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In the human body, many system processes are occurring every second in a living person. Among these processes is the circulation of blood around the body. Blood is the main constituent in which nutrients, waste products, hormones, and gases are transported. This paper will discuss the composition of blood, transportation of oxygen and carbon dioxide as well as how gaseous exchange occurs between the blood and cells of the body.

‘Blood is a connective tissue. Like all connective tissues, it is made up of cellular elements and an extracellular matrix. The cellular elements referred to as the formed element include red blood cells (RBCs), white blood cells (WBCs), and cell fragments called platelets. The extracellular matrix, called plasma, makes blood unique among connective tissues because it is fluid. This fluid, which is mostly water, perpetually suspends the formed elements and enables them to circulate throughout the body within the cardiovascular system’ (Cnx.org, 2013).

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According to Cnx.org, (2013), to understand the mechanisms of gas exchange in the lung, it is important to understand the underlying principles of gases and their behavior. The greater the partial pressure of the gas, the greater the number of gas molecules that will dissolve in the liquid. The concentration of the gas in a liquid is also dependent on the solubility of the gas in the liquid. For example, although nitrogen is present in the atmosphere, very little nitrogen dissolves into the blood, because the solubility of nitrogen in blood is very low. Gas molecules establish equilibrium between those molecules dissolved in liquid and those in air.

However, there are two important aspects of gas exchange found in the lung. These are ventilation and perfusion. Ventilation is the movement of air into and out of the lungs, and perfusion is the flow of blood in the pulmonary capillaries. For gas, exchange to be efficient the volumes involved in ventilation and perfusion should be compatible. The partial pressure of oxygen in alveolar air is about 104 mm Hg, whereas the partial pressure of oxygenated blood in pulmonary veins is about 100 mm Hg (Cnx.org, 2013). When ventilation is sufficient, oxygen enters the alveoli at a high rate, and the partial pressure of oxygen in the alveoli remains high. In contrast, when ventilation is insufficient, the partial pressure of oxygen in the alveoli drops. Without the large difference in partial pressure between the alveoli and the blood, oxygen does not diffuse efficiently across the respiratory membrane.
The body has mechanisms that counteract this problem. In cases when ventilation is not sufficient for an alveolus, the body redirects blood flow to alveoli that are receiving sufficient ventilation. This is achieved by constricting the pulmonary arterioles that serves the dysfunctional alveolus, which redirects blood to other alveoli that have sufficient ventilation. At the same time, the pulmonary arterioles that serve alveoli receiving sufficient ventilation vasodilate, which brings in greater blood flow. Ventilation is regulated by the diameter of the airways, whereas perfusion is regulated by the diameter of the blood vessels. The diameter of the bronchioles is sensitive to the partial pressure of carbon dioxide in the alveoli. A greater partial pressure of carbon dioxide in the alveoli causes the bronchioles to increase their diameter as will a decreased level of oxygen in the blood supply, allowing carbon dioxide to be exhaled from the body at a greater rate. As mentioned above, a greater partial pressure of oxygen in the alveoli causes the pulmonary arterioles to dilate, increasing blood flow (Cnx.org, 2013).

According to Mackean (1995), gas exchange occurs at two sites in the body: in the lungs, where oxygen is picked up and carbon dioxide is released at the respiratory membrane, and at the tissues, where oxygen is released and carbon dioxide is picked up. Moreover, gas exchange occurs in two ways, that is external and internal respiration. External respiration is the exchange of gases with the external environment, and occurs in the alveoli of the lungs. Internal respiration is the exchange of gases with the internal environment, and occurs in the tissues. The actual exchange of gases occurs due to simple diffusion. Energy is not required to move oxygen or carbon dioxide across membranes. Instead, these gases follow pressure gradients that allow them to diffuse. The anatomy of the lung maximizes the diffusion of gases: The respiratory membrane is highly permeable to gases; the respiratory and blood capillary membranes are very thin; and there is a large surface area throughout the lungs. The pulmonary artery carries deoxygenated blood into the lungs from the heart, where it branches and eventually becomes the capillary network composed of pulmonary capillaries.
In external respiration, the pulmonary capillaries create the respiratory membrane with the alveoli. As the blood is pumped through this capillary network, gas exchange occurs. Although a small amount of the oxygen is able to dissolve directly into plasma from the alveoli, most of the oxygen is picked up by erythrocytes (red blood cells) and binds to a protein called haemoglobin, a process described later in this paper. Oxygenated haemoglobin is red, causing the overall appearance of bright red oxygenated blood, which returns to the heart through the pulmonary veins. Carbon dioxide is released in the opposite direction of oxygen, from the blood to the alveoli. Some of the carbon dioxide is returned in haemoglobin, but can also be dissolved in plasma or is present as a converted form.

