Health care is expensive in the United States, where many of us have paid a medical bill only to wonder why the cost of that seemingly small procedure is so high. It makes one wonder how much the most-expensive surgeries must cost. Here's a list of the top 10 most expensive medical procedures according to the 2017 edition of the triennial Milliman Health Cost Guidelines report, along with the reasons why they cost so much.

1. Heart Transplant

Cost: $1,382,400

With heart disease as the leading cause of death in the U.S., open heart surgery is a more common procedure than expected. Part of the high cost of open heart surgery is because it's often an urgent medical procedure that is usually followed by complications. Longer care and follow-up needed after surgery add to the price tag. Meanwhile, heart transplants are among the most complicated of procedures, carrying great risk. Waiting lists are long, and preparation for the surgery is lengthy and expensive. Add to this the expensive procurement of the organ, and you can see why the cost is so great.

2. Double Lung Transplant

Cost: $1,190,700

When other therapies don't work, lung transplants are the last resort for patients of lung disease like emphysema and cystic fibrosis. As with other transplant surgeries, wait lists are long and the cost is high because of the lengthy hospital stay.

3. Intestine Transplant

Cost: $1,147,300

A transplant of the intestine is done to replace dead intestinal tissue with live tissue from a donor, often because of disease or the presence of a tumor. Because an intestinal disease is sometimes accompanied by the liver or other organ failures, intestine transplants can be done in conjunction with another organ, adding more than $400,000 to the bill.

Lack of insurance among more than 44 million Americans drives up the cost of medical procedures.

4. Allogeneic Bone Marrow Transplant

Cost: $892,700

Bone marrow transplants done with a donor's marrow are called allogeneic transplants. The nature of this procedure is risky, and there’s a lengthy prep time in the hospital, as well as an extensive recovery period—all of which makes for a hefty medical bill.

5. Single Lung Transplant

Cost: $861,700

Like a double lung transplant, the wait list can be long and it’s still very expensive to merely replace a single lung. Coupling a lung and a heart transplant together is one of the most expensive procedures a patient can receive, totaling nearly $2.6 million.

6. Liver Transplant

Cost: $812,500

As with a heart transplant, liver transplants are high risk and high cost, with an accompanying waiting list. Criteria are high, which means administrative and prep procedure costs add to the bottom line.

7. Kidney Transplant

Cost: $414,800

Kidney transplants, like the other transplants on this list, are expensive due to the risk, recovery, and prep expense. The one difference is that, with kidney transplants, the old kidney isn't removed because it's been shown it reduces risk that way; surgeons find a different blood supply to attach the new kidney to.

8. Autologous Bone Marrow Transplant

Cost: $409,600

A bone marrow transplant done with one’s own bone marrow is called an autologous transplant, which costs much less than an allogeneic transplant. It’s cheaper because, with autologous transplants, you have to find a donor, which can be difficult, plus complications after the procedure are very common.

9. Pancreas Transplant

Cost: $347,000

Transplants of the pancreas are usually needed when a patient has type 1 diabetes or renal failure. It is often done in tandem with a kidney transplant, increasing the cost of the surgery by nearly 80% and coming in at $618,000.

10. Cornea Transplant

Cost: $30,200

Vision problems stemming from a thinning of the cornea can be fixed with a cornea transplant. Although the survey is done with the patient awake, it can be pricey because of the precision skills required for this procedure.

The Bottom Line

If you think these costs are high, consider patients with a chronic disease that affects more than one organ. These patients often need multiple organ transplants, with bills exceeding a million dollars. Why the whopping price tags for all these surgeries?

In the case of transplants, the hospital stay before and after the surgery makes up the majority of the bill. Some of the cost comes from liability insurance, those high premiums hospitals and doctors have to pay to cover themselves in case of lawsuits. A side-effect of a large number of liability lawsuits is that doctors often order more (expensive) tests than needed to cover themselves, a practice called defensive medicine.

Lack of insurance among more than 44 million Americans drives costs up, since the uninsured use emergency rooms, and often when they've waited so long that pricey emergency surgery is the only option. Add this to the high cost of medication and medical equipment, and you can see how these price tags balloon.

Overall, health insurance coverage is vital for someone who needs one of the above life-preserving and extremely expensive procedures.