Goleta-based Inogen has raised $70.5 million in an initial public offering of 4.4 million shares priced at $16, the bottom of the expected $16 to $18 a share range, according to IPO investment firm Renaissance Capital.

The next few days will be critical to gauging investor confidence in Inogen, which relies heavily on Medicare and other insurance reimbursements for revenues, as shares begin to trade on the open market and investors watch for the price to rise. Inogen makes portable oxygen concentrators for medical patients and was spun out of UC Santa Barbara in 2001 by a group of undergraduate students.

The $70.5 million raised is lower than the $86.2 million the Inogen said it might raise in part because of the lower share price but also because it does not include 661,764 shares that the firm’s underwriters have an option purchase. Those shares are worth a possible $10.6 million, which would bring the total raised to $81.1 million. Inogen has not yet said whether the underwriters — J.P. Morgan, Leerink Partners, William Blair and Stifel — have exercised their options.

In addition to selling shares, the company changed some of the disclosures it made to investors, pointing out its steep reliance on government and private insurance programs for 41 percent of its total revenue in 2012.

“We depend upon reimbursement from Medicare, private payors and Medicaid for a significant portion of our revenue, and if we fail to manage the complex and lengthy reimbursement process, our business and operating results could suffer,” the company said in its disclosures.

The company has come to depend especially heavily on Medicare. In 2008, it bought an obscure firm called Comfort Life Medical Supply that had a Medicare billing number and few other assets. By the first three quarters of 2013, about 28.7 percent, or $15.9 million, of Inogen’s revenue came specifically from Medicare in the form of reimbursements for rentals of its units.

Inogen is also the first company in the Tri-Counties to go public under new, looser disclosure rules brought about by the JOBS Act. The company took advantage of provisions that allowed it to file a secret draft of its filing with the U.S. Securities and Exchange Commission before announcing its IPO and is disclosing less data about executive pay than it otherwise would have to.

Inogen has disclosed that it has serious problems with its internal financial controls. Under the JOBS Act, however, it will not be required to have an accounting firm test those controls once they are fixed.

Inogen notched 354 percent revenue growth between 2009 and 2012, bringing 2012 sales to $48.6 million and eking out a modest profit. For the first nine months of 2013, revenue expanded to $55.7 million, and operating profits stand at $3.9 million.

Inogen underwent a major overhaul in 2009, switching to selling and renting its devices directly to consumers. In its filings, the company said it is the only portable oxygen concentrator in the U.S. that sells directly to its customers.