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This article seeks to explain the equilibrium between oxygen and carbonic acid in blood and the effect that carbonic acid has on haemoglobin's affinity for oxygen, if any. Results from Lawrence, Henderson, Bohr, Hasselbalch, Christiansen, Douglas and Haldane have been presented in this article.
The mass law equation has been utilised to determine what effect carbonic acid has on the haemoglobins affinity for the uptake and binding of oxygen. It has been inferred that the acid does indeed affect the protein portion of the haemoglobin. There might be acid or base radicals present in the protein portion of the haemoglobin that might have an effect on the reaction.
It became clear throughout the course of this study that the values obtained fluctuated in such a way that was too inconsistent to account for the variation of bicarbonate with the varying hydrogen ion. The binding of the acid with haemoglobin is unaffected by the presence of oxygen. Further investigation into this is recommended.
All red blood cells contain around 280 million molecules of haemoglobin. Haemoglobin is an intricate protein composed of four polypeptide chains that all have the aptitude for binding oxygen. It is this ability that makes it essential in the process of oxygen transportation. These molecules possess this aptitude because each molecule contains a sole iron atom that has a weak affinity to oxygen molecules. This affinity is not a true bond but rather an agreement between the full shell of the oxygen and the partially filled shell of iron. Oxygen can also be transported when it is dissolved in the blood plasma. The quantity of oxygen that can be carried in both forms is predominately dependent on the partial pressure (PaO2) of which each form is exposed to. A number of homeostatic mechanisms are responsible for carefully regulating blood pH, HCO3, partial pressure of carbon dioxide (PaCO2) and partial pressure of oxygen (PaO2) in the body. These mechanisms then influence the respiratory and urinary systems so as to control the acid base balance. Blood must be controlled in such a way so it remains within the restricted pH range of 7.35 to 7.45, therefore it is slightly alkaline. As oxygen is not particularly soluble in water, therefore, transportation of oxygen via blood plasma is not the most efficient mode of oxygen transportation. Haemoglobin is a protein that reversibly binds oxygen and then unloads it at body tissues. When oxygen binds to haemoglobin, it becomes oxyhaemoglobin. However, when it unloads the oxygen it becomes deoxyhaemoglobin and picks up carbon dioxide and H+ . It is this substance that gives red blood cells their red hue. http://www.google.com.au/url?source=imgres&ct=img&q=http://www.sciencecases.org/tazswana/hemoglobin.gif&sa=X&ei=zg1uTfXgCoeyvwPB_IAz&ved=0CAQQ8wc&usg=AFQjCNFmeO3HbwWDJ3_aS2itupy8UiVK_w
The fact that deoxygenated blood has more of an affinity for carbon dioxide than oxyhaemoglobin is called the Haldane effect. This further confirms that the tissues are responsible for carbon dioxide pick up while oxygen pickup in the lungs is responsible for carbon dioxide release. This process is simultaneously occurring in reverse, thus completing the Haldane Effect.
There are 3 modes of carbon dioxide transport.
1) In blood plasma as dissolved carbon dioxide
Image reproduced from
Online Biology Book
taken from Life: The Science of Biology
http://www.elp.manchester.ac.uk/pub_projects/2001/MNQC7NDS/carbon_dioxide.htm2) Bound to haemoglobin and plasma proteins (carbaminohaemoglobin)
3) As bicarbonate ions
Around 5% of carbon dioxide is transported in its original state, just dissolved in the blood plasma. This is because carbon dioxide is far more soluble than oxygen. Carbohaemoglobin is the compound that is formed when carbon dioxide reversibly combines with haemoglobin. Carbon dioxide can also bind to amino groups on polypeptide chains of plasma proteins. About 10% of carbon dioxide is transported in these ways. However, a majority of carbon dioxide is transported in the form of bicarbonate ions. These are formed when carbon dioxide combines with water to form carbonic acid (H2CO3) in the red blood cells. This reaction is catalysed by carbonic anhydrase. Bicarbonate ions (HCO3-) and hydrogen ions (H+) are then formed when carbonic acid dissociates. This reaction can occur outside of the red blood cells but does not take place anywhere near as quickly as there is a lack of carbonic anhydrase.
