- Written by Nadia O'Carroll
Breathing is part of the process of respiration, we require oxygen to create energy and we produce carbon dioxide (CO2) as a waste product, so basically with every breathe we must renew our supply of oxygen and remove our waste of carbon dioxide. Breathing is controlled, consciously, as in breath holding, and unconsciously, as in normal breathing, and we have specialised centres in the brain, which control this function.
When we hold our breath, the level of CO2 in our blood increases, more carbonic and lactic acid is produced, and this changes the pH of our bloodstream. The sudden drop in pH stimulates chemical receptors in the blood system, which in turn sends nerve impulses to the respiration centre in the medulla oblongata and pons in the brain, which in turn sends impulses through the phrenic and thoracic nerves to the diaphragm. It is this acidic change in the bloodstream’s chemistry due build up of CO2, which elicits that feeling of desperate need for breath. The same process makes us breathe more quickly when we exercise, because the aerobic activity of our muscles produce more CO2 to create the energy that moves our body, and this causes a drop in blood pH.
There are many other mammals, which have adapted to aquatic life, eg seals, whales, dolphins, otters, beavers and dugongs. They like us, are all still breath-hold divers, who depend on breathing air, yet they are able to dive to great depths and remain active and submerged for a considerable time. Why they are able to do this is not totally understood, but it is not increased lung capacity, in fact the lung volume of many aquatic animals is not particularly great. It is other physiological adaptations such as circulation, acid tolerance, muscle myoglobin levels that play a key role.
Even we humans can weakly experience some of these adaptations ourselves in a self-preservation technique called mammalian diving reflex (actually this reflex also applies to birds and reptiles). If the face of a reptile, bird or mammal is submerged in water colder than 70 degrees F, the receptors in the nasal cavity trigger a response in the brain and nervous system which causes 3 main responses. It slows the heart; it slows then stops blood circulation to the extremities so that the heart brain circuit is preserved, thirdly at depth it causes a blood shift, which allows plasma and water to pass through the thoracic cavity, to protect from the effects of pressure. Consequently, even land dwellers such as humans; can survive longer underwater without oxygen, than they can on land, whether they are conscious or unconscious.