly and is too weak to pump enough blood around his body without assistance.
The pontiff was provided with "all the appropriate therapeutic provisions and cardio-respiratory assistance", a statement by Mr Navarro-Valls said on Friday morning.
"This morning the condition of the Holy Father is very serious," the statement said, but the 84-year-old pontiff remained "conscious, lucid and tranquil", the spokesman said.
Professor Peter Weissberg, Medical Director of the British Heart Foundation, said: "Septic shock puts a phenomenal strain on the heart. Even the fittest patients need specialist care and medicine to survive.
"Those already suffering from heart disease - including those with heart failure - are even more susceptible to septic shock.
"Infection triggers a profound loss of blood pressure - depriving organs around the body of their vital blood supply and putting an enormous strain on the heart."
What other health problems does the Pope have?
The Pope has been plagued by ill health for some time.
He has suffered from Parkinson's disease for several years, and painful arthritis in his legs means he now finds it difficult to stand un-aided.
On February 24 this year he was taken to hospital with flu-related symptoms.
His doctors decided to do an elective procedure called a tracheotomy to help him breathe more easily.
For this procedure a surgeon makes a hole in the air passage. A tube is passed through the hole, down the throat and into the main airway at the top of the lungs.
Days ago his doctors decided he also needed extra help with his nutrition.
They passed a feeding tube through his nose and into his stomach to boost his nutritional intake.