Seawater is has a density eight hundred times higher than the density of air. This means that the resistance of water is also eight hundred times greater. Thanks to their streamlined bodies, without protruding limbs such as ears, shoulders, a long tail or genitals, marine mammals, including seals, are able to move through the water without much effort. The streamlined body is perfected by a thick layer of blubber. It covers all the body parts that would otherwise be sticking out. This is how seals are able to reach speeds of over 30 km per hour in the water.
Seals have to breathe regularly like other mammals to release carbon dioxide and to inhale oxygen. In order to stay under water longer, marine mammals (such as seals) have adjusted to particular laws of nature.
Firstly, they store more oxygen (seals have almost twice the amount of blood that humans have). Seals are able to store a much larger amount of oxygen in their blood, because their blood is higher in red blood cells and has a high concentration of haemoglobin (a molecule in red cells that holds oxygen). For instance, a litre of Weddell seal blood holds five times more oxygen than a litre of human blood. The blood of a common seal holds about two and a half times more oxygen than the blood of a human. Also, seals have a lot of myoglobin in their muscles, which is a molecule that holds oxygen in the muscles. This molecule turns seals’ muscles into a deep red, almost black colour, whereas the muscles of land mammals are light-red.
Secondly, seals use oxygen sparingly when they are under water. Only the heart, brains and vital organs are supplied with oxygen. Their body temperature and metabolism drop drastically and their heart rate slows down to a tenth of its normal pace. Through these adjustments a common seal is able to stay under water for up to 5 or 6 minutes at a time whilst hunting for food. If necessary, they can stay under water for 30 minutes without coming up for air.
How does a seal dive?