Furthermore, in internal respiration gas exchange occurs at the level of body tissues. Similar to external respiration, internal respiration also occurs as simple diffusion due to a partial pressure gradient. This creates a pressure gradient that causes oxygen to dissociate from haemoglobin, diffuse out of the blood, cross the interstitial space, and enter the tissue. Haemoglobin that has little oxygen bound to it loses much of its brightness, so that blood returning to the heart is more burgundy in color. Considering that cellular respiration continuously produces carbon dioxide, the partial pressure of carbon dioxide is lower in the blood than it is in the tissue, causing carbon dioxide to diffuse out of the tissue, cross the interstitial fluid, and enter the blood. It is then carried back to the lungs either bound to haemoglobin, dissolved in plasma, or in a converted form. The blood is then pumped back to the lungs to be oxygenated once again during external respiration.

The other major activity in the lungs is the process of respiration, the process of gas exchange. The function of respiration is to provide oxygen for use by body cells during cellular respiration and to eliminate carbon dioxide, a waste product of cellular respiration, from the body. In order for the exchange of oxygen and carbon dioxide to occur, both gases must be transported between the external and internal respiration sites. Although carbon dioxide is more soluble than oxygen in blood, both gases require a specialized transport system for the majority of the gas molecules to be moved between the lungs and other tissues.

The majority of oxygen molecules are carried from the lungs to the body’s tissues by a specialized transport system, which relies on the red blood cell. Erythrocytes contain a metalloprotein, haemoglobin, which serves to bind oxygen molecules to the erythrocyte. Heme is the portion of haemoglobin that contains iron, and it is heme that binds oxygen. As oxygen diffuses across the respiratory membrane from the alveolus to the capillary, it also diffuses into the red blood cell and is bound by haemoglobin. The following reversible chemical reaction describes the production of the final product, oxyhaemoglobin (Hb–O2), which is formed when oxygen binds to haemoglobin. Oxyhaemoglobin is a bright red-colored molecule that contributes to the bright red color of oxygenated blood.

Hb + O2 ? Hb ? O2
In this formula, Hb represents reduced haemoglobin, that is, haemoglobin that does not have oxygen bound to it. This is because gases travel from an area of higher partial pressure to an area of lower partial pressure. In addition, the affinity of an oxygen molecule for heme increases as more oxygen molecules are bound. As a result, the partial pressure of oxygen plays a major role in determining the degree of binding of oxygen to heme at the site of the respiratory membrane, as well as the degree of dissociation of oxygen from heme at the site of body tissues (Dupree ; Dupree, 2014).
Carbon dioxide is transported by three major mechanisms. The first mechanism of carbon dioxide transport is by blood plasma, as some carbon dioxide molecules dissolve in the blood. The second mechanism is transport in the form of bicarbonate (HCO3–), which also dissolves in plasma. The third mechanism of carbon dioxide transport is similar to the transport of oxygen by erythrocytes. Although carbon dioxide is not considered to be highly soluble in blood, a small fraction—about 7 to 10 percent—of the carbon dioxide that diffuses into the blood from the tissues dissolves in plasma. The dissolved carbon dioxide then travels in the bloodstream and when the blood reaches the pulmonary capillaries, the dissolved carbon dioxide diffuses across the respiratory membrane into the alveoli, where it is then exhaled during pulmonary ventilation as shown in the diagram below (Clegg ; Mackean, 2000).

In conclusion, for oxygen and carbon dioxide to be transported, understanding the principle of gases and their behavior such as solubility and partial pressure is considered. Oxygen is transported mainly via blood while carbon dioxide is transported via blood, as a bicarbonate and by the red blood cells like oxygen. Furthermore, processes such as ventilation and perfusion aid in the transportation and exchange of these gases in the body. Although, for gas exchange to be efficient the volume involved in ventilation and perfusion should be compatible. Lastly, all of the above occurs due to simple diffusion across a respiratory membrane.

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