How Are Cells Supplied With Oxygen and Relieved of Carbon Dioxide - Coursework Example

The term respiration is normally employed to indicate and interchange of two gases, oxygen and carbon dioxide which are transported to and from the cells. An average adult absorbs and utilizes 250 ml of O2 and produces and eliminates about 200 ml of CO2 per minute.1 The lungs and heme proteins in the blood play a very important role in this exchange process. About 99% of the O2 dissolves in the blood and combines with the oxygen carrying protein hemoglobin and about 94.5% of CO2 which dissolves enters into a series of reversible chemical reactions that convert it into other compounds.2 Oxygen Transport O2 delivery to a tissue depends on a number of factors such as the amount of oxygen entering the lungs, the adequacy of pulmonary gas exchange, the blood flow to the tissue which depends on the degree of constriction of blood vessels in the tissue and cardiac output, andon the capacity of the blood to carry oxygen. The amount of oxygen in the blood is determined by the amount of dissolved oxygen, the amount of hemoglobin in the blood which is the primary oxygen carrying heme protein and the affinity of hemoglobin for oxygen. Apart from this the partial pressure of oxygen, which is the pressure exerted by oxygen on any given system also plays an important role in gas movement 2. Reaction of hemoglobin and oxygen. 1 L of whole blood contains 150g of hemoglobin.The primary function of hemoglobin is to transport oxygen from the tissues to the lungs. The hemoglobin protein is made up of 4 subunits, each of which contains a heme moiety attached to a polypeptide chain which is 4 in number, namely 2 α and 2 β chains in humans. Heme is a complex made of a porphyrin and 1 atom of ferrous ion. Each of the 4 iron atoms can bind reversibly to one oxygen molecule.2 Hemoglobin forms a dissociable complex with oxygen. Deoxyhemoglobin (Hb) + 4O2    Oxyhemoglobin (Hb.4O2) The above reaction goes to the right with an increase in oxygen pressure (as in the lungs) and to the left with a decrease in oxygen pressure (as in the tissues) 3. Affinity of hemoglobin to oxygen The quaternary structure of hemoglobin determines its affinity for oxygen. When hemoglobin takes up oxygen, the 2 β chains move close together; when oxygen is given up, they mover father apart. Change in the position of the heme moieties assume a relaxed or R state that favors oxygen binding or a tense, T state that decreases oxygen binding 2. The oxygen binding dissociation curve, which relates the percentage saturation of hemoglobin with the partial pressure of oxygen, PO2 , has a characteristic sigmoid shape. This is because hemoglobin shows cooperative oxygen binding kinetics. When hemoglobin takes up a small amount of oxygen, the R state is favored. When the first heme moiety in hemoglobin combines with oxygen it increase the affinity of the second heme for oxygen, and oxygenation of the second increases the affinity of the third and so on, so that the affinity of hemoglobin for the fourth oxygen is much than the first.2 Partial pressure of oxygen and its diffusion into tissues. Owing to the partial pressure of oxygen it flows downhill from the atmospheric air through the alveoli and blood into the tissues by the simple process of diffusion. Diffusion is defined as the movement of molecules across tissues or membranes from a region of higher concentration to a region of lower concentration in order to equalize the concentration on both sides of the tissue. The PO2 of ambient air is about 159 mmHg and that of the lung alveoli is about 104mmHg. This difference in partial pressure results in the entry of atmospheric air into the lung alveoli. However there is a residual volume of oxygen left in the alveoli as oxygen is not completely exhaled during the process of expiration. As the ambient air mixes with the residual air in the alveoli, its partial pressure decreases to 104mmHg, equal to that of the partial pressure of alveoli air. For oxygen to diffuse across the alveolar tissue into the blood, the partial pressure of blood must be lower than the alveoli. The blood in the alveolar capillary is about 40mmHg, as the pulmonary blood supply returning to the alveoli is deoxygenated due to the use of oxygen in