In animals especially vertebrates, the blood is a specialized body fluid containing among other components blood cells that are found in plasma. The blood cells are mainly the red blood cells and the white blood cells but the most abundant cells are the red blood cells bearing, which carry the haemoglobin; a ferrous protein molecule important in the transportation of respiratory gases. The haemoglobin binds to gases in a reversible manner, which enhances their (gases) solubility in the blood. At the end of the respiration process, cells give out carbon dioxide and take in oxygen because oxygen is essential in cells’ functions especially in the process of making Adenosine Triphosphate (ATP).
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All the body cells need oxygen for respiration and give out carbon dioxide as a by-product. Respiration being a continuous process for life sustenance, oxygen supply and carbon dioxide removal should remain constant. The blood facilitates the constant flow of these respiratory gases.
The mechanisms of respiratory gases transportation through blood
The atmospheric air contains several types of gases, but of importance in respiration are oxygen and carbon dioxide. Before the oxygen is available for transportation to the respiring cells, it enters the lungs from the external environment through the inhalation process. On the other hand, carbon dioxide expelled exits the lungs through the exhalation process. During inhalation, air goes through the nostrils first, then into the nasal cavity for filtering, warming and humidification. The air then goes through the pharynx, larynx, trachea, bronchus and bronchiole into the alveolus of the lungs. The alveolus has a thin wall, which enables gas exchange with the equally thin-walled blood capillaries. For proper functioning of the alveolus, they remain moist due to a chemical called surfactant excreted by special cells of the alveolus. The close proximity of the epithelial cells of the alveolus and the endothelial cells of the capillaries allows the permeability of gases which results in gaseous exchange whereby, oxygen permeates the blood capillaries of the alveolus against a concentration gradient while carbon dioxide permeates into the alveolus in the same mode. Oxygen is now ready for transportation.
Red blood cells in the capillary blood contain haemoglobin, which binds the oxygen molecules permeating from the alveolus. Haemoglobin has high oxygen-binding capacity thus upon binding, it increases the blood oxygen solubility. The bound oxygen finally gets to the respiring cells as oxy-haemoglobin molecules through the circulation system, where it dissociates upon reaching the target cells. Oxygenated blood contains oxy-haemoglobin molecules.
As blood streams through the capillaries, carbon dioxide moves from tissues into the blood against a concentration gradient. Blood plasma carries carbon dioxide in the equilibrium of bicarbonates and carbonic acid where carbonic acid quickly changes into bicarbonate ions. Upon reaching the walls of the alveolus, the bicarbonate ions diffuse as carbon dioxide excrete. The carbon dioxide gas collected in the alveoli of the lungs then exits to the environment through the exhalation process.
Transportation of the respiratory gases would not be possible without the concerted efforts of both the respiratory system, which avails the respiratory gases for transportation and the circulatory system, which provides the components of the actual transportation. Blood from the circulatory system plays a major role as the transportation medium. The presence of red cells with haemoglobin in blood makes it best suitable for the transportation of oxygen and carbon dioxide in the body. Moreover, the formation of easily dissociable oxy-haemoglobin molecules, helps in releasing the oxygen molecules upon reaching the target cells.