The 94-year-old actor was admitted to the hospital in suburban Bandra on Wednesday morning due to dehydration and urinary tract infection.
Doctors said Kumar's kidneys were not functioning properly.
"He is stable at the moment. With the help of medicines, he has passed urine. Creatinine and potassium levels are high. He is alert and conscious. He is responding well to the treatment. Cardiologist Dr Nitin Gokhale and nephrologist Dr Arun Shah are treating him," Ajaykumar Pande, Vice President Lilavati Hospital told PTI.