Below the windows, ahead of the seat, there are small grilles. Look:
What are those for?
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The answers to this question indicate that these are the exhaust vents for the cabin air circulation. I think they appear to be much too large for just that purpose. I don't have a technical reference for this, but I believe those vents also prevent the cabin floor buckling in the event of a sudden pressurization failure.
The cargo hold is beneath the passenger cabin, and both are pressurized because the cylindrical hull can resist pressure better than the flat cabin floor. If the cabin suddenly loses pressure the air in the hold could buckle the floor upward, or if the hold loses pressure the air in the cabin could buckle the floor downward. The floor must be generously vented to prevent this.
US patents # 6129312 and # 5137231 claim systems for combining the cabin air circulation vents with the depressurization relief vents. From the '9312 patent:
An air return grille mounted below the passenger cabin sidewall comprises openings to allow return airflow from the air conditioning system. The grille openings are covered by a single thin baffle with cutouts within its periphery that slip over and are held in place by fasteners on the molded grille. During normal operation, the thin baffle restricts flow through a small opening in the grille. During a sudden decompression of the cargo compartment, the baffle is dislodged from the fasteners. This allows the airflow to pass through all of the molded grille openings in order to achieve rapid pressure equalization.
In short: They are special air paths named the dado panels or decompression panels:
In normal conditions they are partially open to allow air entering the cabin from the ceiling (crown) to exit into the cargo holds.
In case of rapid decompression from a hole in the cargo holds, they fully open to accelerate pressure equalization between the two levels and prevent the cabin floor to collapse down under the effects of high pressure differential.
They are standard safety equipment since a NTSB recommendation after a DC-10 explosive decompression in 1972, due to a cargo door opening in flight. The cabin floor collapsed:
disrupting various control cables which were routed through the floor beams to the rear engine and to the empennage control systems
In the principle they are similar to the louver panels found in restroom doors (doors may be undercut instead). They play a secondary role as firewalls.
In the architecture lingo, the dado is the lower part of a wall below the dado rail. The grille portion of a passenger aircraft wall is called the dado module. It consists on a louvered air grille over a decompression panel (sometimes called the dado panel) which can be open or closed.
Boeing patent for a decompression panel, source
The cargo compartments on a pressurized aircraft are not always fully heated, but they are always pressurized, being part of a single pressure vessel (a cylinder sealed by two bulkheads).
Air from the packs (conditioning system) enters the cabin usually from the ceiling and leaves it by the dado modules, e.g. (A320):
Longitudinal flow, schematic principle, source
There are recirculation ducts in the cargo section with fans drawing air from the cabin. Air is then reconditioned and sent back to the cabin. A fraction is dumped overboard though the outflow valves to regulate pressure in the aircraft.
In case of decompression
In case of rapid decompression in a cargo area, there is a risk of cabin floor collapsing due to the pressure difference above and below it. This happened in 1972, as @krmezljavKuza mentioned. The related NTSB investigation report included this recommendation:
Require the installation of relief vents between the cabin and aft cargo compartment to minimize the pressure loading on the cabin flooring in the event of sudden depressurization of the cargo compartment.
The decompression panels open to act as pressure relief vents and allow a larger quantity of air to flow into the cargo compartment. Pressure can equalize before the floor is damaged.
There are similar decompression panels in the triangle areas indicated in the picture above, between the different sections of the cargo level. There are also louvers in various panels, e.g. lavatory doors.
The grilles are almost certainly meant to equalize pressure between the cabin and cargo bay, particularly in case of rapid decompression. As far as I know, the design feature stems from two consecutive accidents with the DC-10, American Airlines 96 on 12 June 1972, and Turkish Airlines 981 on 3 March 1974. Though the cause of both of these decompressions was a poorly-designed (and poorly-maintained) cargo door, in both cases, the real damage was caused by the collapse of the cabin floor, which severed control cables, and on Turkish Airlines, broke hydraulic lines. After these accidents, the grilles were added to equalize pressure and prevent the floor from collapsing. The floor itself was also strengthened.
If I remember correctly, one of these accidents was the subject of an episode of Air Crash Investigation.
Air Crash Investigation S7E1 "Shattered in seconds" about China Airlines flight 611 on 2002-05-25 (decompression due to fuselage rupture caused by a faulty repair after a tail strike on 1980-02-07) mentions dado panels and an accident with a Turkish Airlines flight in 1974 (together with reconstruction footage of that accident), and it is said that the Turkish Airlines accident led to a NTSB recommendation about adding more dado panels to the cabin. All of this is in sum a good indication that there also is an episode about that particular crash.