Explain The Role Of Haemoglobin In Oxygen And Carbon.docx

Oxygen Transport We take in oxygen during inhalation and it reaches lungs through respiratory tract. Haemoglobin, an intracellular protein is the primary vehicle for transporting 97% of oxygen in the blood. 3% of Oxygen is carried by plasma. Haemoglobin is contained in erythrocytes.

Haemoglobin: The protein inside red blood cells (a) that carries oxygen to cells and carbon dioxide to the lungs is haemoglobin (b). Haemoglobin is made up of four symmetrical subunits and four heme groups. Iron associated with the heme binds oxygen. It is the iron in haemoglobin that gives blood its red colour.

Ox-haemoglobin is haemoglobin bound to oxygen, and it gives oxygen-rich blood a red color. On the other hand, deoxyhaemoglobin is haemoglobin without oxygen bound to it, and it gives oxygen-poor blood a bluish tint. The amount of oxygen bound to the haemoglobin at any time is related to the partial pressure of oxygen to which the haemoglobin is exposed. In the lungs, at the alveolar-capillary interface, the partial pressure of oxygen is high, and therefore the oxygen binds readily to haemoglobin. As the blood circulates to other body tissue in which the partial pressure of oxygen is less, the haemoglobin releases the oxygen into the tissue because the haemoglobin cannot maintain its full bound capacity of oxygen in the presence of lower oxygen partial pressures. Red blood cells in the blood are flattened disc like structures responsible of transporting oxygen and carbon dioxide gases. Red blood cells consist of red ironcontaining pigment called as haemoglobin. Haemoglobin is a respiratory pigment that carries oxygen through red blood cells. Oxygenated blood is carried to tissues. The exchange of gases at tissue level is called as peripheral gas exchange. The capillaries of circulatory system deliver the oxygen rich blood to the tissues of the body. This oxygen diffuses across the walls of the capillaries into tissues. In turn carbon dioxide diffuses into the blood from tissues.

Carbon-dioxide Transport Carbon dioxide molecules are transported in the blood from body tissues to the lungs by one of three methods: 1. Dissolution directly into the blood 2. Binding to haemoglobin 3. Carried as a bicarbonate ion Several properties of carbon dioxide in the blood affect its transport. First, carbon dioxide is more soluble in blood than is oxygen. About 5 to 7 percent of all carbon dioxide is dissolved in the plasma. Second, carbon dioxide can bind to plasma proteins or can enter red blood cells and bind to haemoglobin. This form transports about 10 percent of the carbon dioxide. When carbon dioxide binds to haemoglobin, a molecule called carbaminohaemoglobin is formed. Binding of carbon dioxide to haemoglobin is reversible. Therefore, when it reaches the lungs, the carbon dioxide can freely dissociate from the haemoglobin and be expelled from the body.

Third, the majority of carbon dioxide molecules (85 percent) are carried as part of the bicarbonate buffer system. In this system, carbon dioxide diffuses into the red blood cells. Carbonic anhydrase (CA) within the red blood cells quickly converts the carbon dioxide into carbonic acid (H2CO3). Carbonic acid is an unstable, intermediate molecule that immediately dissociates into bicarbonate ions (HCO3−) and hydrogen (H+) ions. Since carbon dioxide is quickly converted into bicarbonate ions, this reaction allows for the continued uptake of carbon dioxide into the blood, down its concentration gradient. It also results in the production of H+ ions. If too much H+ is produced, it can alter blood pH. However, haemoglobin binds to the free H+ ions, limiting shifts in pH.

The newly-synthesized bicarbonate ion is transported out of the red blood cell into the liquid component of the blood in exchange for a chloride ion (Cl-); this is called the chloride shift. When the blood reaches the lungs, the bicarbonate ion is transported back into the red blood cell in exchange for the chloride ion. The H+ ion dissociates from the haemoglobin and binds to the bicarbonate ion. This produces the carbonic acid intermediate, which is converted back into carbon dioxide through the enzymatic action of CA. The carbon dioxide produced is expelled through the lungs during exhalation. The equation can be seen as follows: CO2 + H2O <- -> HCO3+ H+

What is the oxygen dissociation curve?

The oxygen dissociation curve is a graph that plots the proportion of haemoglobin in its oxygen-laden saturated form on the vertical axis against the partial pressure of oxygen on the horizontal axis. The curve is a valuable aid in understanding how the blood carries and releases oxygen. At high partial pressures of oxygen, haemoglobin binds to oxygen to form oxyhaemoglobin. All of the red blood cells are in the form of oxyhaemoglobin when the blood is fully saturated with oxygen. At low partial pressures of oxygen (e.g. within tissues that are deprived of oxygen), oxyhaemoglobin releases the oxygen to form haemoglobin. The oxygen dissociation curve has a sigmoid shape because of the co-operative binding of oxygen to the 4 polypeptide chains.  Co-operative binding means that haemoglobin has a greater ability to bind oxygen after a subunit has already bound oxygen.  Haemoglobin is therefore most attracted to oxygen when 3 of the 4 polypeptide chains are bound to oxygen.

Which factors affect the oxygen dissociation curve? The oxygen dissociation curve can be shifted right or left by a variety of factors. A right shift indicates decreased oxygen affinity of haemoglobin allowing more oxygen to be available to the tissues. A left shift indicates increased oxygen affinity of haemoglobin allowing less oxygen to be available to the tissues.

pH: A decrease in the pH shifts the curve to the right, while an increase in pH shifts the curve to the left. This occurs because a higher hydrogen ion concentration causes an alteration in amino acid residues that stabilises deoxyhaemoglobin in a state (the T state) that has a lower affinity for oxygen. This rightwards shift is referred to as the Bohr Effect. Temperature: An increase in temperature shifts the curve to the right, whilst a decrease in temperature shifts the curve to the left. Increasing the temperature denatures the bond between oxygen and haemoglobin, which increases the amount of oxygen and haemoglobin and decreases the concentration of oxyhaemoglobin. Temperature does not have a dramatic effect but the effects are noticeable in cases of hypothermia and hyperthermia.

The effect of carbon dioxide on oxygen dissociation curve is known as Bohr Effect. It has been found that increase in concentration of carbon dioxide decreases the amount of oxyhaemoglobin formation. A decrease in carbon dioxide shifts the curve to the left, while an increase in carbon dioxide shifts the curve to the right. According to Bohr Effect, for any particular partial pressure of oxygen, the affinity of haemoglobin toward oxygen decreases and favours dissociation of oxyhaemoglobin when the partial pressure of carbon dioxide increases.

Bohr Effect is very important physiological phenomenon, because uptake of oxygen in lungs and its releases in the tissue is regulated by the concentration of carbon dioxide and H+ ion as well as the partial pressure of oxygen. So, this phenomenon made possible the cellular transport and release of oxygen.