The nasal and buccal (mouth) cavities are separated from each other, see figure three. Although a human can breathe through both cavities, it is more efficient to breathe through the nose as the air is filtered effectively there. Air is warmed and moistened in the nasal cavity. This cavity is lined with small hairs to filter the inhaled air from dust and other small particles. Waugh A. and Grant A. (2001, pg. 241) state that, "The nose is lined with very vascular ciliated columnar epithelium (ciliated mucous membrane), which contains mucus-secreting goblet cells". This helps to keep the cavity moist, trapping any further small airborne particles and also helps to prevent infection by trapping any microbes from the air, see figure four. The mucus and trapped particles are swept to the throat to be swallowed by tiny hair-like structures called cilia.
The filtered air passes through the pharynx and larynx, reaching the trachea. Indge B. et al (2000, pg. 75) explain that “To prevent food or liquid from entering the trachea during swallowing, a flap of tissue, called the epiglottis, temporarily seals off the larynx”, which is the entrance to the trachea. The inner surface of the trachea is lined with ciliated columnar epithelium as with the nasal cavity, the outer wall contains C-shaped cartilage and fibrous and elastic tissue for strength and to keep the passageway open at all times.
The trachea divides off into the left and right bronchi, similar in structure to the trachea, only smaller. These bronchi then divide further into tubes called bronchioles, “contain no cartilage and are held open by the elasticity of the surrounding tissue” (Indge B. et al 2000, pg. 75). The bronchiole tubes branch off several times, the last branched bronchiole tubes are known as terminal bronchioles. Situated at the end of these are Blind-ended air sacs called alveoli, see figure five.
Adult human lungs are said to contain approximately seven hundred and fifty million alveoli. Encyclopaedia Britannica (2002) state that, “The alveoli form clusters, called alveolar sacs, that resemble bunches of grapes”. These clusters enable a large surface area in relation to volume ratio to be achieved. The average surface area in the adult human lung alveoli is around eighty metres squared.
Each alveolus is surrounded by a dense capillary bed, see figure six. Gas exchange occurs between the alveolar sacs and the blood capillaries. Haemoglobin is an oxygen-transporting pigment found within red blood cells. O2 binds to haemoglobin, known as oxyhaemoglobin, and is transported to the body cells. CO2 also binds to haemoglobin, known as carbominohaemoglobin, and is transported back to the lungs to be expelled from the body. As CO2 is much more soluble in fluid than O2, it can also be transported within the blood plasma. The heart controls this transportation, using vessels called arteries and veins, which pump the blood to the lungs to collect O2 and dispose of CO2 and then the blood returns to the heart where it is pumped around the body, see figure seven.
Fick’s law, shown below, can calculate the rate of diffusion.
Ficks Law
Rate of diffusion = Surface are X difference in concentration
Thickness of membrane
(Indge B. et al 2000, pg. 77)
For a fast rate of diffusion, the surface area and difference in concentration have to be as large as possible, also the thickness of membrane, i.e. distance to travel, has to be as small as possible.
A large surface area for diffusion is obtained in humans by the alveoli in the lungs. The structure of alveoli enables them to have a large surface area in relation to volume ratio, see figure eight.
A factor affecting gas exchange with regards to surface area is a condition called emphysema, which is caused by smoking, also from coughing (creating pressure) and a congenital deficiency of elastic tissue in the lungs. It is where "the walls between adjacent alveoli break down" (Waugh A. and Grant A. 2001, pg. 261), reducing the surface area in relation to volume ratio, see figure nine. As a consequence, the rate of diffusion is greatly decreased and the person will experience extremely laboured breathing.
The difference in concentration of O2 and CO2 is maintained by the blood within the capillary beds constantly moving, so as soon as O2 has diffused from the alveolus to the blood, the blood moves onwards and more deoxygenated blood becomes present as a continuous process. The blood is however moving quite slowly in these capillary beds, in order to ensure that as much gas exchange as possible occurs.
