The Respiratory System

1. Adaptations of the Respiratory System
   1. The Blowhole
   2. The Lungs
   3. Transportation of Oxygen
   4. Utilization of Oxygen
2. Preventing the Bends

Dolphins, like other mammals, need air to survive, especially oxygen. Oxygen is one of the main sources of energy in the body and every living thing cannot do without it. Unlike fish, dolphins have to rise to the surface frequently to breathe. When they are underwater, they hold their breath; when they are out of breath, they return to the surface to take in more fresh air.

Unlike other mammals who breathe through their nostrils and mouth, dolphins breathe through the blowhole, which is situated on the top on its head. A reason for this difference is that the blowhole will facilitate the breathing at the surface of the water. Since the blowhole is at the top of the head, only a small region of the head is required to break the surface of the water to inhale air.

The dolphin starts to exhale before reaching the surface and this helps to reduce the amount of time spent breathing at the surface. Dolphins can catch a breath about five times in a minute before diving again, without hindering the progress of their swim. Usually, a dolphin breathes two to four times each minute when it is swimming near the surface. It can hold its breath for seven minutes or more when it is diving.

The lungs of dolphins are not significantly larger or smaller than the land mammals. Obviously, the size of the lungs does not determine the amount of oxygen that can be stored and utilized.

However, the dolphin lungs contain a lot more alveoli (air cells) than human lungs do. Dolphin lungs are made up of two layers of capillaries, and this arrangement increases the efficiency of gas exchange since most mammals have only one layer of capillary. Therefore, this means that the surface area of the lungs have been greatly increased and gas exchange can occur more quickly.

The pleurae of dolphins are thick and elastic. The pulmonary tissue proper contains a generous supply of myoelastic fibers for better elasticity. The bronchial tubes are lined with muscular tissue. Tiny bronchioles are found together with sphincters that cut off the alveoli from the rest of the lung.

These anatomical mechanisms attribute to a more efficient exchange of gas. In dolphins, residual air - the fixed volume of air that always remain inside the lungs - never exceeds 15 percent of total capacity, and their vital capacity - the volume of air that is exchanged frequently - is over 85 percent. In some cases, the vital capacity can even reach 92 percent. Normally in human, only 10 to 20 percent of the air in the lungs are exchanged. But in dolphins, about 80 to 90 percent is renewed, so that their body can get as much oxygen as possible.

The respiratory system of whales certainly has some unusual features, but they are adaptations to prevent water entering the airways: the nasal passages are complex and convoluted, and the larynx (the upper end of the respiratory tube) extends up into the nasal cavity rather than opening into the throat. Powerful muscles form a special plug within the blowhole, preventing water from entering the lungs when the dolphin is underwater.

As mentioned in the earlier section on lungs, the size of the lungs is not the main reason for the dolphin's diving prowess. Even if the lungs are totally filled with air, it will not enable the dolphin to sustain dives as long as fifteen minutes. The main reason why dolphins can hold their breath so long is because of the adaptations made in the circulatory system. These adaptations have greatly increased their capacity to store oxygen and improved the efficiency of oxygen usage.

One adaptation is the amount of oxygen that dolphin blood and muscles can contain. Dolphins draw most of its oxygen from the blood and muscles.

The red blood cells, the main medium of oxygen transportation, are more abundant in dolphins than in humans. Red blood cells, per unit volume of blood is found to be greater. Haemoglobin concentration in the red blood cell is higher too, thereby increasing the tendency of the red blood cells to combine with oxygen and also increasing the amount of oxygen that the blood can carry significantly.

Haemoglobin is also found in the muscular tissues where it is called myoglobin. Myoglobin has a greater affinity for oxygen than blood haemoglobin, and so oxygen in the blood is readily given up to the muscles when the blood passes through. Myoglobin gives the muscles its characteristic dark color and makes muscles one of the main oxygen-storing tissues.

Theoretically, the oxygen that the dolphin takes in is insufficient for it to sustain long dives. Therefore, there must be other mechanisms that enable the dolphin to use oxygen efficiently.

To make full use of the limited oxygen, one of the ways is to provide oxygen only to those organs that need it most urgently. During dives, distribution of blood flow is adjusted such that more blood is supplied to the vital organs such as the brain and the heart. Inessential circulation is reduced, or may be shut down totally. Organs such that the stomach, intestines, kidneys and muscles receive very little blood, or in some cases, no blood flow at all.

