Marine mammals like sperm whales and harbor porpoises must dive in order to forage for their prey. They often eat schooling fish and giant squid. In order to dive great depths, marine mammals often have highly efficient vascular systems, the ability to alter their heart rhythm, and the capacity to constrict their blood vessels. Bradycardia is a significant decrease in heart rate that occurs when marine mammals dive. Highly efficient vascular systems allow marine mammals to store more oxygen. This is evident in the darkened muscles and spleen indicating more densely packed red blood cells and hemoglobin. As a last resort, ischemia and peripheral vasoconstriction can occur during the dive response. This allows for blood to be delivered to the heart, lungs, and brain, however, it restricts blood flow to the liver, gut, kidneys, and rest of the body. The anaerobic metabolism that must occur in the organs with restricted blood flow has detrimental effects. It causes an increased recovery time for the mammal at the surface which makes them more susceptible to predation.
Posted by 
Marine mammals like sperm whales and harbor porpoises must dive in order to forage for their prey. They often eat schooling fish and giant squid. In order to dive great depths, marine mammals often have highly efficient vascular systems, the ability to alter their heart rhythm, and the capacity to constrict their blood vessels. Bradycardia is a significant decrease in heart rate that occurs when marine mammals dive. Highly efficient vascular systems allow marine mammals to store more oxygen. This is evident in the darkened muscles and spleen indicating more densely packed red blood cells and hemoglobin. As a last resort, ischemia and peripheral vasoconstriction can occur during the dive response. This allows for blood to be delivered to the heart, lungs, and brain, however, it restricts blood flow to the liver, gut, kidneys, and rest of the body. The anaerobic metabolism that must occur in the organs with restricted blood flow has detrimental effects. It causes an increased recovery time for the mammal at the surface which makes them more susceptible to predation.