Introduction:
Aquatic mammals, such as dolphins, whales, and seals, do not have gill slits like fish or other marine animals. This is due to their evolutionary adaptation to a semi-aquatic lifestyle, which involves breathing both air and extracting oxygen from the water. Instead of gill slits, these mammals have specialized respiratory systems that enable them to survive in aquatic environments.

Specialized Respiratory Systems:
Aquatic mammals have evolved various adaptations to breathe efficiently in both air and water. Their respiratory systems consist of the following key features:

1. Lungs: Like terrestrial mammals, aquatic mammals have lungs to breathe air. They possess large and highly efficient lungs that allow them to exchange gases with the atmosphere when they come to the surface.

2. Breathing Mechanism: To breathe, aquatic mammals must come to the water's surface. They have a blowhole located on top of their heads, which acts as a specialized nostril. When they surface, they exhale forcefully, expelling stale air from their lungs, and inhale fresh air quickly.

3. Increased Oxygen Storage: Aquatic mammals have a higher concentration of red blood cells and myoglobin, a protein that stores oxygen, in their blood. These adaptations enable them to store more oxygen and enhance their diving abilities.

4. Diving Reflex: When aquatic mammals submerge, their bodies trigger a series of physiological changes known as the diving reflex. This reflex reduces the need for oxygen, as the mammal's heart rate slows down, blood circulation is redirected to essential organs, and oxygen consumption decreases.

5. Oxygen Extraction: Aquatic mammals have a higher affinity for oxygen compared to terrestrial mammals. Their blood cells are designed to extract a higher percentage of oxygen from the air they breathe, allowing them to make the most of each breath.

Conclusion:
Aquatic mammals, such as dolphins, whales, and seals, have evolved specialized respiratory systems to survive in both air and water. While they lack gill slits, their efficient lungs, blowholes, increased oxygen storage, diving reflex, and enhanced oxygen extraction capabilities enable them to thrive in their aquatic habitats. These adaptations highlight the remarkable evolutionary journey of these marine mammals.

to get the same amount of oxygen u need to pass roughly30 - 40 times as much water through gills than air through lungs........ So they doesn't have gill slits