As stated in the habitat section, harp seals spend a lot of their time in the water. A seal can spend approximately 15 minutes submerged without having to come back up for air.  They conserve oxygen by lowering their heart rate by 90%; only the nervous system and sense organs receive a normal flow of blood. After swimming for a while harp seals do not wear out easily because of the lack of lactic acid in their muscles as other animals would. Unfortunately harp seals need time to rest after a swim in order for their body to return to is normal chemistry.
Harp seals, just like all other pinnipeds, have well adapted lungs for the semi aquatic lifestyle. The airways of pinnipeds are supported by smooth muscle and cartilage, as well as alveoli (small sacs under the surface of the membrane in cells) most commonly known as capillaries. They are used as a site of diffusion for gases and nutrients. The alveoli completely collapse in  a harp seals lungs during deep dives. Unlike terrestrial animals, harp seals are able to inflate their lungs even after they collapse due to a pulmonary surfactant system that they have evolved to have. The surfactant acts as an  anti-adhesive during lung collapse and the reopening of alveoli.

 Upside down harp seal 

When migrating, harp seals can spend long periods of time at sea, without ever stepping foot on land. During this time it is not abnormal for a harp seal to sleep while in the water. Scientists have found them sleeping for mere minutes at a time, belly up.
The eyes of harp seals are well adapted to seeing both above and below the surface of the water. Because sight is extremely important to the seals for navigation, their eyes have evolved to be proportionally large, and contains a large spherical lens that helps them focus in on things much more easily.
wet harp seal pupInterestingly enough, in order to deal with the intense glare of arctic ice harp seals have evolved to have a pupil that is mobile. Similar to that of a camel, harp seals also have a membrane that covers the eyes to protect them from the harsh marine waters.
The bodies of harp seals are designed to be good for swimming. Unlike a lot of mammals, the mammary glands and testicles are found under the skins surface to minimize the resistance in the water. It takes far less energy to move without the extra resistance. Flippers are able to easily move their bodies straight through the water. Because water is much more dense than air, the flippers of a harp seal can be much shorter than, for example, the wings of a bird like a penguin.                                

Regulating body temperatures

Harp seals are very good at conserving their body heat. With a thick layer of blubber under their skin, harp seals are able to hold their body heat more easily. This layer of fat also provides a means of buoyancy, stores energy, and gives the seals a shape that is better suited for the aquatic environment where they are often found. In young harp seal pups you can find fur on the surface of their skin in order to keep their small bodies warm.
 Half Leather Half Fur
Another interesting way that the seals keep warm has to do with the same redirect of blood flow that allows them to remain submerged longer. They lower their heart rate by 90%; supplying only the nervous system and sense organs with a normal flow of blood. By redirecting the circulation away from the surfaces, they are preventing a substantial amount of heat loss.    

Skin & Molting

Harp seals are well known for having a fuzzy white coat as a pup. The fur of a pup is well adapted to the lifestyle of harp seals. There is a water repellant layer on the outside to help lessen resistance in water; A Harp Seal's Life Cyclehowever there is still a warm insulating layer underneath that to help with thermoregulation. It was for this very reason that the harp seals were almost hunted to extinction. Hunters found the fur of these creatures to be very valuable. 

Molting for harp seals happens once a year. During the molting period the seals get rid of their worn out skin and replace it with a new one. Most often during this time, harp seals remain on land; when replacing their skin there is a compromise to how well they can retain body heat. So as not to risk hypothermia, harp seals wait for their skin to be ready for the frigid waters of the arctic.

Harp Seal Digestion
Harp seals, being a carnivorous creature, have a relatively short digestive tract in comparison with herbivorous animals. This is due to the fact that a carnivorous diet supplies its consumer with high quality nutrients. Unlike an herbivorous diet that requires far more digestion to get the same amount of nutrients. Because the quality of their diet is so high, harp seals do not need the excess intestine to soak up the nutrients as an herbivore would need. Weirdly enough though, harp seals do have a longer intestinal tract than an animal with a comparable diet, the minke whale. This could be due to the fact that harp seals have to work harder than the whales to get to the locations where their prey are located. Whales are creatures of the sea and only the sea; whereas seals tire and need to be on land from time to time. With the excess intestine seals can receive more nutrients than a whale would eating the same thing, prolonging the time till they need to return to the water to feed again.

To learn about the harp seals reproduction techniques, follow this link!