Gas Exchange Essay Example

of gas to be exchanged as do insects with the internal branching’s of tracheoles increasing their surface area.

The gas exchange system found in insects is more complex than that of the single celled-organism but in its own right is a straight forward process. They have a unique tracheal system that transports oxygen to their respiring cells. The tracheal system consists of spiracles found on their exoskeleton which can open or close to control the level of ventilation or loss of fluid. Leading from the spiracles are tracheae which branch out multiple times to form openings called tracheloes. The smallest tracheoles then diffuse oxygen into every actively respiring cell.

Although this system is essentially the same in all insects the requirements differ depending on the insect. Small insects can meet their requirement of oxygen by diffusion alone but some larger and more active insects have developed ventilation mechanisms. These mechanisms pump air in and out of their bodies by muscular contractions of their abdomen. These contractions expel air from the tracheae and air sacs, and passive inspiration automatically follows. This action is similar to the diaphragm in humans that contracts and relaxes to pump oxygen into and out of the lungs.

The disadvantages to the system used by insects are that the tracheal system means that there is a very high metabolic rate so the insect must remain moving in order to meet their requirements. This opens the way to rapid and sustained movements needed when flying but imposes firm limitations on their size. Fish have a much more efficient gas exchange system than insects and even mammals. This is due to

their habitat of water that consists of less oxygen than air and is a lot denser. The system consists of gills which are out-folding’s of the epithelium suspended in the water.

They are considered internal as they are covered by the operculum. The gills are composed of gill arches that contain the blood vessels. From these branch two rows of filament’s which are covered in lamellae. This structure provides the fish with an extremely large surface area for gas exchange. In fish there are two gas exchange mechanisms, the first of which is the parallel flow mechanism found in cartilaginous fish such as Dogfish. In this process the oxygenated water and the deoxygenated blood flow in the same direction. As the water flows in through the gills the blood is flowing through the gill lamellae in the same direction.

The second mechanism used by most fish is the counter current mechanism found in bony fish such as Cod. In this process the oxygenated water flows in the opposite direction to the deoxygenated blood flowing through the gill lamellae. This mechanism is more efficient at extracting oxygen than the parallel flow mechanism. It extracts around 80% of the available oxygen and allows for the highest percentage of blood oxygen saturation possible. In the parallel mechanism the extraction of blood is around 50% because the diffusion process stops once the concentration gradient reaches equilibrium.

This is due to the fact the water flows in the same direction so the water will not meet any deoxygenated blood. Whereas in the counter current mechanism equilibrium will not be reached as the blood will always come into

contact with water that has a high oxygen concentration. Fish and humans gas exchange systems share the same purpose, but the differing efforts for respiration and contrasting environments mean they had to evolve highly complex systems to extract the oxygen needed for survival compared to that of a single-celled organism.

The system used by fish is far more efficient than that of humans which only extracts about 25% of the oxygen inhaled. The gas exchange systems found in the dicotyledonous plant leaf are very different to those found in any other organism. The structure of a dicotyledonous plant leaf consists of a dense layer of parenchyma cells that are rich in chloroplasts and many supporting tissues that allow this thin layer of cells to perform photosynthesis efficiently. This dense layer of cells is called the palisade mesophyll.

The cells are stacked tightly together with their long axes perpendicular to the top of surface of the leaf. Each one is directly exposed to light at its top and the bottom end is in contact with a loosely packed layer of spongy mesophyll tissue with numerous air spaces between cells. This airy tissue allows for rapid diffusion of atmospheric gases between the leaf and its environment. Unlike animals, plants have no specialized organs for gas exchange but they do have adaptations that enable them to exchange gasses sufficiently.

One is the stomata found on the epidermis of the plant leaf this allows for free unhindered diffusion of carbon dioxide into the leaf. These are each surrounded by two guard cells that regulate the intake of carbon dioxide and the loss of liquid and oxygen.

The conditions within the environment can affect the opening of the stomata if there are drought conditions gas exchange can be affected because the lack of water moving through the plant causes the guard cells to close. Also when the temperature becomes too warm, stomata tend to close.

In some cases the increase in temperature causes an increase in cellular respiration that, in turn, increases carbon dioxide levels. Internal high carbon dioxide concentrations both reverse the carbon dioxide pressure gradient and cause the stomata to close. One of the complicated gas exchange systems it that of humans, in some ways it is similar to that of fish but due to the differing environments humans have developed a very different process in which to exchange gasses with their environment.

Humans have lungs that are a repeatedly branching system of tubes contained within the thoracic cavity. These tubes begin with the trachea that extends down from the nasal passage and mouth cavity where gases are taken in and expelled. Then the trachea splits into bronchi that extend into the left and right lungs and then branch again multiple times forming bronchioles. The system finishes in the alveoli at the end of the bronchioles which are the location of gas exchange. They provide a large surface area through which gases can diffuse.

These gases diffuse a very short distance between the alveolus and the blood because they are only one cell thick and are surrounded by capillaries that are also only one cell thick. There are two stages to breathing in humans the first being inspiration. This process begins with the intercostal muscles contracting and

pulling the ribcage upwards and outwards from the spinal column. At the same time the diaphragm contracts and flattens pushing down on the abdominal organs. These movements increase the volume so will lower the pressure in the thorax.

As this pressure falls below that of the atmosphere air is forced into the lungs to equalize the pressure. When inspiration is complete expiration begins which is a passive process. The muscles in the thorax relax and breathing out follows due to a combination of gravity, the elastic recoil of the connective tissues of the lungs and the pressure exerted by the abdominal organs. We can also consciously force air out of the lungs using our intercostal muscles. Oxygen is carried from the alveoli in the blood by hemoglobin, a conjugated protein that combines with oxygen where it is abundant and then releases it when the concentration falls.

At the alveoli the oxygen molecule enters the capillaries and combines loosely with the heam of hemoglobin. When pressure of oxygen is high, as in the pulmonary capillaries, oxygen binds with the hemoglobin. Once the blood reaches the respiring tissue the pressure of oxygen is lower so the hemoglobin will release the oxygen. An example is muscle human muscle that contains myoglobin that has a higher affinity for oxygen than hemoglobin so can take oxygen from hemoglobin easily. The level at which haemoglobin is adapted to its environment is incredible.

The structure changes depending on the animal’s environment. For high altitude animals, such as llamas the haemoglobin releases oxygen at lower partial pressures than human haemoglobin, so it is easier to cope with the low levels

of oxygen. Crocodile haemoglobin is sensitive to bicarbonate ions, which build up in the blood as the crocodile goes underwater. This causes the haemoglobin to bind tighter to oxygen and only release them at lower partial pressures, effectively stretching out the oxygen supply.