When these inessential areas do not get enough oxygen from the blood, they will stop respirating normally. Instead, they will switch to anaerobic respiration, a form of metabolism that does not require oxygen. It allows cells to be put on hold for some time without oxygen. However, anaerobic metabolism is not as efficient as normal metabolism because it not only produces less energy, it is also a slow process. Furthermore, lactic acid, a byproduct from anaerobic metabolism causes fatigue. An 'oxygen debt' is occurred as well, and could only be settled when it returns to the surface to breathe. A period of time is also required for the dolphin after the dives to break down the lactic acid into a harmless substance. Dolphins can tolerate a high level of lactic acid, compared to some terrestrial mammals.

Another way to use their oxygen supply parsimoniously, is to reduce the heat rate of the dolpins. This can be done by a reflex action of the pituitary gland and the hypothalamus to stimulate the heart to beat slower. This condition is known as bradycardia.

The slower heart rate will restrict the blood flow of the veins to the heart, thereby preventing the heart from sending oxygen-poor blood to the lungs which will cause pressure to build up in the lung. The reduced blood flow will preserve the oxygen supplies in the lungs and blood. After a certain amount of time underwater, the right side of the heart may nearly stop beating; since by sending almost no venous blood to the lungs, more oxygen is saved.

At the same time, the left side of the heart pumps the little reoxygenated blood it receives directly to the brain. The blood will in turn empties its venous blood into specialised "chambers" located primarily in the liver and in a huge dilatation of the vena cava, below the diaphragm.

In dolphins, selective circulation to the brain is made possible by tangled reservoirs of blood known as the retia mirabilia. These massive plexuses consisting of tiny contorted blood vessels, are located mostly beneath the pleura, between the ribs, and on either side of the spine. Retia mirabilia helps to keep alive those areas that would otherwise deteriorate in the presence of venous blood.

The bends, or otherwise known as caisson disease, is the results of the formation of bubbles in the bloodstream and in other tissues when the human divers ascend to the surface too quickly. When human divers venture deep into the sea, the increasing pressure causes nitrogen in the lungs to be dissolved into the blood. The longer their dives and the more frequent they make their dives, more nitrogen will be absorbed into their bloodstream. Nitrogen is harmless when it is dissolved in the blood. The real problem occurs when the divers ascend to the surface.

If the divers surface too quickly, pressure will decrease greatly and suddenly. Due to this decrease in pressure, nitrogen will start to come out of the blood stream. However, if nitrogen comes out too quickly than the lungs can get rid of it, it will form small bubbles in the blood vessels and tissues. These nitrogen bubbles can be lethal because it causes severe pain, and can even cause paralysis and death. Therefore, it is necessary for human divers to take the precaution to prevent the bends. To do so, they must make their ascent slowly, making the appropriate decompression stops, so that nitrogen and other inert gases can be voided gradually through the lungs.

Dolphins, unlike human divers, do not suffer the bends or other decompression sickness even though they dive deeper and more frequent. They suffer no ill effects even if they come to the surface to breathe immediately after diving a considerable depth. There are a few reasons why dolphins can prevent the bends so easily.

One reason is that dolphins do not breathe highly compressed air as they descend. In fact, they hold their breath when they dive. In this way, there will be relatively little amount of nitrogen that can enter their bloodstream, unlike human divers who breathe nitrogen gas constantly when they are underwater.

Some physiologists have found out that nitrogen is highly soluble in the dolphin's blood. Nitrogen-rich blood, when left alone, is also found to restore only a small amount of gas dissolved in it. The dissolved nitrogen in the blood is carried to the large number of nasal sacs and sinuses that act as nitrogen traps that accumulates dissolved nitrogen. These traps release nitrogen gradually as the animal surfaces.

The hydrostatic pressure increases one atmosphere for every 10 metres increase in depth as the dolphin dives. With this increase of pressure, the lungs collapse. In most dolphins, the lungs collapse almost completely when they go deeper than 100 metres. Much of the air in the lungs is forced into nasal passages and air sacs. Since the blood vessels in these passages are fewer and much further away from the surface of the skin, the amount of nitrogen that can be absorbed into the bloodstream is minimised.

Another potentially dangerous condition, called nitrogen narcosis, can also occur if the human divers venture deep underwater for too long. Nitrogen narcosis occurs when nitrogen in the blood bonds to fatty materials, especially the myelin that sheathes nerve fibers, causing similar effects to those caused by having too much alcohol in the bloodstream such as loss of neuromuscular coordination and loss of consciousness. Induced by breathing nitrogen under pressure, it does not affect dolphins because they breathe only at the surface.

Bibliography

Carwardine, Mark. The Book of Dolphins. Dragon's World Ltd, 1996

Cousteau, Jacques. Whales. New York: Harry N. Abrams, Inc, Publishers, 1988.

Harrison, Sir Richard, et. al. Whales, Dolphins and Porpoises. New York: Facts on File, Inc., 1994.