BANGKOK (Reuters) - Thailand's 88-year-old king, the world's longest reigning monarch, has been treated after having water in his lungs and his condition has improved, the palace said in a statement late on Friday.

In a monthly update on King Bhumibol Adulyadej's health, the palace said he had no fever and his breath and blood pressure have gradually improved after a "continuous renal replacement therapy" since Thursday after blood test showed there was water in his lungs and the function of kidneys reduced.

The therapy replaces the normal blood-filtering function of the kidneys. It is used when the kidneys are not working well.

The king's condition showed some improvement in most of August before he had rapid breathing and sticky phlegm on Wednesday, and an examination showed severe infection in his blood, the statement said.

He had a mild fever on Wednesday and a team of physicians had to administer antibiotics after his blood pressure dropped, the palace said.

News about the king's health is closely monitored in Thailand, where King Bhumibol is deeply revered.

The king has been treated for various ailments over the past year at Bangkok's Siriraj Hospital and was last seen in public on Jan. 11, when he spent several hours visiting his palace in the Thai capital.

Anxiety over the king's health and an eventual succession has formed the backdrop to more than a decade of bitter political divide in Thailand that has included military takeovers and sometimes violent street demonstrations.

In August's statement, the palace said the monarch was being treated for a "low fever" and a "possible infection" in his blood.

News about the royal family is tightly controlled in Thailand, where laws protecting the royals from insult make it a crime to defame, insult or threaten the king, queen, heir to the throne or regent.

(Reporting by Khettiya Jittapong; Editing by Antoni Slodkowski and Alison Williams)