The alveolar and capillary walls are both made up of a single layer of epithelial cells. This contributes to a fast rate of diffusion, as the membranes involved are as thin as they possibly could be so that diffusing O2 and CO2 do not have far to travel, see figure ten.
The size of red blood cells that carry O2 and CO2 compared to the lumen size of capillaries also enable a small diffusion path. The lumen is just wide enough for red blood cells to comfortably pass through.
Gases diffuse down a partial pressure gradient. The theory to explain this is Dalton's Law where “each gas in a mixture of gases exerts its own pressure as if all the other gases were not present” (Tortora G. and Grabowski S. 2000, pg. 796). The total pressure exerted is the sum of the pressures exerted independently by each gas in the mixture. Atmospheric gases diffuse down a partial pressure gradient, see figure eleven.
Atmospheric air is a mixture of O2, CO2, nitrogen, water vapour and some inert gases. Atmospheric pressure is approximately 760 mm Hg at sea level. Marieb E. (1998, pg. 822) explains that atmospheric pressure falls at high altitudes and all partial pressure values decrease in direct proportion to the atmospheric pressure. Below sea level the atmospheric pressure increases, so the partial pressure of gases increases as a result of this, see figure twelve.
The solubility of the gases exerts an important influence on gas exchange. Tortora G. and Grabowski S. (2000, pg. 797) states that "The quantity of a gas that will dissolve in a liquid is proportional to the partial pressure of the gas and its solubility coefficient". If the partial pressure in the gaseous phase (in the alveolus) is greater than in the fluid (blood plasma), gas will move into the fluid. If it is less, gas will move out of the fluid until equilibrium is reached.
The various gases present in air have very different solubilities in water (or plasma). CO2 is the most soluble, being approximately twenty four times more soluble than O2. This is why CO2 can also dissolve in plasma to be transported as well as in haemoglobin. “Nitrogen, which is only half as soluble as oxygen, is nearly insoluble” (Marieb E. 1998, pg. 822), and therefore has no known effect on the body at sea level.
As altitude affects partial pressures and the solubility of gases, there are conditions that will affect gas exchange in humans. At high altitudes, the partial pressure of gases decreases and so O2 diffuses into the blood more slowly. This causes altitude sickness, “shortness of breath, headache, fatigue, insomnia, nausea, dizziness, and disorientation…due to the low O2 content of the blood” (Tortora G. and Grabowski S. 2000, pg. 798).
At low altitudes, as the atmospheric pressure is high, the solubility of gases increases along with their partial pressures. This means that the normally non-harmful nitrogen can cause a threat to deep-sea divers for example. So much nitrogen is forced into the blood that it causes a narcotic effect called nitrogen narcosis. Nitrogen is more soluble in lipids than water, so it tends to affect lipid-rich tissues like the central nervous system, bone marrow and fat deposits. “The diver becomes dizzy, giddy, and appears to be intoxicated” (Marieb E. 1998, pg. 830), which causes potential danger to the life of the diver under water. Any further danger can be avoided by a gradual ascend to the surface, so that the nitrogen has time to driven out of the tissues once again without causing any problems.
In conclusion to this essay, it is clear that both the respiratory and cardiovascular systems work together to create the best environment for gas exchange and the most efficient organs to do so. The air is conditioned throughout the respiratory system in preparation for contact with the lower respiratory tract.
The body is also adapted for as fast a rate of diffusion as possible. A large surface area in relation to volume ratio is gained by the structure of the alveoli, the difference in concentration is maintained by the constant flow of blood through the capillaries transporting the gases to and from the alveoli, and the distances for the gases to diffuse across is at an absolute minimum, using single celled layers.
It is also clear that the human body is adapted for efficient gas exchange at sea level. Differing altitudes have an effect on the partial pressures and solubility of gases and so this affects gas exchange.
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