Sorry about the extended cut & paste. Explains it way better than I could. And is fascinating.
Btw, Bradycardia = low or very low heart rate.
1990 The Pinnipeds: Seals, Sea Lions, and Walruses by Marianne Riedmann. University of California Press: Berkeley, L.A., and Oxford.
pp. 33-34: Breath-hold capacity can be increased by taking more oxygen down with each dive. A human diver will breathe deeply -- hyperventilate -- before diving, and descend with lungs full of air. There are disadvantages in this. For a start, full lungs increase buoyancy and so make it more difficult to descend; secondly, there are pressure problems associated with diving with full lungs. Seals hyperventilate before diving, but they expel most of the air from their lungs before they go down, and hence they do not depend on air from the lungs for their oxygen when they dive.
Besides being stored as a gas in the lungs, oxygen can be carried in physical solution in the blood and tissue fluids, or chemically bound to haemoglobin in the red blood cells or to another respiratory pigment, myoglobin, in the muscles. In seals the major store is carried in the blood. Seals have greater blood volumes than terrestrial mammals. A Weddell seal, for example, has about 150 ml of blood per kilogram bodyweight, about twice the value for Man. Furthermore, seal blood contains more haemoglobin than human blood -- about 1.6 times as much. The combined result of this is that the blood oxygen stores per unit of body weight is about three or three and a half times that of Man.
pp. 28-29: Bradycardia and the channeling of blood to critical organs save oxygen. Since muscles are deprived of their usual supply of blood and oxygen, they compensate by containing large amounts of a special compound called myoglobin, an iron-bonding pigment related to hemoglobin that stores oxygen. Myoglobin helps the seal to tolerate the large accumulation of carbon dioxide in its bloodstream that prompts breathing during periods of prolonged breath-holding. Myoglobin therefore helps the animal to conserve oxygen during high-speed movement or deep dives. In fact, cetacean and pinniped muscles appear nearly black when they are exposed to the air. This dark coloration is due to the extremely dense concentrations of deep red myoglobin in the muscle tissues.
A seal's muscles are also able to handle the high amounts of lactic acid that accumulates in its system during periods of heavy or prolonged activity (such as diving), which causes muscle exhaustion in land mammals.
The muscles of marine mammals can function with insufficient oxygen for many hours. Most of the time, the muscles function aerobically (with oxygen), even though the seal may be diving and exercising heavily. They function anaerobically when oxygen supplies are depleted by prolonged submergence. When a seal surfaces from a long dive, it takes several rapid and deep breaths until the body's oxygen supply is replenished; at the same time, at least in harbor seals, a pronounced tachycardia (extremely rapid heartbeat) occurs (Fedak et al. 1985). Seals and whales even have an automatic mechanism that cuts off breathing if they are knocked unconscious, so that their lungs will not fill with water.
In pinnipeds, the oxygen-transporting circulatory system is very large. A seal total blood volume in relation to its body weight is 1.5-2 times greater than that of other mammals. According to Scheffer (1976), "In a full-grown walrus the forked veins which drain the lower body are so enormous that a man can draw them over his legs like a pair of pants!" Wickham et al. (1985) and others (e.g., Lenfant et al. 1970) have identified a number of characteristics of phocid seal blood that appear to help the seal to cope with the hypoxia, or oxygen deficiency, of long dives. Phocid blood has a greater capacity than the blood of terrestrial mammals to store oxygen because phocids have fewer and larger red blood cells with a higher concentration of hemoglobin, which stores and carries oxygen.
End of paste.
So it seams seals don't so much hold their breath as hold onto lots of O2 in their blood and muscle, and other tissue.