the mitochondria. The lower PO2 of oxygen in the capillary results in diffusion of oxygen from the alveoli to the pulmonary capillaries thereby increasing the oxygen concentration in the blood to a maximum of 104mmHg 4. In vivo, the hemoglobin in the blood at the ends of the pulmonary capillaries is about 97.5% saturated with oxygen, while the remaining is in solution; the PO2 in the arterial blood is about 97mmHg. The arterial blood therefore contains a total of about 19.8 ml of oxygen per dl: 19.5 ml bound to hemoglobin and 0.29 ml in solution 2. The removal of oxygen from hemoglobin at the tissue site is called oxygen dissociation. The major factors that aid in the dissociation is explained as follows: As temperature rises, oxygen is removed more efficiently. During metabolism in tissues some of the energy produced is released as heat, which helps in oxygen dissociation to the tissues. When we exercise the heat produced is more due to increased metabolism resulting in increased uptake of oxygen. A decrease in pH (increase in acidity) in the tissues, increases the dissociation of oxygen. Again during exercise pH decreases causing improved dissociation of oxygen from hemoglobin. Another factor that affects the dissociation of oxygen is 2, 3 diphosphoglycerate, which is formed form 3-phosphoglyceraldehyde, which is a product of glycolysis. It is a highly charged anion that binds to the β chains of deoxyhemoglobin. Hence in its presence the deoxygenation of hemoglobin is favored. Also an increase in PCO2 is accompanied by a decrease in pH, thereby causing dissociation of oxygen from hemoglobin. Hence under increased CO2 concentration hemoglobin prefers CO2 to O2. This is also reffered to as the Bohr effect. 4, 2. In the venous blood at rest, the hemoglobin is 75% saturated and the total oxygen content is about 15.2 ml per dl: 15.1 ml bound to hemoglobin and 0.12 ml in solution. Thus at rest the tissues remove about 4.6 ml of blood from each dl of blood passing through them. In this way, 250ml of oxygen per minute are transported from the blood to the tissues at rest 2. Carbon dioxide transport The partial pressure of CO2 causes it to flow downhill from the tissues to the alveoli. The solubility of CO2 in blood is about 20 times than of O2. The PCO2 of arterial blood is about 40mmHg. The high amount of CO2 produced in the tissues as a result of metabolism diffuses into the red blood cells where it is rapidly hydrated to H2CO3, due to the presence of carbonic anhydrase. This complex inturn dissociates into H+ and HCO3- . This H+ is buffered by hemoglobin and the HCO3- enters the plasma. Deoxygenated hemoglobin binds more H+ than the oxygenated counterpart, resulting in a decline in the O2 saturation of hemoglobin as it passes through the tissues 2. The excess of HCO3- in the red blood cells is exchanged for Cl- by a process called the chloride shift, mediated by a Band 3 membrane protein. Some of the CO2 in the red cells reacts with the amino groups of proteins, principally hemoglobin, to form carbamino compounds. Hence CO2 is transported in the following different forms as in plasma it exists as dissolved, carbamino compounds with plasma proteins and hydrated, H+ buffered, HCO3- in plasma. In red blood cells it exists as dissolved, carbamino – Hb, hydrated, H+ buffered, 70%of HCO3- enters the plasma and Cl- shift into cells.Of the approximately 49 ml of CO2 in each dl of arterial blood, 2.6 ml are dissolved, 2.6 ml are in carbamino compounds, and 43.8 ml are in HCO3-. In the tissues, 3.7 ml of CO2 per dl of blood is added of which, 0.4 ml stays in solution, 0.8 ml forms carbamino compounds and 2.5 ml forms HCO3-. Owing to the lower partial pressure of PCO2 in the lung alveoli, the 3.7 ml of CO2 are discharged into the alveoli. In this fashion, 200 ml of CO2 per minute at rest and much larger amounts during exercise are transported from the tissues to the lungs for excretion 2. References: 1. West E.S, Todd W.R, Mason H.S, and Van Bruggen J.T. 1966.Textbook of biochemistry. 4th ed. MacMillan, New York. 2. William. F.Ganong. 1995.Review of Medical Physiology. 17th ed. Appleton and Lange. New York. 3. Bhagavan. N.V. 2001.Medical Biochemistry, 4th ed. Academic Press.USA. 4. http://www.utpb.edu/courses/jeldridge/Cardiopulm/Unit6_2a.htm Read More