As mentioned earlier, haemoglobin is anÂ iron-containing oxygen-transport metalloprotein that transports oxygen from the lungs to the tissues and carbon dioxide from the tissues to be expelled from the lungs. Haemoglobin functions most efficiently when it binds oxygen in the oxygen rich environment of the lungs, so as to release oxygen quickly in the comparatively oxygen poor atmosphere of the body tissues. The haemoglobin molecule is made up of four polypeptide chains. These consist of 2 alpha chains that each have 141 amino acids, and two beta chains, each with 146 amino acids. Each of these units is a long stretch of protein that is mainly coiled up into eightÂ alpha helices. In mammals, haemoglobin is located entirely within specialised cells called erythrocytes, which do not contain a nucleus or mitochondria. Having haemoglobin in these cells avoids problems that would be cause by the presence of the osmotically active material in plasma, and provides the haemoglobin with a stable and regulated environment, supplying it with all the appropriate ions and enzymes. Oxygen loading occurs in the lung and is distributed to the tissue cells. As oxygen deficient blood moves through the lungs, oxygen diffuses from the air sacs of the lungs into the blood and then into the erythrocytes, where it binds to haemoglobin. When oxygen binds to iron, the haemoglobin, now called oxyhaemoglobin
How does the internal environment of the red blood cell achieve equilibrium when negatively charged bicarbonate ion diffuses out of it?
This is justified because it entails additional equilibrium within the internal environment of red blood cells.
ANSWERSummary diagram of the biochemical reactions
As the bicarbonate exits the cell, pH will drop, therefore, causing oxygen to dissociate from haemoglobin. This means that equilibrium will once again be achieved due to the fact that the levels of oxygen will rise to compensate for the absence of carbon dioxide. There are many proteins in the blood, haemoglobin, plasma proteins and phosphate. (http://www.anaesthesiamcq.com/AcidBaseBook/ab2_2.php)
These act as buffers for pH as they have active sites on the proteins, or branches coming off the main chain that are able to dissociate both as either acids or bases. They do so by giving off or taking up H+ protons. Haemoglobin plays the greatest role in this.
Why do the dissociation curves of Barcroft's blood show such a dramatic increase in saturation of O2 at a certain point?
It displays the buffering effect that haemoglobin has upon acids.
This dissociation curve describes the relation between the partial pressure of oxygen (x axis) and the oxygen saturation (y axis).
As more molecules of oxygen attach, haemoglobin's affinity for oxygen climbs. So, when the pressure increases, more molecules will bind up until the point where the maximum amount that can bind has been reached. (http://www.ventworld.com/resources/oxydisso/dissoc.html)
As this limit is approached, not much more binding will occur due to the haemoglobin being saturated with oxygen. The curve is then comparatively flat at pressures above approximately 60mmHg. This indicates that the oxygen content will not change dramatically even lwith large increases in the partial pressure. In order to obtain more oxygen, the tissue would need blood transfusions or supplemental oxygen so as to increase the haemoglobin count.
What is so important about the chemistry of haemoglobin?
This concept should be explained so as to not confuse haemoglobin's properties with those properties of all other proteins.
In what way does haemoglobin act as a buffer against changes in blood pH?
As well as binding oxygen. Haemoglobin can also bind CO2 where it collects it from the tissues to be transported to the lungs. However, if haemoglobin was unable to perform this essential task, carbon dioxide would form carbonic acid, therefore, causing the blood to have a lower pH than that coming from the lungs. 
Haemoglobin can bind and extra protons let go by carbonic acid. Haemoglobin regulates pH by binding excess