Camped on the baking concrete outside a gleaming transplant center, Karachi's sick have come from miles around in desperate hope for a last chance at life.

There, Dr Adibul Rizvi provides free medical care to hundreds of thousands of people each year, providing a much-needed alternative to Pakistan's public health sector, which critics dismiss as chaotic, corrupt and vastly under-resourced.

Rizvi, with a thatch of white hair as springy as his step, roams the crowded halls of his life's work every day from 8 am until midnight or later, visiting the bedsides of patients - children, criminals, VIPs alike.

At 79 he is fueled by love of his job, his enthusiasm evident as he recounts his 42-year odyssey from starting with an eight-bed ward to building one of the largest treatment networks in South Asia.

The achievement cannot be understated. His Sindh Institute of Urology and Transplantation is funded largely by charitable donations, some as small as 100 rupees ($1) at a time, and has treated millions of people over the last four decades.

More than 300 transplants and 260,000 dialysis sessions were carried out in 2015 alone, with follow-up treatments and medications provided for as long as it takes - all entirely for free.

"Governments in a developing country ... they cannot afford a state-of-the-art health service," says Rizvi, describing the unimaginable poverty that staff in some of his 10 health centers across the country encounter.

The government does give money, its funding accounts for some 30 percent of SIUT's budget, but not enough to fulfill Rizvi's philosophy that "every human ... has got a right to access healthcare, to live with dignity".

Also, a lack of awareness in Pakistan about the value of organ donation is "pushing back our progress", Rizvi admits.

Pakistanis who enter SIUT's main center in Karachi are taken aback by its spotlessness and efficiency, traits virtually absent in many of the country's other medical facilities.

But bustling through the hospital, Rizvi insists he has achieved nothing yet, and vows to take on no less an enemy than cancer next, calling it "doable".

He brushes off any suggestion of retirement.

"My colleagues, they are working (the) same way. Most of them have become hypertensive, I have not," he laughs.